mil std 120 change notice 1

8/11/2019 Mil Std 120 Change Notice 1

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MIL-STD-I20

12 December 1950

W/CHG. NOTICE #1

9 S E P T 1 9 6 3

MILITARY STANDARD

GAGE INSPECTION

UNITED STATES

GOVERNMENT PRINTING OFFICE

WASHINGTON : 1951

For sale by the Superintendent of Documents, U.S. Government Printing OfficeWashington 25, D. C. - Price $1.00

Downloaded from http://www.everyspec.com


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MIL-STD-12012 December 1950

MUNITIONS BOARD STANDARD AGENCY

DEPARTMENT OF DEFENSE

WASHINGTON, D. C.

12 D E C E M B E R 1950Gage InspectionMIL-STD-120

1. This sta nda rd ha s been a pproved by the Depart ments of the Army, th eNavy, and the Air Force for the purpose of establishing uniform practicesthroughout the Mili tary Services for the care and use of inspection gagesand special measuring devices used in connection with the dimensional controlof all products a nd equipment procured by the U . S. Armed Forces.

2. The Munitions Board Standards Agency approved this standard forprinting a nd inclusion in the MI L series of stan da rds on 12 December 1950.

3. In a ccordance with esta blished procedure, the Munitions B oard S ta nd-ards Agency has designated the Ordnance Corps, the Bureau of Ordnance,an d t he Air Force, respectively, as joint custodians of this sta nda rd.

4. When repeated deviations from this standard are required by a depart-ment or technica l service, a report sha ll be submitt ed to the Munitions Boa rdStandards Agency by the activity concerned explaining the reason for thedeviation.

II



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MlL-STD-12012 December 1950

Paragraph

4.6.24.7

4.7.1

4.7.2

4.7.34.7.44.84.8.14.8.24.8.34.8.44.8.54.8.65

5.1

5.1.1

5.1.1.1

5.1.1.25.1.25.1.35.1.45.1.55.1.65.2

5.2.1

5.2.1.15.2.1.2

5.2.1.3

5.2.1.4

5.2.1.5

5.2.1.6

5.2.25.2.35.2.4

5.2.55.2.65.2.6.1

5.2.6.25.2.75.2.85.2.95.2.10

IV

Elect ric compa ra tors ---------------Optical instruments and com-

pa r a t or s - -- -- -- -- -- -- -- -- -Opt ica l fla t s--------------------------Optical projection machines----Tool ma ker s microscope-----Opt ica l div iding hea d--------------Auxili a ry eq ui pm en t --------------Sine ba rs a nd pla t es---------------Dia l indica tors-----------------------Surfa ce la ces -------------------------Tool ma kers fla t --------------------Ha rd ened st eel sq ua re------------Pipe thread checking block------TYP E S OF G AG E ---------------G en er a l -----------------------S cope-------------------------------St a nda rd ga ge bla nks -------------

Standard screw thread gages---G o ga ges-----------------------------N ot G o ga ges----------------------Accepta nce check ga ges -----------Wear lim it check ga ges -----------Ma st er set t in g g a ges --------------P lug ga ges-----------------------------Plain cylindrical plug gages-----S t a nda rd desi gn s-------------------Cylindrical plug gage, single-

en d, solid ----------------------------Cylindrical plug gage, single-

end, progressive------------------

Cylindrical plug ga ge, double-e n d -------------------------Cylindrical plug ga ge, repla ce-

a ble--------------------------------Cylindrical plug gage, revers-

ible-----------------------------P la in t a per plug ga ges-----------Threa d plug ga ges -----------------Ta p e r t h r e a d e d p i p e p l u g

ga ges----------------------------Taper plain pipe plug gages-----S pl in e plu g ga ges ------------------Involute spline plug gages------

Straight-sided spline plug gage-Alin em en t plu g ga ges-------------G ra du a t ed plug ga ges------------Fla t plug ga ges----------------------Miscellaneous plug gages---------

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Paragraph5.3

5.3.1

5.3.25.3.35.3.4

5.3.55.3.65.3.75.3.85.4

5.4.1

5.4.25.5

5.5.1

5.5.25.5.3

5.5.4

5.5.4.15.5.4.25.5.4.3

5.5.4.4

5.6

5.6.1

5.6.25.6.35.6.45.6.5

5.6.65.6.75.7

5.7.1

5.7.25.7.35.8

5.8.1

5.8.25.8.35.8.46

6.16.26.2.16.2.26.2.3

Rin g ga ges-----------------------------P la in r in g ga ges---------------------Twi n rin g ga ges ---------------------P rog res siv e ri ng ga ges ------------Taper r in g ga ges --------------------Threa d ri ng ga ges ------------------Taper threaded pipe ring gages-Taper plain pipe ring gages----S pl in e ri ng ga ges -------------------Triroll ga ges ---------------------------Threaded pipe triroll gages-----Taper plain pipe triroll gages----Sna p ga ges----------------------------Sna p ga ge const ru ct ion------------Adjustable length gages----------Combinat ion r ing and snap

ga ges-----------------------------Thre a d sn a p ga ges-----------------

Roll t hr ea d sna p ga ges ------------Flat-anvil thread snap gages--S i n g l e - p o i n t t h r e a d s n a p

ga ges ----------------------------Segmen t ro l l t h r ead snap

g a g es -------------------------Miscel laneous and specia l

ga ges ----------------------------Flush pin ga ges----------------------S pa nner ga ges -----------------------Fixture ga ges -------------------------Concen t ri cit y g a ges----------------Template or profile gages-------

Funct ion a l ga ges--------------------Cha mber ga ges----------------------Air ga ges -------------------------------P res su re t ype a ir ga ge----------Flow type a ir ga ge------------------Air sna p ga ge-----------------------B ore in spect ion dev ices-----------Sta r ga zes -----------------------------D ia l ind ica tor bore ga ges-------H or os cop es -------------------------Mirr or a nd la mp devices-------C A R E , U S E , A N D M A I N -

TENANCE -------------------------

G en er a l----------------------------S ur veill a nce of ga ge s--------------Id ent ifica t ion ma rkin g-----------Loa n of gages t o contra ctors---C le a ning---------------------------

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Paragraph

6.2.46.2.5

6.2.5.16.2.5.2

6.2.5.36.2.6

6.2.6.16.2.6.26.2.6.36.2.6.46.36.3.16.3.2

6.3.3

6.3.4

6.3.4.16.3.4.26.3.4.36.3.4.46.3.4.56.3.4.66.3.4.7

6.3.4.8

6.3.4.9

6.3.4.106.3.4.116.3.4.126.3.4.136.3.4.146.3.4.15

6.3.4.16

6.3.4.17

7

7.17.1.17.1.27.1.3

P reser va t ion pr ocedu re -----------Maintenance and control pro-

ced u r es -------------------------G a ge records--------------------------Time limit checking system------

Use of checker master sett ingg a g es -------------------------Damage control and correc-

t i on ------------------------Removal of nicks and burrs----Control of w ea r-----------------------Seiz ur e and ga lling -----------------Ins pection of dropped ga ges---Applica t ion procedures------------Forcin g of ga ges ---------------------M e t h o d o f a p p l y i n g s n a p

g a g es -----------------------Me thod o f app ly ing sma l l

plug ga ges--------------------------Methods of applying thread

g a g es -------------------------Function of thread gages---------G o th rea d plu g ga ges ----------Not Go thread plug gages------G o th rea d ri ng g a ges ------------Not Go thread ring gages---Roll thread snap gages------Standard taper p ipe thread

g a g e s - - - -- - - -- - - -- - - -- - --A e r o n a u t i c a l t y p e t a p e r

threaded pipe ANPT gages---

L 1 and L 3 taper threaded pipeplug ga ges -------------------------

Taper plain pipe plug gage----Threaded pipe triroll gage------Ta per pla in tr iroll g a ges-------Threaded pipe ring gages--------Ta per pla in ri ng ga ges-----------Special threaded pipe gaging

con dit ions--------------------------Check or master set t ing thread

plug ga ges--------------------------C h e c k or s e t t i n g t a p e r

threaded pipe plug gages-----

S U R FA C E R O U G H N E S SME ASU REME NT---------------

G e n e r a l - - - -- -- - -- -- - -- -- -D ef in it io n ---------------------S t a n d a r d i z a t i o n - ---------Appli ca t ion t o ga ges---------------

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Paragraph

7.2

7.2.17.2.27.2.2.17.2.2.27.2.2.37.2.2.47.2.2.5

7.2.2.67.2.2.77.2.37.2.3.17.2.3.27.2.3.37.2.3.47.2.3.5

7.2.3.6

7.2.3.77.2.3.87.2.3.9

7.2.4

8

8.18.1.1

8.1.2

8.1.38.28.2.1

8.2.28.2.38.2.48.2.5

8.3

8.3.1

8.3.28.3.38.3.48.3.5

8.3.68.3.6.18.3.6.2

Surface roughness measure-m e n t -----------------------

Tracer type instruments--------P rofilom et er -----------------------Tra cer unit ----------------------------Microin ch met er ---------------------Mot ot ra ce----------------------------G la ss rough ness specimen------Interpretation of meter read-

ings--------------------------------U sin g t he p rof ilometer ------------P rofilomet er rota ry pilotor-----B ru sh su rfa ce a na lyz er-----------P ick-up a rm---------------------------Surfa ce pla te--------------------------Drive hea d-----------------------------Am pl ifi er ---------------------------Direct inking oscillograph--------

Interpreting the oscil lographc h a r t - - -- - - -- - - -- - -- - - -- -

Root mean square meter-------Glass calibration standard-------Using the brush surface ana-

l y z e r - - - - - - - - - - - - - - - - -Surface roughness comparator

blocks-------------------------G A G E I N S P E C T I O N

ME THOD S ---------------------G en er a l-----------------------------P u r pos e--------------------------S cope--------------------------------

Ar r a n gem en t --------------------La bora tory cond it ions------------Laboratory temperatures--------Atmospheric conditions-----------Clea nlines s--------------------------Lig h t ing-----------------------------Location of precision instru-

m er it s --------------------------Genera l inspect ion ins t ruc-

t ion s ----------------------------D r a w in g s -----------------------M a r k in g ------------------------G a ge records ca rds ------------------

P reca ut ions------------------------Selecting the inspection m eth-

od -----------------------------Steps in the inspection process--C le a n in g ------------------------H a rd nes s mea sur emen t ----------

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Paragraph

8.3.6.38.3.6.48.3.6.5

8.3.6.6

8.3.6.78.3.6.88.3.78.3.7.18.3.7.28.4

8.4.18.4.28.4.2.18.4.2.28.4.2.3

8.4.38.4.3.18.4.3.28.4.3.38.4.3.4

8.4.48.4.58.4.5.18.4.5.2

8.4.5.38.4.5.4

8.4.5.58.4.5.68.4.68.4.78.4.88.4.98.5

8.5.18.5.28.5.38.5.4

8.5.58.5.5.18.5.5.28.5.68.5.6.18.5.6.2

VI

D em a gnet iz in g---------------------Visua l impa ct ion --------------------Checking loosely toleranced di-

men sion s-------------------------Checking for function-------------Checking closely tolerance di-

men sions-------------------------S ea li ng-----------------------------Finis hes------------------------------Req uir em en t s--------------------Inspect ion ----------------------------Care and use of inspect ion

t ools--------------------------------G en er a l----------------------------Microme ter ca liper s---------------Outside micrometers--------------Inside micrometers----------------Micrometer depth gages----------

Vernie r g a ges ------------------------Ver nier ca lipers---------------------Ver ni er d ept h ga ges--------------Verni er he igh t ga ges -------------Vernier height gage with test

in dica t or -------------------------Dia l in di ca t or ga ges --------------Surface plate and accessories--Surfa ce pla te-------------------------Angle iron and tool maker s

knee ------------------------------P a ra ll el s----------------------------P reci sion s t ra igh t edge-----------

Universal precision square----P la ner g a ge---------------------------Roll holder ----------------------------H old -dow n f ix t ur e------------------C ha nnel-ba r fix t ur e---------------Loca t in g pla t e-----------------------C a r e a n d u s e o f p r e c i s i o n

measuring instruments-------G en er a l----------------------------G a ge blocks a nd a ccessories---Sine bars and sine plates -----Light wave measuring equip-

men t ------------------------------

Opt ica l in st ru men t s---------------C on tour pr ojector ------------------Tool ma kers m icr oscop e---------Direct mea suring equipment--S uper micr om et er -----------------Universal measuring machine-

Page

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7878

7879797979

797979798080

81818181

82848585

858687

878787888889

89899092

94979799

100100102

Paragraph

8.5.7

8.5.7.18.5.7.28.5.7.3

8.5.7.48.5.7.58.5.7.68.5.7.78.5.88.5.8.18.5.8.2

8.5.98.6

8.6.1

8.6.28.6.3

8.78.7.1

8.7.2

8.7.3

8.7.4

8.7.5

8.8

8.98.9.18.9.1.18.9.1.28.9.1.38.9.1.48.9.1.58.9.1.68.9.28.9.2.1

8.9.2.2

Mechanical and electrical com-pa ra t or s-------------------------

Metron ga ges-------------------------E lect olimit ga ges -------------------Vis ua l ga ges --------------------------Supersensitive comparator-----Dia l compa ra tor---------------------D ia l bore ga ge---------------------U sing compa ra t ors -----------------Indexing equipment--------------P recis ion ind exin g hea d--------Bench cen te r w i th s ine ba r

in de xin g fa ce pla t e-------------H a rdnes s t es t er ---------------------Gage inspection of plain plug

ga ges ---------------------------Measu remen t w i th a bench

microm et er -----------------------

Measurement wi th a mechan-ica l compa ra tor ------------------Measurement w ith a precision

mea su rin g ma chin e-----------Inspection of tapered plugs---Measur ing s t ra ightness and

a ngle of t a per--------------------Checking ga ges when front fa ce

is n ot squa re----------------------Checking diameters of taper

plugs w it h r oll s------------------Measuring the diameters of

tapered plugs with roll and

sin e ba r-----------------------------C h e c k i n g t h e d i a m e t e r o f

tapered p lugs wi th masterr ings-----------------------------

Alinement and concentricitypl u g s --------------------------

Rin g a nd receiver ga ges---------P la in r in g ga ges---------------------Check pl ugs--------------------------Int er na l ca lipers--------------------Dia l bor e g a ges----------------------Ca st s--------------------------------In ternal measur ing ma chine

G a ge blocks---------------------------Receiver ga ges ----------------------Check plug a nd blueing (an gle

ch ec k )- - - -- - - -- - - -- - - -- - - -Pencil lines and check plug

(a ngle check)----------------------

Page

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107107

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Paragraph

8.9.2.3

8.9.2.4

8.9.2.5

8.9.2.6

8.108.10.18.10.28.10.38.11

8.11.1

8.11.2

8.11.3

8.11.4

8.11.58.11.68.11.78.11.88.11.98.12

8.12.18.12.28.12.38.12.4

8.12.58.13

8.13.18.13.28.13.38.13.48.13.58.14

8.15

8.16

8.16.1

Sine bar and angle plate (anglech eck )--------------------------

Precision balls (angle and di-a met er ch eck)-------------------

Ta p e r e d c h e c k plugs (d i -

a met er ch eck )------------------Calibrated tapered plugs (di-a met er ch eck )------------------

In spect ion of sn a p ga ges -------Plain and built-up snap gages---Adjus t a ble s na p ga ges -----------Large snap gages (all types)----Threa d plug ga ges----------------L ea d -----------------------------Check ing ang le w i th a t e s t

t o o l - - - - -- - - - -- - - - -- - - - -Checking angle with an optical

com pa r a t or -----------------

Checking angle with a toolma kers micr oscope-----------Roo t -------------------------------P it ch dia met er ---------------------Ma jor dia met er -------------------C oncen t r ici t y----------------------F in ish ----------------------------I n s p e c t i o n o f t h r e a d r i n g

g a g es -- -- - - -- - -- - - -- - - -- - -Thr ea d for m------------------------Lea d--------------------------------P it ch dia met er ---------------------C o n c e n t r i c i t y o f p i t c h a n d

min or d ia met er s---------------Min or dia met er -------------------Inspection of tapered thread

pl ug g a ges -----------------------Angle a nd r el ie f-------------------L ea d-------------------------------P itch d ia met er a nd t a per-------Ma jor dia met er -------------------C on volu t ion ----------------------Inspection of tapered thread

r in g ga ges -----------------------I n s p e c t i o n o f t h r e a d s n a p

g a g e s -----------------------

Inspection of qualifying threadg a g es ---------------------Check ing qua l i fy ing th read

gages with a contour projec-t or ---------------------------

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Paragraph

8.16.2

8.17

8.18

8.19

8.208.20.18.20.2

8.20.3

8.20.48.20.5

8.20.6

8.20.78.218.21.1

8.21.1.1

8.21.1.2

8.21.1.3

8.21.1.4

8.21.2

8.21.38.21.3.1

8.21.3.2

8.21.3.3

8.21.3.4

8.22

8.22.1

8.22.28.23

8.23.18.23.2

Checking qualifying threadswit h a th rea d t emplat e------

Inspection of segmental threadg a g e s - - -- - -- - -- - -- - -- - -

Inspect ion of Acme thread

g a g es -----------------------Inspection of modified squaret hrea d ga ges --------------------

Inspection of flush pin gages---G en er a l---------------------Depth measur ing f lush p in

g a g e------------------------Length measuring flush pin

g a g es ------------------------P ost ty pe flush pin g a ges-------Flush pin ga ges for locat ion of

h ol es -----------------------Ta pered f lus h pi n g a ges ---------

G ooseneck flush pin g a ges-----Inspection of profile gages------Template gages for profile of

r a d iu s ----------------------Checking radii where great ac-

curacy is not required-----------Checking radii where accuracy

is r eq uir ed -----------------------C h e c k i n g r a d i i w i t h g a g e

blocks, tool ma ker s but tonsa nd pa ra ll els--------------------

Checking template gages w ithspecial indicator fixture-----

Template gages for profile ofa n g l e -----------------------P rofi le pl ug ga ges -----------------Measur ing p lug wi th p l a in

r a d iu s ----------------------Measuring profile ogive check

plugs with single radius---------Measu ring profile ogive check

plugs with double radius-----Measuring plugs with convex

a nd conca ve r a dii --------------Inspection of fixture gages------Flush pin type fixture gages----

Indicator type fixture gages----Inspection of miscellaneousg a g es -- - -- - - -- - -- - - -- - - --

Dia l ind ica tor ga ges--------------Special indicator snap gages---

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Paragraph

8.23.38.23.48.23.58.23.68.23.78.23.88.23.9

8.248.24.18.24.28.24.38.24.48.24.58.24.68.24.78.24.88.24.98.25

S pa nner ga ges ----------------------Dovet a il ga ges ----------------------S pl in e ga ges -------------------------S plin e r in g ga ges ------------------Ta per ed sp lin e g a ges-------------La rge ga ges --------------------------Ca ms------------------------------------

Checking gages for bear---------P la in plug g a ges --------------------P la in r in g ga ges --------------------Threa d pl ug ga ges -----------------Threa d r in g ga ges -----------------Thread concentricity gages-----Sna p ga ges----------------------------Indica t or ga ges ---------------------F lu sh pin ga ges---------------------Fix t ure ga ges ------------------------Cal ibra t ing thread measur ing

w ir es---------------------------

Figure 1.Figure 2.Figure 3.Figure 4.Figure 5.Figure 6.Figure 7.Figure 8.Figure 9.Figure 10.Figure 11.

Figure 12.Figure 13.Figure 14.Figure 15.Figure 16.Figure 17.Figure 18.Figure 19.Figure 20.Figure 21.Figure 22.Figure 23.Figure 24.

Figure 25.Figure 26.Figure 27.

Page

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187187188188188189189189189189

190

Paragraph

8.268.26.1

8.26.28.26.3

8.27

8.27.18.27.2

8.27.3

8.27.4

8.27.5

8.27.6

LIST OF FIGURES

Unilateral tolerances.Bilateral tolerances.Unequal bilateral tolerances.Plug gage tolerance.Built-up snap gage tolerances.Ring gage tolerances.Steel rule.Hook rule.Steel rule with holder.Rule depth gage.Adjustable steel square.

Combination set .Precision gage block set.Gear and thread measuring wires.Cylindrical square.Vernier caliper.Vernier scale.Vernier depth gage.Vernier height gage.Gear tooth vernier caliper.Vernier bevel protractor.Vernier on bevel protractor.Micrometer caliper.Micrometer barrel and thimble scales.

Vernier micrometer scales.Examples of micrometer vernier readings.Screw thread micrometer caliper.

Ca libr a t in g ga ge blocks---------Sugges t ions in re inspect ing

ga ge blocks-------------------------A pr ecisi on ga ge b locks ------B precision gage blocks or

us ed ga ge blocks ---------------Helix a ngles--------------------------

Tables for American nationalcoarse and fine thread series-Chart for determining helix

a ngles----------------------------Thread flank angle correction

f a ct or s --------------------Tables for thread lead check-

ing---------------------------------Tables of equivalent hardness

n um ber s -----------------------Key to table of equivalent

ha rd ness nu mb ers ------------

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Figure 28.Figure 29.Figure 30.Figure 31.Figure 32.Figure 33.Figure 34.Figure 35.Figure 36.Figure 37.Figure 38.Figure 39.Figure 40.Figure 41.Figure 42.Figure 43.Figure 44.Figure 45.Figure 46.Figure 47.Figure 48.Figure 49.Figure 50.Figure 51.Figure 52.Figure 53.Figure 54.Figure 55.Figure 56.Figure 57.Figure 58.Figure 59.Figure 60.Figure 61.Figure 62.Figure 63.Figure 64.Figure 65.Figure 66.Figure 67.Figure 68.Figure 69.Figure 70.Figure 71.Figure 72.Figure 73.Figure 74.Figure 75.Figure 76.Figure 77.

Inside micrometer (caliper-type).Inside micrometer (rod-type).Micrometer depth gage.Supermicrometer.Universal measuring machine.Electrolimit comparator.Visual gage.Dial comparator.Optical flats.Optical comparator.Contour measuring projector.Tool makers microscope.Optical dividing head.Sine bar.Sine bar set upon gage blocks.Sine plate.Sine plate with base plate.Dia l indicat or (gear t ra in type).Dial test indicator.Surface plate.Tool makers flat.Hardened steel square.P ipe threa d ga ge checking block.Cylindrical plug gage, single-end solid.Cylindrical plug gage, single-end progressive.Cylindrical plug gage, double-end.Cylindrical plug gage, replaceable.Cylindrical plug gages, reversible.Plain taper plug gages.Thread plug gages.Taper threaded pipe plug gages.Taper plain pipe plug gages.Involute spline plug gage.Straight-sided spline plug gage.Alinement plug gages.Graduated plug gage.Flat plug gage.Miscellaneous plug gages.Plain r ing gages.Twin ring gages.Progressive ring gages.Thread ring gages.Taper threaded pipe ring gage.Taper plain pipe ring gage.Spline ring gages.Threaded pipe triroll gage.Ta per plain pipe triroll ga ge.Adjustable snap gages.Adjustable length gage.Combination ring and snap gage.

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MIL-STD-12012 December

Figure 78.Figure 79.Figure 80.Figure 81.Figure 82.Figure 83.Figure 84.Figure 85.Figure 86.Figure 87.Figure 88.Figure 89.Figure 90.Figure 91.Figure 92.Figure 93.Figure 94.

1950

Roll thread snap gage.Flat-anvil thread snap gage.Single-point thread snap gage.Segment roll thread snap gage.Flush pin gage.Spanner ga ge.Fixture gage.Concentricity ga ge.Templat e or profile gage.Functional gage.Chamber gage .Air gage.S t a r gage.Dia l indicat or bore gage.Proper method of insert ing Go or Not Go thread plug gage.Improper method of insert ing Go or Not Go thread plug gage.Aline bolt w ith G o rolls and lower int o gage. Black line on screw shows orientation

of gage.

Figure 95. Not Go rolls stop bolt when i t passes through Go rolls .Figure 96. Bolt is turn ed turn at Not G o rolls to check for out-of-roundness. Note t he new position of the black line on bolt.

Figure 97. Raise bolt back through Go rolls and check further for out-of-roundness.Figure 98. Assembled th readed pipe and coupling at ha ndtight engagement , showing dimensions

L 1, L 2, and L 3.Figure 99. L 1 gage screwed into product. The gages basic notch has stopped at fittings face.Figure 100. Pipe fi t t ing shown on figure 99 is next checked with L 3 gage, and here s tops wi th

basic notch with in turn (thread ) from the product face.Figure 101. Taper plain plug gage is used next on product shown on figures 99 and 100. Here

it has stopped with basic notch at face of product. Fit t ing is acceptable.Figure 102.Figure 103.

Figure 104.


Figure 116.Figure 117.Figure 118.Figure 119.Figure 120.Figure 121.

X

Excessive shake or play of taper plain pipe plug gage is cause for rejection of product.Two fittings cross-sectioned for inspection, Center object is proof cast.

Fit t ing screwed into threa ded pipe tr iroll gage. Small end of f i t t ing is f lush withbasic step.

Taper plain triroll gage with fitting inserted. Flush pin shows fi t t ing t o be basic.Sett ing plug gage having full a nd trun cated thr eads on ea ch member.Ta per pipe threa d sett ing plug gage.Threa ded pipe triroll gag e set correctly.Ta per plain sett ing pipe thr eaded plug ga ge.Taper plain triroll gage.P rofilometer w ith mototra ce.Tracer unit with interchangeable skid mounts.Microinch meter a nd sca le-selector sw itch.G lass roughness specimen.P rofilometer r otary pilotor.

Brush surface analyzer.Pick-up arm.Amplifier.Direct inking oscillograph.Root mean square meter.Glass ca l ibra t ion s tandard .



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Figure 122.Figure 123.Figure 124.Figure 125.Figur e 126.Figure 127.Figure 128.Figure 129.Figure 130.Figure 131.Figure 132.Figure 133.Figure 134.Figure 135.Figure 136.Figure 137.Figure 138.Figure 139.Figure 140.Figure 141.Figure 142.Figure 143.Figure 144.Figure 145.Figure 146.Figure 147.Figure 148.Figure 149.Figure 150.Figure 151.Figure 152.

Figure 153.



Surface roughness comparator blocks.Adjusting micrometer depth gage.Use of vernier height gage.Use of vernier height ga ge with t est indicat or.Er ror in test indicator.Method of placing hea vy object on a surface plate.Tool ma ker s kneeuniversal a ngle iron.Ch ecking tool maker s knee for squa reness.Checking sq ua reness by mea ns of tool ma ker s butt ons.Un iversa l precision sq ua re.Pla ner gage.Reversible roll holder.Conventional roll holder.Hold-down fixtures.Channel bar f ixture.Locat ing plat e.Checking profile gage on locating plate.Sna p gage ma de of gage blocks.Sine plate with centers.Use of a sine plate.Measurement by mean s of l ight w a ves.Computing error in f latness.Interference pattern.pleasuring height with an optical f lat .02" l ight w ave micrometer w ith roll anvil , etc.Aligning fixture for projector.P ositioning fixtur e for tool ma ker s microscope.Setting up bench micrometer for measurement over rolls.Metron compar a tor.Electrolimit compa ra tor.Electrolimit height gage.

Pressure regulator for use with visual gage thread checking attachment (cover re-moved to show construction).Universal external visual gage.Supersensit ive comparator.Dial comparator.Dia l bore gage.Precision indexing head.B ench center w ith sine bar indexing face plat e.Rockwell hardness tester.Measuring small plug gages.Measuring large plug gages.Checking front face of plug for perpendicularity.Checking taper on sine plate.

Measuring taper on special sine plate fixture.Measuring taper and diameter of tapered plug.Set-up for measuring taper.Tapered plug-drawing specifications.Computing tapered plug diameter from measurements over rolls.Finding location of dat um diam eter.

XI



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Figure 171.Figure 172.Figure 173.Figure 174.Figure 175.Figure 176.Figure 177.Figure 178.Figure 179.Figure 180.Figure 181.Figure 182.Figure 183.Figure 184.Figure 185.Figure 186.Figure 187.Figure 188.

Figure 189.Figure 190.Figure 191.Figure 192.Figure 193.Figure 194.Figure 195.Figure 196.Figure 197.Figure 198.Figure 199.Figure 200.Figure 201.

Figure 202.Figure 203.

Checking diameter with roll and sine bar.Method of clamping ga ge over sine bar.Roll for checking ta per plug.Lengt h of block for odd size roll.Checking tapered plug with check rings.Computing diam eter of ta pered plug.Computing location of intersection (small taper over large taper).Computing location of intersection (large taper over small taper).Computing dista nces from ba ck face to intersections of ta pers.Special f ixture for checking ta per and diam eter of ta pered plugs.Checking taper with special fixture.Checking length to datum diameter with special f ixture.Checking tapered ring on sine bar.Set-up for mea suring the a ngle and t he diameter of an int ernal conical surface.Computing internal angle from measurements over two balls .Computing diamet er at large end of ta per from measurement over ball .Measuring length to datum diameter by means of calibrated plug.Checking parallel ism of anvils on large snap gages.

Checking th read a ngle by means of a test t ool.Test tools and correction spacers for checking thread angle.Relationship between a xial plane a nd pla ne of projection.Computation of helix angle.Correction factor for t hread an gle.Common visual errors found in thread gages.Root inspection of th read ga ges.Three-wire method of measuring pitch diameter of thread plug gages.Pit ch line.Best size wire.Height of wire above vertex.Height of pitch line above vertex.Method of making cast of threa d ring ga ge.

Making lead cast of thread ring.American gage design standard thread ring locking device. 1, Locking screw; 2,Sleeve; 3, Adjusting screw; 4, Body; 5, Adjusting slots; 6, Adjusting slot terminal hole; 7,Locking slot.

Figure 204. Measurement of pitch diameter of taper thread gages by the two-wire method.Figure 205.Figure 206.Figure 207.Figur e 208.Figure 209.Figure 210.Figure 211.Figure 212.


Locating height of fixed measuring wire.Computing pitch diameter a nd ta per of ta per thread plug.Specifica tion- -14 N. P . T. thr ead plug ga ge.Set-up for measuring pitch diameter with taper fixture.Calibrating taper thread fixture.Taper thread checking fixture.Hold-down wire for taper thread checking fixture.Measuring major diameter over rolls.

Measuring ma jor diameter pipe threa ds by roll and gage block method.Thread snap gages.Set t ing thread snap gage .Set -up for checking qua lified th rea ds on a projector.Measuring length to reference surface.Thread checking template.

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Figure 219.Figure 220.Figur e 221.Figure 222.Figure 223.Figure 224.Figure 225.Figure 226.Figure 227.Figure 228.Figure 229.Figure 230.Figure 231.Figure 232.Figure 233.Figure 234.Figure 235.Figure 236.

Figure 237.Figur e 238,Figure 239.Figure 240.Figure 241.Figure 242.Figur e 243.Figur e 244.Figure 245.Figur e 246.Figur e 247.Figur e 248.Figure 249.

Figur e 250.Figure 251.Figure 252.Figure 253.Figure 254.Figur e 255.Figure 256.Figure 257.Figure 258.Figure 259.Figure 260.Figure 261.Figure 262.

Figur e 263.Figure 264.Figure 265.Figure 266.Figure 267.Figure 268.

Special angle iron for supporting thread template.Set-up for checking qualifying thread gage (top view).Measuring major diameter of segmental thread gage.Set-up for checking depth measur ing flush pin ga ge.Set-up for checking length measuring flush pin gagepin method.

Checking length measuring flush pin gagepin method.Set-up for checking length measuring flush pin gagemicrometer method.Checking length measuring flush pin gagemicrometer method.P ost type flush pin gage.Conditions commonly found on t a pered pins.Flush pin for measuring location of holes.Set-up for checking small conical flush pin gage.Computing length t o apex on t apered flush pin gage.Measuring radius with projector.Set-up for checking ra dius w ith indicat or.Checking ra dius with fixed butt on and ga ge blocks.Checking u niformity of radius.Checking inside radius (with no precision reference surface).

Checking inside radius (where radius is tangent to straights surface).Checking inside radius (where radius is not tangent to straight surface).Computing dista nce from origin to stra ight surfa ce.Computing vertical distance from origin to center of roll.Computing dista nce from para llel to roll.Setting template gage in position.Checking radius on template gage.Checking outside radius (where radius is tangent to straight surface).Checking outside radius (wh ere radius is not ta ngent t o straight surfa ce).Set-up for checking a large radius with a special indicator fixture.Checking a large radius with a special indicator fixture.Set-up for checking a small radius with a special indicator fixture.Checking a small radius with a special indicator fixture.

Checking t emplate ga ge for location of intersection.Measuring distance to apex on template gage.Checking t emplate ga ge for a ccura cy of an gles.Checking outside radius (where there is no precision reference surface).Measuring profile ogive check gage.Measuring profile check plug (single radius).Measuring second radius of profile check plug.Computing point of ta ngency of double rad ius.Computing variable angle in double radius measurements.Computing position of roll in double radius measurement.Computing measurement over rolls in double radius measurement,Measuring outside radius of profile plug with outside and inside radii.Measuring inside radius of profile plug with outside and inside radii.Checking flush pins and steps on fixture gage.Checking angle of dovetail ga ge.Checking female dovetail gage.Checking male dovetail gage.Measuring w idth of tapered doveta il on a n indexing plat e.Computing relationship of angles of tapered dovetail gage.

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Figure 269.Figure 270.Figure 271.Figure 272.Figure 273.

Figure 274.Figure 275.Figure 276.Figure 277.Figure 278.Figure 279.Figure 280.Figure 281.

Checking t a per doveta il on sine plat e.Measuring distance between apexes of tapered dovetail-ball method.Measuring an gles, of sides of ta pered dovetail-ball met hod.Computing distance between apexesball method of measurement.Checking spline ring gage.

Checking large locating ring gage.Checking a ca m.Set -up for checking pa ra llelism of gage blocks.Horizontal check for gage block parallelism.Vertical check for ga ge block par a llelism.Rough check of difference in length of two gage blocks.Check of difference in length of two gage blocks.Cha rt for determining helix angles of screw t hrea ds.

P a g e 3 Ta b l e IPage 194 Table IIPage 194 Table IIIPage 195 Table IVPage 195 Table VP a ge 197 Ta ble VIPage 197 Table VII

XIV

LIST OF TABLES

Tolerances for plain cylindrical plug and ring gages.American national coarse thread seriesAmerican national fine thread series60 degree included threa d a ngle29 degree included thread angleThread lead checkingEquivalent hardness numbers



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MIL-STD-120CHANGE NOTICE 19 SEPTEMBER 1963

MILITARY STANDARD

GAGE INSPECTIONTO ALL ACTIVITIES :

1 . THE FOLLOWING PAGES OF MIL-STD-120 HAVE BEENSEDE THE PAGES LISTED:

S U P E R S E D E DNE W P AG E S D ATE P AG E

89 9 SEPTEMBER 1963 8990 " " " 90

169 " " " 169

REVISED AND SUPER-

DATE

12 DECEMBER 1950 " " "

" " "

2. RETAIN THIS NOTICE AND INSERT THE TABLE OF CONTENTS.



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1

PURPOSE AND SCOPE1.1 GENERAL

1.1.1 Scope. This standard is to providecorrelated technical information applicable tothe inspection of gages, special tools, andmeasuring devices. The principal subjects cov-ered are nomenclature, tolerances and fits,measuring tools and equipment, gages, andmethods of measurement and inspection. De-tails of gage design are not included.

1.1.2 Purpose. The purpose of gaging isto determine compliance of the componentparts of a mechanical product with the dimen-sional requirements of contracts, drawings, andspecifications, whether such product is madeby a single manufacturer or whether parts tobe assembled are made by several manufac-turers variously located. Gaging thereby as-sures the proper functioning and interchange-ability of parts, that is, one part will fit inthe same place as an-y similar part and performthe same function, whether the part is for theoriginal assembly or replacement in service.

1.1.3 Classification of gages and measur-

ing equipment. Gages and measuring equip-ment are classified as follows:1.1.3.1 Length standards. Standards of

length and angle from which all measurementsof gages are derived consist of precision gageblocks, end measuring rods, line-graduatedstandards, master disks, calibrated wires androlls, precision squares , graduated circles, andsimilar items.

1.1.3.2 Master gages. Master gages aremade to their basic dimensions as accuratelyas possible and are used for reference, such asfor checking or setting inspection or manufac-

turers gages.1.1.3.3 Inspection gages. Inspection gagesare used by the representative of the purchas-ing agency to inspect products for acceptance.

These gages are made in accordance with estab-lished design requirements. Inasmuch as in-spection gages are subjected to continuous use, agage makers tolerance will always be appliedand a wear allowance, where applicable, maybe included in the design of these gages. Tol-erances of inspection gages are prescribed byspecified drawing limits.

1.1.3.4 Manufacturers gages. Manufac-turers gages are used by the manufacturer orcontractor for inspection of parts during pro-

duction. In order that the product will bewithin the limits of the inspection gages, man-ufacturers or working gages should havedimensional limits, resulting from gage toler-ances and wear allowances, slightly fartherfrom the specified limits of the parts inspected.

1.1.3.5 Nonprecision measuring equip-ment. Nonprecision measuring equipment aresimple tools such as rules and plain protractorsused to measure by means of line graduationssuch as those on a scale.

1.1.3.6 Precision measuring equipment.Precision measuring equipment are tools usedto measure in thousandths of an inch or finerand usually employ a mechanical, electrical oroptical means of magnification to facilitatereading.

1.1.3.7 Comparators. Comparators are pre-cision measuring equipment used for compara-tive measurements between the work and a con-tact standard, such as a gage or gage blocks.

1.1.3.8 Optical comparators and gages.Optical comparators and gages are those whichapply optical methods of magnification ex-clusively. Examples are optical flats, tool

maker's microscopes, and projection compara-tors. Projection comparators usually providefor absolute measurements as well as for com-parative measurements.

1



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MIL-STD-l2012 December 1950

2.3.3.1.1 Class XX (plug gages only).Class XX gages are precision lapped to thesmallest practicable tolerances and are com-monly applied to master gages and set-upstandards.

2.3.3.1.2 Class X (plug and ring gages).Class X gages are precision lapped to close tol-erances for many types of masters and the high-est quality working and inspection gages.

2.3.3.1.3 Class Y (plug and ring gages).Class Y gages have a good lapped finish withslightly larger tolerances and are commonlyapplied to working gages. N O T E : X and Yclassification and tolerances for plain cylindri-cal plug and ring gages should not be confusedwith X and Y classification and tolerances forthread gages.

2.3.3.1.4 Class Z (plug and ring gages).

Class Z gages have a commercial finish, groundand polished, but not fully lapped, and are usedfor working gages where part tolerances arelarge.

2.3.3.2 Tolerance for thread g a g es .Standard tolerances for thread plug and ringgages and threaded setting plugs are tabulatedin the current issue of National Bureau of Standards Handbook H28, Screw ThreadStandards for Federal Services, and are asfollows :

2.3.3.2.1 W thread gages. W thread gagetolerances represent the highest commercial

grade of accuracy or workmanship. They areespecially applicable t o truncated setting plugsfor thread ring gages and to gages used forthreaded products made to special closetolerances.

2.3.3.2.2 X thread gages. X thread gagetolerances are larger than W tolerances and aresuitable for inspection and setting gages ex-cept where W tolerances may be more desir-able.

2.3.3.2.3 Y thread gages. Y thread gagetolerances are larger than X thread gage tol-erances and in addition provide standard wearallowances. Y gages are considered suitablefor inspection and working gages for classes 1and 2 threads, inch diameter and larger.They may also be used as working gages forclasses 2 and 3.

23.4 Gage wear allowance. A wear allow-

4

ance on a go gage is an intentional differencebetween the nominal or specified size of the gageand the basic or design size of the part. Itspurpose is to provide for a moderate amount of wear to occur before the gage is worn beyondthe product limit. Wear allowance, when re-quired, is applied on all go gages where thepart rubs or presents a wearing action to thegage. When part tolerances are very small, itis not feasible to provide a wear allowance be-cause the wear allowance, plus the requiredgage tolerance, would deprive the part manu-facturer of too much of his tolerance. otherexceptions are dial indicator gage assembliesand adjustable snap gages which may be resetas soon as they wear materially, and flush pingages which may have the step reground (seefigs. 4,5, and 6).

Figure 4. Plug gage tolerance.

2.3.5 Relation of gage limits to partlimits. The limits of the part, as specified onthe drawings, shall not be exceeded as a resulteither of tolerance or wear of the gages. There-fore, the extreme sizes for all gages shall notexceed the extreme limits of the part to be



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gaged. All variations in the gage, whatevertheir cause or purpose, shall bring the gagewithin these extreme limits. Thus a gage thatchecks a minimum dimension may be larger, butnever smaller, than the minimum size specifiedfor the part to be gaged. Likewise, the gagethat checks the maximum dimension may besmaller, but never larger, than the maximumsize specified for the part to be gaged.

Figure 5. Built-up snap gage tolerances.

Figure 6. Ring gage tolerances.

2.3.6 Disputed rejections. Any part whichis so close t o either rejection limit as to be im-properly rejected, either as a result of toleranceor wear of the inspection gages, will be rein-spected, using a gage as close to the productlimit as is practicable, or by use of precisionmeasuring instruments. Any observationalerrors in the reinspection must, however, be inthe direction of safety rather than in the direc-tion of danger of acceptance of improper parts.

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3

NONPRECISION MEASURING EQUIPMENT

3.1 GENERAL This section describesvarious commercially available nonprecisionmeasuring equipment. Such equipment nor-mally are tools used in the inspection of partswhen quantities are small and the tolerances aresufficiently large to permit satisfactory appli-cation. For example, line-graduated handmeasuring tool, such as a rule, does not controlaccuracy of measurement beyond the smallestgraduation on its scale. The scale is usuallyread to its nearest graduation.

3.2 DESCRIPTION3.2.1 Steel rule. The steel rule (see fig.

7) is a line-graduated measuring instrumentwhich is read by the comparison of its scalewith the edge or surface to be measured. Steelrules are made in various shapes or sizes andwith various attachments and refinements. The6-inch steel rule is most frequently used in theshop. Rules in the inch system are graduated in binary fractions down to inch, in decimal

spring steel allow measurements to be takenover curved or warped surfaces. Rules havingend scales facilitate measurements in restrictedspace.

Fillet rules which have one end cornertrimmed at an angle allow measurements to bemade across corner fills and fillets.

Figure 7. Steel rule.

3.2.2 Hook rule. The hook rule (see fig. 8)is a standard rule with a fixed shoulder attachedto one end. This rule facilitates accurate meas-urements from an end surface on a part. It isespecially useful in positions where the end of the rule is hidden or out of reach and the scalegraduations cannot be accurately alined withthe edge of the part. It is frequently used forsetting inside calipers and dividers.

Figure 8. Hook rule.

fractions down to one-hundredth of an inch.They are also available in the metric system.A large number of combinations of scales isavailable. A rule commonly has four scales, acoarse and a fine scale on each side. The scaleson one side normally run in reverse to those onthe other. Flexible rules in thin tempered

3.2.3 Steel rule with holder. The steelrule with holder (see fig. 9) consists of a set of special short rules and a holder which is slottedat one end to allow insertion of the rules.Usually there is a set of five rules of varyinglength ( to 1 inch) with each holder. Therules are graduated on both sides.

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in sets of various widths, lengths, and configu-rations.

3.2.6 Combination set and combinationsquare. The combination set consists of aheavy graduated blade, a square, a center head,and a protractor (see fig. 12). The combina-tion square is the same as the combination setbut without the protractor. The combination

Figure 9. Steel rule with holder.

set is practically a universal measuring instru-ment for nonprecision line measurement andlayout. The square of the combination can bemoved along the blade and clamped in any

Figure 10. Rule depth gage.

3.2.4 Rule depth gage. The rule depthgage (see. fig. 10) is an ordinary rule equippedwith a sliding shoulder or head so that it can beused to check the depth of holes or slots. Therule may be in the form of a rod. A modifica-tion of the rule depth gage has the rule equippedwith a fixed shoulder to facilitate the measure-ment of holes which extend completely througha piece.

3.2.5 Adjustable steel square. The adjustable steel square (see fig. 11) is a type of beam square, having a sliding blade insteadof a fixed blade. The blade fits into a groovein the base of the beam and can be locked inplace by a thumb screw in the head of the beam.Blades for the adjustable square usually come

position by means of a set screw. When it isclamped in position, one side of the squaremakes a 90 degree angle with the blade and theother side makes a 45 degree angle. It usually

Figure 11. Adjustable steel square.

7



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incorporates a spirit level set at right angles tothe blade. The center head forms a centersquare when clamped to the steel rule. It hastwo flat measuring surfaces intersecting at 90degrees. One of these surfaces is slotted sothat when the head is placed on the blade, the

blade bisects the angle of the head. The pro-tractor of the combination set is provided witha swivel or turret to which the steel rule clamps.The revolving turret is graduated from O to 90degrees in either direction. The protractorhead may be either single with the shoulder

extending from only one side of the blade, ordouble with the shoulder extending from bothsides. Most protractor heads are equippedwith a spirit level. Note: The type of pro-tractor shown in figure 12 is called a plain pro-tractor by some tool makers, and a bevel pro-

tractor by others. The terms plain protractorand reversible protractor are also used in con-nection with this instrument to denote singleand double type heads. This tool should not beconfused with the vernier type protractor whichis a precision measuring instrument.

Figure 12. Combination set.

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4

PRECISION MEASURING EQUIPMENT

4.1 GENERAL. Precision measurement is

a process of accurately deriving the numericalvalue or size of a physical dimension in stand-ard units of measurement such as inches, centi-meters, degrees, minutes and seconds, by com-

paring the dimension with a precision standardof length or angle. This section describes vari-ous commercially available precision measuringequipment and the length or angle standardswith which they are associated. Such tools andstandards are used primarily for the measure-ment of gages, but they are also used, to a con-siderable extent, in the inspection of manufac-tured parts.

4.2 LENGTH STANDARDS4.2.1 Line-graduated standards. Line-

graduated standards used in gage inspectioncommonly have the form of accurately ruledscales and dials embodied in various types of precision measuring instruments.

4.2.2 Precision gage blocks. Prec is iongage blocks (see fig. 13) are square or rectangu-lar pieces of hardened alloy steel or sinteredcarbide, having lapped, flat and parallel meas-uring surfaces on opposite ends. This permitscombining them, by wringing, to produce any

desired size. They are usually measured bycomparison with standard blocks, which havebeen calibrated and certified by the NationalBureau of Standards, using comparator read-ings in millionths of an inch. original or abso-lute measurements of gage blocks are derivedby comparison with standard wave lengths of light by means of an interferometer. Gageblocks are most commonly used as length stand-ards in gage inspection and are usually suppliedin sets varying from 28 to 103 blocks of differ-ent lengths. The most generally used set of the inch system has 81 blocks with possibly two

additional carbide wear blocks. Gage blocksare made in several degrees of accuracy, of which the most generally used is 0.000004 inchand designated as A quality The highest

9

or laboratory grade, designated as AA, has a

guaranteed accuracy of 0.000002 inch for allblocks up to 1 inch in length and 0.000002inch per inch of length for larger blocks. Otherlower grades are also available. Variations inthe lengths of blocks have a random distribu-tion between plus and minus. The total errorin a block combination, therefore, seldom ex-ceeds twice the error in a single block, andoften is actually less than that of a singleblock. If the error were permitted to be inone direction only, the accumulated error in acombination might be considerable. Althoughprecision gage blocks are most widely used asmasters for checking other gages, their use inthe shop for setting of tools and in layout work is increasing. Chromium plated and carbideblocks are used where blocks are subject toconsiderable wear.

Figure 13. Precision gage block set.

4.2.3 End measuring rods. End measur-ing rods normally are made of tool steel,hardened on the ends, drawn and seasoned, andsometimes tipped with a suitable wear-re-



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sisting material. They are commercially avail-able in lengths from 1 to 23 inches and aresuitable as standards for measurements to0.0001 inch. They are cylindrical in shapeand have either flat and parallel end surfaces orspherical end surfaces of radius equal to or

slightly less than one-half the length of therod. The body is equipped with suitable in-sulating grips to permit handling the rod withminimum heat absorption.

4.2.4 Master disks. Master disks are cus-tomarily made of tool steel in a range of sizesfrom 0.105 to and including 8.010 inches indiameter, and are accurate within 0.00001inch per inch of diameter. They are given thesame hardening, seasoning, and lapping processas that for precision gage blocks, and are usedas standards for comparators, supermicrom-eters, and similar instruments. (See 5.1.6 andfig. 31.)

4.2.5 Wires and rolls. Wires and rolls of hardened steel or sintered carbide are used be-tween measuring contracts and the work whena dimension on the work to be measured is notdirectly accessible to flat measuring contacts.They are used, for example, for measuring thepitch diameter of thread plug gages, or formeasuring a dimension at a designated pointon a sloping surface. They are available in

binary fractional inch sizes and in certain deci-mal inch sizes suitable for the measurement of screw threads and gears. Thread wires arefurnished in sets of three wires of a givendiameter; gear wire sets contain two wires; thediameters of these wires being the best size

for a given pitch (see fig. 14). They are re-quired to be within 0.0001 inch of the nominalsizes and the wires of a set must be equal insize, round, and straight within 0.00002 inch.

4.2.6 Solid angle standards and precisionsquares. Solid angle standards have a varietyof forms such as angle gage blocks, universalangle gages, graduated cone points, and pre-cision squares. Angle gage blocks function inthe same manner as gage blocks for lengthmeasurements, and may be combined by wring-ing. Blocks may be assembled to serve as thecomplement of the angle being gaged. Thus,a set of 14 or 16 blocks suffices to form anyangle to 1 second of arc from 1 second to 360degrees. Cone points consist of hardened steel,short, cylindrical rods, each having a coaxialconical point at one end. A set of such pointsin increments of 5 minutes is particularly usefulin the measurement of flank angles of largethread gages. The cylindrical square (see fig.15) is a master square against which squaresused in inspection are checked.

Figure 14. Gear and thread measuring wires.

.

Figure 15. Cylindrical square.

It is an alloy steel cylinder, ground and lappedto be an accurately true cylinder and to havethe end planes at 90 degrees to the longitudinalaxis. Either end of the cylinder may be usedas the base. The end surfaces are in relief so

10



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that the base plane consists of a narrow peri-pheral notched ring so dust particles can beworked out by rotating the square when restingit on a flat surface.

4.3 SCALE AND VERNIER. Precisionmeasuring equipment which embody line-grad-uated standards, scales, or graduated circles,equipped with verniers for subdividing scaledivisions, are grouped into the following types:

4.3.1 Vernier caliper. The vernier caliper(see fig. 16) consists of a solid L-shaped frameand a sliding jaw which can be moved alongthe long arms of the L-shaped frame. Thesliding jaw is in two sections joined by a hori-zontal screw. The larger section carries themeasuring jaw and vernier scale; the smallersection or binding clasp has an adjusting nutacting on the horizontal screw. Both sectionscan be clamped independently to the long armof the L-shaped frame. Thus the sliding jawcan be moved into an approximate position, theclasp clamped, and the larger section and meas-uring jaw moved to the exact position by move-ment of the adjusting nut. The blade of thesolid L-shaped frame usually is graduated onboth sides, one side for outside measurementsand the other for inside measurements. Somecalipers have only one scale, and it is necessaryto add the width of the gaging jaws to the read-ing when an inside measurement is taken.


The main scale is divided into 1.0-, 0.1-, and0.025-inch intervals, but only the 1.0 and 0.1 in-tervals are numbered. The vernier scale has25 divisions. A point is placed on the slidinghead and another one of the frames so thatdividers can be set to transfer distances. Forsome applications the vernier caliper has defi-nite advantages over the micrometer caliper (see4.4. 1) because of its wide range and because itcan be used for measuring both inside and out-side dimensions. It does not, however, havethe accuracy of a micrometer caliper. A ver-nier caliper should show any l-inch interval of length within an accuracy of 0.001 inch. Inany 12 inches its accuracy should be within0.002 inch, and the error should not increaseby more than about 0.001 inch for every 12inches thereafter.

4.3.1.1 Directions for reading the verniercaliper. The vernier scale is a short auxiliaryscale which either slides along the main scaleor is stationary while the main scale moves.It allows readings to be made between the grad-uations on the main scale. The principle of thevernier is applied to many types of measuringinstruments. A vernier exhibits an ordinarilyimperceptible difference in length or movement.The vernier scale usually has one or more grad-uations in the same length as the main scale.If the whole vernier scale contains one more

Figure 16. Vernier caliper.

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Figure 18. Vernier depth gage.

taken by the corners of the jaws, causing rapid

wear of the tool.4.3.5 Vernier bevel protractor. The ver-

nier bevel protractor (see fig. 21 ) also calleda universal bevel protractor, is an angle meas-uring instrument designed for the layout andchecking of angles to an accuracy greater than1 degree. It consists of two members, a baseand a blade, which can be set in any angularrelationship to each other. The position of one member relative to the other is indicated ona circular dial which has four O- to 90-degreescales. By means of a vernier, the scale can beread to the nearest 5 minutes or degree. The

blade is adjustable and, by means of a clamp-ing mechanism and thumb nut, can be lockedto the central connecting arm at any positionalong its axis. The ends of the blades areusually beveled to allow the protractor to beused in measuring from square to undercutshoulders where a square end blade would notenter. The blade assembly (blade and centralarm) is locked to the scale and base by a largecentral locking nut. Either a separate or in-tegral means of fine adjustment is usually pro-vided to allow controlled final movement of the blade through a small arc. The instrument

shown in figure 21 has an adjustable arm oracute angle blade for the purpose of measuringsmall angles on pieces so shaped that they

13

would be difficult to measure from the fixed

base. The back of the vernier bevel protractoris flat so that the tool may be placed flat ona surface plate or on the work being checked.

4.3.6 Directions for reading the vernierprotractor. The vernier indicates every 5minutes or degree. When the zero on thevernier exactly coincides with a graduation onthe scale, the reading is in exact degrees, as inthe upper scale shown on figure 22, in whichthe reading is 17 degrees O minutes. When thezero graduation of the vernier does not coincideexactly with a graduation on the scale, thenumber of degree or 5 minute intervals to

be added to the whole degree reading is deter-mined by counting the number of divisionsfrom the zero of the vernier to the first line onthe vernier that coincides with a line on thescale. As each division on the vernier repre-sents 5 minutes, the number of these divisionsmultiplied by 5 will be the number to be addedto the whole number of degrees. For example,the lower scale shown on figure 22 shows thezero on the vernier between 12 and 13 on thescale. The zero on the vernier has, therefore,moved 12 whole degrees to the right from zeroon the scale. In the same direction, the tenth

line of the vernier, representing 50 minutes isthe line which exactly coincides with a line onthe scale. The reading, then, is 12 degrees 50



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Figure 19. Vernier height gage.

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Figure 20. Gear tooth vernier caliper.

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Figure 21. Vernier bevel protractor.

Figure 22. Vernier on bevel protractor.

minutes. Since the divisions, both on the scaleand on the vernier, are numbered both to right

and left from zero, any size of angle can bemeasured. The readings on the scale and onthe vernier are taken either to the right or left,according to the direction in which the zeroon the vernier is moved.

4.4 MICROMETERS4.4.1 Micrometer caliper. The micrometer

caliper (usually called a micrometer or mike)is second only to the steel rule in the extent of its use by an inspector. It utilizes the relation-ship between the axial and circular motions of a screw as a means of precision measurement.The micrometer caliper consists essentially of

a U-shaped frame and a screw (see fig. 23).One end of the U-shape holds the anvil, whichis a hardened steel button pressed or screwedinto the frame, and the other end of the U-shapecarries the hollow steel barrel in which thescrew turns. The threads of the screw engage

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micrometer scale which shows the axial move-ment of the screw. The thimble graduations,therefore, subdivide the relatively larger divi-sions of the barrel scale. Some micrometercalipers have a ratchet on the top of the thimblecap connected to the thimble and screw throughan over-riding clutch held together by a spring.The diameter of the nut which engages thescrew can be adjusted to compensate for wearby tightening the ring nut. Some micrometercalipers have a clamp ring or lock nut set in theU-shaped frame at the spindle bearing. Thisdevice acts as a caroming roll, which operatesagainst a split nut to bind the spindle bearing.Turning the clamping ring until it is just tighteffectively locks the spindle in position. Themost common size micrometer caliper measuresfrom 0 to 1 inch. The larger size micrometercalipers also measure within a range of 1 inch.Thus, for example, 1 inch must be added to thebarrel reading of a 2-inch micrometer caliperto give the correct distance between anvil andspindle. Most micrometer calipers are gradu-ated to read to thousandths of an inch and maybe read to ten-thousandths by estimating tenthsof the thimble graduat ions. If they areequipped with a vernier scale, they can be readdirectly to a ten-thousandth of an inch. Thisis approximately the limit of accuracywhich is built into an ordinary commercialmicrometer.

4.4.1.1 Directions for reading a microme-

ter caliper. The distance between the anviland the unthreaded part of the screw, calledthe spindle, is determined by the number of whole turns and fractions of a turn of the screw.The screw of a micrometer caliper reading inthe inch system has 40 threads per inch, thuseach revolution of the screw opens the microm-eter caliper or 0.025 inch. The barrel isgraduated to show the revolutions of the screw.There are 40 graduations on the barrel of amicrometer caliper having a range of 1 inch.To simplify the reading, every fourth gradua-tion representing tenths of an inch, is numbered.The thimble is graduated so that each of aturn, or 0.001 inch) is indicated byone graduation on the thimble. The thimblethus has 25 graduations, every fifth graduation

being numbered for convenience in reading asshown on figure 24. A measurement in thou-sandths on the micrometer caliper is to be readas follows:

a. Note the number of graduations on thebarrel uncovered by the thimble travel and mul-tiply this number by 0.025.

b. Note the thimble graduation which coin-cides most closely with thebarrel.

BARREL

reference line on the

SCALE

Figure 24. Micrometer barrel and thimble scales.

c. Add this reading in thousandths of aninch to the barrel reading. This method is ap-plied to the example shown on figure 24, asfollows :

Take care in reading a micrometer caliper whenthe thimble is approaching the completion of a revolution. If the thimble zero has not passedthe index line on the barrel, do not read the

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barrel for an extra 0.025 inch. The next gradu- thimble and to add that vernier graduationation may begin to show but has not actually number in ten-thousands to the regular microm-become a portion of the barrel reading. That eter reading. The numbers on the coincidingportion of the graduation that starts to show graduation of the thimble scale are to be com-near the end of the revolution is accounted foras the thimble reading. The accuracy of meas-urements made with the micrometer dependson the contact load (or in common parlance,measuring pressure) used. It is necessary todevelop a sense of feel in adjusting the mi-crometer to the work. This can best be doneby taking measurements of precision gageblocks or other accurately known standards.Also, a sensitive feel is best developed by grasp-ing the smooth portion of the thimble rather

pletely disregarded in the use of the vernierscale. Never use the number of a thimble scalegraduation that coincides with a vernier gradu-ation to obtain the vernier reading. Examplesof micrometer vernier readings are shown onfigure 26. In example A, the barrel reading is0.45. The thimble reading lies between 0.019and 0.020. The seventh graduation of thevernier scale coincides with a graduation of thethimble scale, showing that the exact. readingof the thimble scale is 0.0197. The micrometer

than the knurled portion. reading for example A is, therefore, 0.4697.4.4.1.2 Directions for reading the vernier In example B, the barrel reading is 0.45. The

micrometer caliper. The vernier micrometer thimble reading is exactly 0.019. This is veri-

caliper is a standard micrometer equipped with fied by the fact that only the zero graduationsa vernier scale. The vernier micrometer scale of the vernier coincide with thimble scale(see fig. 25) is etched on the barrel in line withand contiguous to the thimble scale. It con-sists of 11 equally spaced lines, the first 10 linesnumbered from O to 9, the last graduationmarked with a second O. The 10 divisions of the vernier scale occupy the same space as 9divisions on the thimble. One vernier divi-sion therefore is inch. Thedifference between the thimble and vernierdivision are thusallowing a reading of inch. Thus, when

a thimble graduation does not coincide with theindex line on the barrel, it is necessary onlyto determine which vernier graduation on thebarrel coincides with a graduation on the

graduations. The micrometer reading for ex-ample B is therefore 0.4690. The vernier scaledoes not enter into this reading except to con-firm the apparent position of the thimble scaleon the index line.

Figure 26. Examples of micrometer vernier readings.

Figure 25. Vernier micrometer scales.

4.4.2 Screw thread micrometer caliper.The screw thread micrometer caliper (see fig.27) is a micrometer caliper adapted for meas-uring pitch diameters of threads. The spindlehas a cone-shaped tip which is truncated so thatit does not contact the root of the thread. Theanvil is swiveled and has a truncated V-grooveto fit a thread. The contacts occur at the sidesof the thread. The point on the spindle couldbe used for any pitch but the V-shaped anvil islimited in its range. For this reason, six differ-ent micrometer calipers are necessary to cover

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M I L - S T D - 1 2 01 2 D e c e m b e r 1 9 5 0

a range of 4 to 64 threads per inch in a givendiameter range. when the micrometer caliperis set to zero, the pitch lines ofthe spindleand anvil coincide. When open, the readinggives the pitch diameter. Thread micrometersare convenient to use but are subject to certain

limitations. When the micrometer is set toread zero with the thimble and anvil together,as is usually done, the reading over the threadsis always slightly distorted because, the threadsurfaces being warped, contact of the anvil onthe thread is not in the same axial plane whichthe spindle cone contacts. The readings arenot distorted when thread micrometers are setto a standard thread plug gage of known pitchdiameter and used for measuring threads of thesame p0itch and diameter as the plug. When soset, however, the tool will not read exactly zerowhen spindle and anvil are brought together.

Figure 28. Inside micrometer, caliper-type.

4.4.3.2 Rod-type. The rod type inside mi-cromer (see fig. 29) is used to measure insidedimensions larger than those covered by thecaliper type inside micrometer. It consists of a micrometer head and a series of interchange-able extension rods of varying lengths. Anespecially short inside micrometer with a rangeof from 1 to 2 inches may be obtained, and by

means of extension rods, a length up to 100inches or more may be measured. The rangeof the micrometer screw itself is short. Thesmallest size micrometer has inch length of screw and the largest only a 1 inch length of screw. some sets contain a collar by which theinch steps between rods may be split up whenthe screw is inch. The collar extends thelength of any rod inch.

Figure 27. Screw thread micrometer caliper.

4.4.3 Inside micrometers. Inside microm-

eters are of two types, the caliper-type and therod-type. Both types read in thousands of ani n c h . 4.4.3.1 Caliper type. The caliper type in-side micrometer is designed for measuring smallinside dimensions. It is made commercially inranges of from 0.2 to 1 inch, to 1 inchesand 1 to 2 inches. The minimum dimension isdetermined by the width of the measuring jaws.These are gound with a small radius to insureline contact in a hole. Some caliper-type insidemicrometers have the barrel scale on the spindle,which does not turn (see fig. 28). They areread in the same way as an ordinary outsidemicrometer caliper. Others have the barrelscale on the barrel in the same position as on anoutside micrometer caliper but the scale readsfrom the right to left instead of left to right.

Figure 29. Inside micrometer, rod-type.

4.4.4 Micrometer depth gage. The mi-crometer depth gage (see fig. 30) consists of aflat shoe or shoulder attached to the barrel of a micrometer head. The base of the shoulderis hardened and ground in order to be at rightangles to the axis of the spindle. The spindle,which protrudes from the shoulder, may bea flat strip or round rod and may be eithergraduated or ungraduated. The range of measurement of the type of gage shown onfigure 30 can be increased by the use of inter-changeable extension rods of different lengths.

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similar to that found on micrometer calipers.Alongside the graduated drum is a vernier scaleby which readings to 0.0001 inch can be made.The micrometer screw of the supermicrometerhas a range of from 0 to 1 inch. but the instru-ment itself covers a range of 8 inches in 1-inchsteps by the use of l-inch master disks as shownon figure 31. The tailstock can be moved to

any position along the bed. The pressuremechanism can be adjusted by means of knurledknob for a contact load of 1 or 2 pounds.These are the two loads recommended by theNational Bureau of Standards for measuringthreads by the three-wire method.

4.5 MEASURING MACHINES

4.5.1 Universal measuring machine. Theuniversal measuring machine (see fig. 32) gen-erally used in the United States is a bench typemeasuring machine consisting of a heavy basecontaining accurate ways upon which a mi-crometer headstock with microscope can bepositioned in relation to a master bar graduatedat each l-inch interval. A pressure tailstock,which is also adjustable on the ways, providesmeans of adjusting and controlling electricallythe measuring load from 1 to 2 pounds, in

increments of 8 ounces. It is similar to thesupermicrometer in appearance and operation,but the accuracy is greater as direct measure-ments can be taken to 0.00001-inch. The ma-chine is furnished in various sizes having capac-ities from 0 to 12, 24, 26, 48, 80, or 120 inches.The master bar of this type of measuring ma-chine is a line-graduated standard which runs

the length of the machine at the rear. It ismade of seasoned steel and bears microscopicreference lines on highly polished plugs. Thesegraduations are placed 1 inch apart and areused for setting the measuring head at eveninch positions. The measuring head includesa microscope, with a double cross-hair, to seton the inch graduations of the master bar. Thepressure tailstock makes possible the duplica-tion of exact measuring load at all times.Changes in load are made by turning the knobat the rear of the tailstock until the desiredload is reached, as indicated on a graduatedscale which is calibrated in 8-ounce divisionsfrom 1 to 2 pounds. A milliammeter,located.on the measuring head, shows when the cor-rect measuring load has been reached. By ad-vancing the anvil in the measuring head, thework is pressed against the tailstock anvil until

Figure 32. Universal measuring machine.

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the pointer of the milliammeter reaches thecenter of the scale. The size is the sum of thenumber of the inch graduation of the masterbar to which the microscope is set and thereading of the micrometer head.

4.5.2 Special measuring machines. Meas-

uring machines of special design for specificpurposes are available commercially. Amongthese are lead measuring machines for deter-mining errors in pitch of screw thread gagesand internal measuring machines for measur-ing the diameters of ring gages.


4.6 COMPARATORS. Comparators areused for determining dimensions by measur-ing the difference in length between a cent actlength standard, such as a master disk or gageblock combination, and the work.

4.6.1 Vertical comparators. Most com-

parators are of the vertical bench type, consist-ing of a ribbed cast iron base upon which ismounted an anvil for supporting the work anda vertical hollow steel shaft or column whichsupports the measuring head. The measuringhead contains the measuring spindle and the

Figure 33. Electrolimit comparator.

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4.7 OPTICAL INSTRUMENTSCOMPARATORS. For certain typesspection. instruments and comparators

ANDof in-based

on optical principles are more advantageouslyapplicable than mechanical means. suchequipment includes optical flats, optical com-parators, tool makers microscopes, and opticaldividing heads.

4.7.1 Optical flats. Optical flats (see fig.36) are transparent disks of glass, quartz, orsemiquartz. One side of the disk is an opticallyflat surface, being within 1 or 2 millionths inchof a true plane. Interference fringes, or lightand dark bands, are produced when an opticalflat is superimposed on another flat surface insuch a way that there is a very small angle be-tween the surfaces and the surfaces are suitablyilluminated with approximately monochro-matic light. The straightness of the fringesis a measure of the flatness of the surface beingtested, one fringe corresponding approximatelyto 0.00001 inch in great light and 0.000012 inchin yellow light. In addition to checking flat-ness, optical flats can be used for comparingthe length and parallelism of nominally equalprecision gage blocks.

Figure 36. Optical flats.

4.7.2 Optical projection machines. Theoptical comparator ( see fig. 37 ) and contourmeasuring projector ( see fig. 38) are both opti-

cal projection machines for measuring or com-paring objects by means of a magnified shadowimage. A concentrated, parallel beam of lightis projected across the object to be measuredand by a system of optical magnification thecontour of the part is projected onto a screen.

25

Figure 37. Optical comparator.

The shadow is compared with lines on a chartcorresponding to the limits of size of the con-tour of the part being checked. The magnifi-cation of the comparator is the ratio of the sizeof the shadow on the screen to the size of the

object. The magnification is dependent pri-marily upon the projection lens system and onthe screen distance. Interchangeable lens sys-tems are available, with magnifications of 10,20, 25, 3l, 50, 62, and 100 times. The pro-tractor screen consists of a circular disk of finely



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Figure 38. Contour measuring projector.

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ground glass mounted in a ring which is grad-uated and provided with a vernier which per-mits the measurement of all angles to a mini-mum reading of 1 minute of arc. The groundglass of the screen is provided with fine brokenreference lines, etched in the glass. These linesare filled with black pigment and can bematched with the image when taking measure-ments. The work table has a longitudinal andtransverse travel and is equipped with microm-eters having a minimum reading of 0.0001 inch.

4.7.3 Tool makers microscope. The toolmakers microscope, (see fig. 39) is an opticalinstrument for measuring linear and angulardimensions. It consists of a microscope with aprotractor head, mounted on a support columnwith adjustable tilt, and a mechanical stagepermitting longitudinal, transverse, and an-gular measurement. The object being meas-ured is seen through the microscope enlarged30 or more times, depending on the objectivelens which is used. The protractor head givesangular readings by vernier to 1 minute, andhas two rotatable interrupted cross lines. Themicrometer stage is operated by micrometerscrews which have drums graduated to read0.0001 inch per division. The full range of thestage is 4 inches of longitudinal motion and 2inches of transverse. Of the longitudinal mo-tion, 2 inches are by screws and 2 additionalinches by insertion of gage blocks.

4.7.4 Optical dividing head. The opticaldividing head (see fig. 40) is a final inspectioninstrument designed to give the greatest ac-curacy possible in circular measurement of angular spacing. The complete unit consistsof the dividing head, surface plate, and tail-stock. The measuring elements consist of aprecision graduated circle, which is read bymeans of a microscope, and an auxiliary gradu-ated drum which permits reading to 6 secondsor 1 second of arc. Accuracy within 2 secondsof arc may be obtained with this instrument.The dividing head may be equipped with astandard driver designed for work of varioussizes and characteristics, a chuck having in-dividually adjustable jaws, or a precision faceplate. The surface plate is made of fine-grain

way for maintaining accurate alinement of tail-stock to the dividing head by means of a keywhich fits into the keyway in the surface plate.

4.8 AUXILIARY EQUIPMENT. Someof the more important items of equipment,which are auxiliary to precision measuring in-struments and comparators, are:

4.8.1 Sine bars and plates. The sine barand sine plate are tools designed for measuringangles accurately or for locating work to agiven angle within very close limits. The sinebar (see figs. 41 and 42) consists of an accuratestraightedge in which are set two hardened andground plugs or buttons. These plugs or but-tons must be of the same diameter, and theircenter distance is commonly an even 5, 10, or20 inches to facilitate calculations. All ref-erence surfaces must be parallel with eachother and with the centerline of the plugs orbuttons. The sine plate (see figs. 43 and 44)must meet the same requirements as the sinebar and differs only in that the reference sur-face is in the form of a plate. Some are fur-nished with a base plate and are hinged.

4.8.2 Dial indicators. A dial indicator,also called a dial gage, is a mechanical leversystem or gear train used for amplifying smalldisplacement and measuring it by means of apointer which traverses a graduated dial. Itis not a complete gage unless it is mounted onan arm extending from the frame of a ma-

chine, or on some type of pedestal or gagingfixture. The dial indicator. when so mounted,can then measure or indicate the differencebetween the piece being checked and the masteror standard by which it was set.Indicators are of two types, the dial indicator(gear train type) (see fig. 45) and the dialtest indicator (see fig. 46). A sharp distinctionbetween dial indicators and test indicators can-not be made. The dial indicator, however, willoperate through a longer range, and trans-forms a small linear motion of a spindle, bymeans of a rack and pinion and a gear train,into a greatly magnified circular motion of ahand on a dial, while the test indicator em-ploys a lever or combination of levers insteadof a rack and pinion for magnification. Stand-

cast iron alloy and mounted on a three point ard dial indicators are made in sizes rangingsupport. It is provided with a precision key- from to 3 inches in diameter. The

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Figure 39. Tool maker's microscope.

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Figure 40. Optical dividing head.


Figure 41. Sine bar.

Figure 42. Sine bar set upon gage blocks.

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