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Measurement of Individual Elements (Metrology)

15.7.

15.7.1. Measurement of tooth thickness.

The permissible error or the tolerance on thickness of tooth is the variation of actual thickness of tooth from its

theoretical value. The tooth thickness is generally measured at pitch circle and is therefore, the pitch line thickness of

tooth. It may be mentioned that the tooth thickness is defined as the length of an arc, which is difficult to measure

directly. In most of the cases, it is sufficient to measure the chordal thickness i.e., the chord joining the intersection of the

tooth profile with the pitch circle. Also the difference between chordal tooth thickness and circular tooth thickness is

very small for gear of small pitch. The thickness measurement is the most important measurement because most of the

gears manufactured may not undergo checking of all other parameters, but thickness measurement is a must for all

gears. There are various methods of measuring the gear tooth thickness.

(i) Measurement of tooth thickness by gear tooth vernier calliper, (ii) Constant chord method. (iii) Base

tangent method, (iv) Measurement by dimension over pins.

The tooth thickness can be very conveniently measured by a gear tooth vernier. Since the gear tooth thickness varies

from the tip of the base circle of the tooth, the instrument must be capable of measuring the tooth thickness at a specified

position on the tooth. Further this is possible only when there is some arrangement to fix that position where themeasurement is to be taken. The tooth thickness is generally measured at pitch circle and is, therefore, referred to as

pitch-line thickness of tooth. The gear tooth vernier has two vernier scales and they are set for the width (w) of the toothand the depth (d) from the top, at which w occurs.

Considering one gear tooth, the theoretical values of w and d can be found out which may be verified by the instrument.In Fig. 15.14, it may be noted that w is a chord ADB, but tooth thickness is specified as an arc distance AEB. Also the

distance d adjusted on instrument is slightly greater than the addendum CE, w is therefore called chordal thickness and dis called the chordal addendum.

In Fig. 15.14, w =AB = 2ADNow, AdD = 8 = 36074N, where N is the number of teeth,

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Fig. 15.14

Any error in the outside diameter of the gear must be allowed for when measuring tooth thickness.In the case of helical gears, the above expressions have to be modified to take into account the change in curvature

along the pitch line. The virtual number of teeth Nv for helical gear = Mcos3 a (a = helix angle)

These formulae apply when backlash is ignored. On mating gears having equal tooth thickness and without addendum

modifications, the circular tooth thickness equals half the circular pitch minus half the backlash.

Gear Tooth Calliper.

(Refer Fig. 15.15). It is used to measure the thickness of gear teeth at the pitch line or chordal thickness of teeth and the

distance from the top of a tooth to the chord. The thickness of a tooth at pitch line and the addendum is measured by an

adjustable tongue, each of which is adjusted independently by adjusting screw on graduated bars. The effect of zeroerrors should be taken into consideration.

This method is simple and inexpensive. However it needs different setting for a variation in number of teeth for a given

pitch and accuracy is limited by the least count of instrument. Since the wear during use is concentrated on the two jaws,

the calliper has to be calibrated at regular intervals to maintain the accuracy of measurement.

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Fig. 15.15. Gear Tooth Vernier Calliper.

15.7.2. Constant Chord Method.

In the above method, it is seen that both the chordal thickness and chordal addendum are dependent upon the number

of teeth. Hence for measuring a large number of gears for set, each having different number of teeth would involve

separate calculations, inus tne proceaure Decomes laborious and time-consuming one.The constant chord method does away with these difficulties. Constant chord of a gear is measured where the tooth

flanks touch the flanks of the basic rack. The teeth of the rack are straight and inclined to their centre lines at the

pressure angle as shown in Fig. 15.16.

Also the pitch line of the rack is tangential to the pitch circle of the gear and,- by definition, the tooth thickness of therack along this line is equal to the arc tooth thickness of the gear round its pitch circle. Now, since the gear tooth and

rack space are in contact in the symmetrical position at the points of contact of the flanks, the chord is constant at this

position irrespective of the gear of the system in mesh with the rack.

Fig. 15.16

This is the property utilised in the constant chord method of the gear measurement.

The measurement of tooth thickness at constant chord simplified the problem for all number of teeth. If an involute tooth

is considered symmetrically in close mesh with a basic rack form, then it will be observed that regardless of the number

of teeth for a given size of tooth (same module), the contact always occurs at two fixed point A and B. AB is known as

constant chord. The constant chord is defined as the chord joining those points, on opposite faces of the tooth, whichmake contact with the mating teeth when the centre line of the tooth lies on the line of the gear centres. The value of AB

and its depth from the tip, where it occurs can be calculated mathematically and then verified by an instrument. The

advantage of the constant chord method is that for all number of teeth (of same module) value of constant chord is

same. In other words, the value of constant chord is constant for all gears of a meshing system. Secondly it readily lends

itself to a form of comparator which is more sensitive than the gear tooth vernier.

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15.7.3. Base Pitch.

This is defined as the circular pitch of the teeth measured on the base circle. In Fig. 15.17, AB represents the portion of

a gear base circle, CD andEF the sides

of two teeth, FD being the base pitch. From the property of involute, if any line as GH is drawn to cut the involutes and

tangential to the base circle, the GH = FD.

Thus base pitch could also be defined as equal to the linear distance between a pair of involutes measured along a

common generator.Base circumference = 2kRb

.-. Base pitch = 2kRb/N

If § is the pressure angle, then

RB = P.C.R. x cos <t> = (P.C.D./2) cos <J> .-. Base pitch = (2nlN) x (P.C.D./2) x cos (j) = nm cos (|>

This is the distance between tangents to the curved portions of any two adjacent teeth and can be measured either with

a height gauge or on an enlarged projected image of the teeth. This principle is utilised in ‘David Borwn’ tangent

comparator and it is the most commonly used method.

Fig. 15.17

15.7.4. Base Pitch Measuring Instrument.

This instrument has three tips. One is the fixed measuring tip, other one is the sensitive tip whose position can be

adjusted by a screw and the further movement of it is transmitted through a leverage system tothe dial indicator ;

and the third tip is the supplementary adjustable stop which is meant for the stability of the instrument and its position

can also be adjusted by a screw. The distance between the fixed and sensitive tip is set to be equivalent to the base

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pitch of the gear with the help of slip gauges. The properly set-up instrument is applied to the gear so that all the threetips contact the tooth profile. The reading on dial indicator is the error in the base pitch.

Fig. 15.18

15.7.5. The Base Tangent Method. (‘David Brown’ tangent comparator).

In this method, the span of a convenient number of teeth is measured with the help of the tangent comparator. This uses

a single vernier calliper and has, therefore, the following advantages over gear tooth vernier which used two vernier

scales :

(i) the measurements do not depend on two vernier readings, each being function of theother.

(ii) the measurement is not made with an edge of the measuring jaw with the face. Consider a straight generator (edge)

ABC being rolled back and forth along a base circle

(Fig. 15.19). Its ends thus sweep out opposed involutes A2 AA^ and C2 CCi respectively. Thus the measurements

made across these opposed involutes by span gauging will be constant (i.e. AC = AiCi = A2C2 = A0Cq) and equal to

the arc length of the base circle between the origins of involutes.

Further the position of the measuring faces is unimportant as long as they are parallel \ and on an opposed pair of the

true involutes. As the tooth form is most likely to conform to a true involute at the pitch point of the gear, it is alwayspreferable to choose a number of teeth such that the measurement is made approximately at the pitch circle of the gear.

The value of the distance between two opposed involutes, or the dimension over parallel faces is equal to the distance

round the base circle between the points where the corresponding tooth flanks cut i.e., ABC in Fig. 15.19. It can be

derived mathematically as follows :

Fig. 15.19. Generation of pair of involutes by a common generator.

The angle between the points A and C on the pitch circle where the flanks of the opposed involute teeth of the gear cutthis circle can be easily calculated.

Let us say that the gear has got W number of teeth and AC on pitch circle corresponds to ‘S’ number of teeth. (Fig.

15.20); .-. Distance AC = (S – 1/2) pitches

.-. Angle subtended by AC = (S- 1/2) x 2rc/A7 radians.

Angles of arcs BE and BD

It may be noted that when backlash allowance is specified normal to the tooth flanks, this must be simply subtracted

from this derived value.

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Tables are also available which directly give this value for the given values of S, N and m.

This distance is first calculated and then set in the ‘David Brown’ tangent comparator (Fig. 15.21) with the help of slip

gauges. The instrument essentially consists of a fixed anvil and a movable anvil. There is a micrometer on the moving

anvil side and this has a very limited movement on either side of the setting. The distance is adjusted by setting the fixed

anvil at desired place with the help of looking ring and setting tubes.

15.7.6. Tangential Gear Tooth Calliper.

It is utilised for measuring variations on the basic tooth profile from the outside diameter of spur and helical gears. The

instrument consists of body, on the underside of which there are two slides having the tips acting like measuring

contacts. The extended spindle of a dial indicator with the contact point A

Fig. 15.20

passes between the two tips along the vertical axis of symmetry of the instrument. The measuring tips are spread apart

or brought together simultaneously and symmetrically in reference to the central axis by a screw which has a right-hand

and a left-hand thread. The contact faces of the measuring tips are flat and arranged at angles of

Fig. 15.21. ‘David Brown’ Base Tangent Comparator.

14.5° or 20° with the central axis. The calliper is set up by means of a cylindrical master gauge of proper diameterbased on the module of the gear being checked. After adjusting the tips by the screw, these are locked in position by

locking nuts. The properly set up instrument is applied to the gear tooth and the dial indicator reading shows how much

the position of the basic tooth profile deviates in reference to the outside diameter of the gear.

15.7.7. Test Plug Method for Checking Pitch Diameter and Tooth Spacing. Measurement over the rollers placed in the

space between a pair of gear teeth gives a convenient method for checking tooth spacing and the pitch diameter. The

special case of the roller with its centre on the pitch circle simplifies the problem. It is, therefore, considered desirable to

find the diameter on the roller whose centre will lie on the pitch circle and to derive an expression for the distance over

the rollers placed in opposite teeth spaces with the centres of rollers lying on the pitch circle.

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Fig. 15.22

Fig. 15.23

In Fig. 15.23 a rack tooth is shown symmetrically in mesh with a gear tooth space, the curved sides of the gear tooth

touching the straight rack sides at A and B on the line of action. Let us assume that the centre of the roller lies on the

pitch point O. Now, if the rack tooth be considered as an empty space bounded by its outline, a circle with centre O

and radius OB would fit and touch the rack space at A and B since OA and OB are perpendicular to the sides of the

rack tooth. Thus the circle would touch the gear teeth at A and B.

In A OBD, OB is the radius of roller

OD = Circular pitch/2 = (7t/4)m

OBD =90°, BdD = <|> = pressure angle, .’. OB = OD cos <j> = (re/4) m cos <(>

Dia. of roller = 2 x OB = 2 x (ron/4) cos <|) = (rc/4) m cos <|>.

This is the diameter of a roller which will rest in tooth space and lie with its centre on the pitch circle. This value is

constant for all gears of same pitch and pressure angle.

For gears with even number of teeth, a direct measurement by placing two rollers in exactly opposite tooth spaces ispossible. In this case, the gauging diameter over the rollers

If the gear has an old number of teeth, a radial measurement with the gear between centres can be carried out, using a

comparator with the gear. The accuracy of the spacing over any number of teeth may be checked by finding the angles

subtended at the centre and comparing this with that obtained from a chordal check of the plugs.

As already indicated, precision gears and other gears are generally checked for tooth thickness by dimension over pins,

as the dimensions over pins reading is mathematically related to the tooth thickness. This also verifies the correctness of

profile and other elements of gear. Under this method two pins of equal diameter are placed in two opposite tooth

spaces

for gears having odd number of teeth.

where M = Required size over the wires, m = Module, (j) = Pressure angle, d = Diameter of wire = 1.728 x m, §m =

Pressure angle at centre of pin and is given by the relation

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Using the above equations, the size ‘AT over wires can be calculated. In case of helical gears the formulae used are as

below :

The helical gears with odd number of teeth should not be measured with two wires, because in this case the correction

factor cos (90/iV) is not valid and can result in serious errors. In such cases the gear can be mounted on an arbour and

a radial measurement made from the top of the wire to the axis of gear.

Size over wires/balls for helical gears

15.7.8. Checking of Profile of Involute Shape of Gear.

Profile is the portion of tooth flank between the outside circle or start of tooth tip and the specified form circle of

diameter approximately equal to pitch circle diameter minus twice the module.

Profile error is the deviation of the actual tooth from the theoretical profile in the designed reference plane of rotation.

For testing profile, tip relief and any portion of the tooth surface below the active profile is not to be considered. The

tolerance on the profile error is permitted as per the following table.

Table

Accuracy Class or Grade of GearProfile Tolerance in Microns

1 2.0 + 0.06 x *

2 2.5 + 0.10 xk

3 3.0 + 0.16 x A;

4 4.0 + 0.25 x h

5 5.0 + 0.40 x k

6 6.3 + 0.63 x k

7 8.0 + 1.0 x k

8 10 + 1.6 x k

9 16 + 2.5 x k

10 25 + 4.0 x k

11 40 + 6.3 x k

12 63 + 10xA

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Where m is module, and D is the pitch circle diameter in mm.

(a) Inspection of profile using dividing head and height gauge.

This method has been derived from the property of the involute as shown in Fig. 15.24. The distance between points Aand B measured on the generatix tangent to the base circle is equal to the arc from the tangent point B to the origin P of

the involute on the base circle, i.e. arc BP = AB = (Db ji/360) x ez

S.No. Dividing Head Reading ez values Dbxn AB values = „ – . x e, obu

1 ez

2 ez + 10′

CO ez + 20′

4 —

5 —

6 ez-10′

7 e,-20′

CO —

9 —

10 —

When involute profile is rotated on dividing head by small roll angles, then the consecutive profile points A fall on to the

vertical tangent line. It is possible to compute suitable roll angles for any diameter Dz from the corresponding pressure

angle by equations : DJDb = sec <j>z, ez = (180/rc) x tan

Initially for any known diameter; say pitch diameter, the value of ez and so AB is found. The highest gauge is set to zero

at this height above the gear centre by means of slip gauges, then the corresponding position of gear profile is obtained

by rotating the gear tooth towards indicating stylus until zero is obtained. Number of angles is increased or decreased insteps of 10′ or half degree as is convenient and for these values of ez the values of AB are calculated, and the height

gauge is set to this height by means of slips and then the dial reading over the tooth is compared. The deviation of dial

reading gives the error of profile at these points.

This is a very time consuming method but best suited for calibration of master involute. It is therefore useful only for very

precision components and involute master cams.

(b) Gear involute measuring machine. This machine is designed for checking the involute profiles of the spur and

other gears. The machine is suitable for inspection of gear having module from 1 to 10 mm having maximum outside

diameter upto 300 mm. The machine is provided with a measuring stylus. The kinematic design of the machine is such

that when job is rotated the measuring stylus which is initially set at base circle radius of the gear by means of slip gauge,

is also slided along the involute curve. The deviation of the tooth profile from the correct involute is indicated by a dial

indicator of accuracy 0.001 mm connected by lever mechanism with the stylus. A master involute template is also

provided with the machine for setting and calibration of the machine.

(c) Checking of involute shape of gear. As the involute curve is traced by the end A of a straight edge which rolls

without slipping on a base circle diameter cylinder, any point C on the curve will correspond to the position CE of the

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straight edge, which, therefore, always remains tangential to the base circle. Conversely if the base circle cylinder were

to roll on a fixed straight edge, any fixed point e.g., C would move in an involute path, such as CA, as the cylinder rolls

along the straight edge CE.

A straight edge rolled on the edge of a disc will be seen to be the equivalent of this arrangement, and provides theprinciple on which the involute tester operates. The gear to be tested is held on the mandrel m, which carries a ground

disc d having exactly the same diameter as the base circle of the gear under test (Fig. 15.25). A straight edge e is

mounted on a slide on the body of the instrument and in contact with base circle disc, so that as the straight edge moves

along the slide, the base circle disc and gear are rotated without slip. A point on straight edge thus describes the true

involute corresponding to the base circle, and if the top of an indicator of some kind is

mounted exactly in the plane of the edge of the straight edge and in contact with the tooth flank it will register by its

movements any departure of the tooth profile from the theoretical involute resulting either from errors or from deliberate

modification of the profile. The indicator can also be replaced by the sensing element of a recorder so that permanent

records of the gear teeth profiles can be made.

Fig. 15.25. Principle of Involute Tester.

(d) Involute profile testing machine. Such machines can be set to desired base circle radius values, thus doing away

the necessity of having base disc for each gear of different dimensions. This facility is provided by a master base, disc or

involute cam, built into the machine, and coupled to a linkage system which enables the base radius of the generated

curve to be varied by adjustment of the linkage bar positions.

These machines generate the required involute shape, measure the departures from it that exist on the actual tooth flanks

of the gear. The principle of operation is that of a base circle rolling without slip along a straight edge, or vice versa. A

stylus detects the deviations in the metal condition of the tooth flank from the true involute in the form of a continuous

trace on a strip chart recorder. A perfect profile will result in a straight trace parallel to the longitudinal axis of the chart

paper.

15.7.9. Measurement of Gear Pitch. Gear pitch can be measured in the following

ways :

(i) Cumulative circular pitch error over a span of teeth.

(ii) Adjacent pitch error or pitch variation. (iii) Base pitch variation.

Here L = Knm2, where If is the sector of pitches over which pitch error is to be checked.

The pitch error of gear can be easily determined by comparing the span length over a specified number of teeth i.e., the

cumulative error on a sector of predetermined pitches is measured.

The pitch variation is the difference between the longest and shortest circular pitch in the whole gear.

The measurement of cumulative error over a span of teeth and also the pitch variations can be conveniently measuredusing a dividing head and height gauge fitted with dial indicator. The basic method of measurement involves indexing the

gear through single or multiple tooth angles and determining flank position circumferentially by means of a precision

indicator mounted on a radially disposed slide. By means of the dividing head, rotation to the gear is given by the

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amount of theoretical angular pitch, the variation in the position of tooth is measured by the dial indicator. For larger

gears, the angular accuracy of the dividing table must be higher in order that pitch error is determined accurately.

Though dividing tables with ± 10 seconds of arc resolution are common, dividing tables are available which can be read

to the nearest second of arc. Of course, these have to be calibrated using precision polygon and photo-electric

autocollimator. The indicator unit should be capable of measuring reliably upto 0.001 mm over a range of0.025 mm.

For gears having larger cumulative pitch errors, indicators of lower sensitivity have to be used.

Tolerances on pitch errors A. Tolerance over a sector of ‘K’ Pitches

Grade of Accuracy Class of gearPermissible Error in Microns

1 0.25 <L + 0.6

2 0.4a/L + 1.0

3 0.63 VZT +1.6

4 1.0 +2.5

5 1.6 <L +4.0

6 2.5VT+6.0

7 3.55^+8.0

8 5.0 <L+ 12

9 7.1 VT + 17

10 10 +28

11 14 -4L+ 33

12 20VT+83

B. Tolerance on Pitch Variations

Class Permissible Error in Microns

1 0.80 + 0.06 F

2 1.25 + 0.1F

3 2.0 + 0.16F

4 3.2 + 0.25 F

5 5.0 + 0.40 F

6 8.0 + 0.63 F

7 11 + 0.90 F

8 16 + 1.25 F

9 22 + 1.80 F

10 32 +2.50 F

11 45 + 3.55 F

12 63 +5.0 F

The base pitch is the circular pitch of the teeth measured on the base circle, as we know that the base pitch = p x cos §.

This is also the distance between the tangent to the curved positions of any two adjacent teeth. The base pitch can be

very conveniently measured using ordinary base pitch measuring instruments which measure the straight distance

between tangents to the adjacent teeth.Base pitch can be accurately measured using tool makers or universal microscope.

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Here F = m + 0.25 -H5 where D is the pitch circle diameter in mm.Tooth-to-tooth pitch errors can be easily determined by use of two dial gauges by measuring the position of a suitable

point on a tooth after the gear has been indexed through a suitable angle.

The gear is mounted in the centre with indexing arrangement. Two dial gauges are mounted as shown in Fig. 15.26.

There is a spindle below the dial gauge having a small sphere at the end and touching the gear tooth at the pitch circle.The gear is then indexed through successive pitches to give a constant reading on dial gauge A. Any changes in the

reading on dial gauge B indicate that pitch errors are present. The actual error can be determined by deducting the

individual reading on dial B from the mean of the readings.

The pitch of the gear teeth can also be measured by measuring the distance from a point on one tooth to a suitable point

on the next tooth.

Circular Pitch Measuring Machine.

This instru-

Fig. 15.26

ment is used lor checking the circular pitch ol gear tooth. The two measuring contact tips are applied on the same sides

of adjacent teeth of the gear. The left-hand tip is first set up to the required module by means of some suitable

arrangement. The right hand tip is a two armed lever whose one contacts the gear tooth and the other one actuates the

contact point of the dial indicator. Two guide points are also provided for the stability of the instrument.

The pitch variations can also be measured by the instrument shown in Fig. 15.27. It employs a fixed finger and stop for

consistent positioning on successive pairs of teeth, and a movable finger whose movement can be sensed by a dial

indicator.

It may be noted that readings obtained by above instrument will be affected by profile variations and runout of the gear.

In the case of helical gears, measurements may be made in the normal plane of the conjugate rack and divided by the

cosine of helix angle for comparison with standard tolerances.

It may be mentioned that the above method of measuring pitch error can be applied to medium sized gears measuring

from a few cms upto about a metre diameter. Small gears as used in watches and clock are inspected by optical proj

ection in which the enlarged images of some of the teeth are compared

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Fig. 15.27. Schematic arrangement of a pitch checking instrument.

against a master diameter at the screen of the projector, b or thick gears convergent illumination is used to enable optical

focussing at a selected section across the face-width. Effect of Eccentricity of Pitch Error

It may be emphasized that gear pitch error and tooth eccentricity are inseparable because presence of one affects the

other. Since eccentricity can take any position with reference to the tooth under consideration, the effective eccentricity

for various positions of gear rotation at angle 6 is expressed as equal to e sec § sin (cj) + 8), where e = eccentricity and

<|> = pressure angle.

Thus if a gear has been measured for tooth spacing error, the effect of its mounting eccentrically on pitch error can be

taken care of by applying corrections progressively to the observed error values by calculating the correction applicable

to each tooth from the above equation.

15.7.10. Runout. Runout means the eccentricity in the reference or pitch circle.

Gears that are eccentric tend to have a vibration per revolution. A badly eccentric tooth may cause an abrupt gear

failure. The runout in the gears is measured by employing gear eccentricity testers. The gear is held on a mandrel in the

centres and the dial indicator of the tester possesses the special tip depending upon the module of gear being checked.

The tip is inserted in between the tooth spaces. The gear is rotated tooth by tooth. The maximum variation is noted from

the dial indicator reading and it gives the runout of the gear. The runout is twice the eccentricity. The adjoining table

indicates the permissible runouts.

Class or Grade Permissible Runout

in Microns

1 0.224 F + 3.0

2 0.335 F + 4.5

3 0.560 F +7.0

4 0.900 F + 11

5 1AF+ 18

6 2.24 F + 28

7 3.15^+40

8 4.0 F + 50

9 5.CF+63

10 6.3 F + 80

11 8.0 + 100

12 10.0 F+ 125

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15.7.11. Lead.

It is the axial advance of the helix or the worm thread per turn. The control of thread lead is necessary to ensure

adequate contact across the face width. The relationship to the helix angle has already been explained earlier.

The instrument which checks the lead consists of a probe being advanced along a tooth surface, parallel to the axis. The

probe is a suitable dial indicator tip fixed in a suitable device.

When the gear is rotated, the displacement of the probe in one complete revolution of gear is found which is the lead.

In the case of worm thread, the axial pitch of the thread is first measured which multiplied by the number of threads in

the worm gives the lead.

15.7.12. Backlash.

Backlash in the gears is the play between the mating tooth surfaces. For the purposes of measurement and calculations,

backlash is defined as the amount by which a tooth space exceeds the thickness on an engaging tooth. Backlash in the

gear teeth results on account of errors in profile, pitch thickness of teeth etc. It is measured by mounting the gears in

specified position. Backlash should be measured at the tightest point of the mesh. The pinion is held solidly against

rotation and a rigidly mounted dial indicator is placed against the tooth at the extreme heel perpendicular to the surface.

The backlash is determined by moving the gear back and forth. The backlash variation is measured by locating the

points of maximum and minimum backlash in the pair and obtaining the difference. For precision gears the variation

should not exceed 0.02 to 0.03 mm.

15.7.13. Lead Measurement.

In order that tooth load be uniformly distributed across the face width of the gears, it is essential that lead per tooth of

mating gears should be closely matched. Errors in the helix of either gear would result in non-uniform load concentration,

resulting in noisy operation and damage. Irregularities in lead could occur due to either poor manufacture or from the

presence of tooth undulations.

It is important to note that while the helix angle of pinion and gear is same but their lead is different depending on their

diameters. Also the helix angle value increases from the roots to the tips of the teeth.

Lead can be measured either on a point-to-point basis, or by means of continuous generation using special purpose

measuring machines. One type of generating machine incorporates a sine bar mechanism together with a means of

converting a derived linear motion into a rotary motion.

Next post: Composite Method of Gear Checking (Metrology)

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Metrology

What is Metrology

Physical Measurement (Metrology)

Need of Inspection (Metrology)

Measuring Instruments (Metrology)

Selection of Instruments (Metrology)

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