unit55 threads and thread cutting

8/13/2019 Unit55 Threads and Thread Cutting

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Copyright The McGraw-Hill Companies, Inc.Permission required for reproduction or display.

PowerPoint to accompany

Krar Gill Smid

Technology of Machine Tools6thEdition

Threads and Thread

Cutting

Unit 55


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55-2

Objectives

Recognize and state the purposes of six

common thread forms

Set up a lathe to cut inch external Unifiedthreads

Set up an inch lathe to cut metric threads

Set up a lathe and cut internal threads Set up a lathe and cut external Acme

threads


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55-3

Threads

Used for hundreds of years for holding parts

together, making adjustments, and

transmitting power and motion

Art of producing threads continually

improved

Massed-produced by taps, dies, threadrolling, thread milling, and grinding


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55-4

Threads

Thread

Helical ridge of uniform section formed on

inside or outside of cylinder or cone Used for several purposes:

Fasten devices such as screws, bolts, studs, andnuts

Provide accurate measurement, as in micrometer

Transmit motion

Increase force


5/84

55-5

Thread

TerminologyCopyright The McGraw-Hill Companies, Inc.

Permission required for reproduction or display.


6/84

55-6

Thread Terminology

Screw thread

Helical ridge of uniform section formed on

inside or outside of cylinder or cone

External thread

Cut on external surface or cone

Internal thread Produced on inside of cylinder or cone


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55-7

Major diameter

Largest diameter of external or internal thread

Minor diameter

Smallest diameter of external or internal thread

Pitch diameter Diameter of imaginary cylinder that passes

through thread at point where groove and threadwidths are equal

Equal to major diameter minus single depth ofthread

Tolerance and allowances given at pitchdiameter line


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55-8

Number of threads per inch

Number of crests or roots per inch of threadedsection (Does not apply to metric threads)

Pitch

Distance from point on one thread tocorresponding point on next thread, measured

parallel to axis

Expressed in millimeters for metric threads

Lead

Distance screw thread advances axially in one

revolution (single-start thread, lead = pitch)


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55-9

Root

Bottom surface joining sides of two adjacentthreads

External thread on minor diameter

Internal thread on major diameter

Crest Top surface joining two sides of thread

External thread on major diameter

Internal thread on minor diameter

Flank

Thread surface that connects crest with root


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55-10

Depth of thread

Distance between crest and root measured

perpendicular to axis

Angle of thread

Included angle between sides of thread

measured in axial plane

Helix angle Angle that thread makes with plane

perpendicular to thread axis


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55-11

Right-hand thread

Helical ridge of uniform cross section ontowhich nut is threaded in clockwise direction

When cut on lathe, toolbit

advanced from right to left

Left-hand thread

Helical ridge of uniform cross section onto

which nut is threaded in counterclockwise

direction

When cut on lathe, toolbit

advanced from left to right

Copyright The McGraw-Hill Companies, Inc.



12/84

55-12

Thread Forms

April, 1975 ISO came to an agreement

covering standard metric thread profile

Specifies sizes and pitches for various threads

in new ISO Metric Thread Standard

Has 25 thread sizes, range in diameter from 1.6

to 100 mm Identified by letter M, nominal diameter, and

pitch M 5 X 0.8


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55-13

Table 55.1

ISO metric pitch and diameter

combinationsNominal Thread Nominal Thread

Diameter Pitch Diameter Pitch

(mm) (mm) (mm) (mm)

1.6 0.35 20 2.5

2 0.4 24 3

2.5 0.45 30 3.5

3 0.5 36 43.5 0.6 42 4.5

4 0.7 48 5

5 0.8 56 5.5

Portion of table taken

from textbook


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55-14

ISO Metric Thread

60 included angle, crest = 0.125 times pitch

D (depth of thread) is 0.6143 times pitch

Root is 1/4thof pitch,

Flat on root (FR) wider than crest (FC)




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55-15

ISO Metric Thread

x.250

x.125

x54127.0(external)

PFR

PFC

PD




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55-16

American National Standard Thread

Divided into four main series, all havingsame shape and proportions

National Coarse (NC)

National Fine (NF)

National Special (NS)

National Pipe (NPT)

Has 60 angle with root and crest truncatedto 1/8th the pitch

Used in fabrication, machine construction


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55-17


N

PF

NPD

.125orx.125

.6134orx6134.




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55-18

British Standard Whitwork

(BSW) Thread Application is same

as for American

National form threadN

PR

NPD

.1373orx.1373

.6403orx6403.




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55-19

Unified Thread

Developed by U.S., Britain, and Canada forstandardized thread system

Combination of British Standard Whitworth and





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55-20

NPF

NPF

NPD

NPD

.250

orx.250thread)(internal

.125orx.125thread)(external

.5413orx5413.thread)(internal

.6134orx6134.thread)(external




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55-21

American National Acme Thread

Replacing square thread in many cases

Used for feed screws, jacks, and visesD=minimum .500P

=maximum .500P+0.010

F=.3707P

C=.3707P-.0052

(for maximum depth)




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55-22

Brown & Sharpe Worm Thread

Used to mesh worm gears and transmitmotion between two shafts at right angles to

each other but not in same plane

D=.6866P

F=.335P

C=.310P



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55-23

Square Thread

Being replaced by Acme thread because ofdifficulty in cutting it

Often found on visesand jack screws

D= .500P

F = .500P

C= .500P + .002



55 24


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55-24

International Metric thread

Standardized thread used in Europe

F = 0.125P

R= 0.0633P (maximum)

= 0.054P(minimum)

D= 0.7035P (maximum)

= 0.6855P (minimum)



55 25


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55-25

Thread Fits and Classifications

Fit

Relationship between two mating parts

Determined by amount of clearance orinterference when they are assembled

Nominal size

Designation used to identify size of part

Actual size

Measured size of thread or part

Basic size: size from which tolerances are set

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55-26

Allowance

Permissible difference between largestexternal thread and smallest internal thread

Difference produces tightest fit acceptablefor any given classification

The allowance for a 1 in.8 UNC Class 2A and 2B fit is:

Minimum pitch diameter of the

internal thread (2B) = .9188 in.Maximum pitch diameter of the

external thread (2A) = .9168 in.

Allowance = .002 in.

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55-27

Tolerance

Variation permitted in part size

May be expressed as plus, minus, or both

Total tolerance is sum of plus and minus tolerances

In Unified and National systems, tolerance is pluson external threads and minus on internal threads

Maximum pitch diameter of the


Minimum pitch diameter of the


Tolerance = .0068 in.

The tolerance for a 1 in.8 UNC Class 2A thread is:

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55-28

Limits

Maximum and minimum dimensions of part

Maximum pitch diameter of the

external thread (2A) = .9168 in..

Minimum pitch diameter of theexternal thread (2A) = .9100 in.

The limits for a 1 in.

8 UNC Class 2A thread are:

55 29


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55-29

ISO Metric Tolerances and Allowances

ISO metric screw thread tolerance system provides forallowances and tolerances defined by tolerance grades,tolerance positions, and tolerance classes

Tolerance grades specified numerically

Medium tolerance indicated by number 6, number below 6indicates finer tolerance and number above 6 indicates greatertolerance

Tolerance for thread at pitch line and major diameter may beshown on drawing using the following symbols:

External threads:eindicates large allowance

gindicates small allowance

hindicates no allowance

Internal threads:Gindicates small allowance

Hindicates no allowance

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55-30

Tolerance Example:

An external metric thread may be designated as follows:

Pitch Outside

Nominal Dia. Dia.

Metric Size Pitch Tolerance ToleranceM 6 X 0.75 - 5g 6g

The thread fit between mating parts is indicated by

internal thread designation followed by the external

thread tolerance:

M 20 X2 -6H/5g6g

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55-31

Symbols Used to Indicate

Allowance For external threads:

eindicates a large allowance

g indicates a small allowance

hindicates no allowance

For internal threads:

G indicates a small allowance

Hindicates no allowance

55 32


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55-32

Three Categories of Unified

Thread Fits External threads classified as 1A, 2A, and 3A and

internal threads as 1B, 2B, 3B

Classes 1A and 1B Threads for work that must be assembled

Loosest fit

Classes 2A and 2B Used for most commercial fasteners

Medium or free fit

Classes 3A and 3B Used where more accurate fit and lead required

No allowance provided

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55-33

Thread Calculations: Example 1

To cut a correct thread on a lathe, it is necessary first to make

calculations so thread will have the proper dimensions.

D=single depth of thread

P=pitch

in..061.100x.61343

x61343.

in.100.10

11

PD

tpiP

Calculate pitch, depth, minor diameter, and width of flat

for a

10 UNC thread.

in.0125.

10

1x

8

1

8flatofWidth

in.628.

.061)(.061-.75

)(-diaMajordiaMinor

P

DD

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55-34

Thread Calculations: Example 2

P=pitch= 1 mmD=0.54127 x 1

= 0.54 mm

What are the pitch, depth, minor diameter, width of crest and

width of root for an M 6.3 X 1 thread?

mm125.0

1x125.0

x.1250crestofWidth

mm22.5

.54)(.54-6.3)(-diaMajordiaMinor

P

DD

mm25.0

1x0.25

x25.0rootofWidth

P

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55-35

Procedure to Set the Quick-

Change Gearbox for Threading1. Check drawing for thread pitch required

2. From chart on quick-change gearbox, find

whole number that represents pitch in

threads per inch or in millimeters

3. With lathe stopped, engage tumbler lever

in hole, which is in line with the pitch

4. Set top lever in proper position as

indicated on chart

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55 36

5. Engage sliding gear in or out as required

6. Turn lathe spindle by hand to ensure that

lead screw revolves

7. Recheck lever settings to avoid errors

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55 37

Thread-Chasing dial

Lathe spindle andlead screw must bein same relative

position for each cut Thread-chasing dial

attached to carriage forthis purpose

Dial has eight divisions

Even threads use any division

Odd threads either numberedor unnumbered: not both



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55 38

Thread Cutting

Produces a helical ridge of uniform section

on workpiece

Performed by taking successive cuts with

threading toolbit of same shape as thread

form required

Work may be held between centers or inchuck

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55 39

Procedure to Set Up a Lathe for

Threading (60 Thread)1. Set lathe speed to speed used for turning

2. Set quick-change gearbox for required pitchin threads per inch or in millimeters

3. Engage lead screw

4. Secure 60 threading toolbit and check

angle using thread center gage5. Set compound rest at 29 to right; set to left

for left-hand thread

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55 40

6. Set cutting tool to height of lathe center

point7. Mount work between centers

Make sure lathe dog is tight on work

If work mounted in chuck, it must be heldtightly

8. Set toolbit at right angles to work, using

thread center gage9. Arrange apron controls to allow split-nut

lever to be engaged

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Thread-Cutting Operation

Procedure to cut a 60 thread

1. Check major diameter of work for size

2. Start lathe and chamfer end of workpiece

with side of threading tool to just below

minor diameter of thread

3. Mark length to be threaded by cutting

light groove at this point with threading

tool while lathe revolving

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4. Move carriage until point of threading tool

near right-hand end of work5. Turn crossfeed handle until threading tool

close to diameter, but stop when handle is at

3 o'clock position

6. Hold crossfeed handle in this position and

set graduated collar to zero

7. Turn compound rest handle until threading

tool lightly marks work

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8. Move carriage to right until toolbit clears

end of work

9. Feed compound rest clockwise about .003 in.

10. Engage split-nut lever on correct line of

thread-chasing dial and take trial cut along

length to be threaded

11. At end of cut, turn crossfeed handle

counterclockwise to move toolbit away from

work and disengage split-nut lever

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12. Stop lathe and check number of tpi with

thread pitch gage, rule, or center gage

13. After each cut, turn carriage handwheel to

bring toolbit to start of thread and return

crossfeed handle to zero

14. Set depth of all threading cuts with compound

rest handle

See Table 55.2 and Table 55.3

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When tool is fed in at 29, most of the cutting

is done by the leading edge of toolbit.



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Table 55.3: Depth setting for cutting 60

ISO

metric threadsCompound Rest Setting

Pitch (mm)

(mm) 0 30 29

0.35 0.19 0.21 0.21

0.4 0.21 0.25 0.24

0.45 0.24 0.28 0.27

0.5 0.27 0.31 0.310.6 0.32 0.37 0.37

0.7 0.37 0.43 0.43

0.8 0.43 0.5 0.49


from textbook

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Table 55.2 Depth settings for cutting 60

national form threads*

Compound Rest Setting

tpi 0 30 29

24 .027 .031 .0308

20 .0325 .0375 .037

18 .036 .0417 .041

16 .0405 .0468 .046

14 .0465 .0537 .0525

13 .050 .0577 .057

11 .059 .068 .0674


from textbook

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15. Apply cutting fluid and take successive

cuts until top (crest) and bottom (root) ofthread are same width

16. Remove burrs from top of thread with file

17. Check thread with master nut and take

further cuts

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Six Ways to Check Threads

Depends on accuracy required:

1. Master nut or screw

2. Thread micrometer

3. Three wires

4. Thread roll or snap gage

5. Thread ring or plug gage

6. Optical comparator

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To Reset a Threading Tool

Must reset when need to remove partlythreaded work from lathe, tool removed forregrinding, or work slips under lathe dog

Procedure

1. Set up lathe and work for thread cutting

2. Start lathe, toolbit clear of work, engage

split-nut lever on correct line3. Allow carriage to travel until toolbit

opposite any portion of unfinished thread

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4. Stop lathe, leaving split-nut lever engaged

5. Feed toolbit into thread groove using onlycompound rest and crossfeed handles until

right-hand edge of toolbit touches rear side

of thread6. Set crossfeed graduated collar to zero

7. Back out threading tool using crossfeed

handle, disengage split-nut lever, andmove carriage until toolbit clears start of

thread

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8. Set crossfeed handle back to zero and take

trial cut without setting compound rest

9. Set depth of cut using compound rest

handle and finish thread to required depth

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To Convert an Inch-DesignedLathe to Metric Threading

Metric threads may be cut on standard quick-

change gear lathe by using a pair of change gears

having 50 and 127 teeth respectively. Lead screwhas inch dimensions so necessary to convert pitch

in millimeters to centimeters and then into threads

per inch

1 in. = 2.54 cm which gives ration 1:2.54

teeth127

teeth50

50

50x

54.2

1

(spindle or drive gear)

(lead screw)

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Procedure to Cut a 2-mm Metric Thread on

Standard Quick-Change Gear Lathe

1. Mount the 127-tooth gear on lead screw

2. Mount the 50-tooth gear on spindle

3. Convert the 2-mm pitch to threads per

centimeter:

cmP

cmmm

thread/52

10

110

4. Set quick-change gearbox to 5 tpi

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5. Set up lathe for thread cutting

6. Take light trial cut7. Reverse spindle rotation until cutting tool

has just cleared start of threaded section

8. Check thread with metric screw pitch gage

9. Cut thread to required depth

Note: Never disengage the split nut untilthe thread has been cut to depth.

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Procedure to Cuta Left-Hand Thread (60)

Basically same as for right-hand threads,

with few exceptions:

1. Set lathe speed and quick-change gearboxfor pitch of thread to be cut

2. Engage feed-direction lever so that lead

screw will revolve in opposite direction tothat for right-hand thread

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3. Set compound rest to 29 to the LEFT

4. Set up left-hand threading tool and squareit with work

5. Cut groove at left end of section to be

threaded

Gives cutting tool starting point

6. Proceed to cut thread to same dimensions

as for right-hand thread

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Cutting a Thread on

a Tapered Section

Use same setup as for regular thread cutting

Most important that threading tool be set at

90 to axis of work

Do NOT set it square with tapered surface

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Common Methods ofMeasuring Threads

1. Thread ring gage

2. Thread plug gage

3. Thread snap gage

4. Screw thread micrometer

5. Optical comparator6. Three-wire method

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Three-Wire Method ofMeasuring Threads

Recommended by Bureau of Standards andNational Screw thread Commission

Three wires of equal diameter placed in thread,two on one side and one on other side

Wire used should be hardened and lapped to three timesaccuracy of thread to be inspected

Standard micrometer used to measure distanceover wires (M)

Different sizes and pitches of threads requiredifferent sizes of wires

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The Three-Wire Method ofMeasuring 60 Threads



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To Calculate theMeasurement over the Wires

NGDM

5155.13

where M=measurement over the wires

D=major diameter of the thread

G=diameter of the wire size used

N=number of tpi

Any of the following formulas can be used to calculate G:

PN

PN

PN

.505or.505ireSmallest w

.57735or.57735

wiresize-Best

1.010or

010.1

reLargest wi

55-63


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Multiple Threads

Used when necessary to obtain increase in leadand deep, coarse thread cannot be cut

May be double, triple or quadruple, depending on

number of starts around periphery of work Pitch: distance from point on one thread to

corresponding point on next thread

Lead: distance nut advances lengthwise in one

complete revolution Single-start: pitch and lead equal

Double-start: lead twice the pitch

Triple-start: lead three times pitch

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Single ThreadDouble Thread

Triple ThreadCopyright The McGraw-Hill Companies, Inc.Permission required for reproduction or display.

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Multiple Threads Maybe Cut on a Lathe

1. Using accurately slotted driveplate or

faceplate2. Disengaging intermediate gear of end gear

train and rotating spindle desired amount

3. Using thread-chasing dial Only for double-start threads with odd-number

lead

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Procedure to Cut an 8-tpiDouble Thread

1. Set up lathe and cutting tool as for cuttingsingle-start thread

2. Set quick-change gearbox to 4 tpi3. Cut first thread to half depth required for 4 tpi

4. Leave crossfeed handle set to depth of

thread and not reading on compound restgraduated collar

5. Withdraw threading tool from work usingcompound rest handle

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6. Revolve work exactly turn

Remove work from lathe with lathe dogattached

Replace work in lathe with tail of dog in slot

exactly opposite one used for first thread

7. Reset crossfeed handle to same position as

when cutting first thread

8. Cut second thread, feeding compound rest

handle until graduated collar at same

setting as for first thread

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Thread-Chasing Dial Method of

Cutting Multiple Threads Double-start threads with odd-numbered

lead may be cut using thread-chasing dial

1. Take one cut on thread by engaging splitnut at numbered line on chasing dial

2. Without changing depth of cut, takeanother cut at unnumbered line on chasing

dial (second thread will be exactly inmiddle of first thread)

3. Continue cutting thread to depth, takingtwo passes for every depth-of-cut setting

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Square Threads

Found in vise screws,jacks, and other deviceswhere maximum powertransmission required

Square threading toollooks like short cutting-off tool

Both sides of squarethreading tool must be

ground at angle toconform to helix angleof thread


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Square Threading Tool

Helix angle of thread (angle of square threadingtool) depends on two factors:

Changes for each different lead on given diameter

(greater lead, greater angle) Changes for each different diameter of thread for given

lead (larger diameter, smaller angle)


55-71

T C l l t th H li A l f th


71/84

To Calculate the Helix Angles of the

Leading and Following Sides of a

Square Thread

diametermajorofncecircumfere

threadofleadanglefollowingTan

diameterminorofncecircumfere

threadofleadangleleadingTan

55-72


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Clearance

Square toolbit must beprovided with approximately1 clearance on each side

Makes it thinner at bottom

For leading side of tool

Add 1 to calculated helixangle

On following side

Subtract 1 from calculatedangle


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Procedure to Cut Square thread

1. Grind threading tool to proper leading andfollowing angles Width of tool should be ~.002 in. wider than

thread groove

2. Align lathe centers and mount work

3. Set quick-change gearbox for required

number of tpi4. Set compound rest at 30 to right, whichwill provide side movement if necessary toreset cutting tool

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5. Set threading tool square with work and

center6. Cut right-hand end of work to minordiameter for approximately .060 in. long

7. If work permits, cut recess at end of threadto minor diameter

8. Calculate single depth of threadN

500.

9. Start lathe and just touch tool to workdiameter

10. Set crossfeed graduated collar to zero

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11. Set .003 in. depth of cut with crossfeed

screw and take trial cut12. Check thread with thread pitch gage

13. Apply cutting fluid and cut thread to depth,moving crossfeed in from .002 to .010 in.for each cut Depth of cut will depend on thread size and

nature of workpiece

Note: Since thread sides are square, allcuts must be set using crossfeed screw

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Acme Thread

Gradually replacing square thread because

stronger and easier to cut with taps and dies

Used extensively for lead screws because29 angle formed by its sides allows split nut

to be engaged during thread cutting

Provided with .010-in. clearance for bothcrest and root on all sizes of threads

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Procedure to Cut an Acme Thread

1. Grind toolbit to fit

end of Acme thread gage

2. Grind pointof tool flat until

it fits into slots

of gage

Indicates number

of threads per

inch to be cut


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3. Set quick-change gearbox to required

number of threads per inch4. Set compound rest 14 to right

5. Set Acme threading tool on center and

square it with work using gage6. At right-hand end of work, cut section

.060 in. long to minor diameter

7. Cut thread to proper depth by feedingcutting tool, using compound rest

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Measuring Acme Threads

One Wire Method Single wire or pin of correct diameter placed inthread groove and measured with micrometer

Thread correct size when micrometer reading over

wire is same as major diameterof thread and wire is tightin the thread

Diameter of wire to be

used can be calculated

as follows:

Wire diameter = .4872 x pitch


55-80


80/84

Internal Threads

Cutting threads in a hole

Operation performed on work held in chuckor collet or mounted on faceplate

Threading tool similar to boring toolbit exceptshape ground to form of thread to be cut

Procedure

1. Calculate tap drill size of thread

N

1-diametermajorsizedrillTap

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2. Mount work to be threaded in chuck orcollet or faceplate

3. Drill hole approximately 116 in. smallerthan tap drill size in workpiece

4. Mount boring tool in lathe and bore hole totap drill size

5. Recess start of hole to major diameter ofthread for .060-in. length

6. If thread does not go through workpiece, a

recess should be cut at end of thread tomajor diameter; recess should be wideenough to allow threading tool to run outand permit time to disengage split-nut lever

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The compound rest is set at 29 to the left

for cutting right-hand internal threads.


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7. Set compound rest at 29to left, to right forleft-hand threads

8. Mount threading toolbit into boring bar andset it to center

9. Square threading tool with thread center

gage10. Place mark on boring bar, measuring from

threading tool, to indicate length of hole tobe threaded

11. Start lathe and turn crossfeed handle outuntil threading tool just scratches internaldiameter

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7. Set crossfeed graduated collar to zero

8. Set .003-in. depth of cut by feeding

compound rest outand take trial cut9. At end of eachcut on internal thread,

disengage split-nut lever and feed crossfeedhandle into clear thread

10. Clear threading tool from hole and checkpitch of thread

11. Return crossfeed handle back to zero and set

depth of cut by turning compound rest outdesired amount

12. Cut thread to depth; check fit with a screwor threaded plug gage

unit55 threads and thread cutting

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