Thread Micrometer Method

8/10/2019 Thread Micrometer Method

1/16

Measurement of major diameter. For the measurement of major diameter of external threads, a good quality handmicrometer is quite suitable. In taking readings, a light pressure must be used as theanvils make contact with the gauge at points only and otherwise the errors due to

compression can be introduced. It is, however, also desirable to check the micrometerreading on a cylindrical standard of approximately the same size, so that the zero erroretc., might not come into picture.For greater accuracy and convenience, the major diameter is measured by benchmicrometer. This instrument was designed by N.P.L. to estimate some deficienciesinherent in the normal hand micrometer. It uses constant measuring pressure and withthis machine the error due to pitch error in the micrometer thread is avoided. In orderthat all measurements be made at the same pressure, a fiducial indicator is used in

place of the fixed anvil. In this machine there is no provision for mounting the workpiecebetween the centres and it is to be held in hand. This is so, because, generally thecentres of the workpiece are not true with its diameter. This machine is used as acomparator in order to avoid any pitch errors of micrometers, zero error setting etc. Acalibrated setting cylinder is used as the setting standard.The advantage of using cylinder as setting standard and not slip gauges etc., is that itgives greater similarity of contact at the anvils. The diameter of the setting cylinder mustbenearly same as the major diameter. The cylinder is held and the reading of the

micrometer is noted down. This is then replaced by threaded workpiece and againmicrometer reading is noted for the same reading of fiducial indicator. Thus, if the sizeof cylinder is approaching that of major diameter, then for a given

Fig. 13.8. Bench Micrometer. 13.4.2.

Major diameter of internal threads. The measurement of the elements of an internal thread is more cumbersome. Since it isdifficult to approach the elements of internal thread, an indirect approach is followed bymaking a cast of the thread. The main art thus lies in obtaining a perfect cast, because
http://lh5.ggpht.com/_X6JnoL0U4BY/S02UP-dQTQI/AAAAAAAAFNM/D0fFQCFnIFM/s1600-h/tmp578_thumb1.pnghttp://lh3.ggpht.com/_X6JnoL0U4BY/S02UOViwcnI/AAAAAAAAFNE/tG9-cSrIBiA/s1600-h/tmp577_thumb1.pnghttp://lh5.ggpht.com/_X6JnoL0U4BY/S02UP-dQTQI/AAAAAAAAFNM/D0fFQCFnIFM/s1600-h/tmp578_thumb1.pnghttp://lh3.ggpht.com/_X6JnoL0U4BY/S02UOViwcnI/AAAAAAAAFNE/tG9-cSrIBiA/s1600-h/tmp577_thumb1.png


2/16

once good cast is available the various elements can be measured as for externalthreads.Cast may be made by plaster of paris, dental wax, or sulphur. The part whose internalthread is to be measured is first cleaned and brushed with a fine oil. The part is then

mounted between two wooden blocks whose upper surface lie about half-way up thering. Cast material is then poured to a depth less than the radius of part to permit easyremoval of cast without screwing it out. After the plaster is set, it should be taken outwithout rotating, but by pulling up the middle portion of the cast. It may be men-

Fig. 13.9. Measurement of minor diameter. tioned that taking out of sulphur cast is easier than the plaster. Oiling is not necessary incase of sulphur cast.13.4.3. Measurement of minor diameter. This is also measured by a comparative process using small Vee-pieces which makecontact with a root of the thread. The Vee-pieces are available in several sizes havingsuitable radii at the edges. The included angle of Vee-pieces is less than the angle of

the thread to be checked so that it can easily probe to the root of the thread. Tomeasure the minor diameter by Vee-pieces is suitable for only Whitworth and B.A.threads which have a definite radius at the root of the thread. For other threads, theminor diameter is measured by the projector or microscope.The measurement is carried out on a floating carriage diameter measuring machine inwhich the threaded work-piece is mounted between centres and a bench micrometer isconstrained to move at right angles to the axis of the centre by a Vee-ball slide. Themethod of the application of Vee-pieces in the machine is shown diagrammatically inFig. 13.9. Thedimensions of Vee-pieces play no important function as they are interposed betweenthe micrometer faces and the cylindrical standard when standard reading is taken.It is important while taking readings, to ensure that the micrometer be located at rightangles to the axis of the screw being measured. The selected Vees are placed on eachside of the screw with their bases against the micrometer faced. The micrometer head isthen advanced until the pointer of the indicator is opposite the zero mark, and note
http://lh3.ggpht.com/_X6JnoL0U4BY/S02URDpN9kI/AAAAAAAAFNU/Fyyb2yvTN0k/s1600-h/tmp579_thumb1.png


3/16

being made of the reading. The screw is then replaced by standard reference disc or aplain cylindrical standard plug gauge of approximately the core diameter of the screw tobe measured and second reading of the micrometer is taken.If reading on setting cylinder with Vee-pieces in position = Ri and reading on thread =

i?2 and diameter of setting cylinder = Dlt Then minor diameter = Di + (R2 Ri).Readings may be taken at various positions in order to determine the taper ovality.Before proceeding to the measurement of effective diameter, the screw diametermeasuring-machine is first described in brief here. The machine is shown in Fig. 13.10.

Also refer Fig. 13.16 for schematic sketch. If consists of three main units. A basecasting carries a pair of centres, on which the threaded work-piece is mounted. Anothercarriage is mounted onit and is exactly at 90 to it. On this is provided another carriage capable of moving

towards the centres. On this carriage one head having a large thimble enabling readingupto 0.002 mm is provided. Just opposite to it is a fixed anvil which is spring loaded andits zero position is indicated by a fiducial indicator. Thus the micrometer elements areexactly perpendicular to the axis of the centres as the two

Fig. 13.10. Screw measuring machine. carnages are located perpendicular to each other. On the fixed carriage the centres aresupported in two brackets fitted on either end. The distance between the two centrescan be adjusted depending upon the length of the threaded job. After job is fittedbetween the centres the second carriage is adjusted in correct position to takemeasurements and is located in position. The third carriage is then moved till thefiducial indicator is against the set point. The readings are noted from the thimble head.It is now obvious that the axis of the indicator and micrometer head spindle is same andis perpendicular to the line of two centres. The indicator is specially designed for thisclass of work and has only one index line, against which the pointer is always to be set.This ensures constant measuring pressure for all readings. Sufficient friction is providedby the conical pegs to restrain the movement of carriage along the line of centres. Theupper carriage is free to float on balls and enables micrometer readings to be taken ona diameter without restraint. Squareness of the micrometer to the line of centres can be
http://lh4.ggpht.com/_X6JnoL0U4BY/S02UYrnAhEI/AAAAAAAAFNc/aAc8i-Vu74o/s1600-h/tmp581_thumb1.png


4/16

adjusted by rotating the pegs in the first carriage which is made eccentric in itsmounting.

Above the micrometer carriage, two supports are provided for supporting the wires andVee-pieces for measurement of effective diameter etc.

13.4.4. Minor diameter of internal threads. Minor diameter of internal threads can be measured conveniently by the followingmethods : (i) Using taper parallels. The taper parallels are pairs of wedges having radiused andparallel outer edges. The diameter across their outer edges can be changed by slidingthem over each other as shown in Fig. 13.11. The taper parallels are inserted inside thethread and adjusted until firm contact is established with the minor diameter. The

diameter over the outer edges is measured with a micrometer. This method is suitablefor smaller diameter threads.(ii) Using rollers. For threads bigger than 10 mm diameter, precision rollers areinserted inside the thread and proper slip gauge inserted between the rollers as shownin Fig.

Fig. 13.11. Measuring minor diameter of internal thread using taper parallels.

Fig. 13.12. Measurement of minor diameter of internal thread using rollers. 13.12, so that firm contact is obtained. The minor diameter is then the length of slipgauges plus twice the diameter of rollers.13.4.5. Effective diameter measurements.
http://lh5.ggpht.com/_X6JnoL0U4BY/S02Ue7qw85I/AAAAAAAAFNs/j6z9HxFvVPY/s1600-h/tmp583_thumb1.pnghttp://lh3.ggpht.com/_X6JnoL0U4BY/S02UaDfxBHI/AAAAAAAAFNk/I9jfkpHlr9o/s1600-h/tmp582_thumb1.pnghttp://lh5.ggpht.com/_X6JnoL0U4BY/S02Ue7qw85I/AAAAAAAAFNs/j6z9HxFvVPY/s1600-h/tmp583_thumb1.pnghttp://lh3.ggpht.com/_X6JnoL0U4BY/S02UaDfxBHI/AAAAAAAAFNk/I9jfkpHlr9o/s1600-h/tmp582_thumb1.png


5/16


6/16

Fig. 13.14. One-wire method.

Fig. 13.15 (b). Two wire method. For correct results it is necessary to use a separate thread micrometer for every size ofscrew thread to be gauged, otherwise there will always be a small amount of errorinherent in thread micrometer.

A big advantage of thread micrometer is that this is the only method which shows thevariation for the drunken thread.13.4.5.2.

One-wire method. In this method, one wire is placed between two threads at one side and on the otherside the anvil of the measuring micrometer contacts the crests as shown in Fig. 13.14.First the micrometer reading is noted on a standard gauge whose dimension is nearlysame as to be obtained by this method. Actual measurement over wire on one side andthreads on other side = size of gauge difference in two micrometer readings.
http://lh3.ggpht.com/_X6JnoL0U4BY/S02UimXvkZI/AAAAAAAAFOE/ZBSHtGDmoRc/s1600-h/tmp586_thumb1.pnghttp://lh5.ggpht.com/_X6JnoL0U4BY/S02UhRhu2EI/AAAAAAAAFN8/iZyE2mOZy0M/s1600-h/tmp585_thumb1.pnghttp://lh3.ggpht.com/_X6JnoL0U4BY/S02UimXvkZI/AAAAAAAAFOE/ZBSHtGDmoRc/s1600-h/tmp586_thumb1.pnghttp://lh5.ggpht.com/_X6JnoL0U4BY/S02UhRhu2EI/AAAAAAAAFN8/iZyE2mOZy0M/s1600-h/tmp585_thumb1.png


7/16


8/16

the axis of the screw under test. In Fig. 13.16, the lower slide (wrongly indicated aslower side in Fig. 13.16) is capable of movement parallel with the axis of thread whilethe top slide moves at 90 to the thread axis.

Fig. 13.16. Thread measuring machine. 13.4.5.4. Three wire method. This method of measuring the effective diameter is anaccurate method. In this three wires or rods of known diameter are used : one on oneside and two on the other side [Fig. 13.17 (a) and (6)]. This method ensures thealignment of micrometer anvil faced parallel to the thread axis. The wires may be eitherheld in hand or hung from a stand so as to ensure freedom to the wires to adjustthemselves under micrometer pressure.

Fig. 13.17.Three wire method of measuring effective diameter .

(Also refer Problem 13.5 at the end of this chapter for rake correction andcompression correction). 13.4.6. Best Size Wire. This wire is of such diameter that it makes contact with the flanks of the thread on theeffective diameter or pitch line. Actually effective diameter can be measured with any
http://lh6.ggpht.com/_X6JnoL0U4BY/S02UtoLaZ-I/AAAAAAAAFOs/PA6xbMj8L9c/s1600-h/tmp592_thumb1.pnghttp://lh6.ggpht.com/_X6JnoL0U4BY/S02UrnrWQAI/AAAAAAAAFOk/sonlrgnRwHM/s1600-h/tmp591_thumb1.pnghttp://lh3.ggpht.com/_X6JnoL0U4BY/S02UmxvuNsI/AAAAAAAAFOc/1w-mMvP-jnI/s1600-h/tmp590_thumb1.pnghttp://lh5.ggpht.com/_X6JnoL0U4BY/S02UltXNIBI/AAAAAAAAFOU/mIx0Vz_Heuc/s1600-h/tmp589_thumb1.pnghttp://lh6.ggpht.com/_X6JnoL0U4BY/S02UtoLaZ-I/AAAAAAAAFOs/PA6xbMj8L9c/s1600-h/tmp592_thumb1.pnghttp://lh6.ggpht.com/_X6JnoL0U4BY/S02UrnrWQAI/AAAAAAAAFOk/sonlrgnRwHM/s1600-h/tmp591_thumb1.pnghttp://lh3.ggpht.com/_X6JnoL0U4BY/S02UmxvuNsI/AAAAAAAAFOc/1w-mMvP-jnI/s1600-h/tmp590_thumb1.pnghttp://lh5.ggpht.com/_X6JnoL0U4BY/S02UltXNIBI/AAAAAAAAFOU/mIx0Vz_Heuc/s1600-h/tmp589_thumb1.pnghttp://lh6.ggpht.com/_X6JnoL0U4BY/S02UtoLaZ-I/AAAAAAAAFOs/PA6xbMj8L9c/s1600-h/tmp592_thumb1.pnghttp://lh6.ggpht.com/_X6JnoL0U4BY/S02UrnrWQAI/AAAAAAAAFOk/sonlrgnRwHM/s1600-h/tmp591_thumb1.pnghttp://lh3.ggpht.com/_X6JnoL0U4BY/S02UmxvuNsI/AAAAAAAAFOc/1w-mMvP-jnI/s1600-h/tmp590_thumb1.pnghttp://lh5.ggpht.com/_X6JnoL0U4BY/S02UltXNIBI/AAAAAAAAFOU/mIx0Vz_Heuc/s1600-h/tmp589_thumb1.pnghttp://lh6.ggpht.com/_X6JnoL0U4BY/S02UtoLaZ-I/AAAAAAAAFOs/PA6xbMj8L9c/s1600-h/tmp592_thumb1.pnghttp://lh6.ggpht.com/_X6JnoL0U4BY/S02UrnrWQAI/AAAAAAAAFOk/sonlrgnRwHM/s1600-h/tmp591_thumb1.pnghttp://lh3.ggpht.com/_X6JnoL0U4BY/S02UmxvuNsI/AAAAAAAAFOc/1w-mMvP-jnI/s1600-h/tmp590_thumb1.pnghttp://lh5.ggpht.com/_X6JnoL0U4BY/S02UltXNIBI/AAAAAAAAFOU/mIx0Vz_Heuc/s1600-h/tmp589_thumb1.png


9/16

diameter wire which makes contact on the true flank of the thread, but the values soobtained will differ from those obtained with best size wires if there is any error in angleor form of thread. It is recommended that for measuring the effective diameter, alwaysthe best size wire should be used and for this condition the wire touches the flank at

mean diameter line within 1/5 of flank length (Refer Solved Problem 13.2). With bestsize wire, any error on the measured value of simple effective diameter due to error inthread form or angle is minimised.

13.4.7. Measurement of Effective Diameter of Tapered Threads. The measurement of the effective diameter of taper threads is not made perpendicularto the axis, but at an angle depending on the taper. The measurement is made at agiven point or distance from the end of the thread, and in the three wire method, thesingle wire is placed at this point. The other two wires are placed in the two oppositegrooves and care must be taken to ensure that the micrometer or measuring anvilsmake contact with each of the three wires.

13.4.8. Effective Diameter of Internal Threads (Fig. 13.18).

Thread comparator. In this case a pair of ball tips engage the flanks of the threads in thework and measure the effective diameter only.The ball tip on the right is fixed at the end of a measuring jaw attached to a floatinghead in the sliding bracket (B). The floating head has extension in contact with thespindle of the dial indicator and the movement of floating head towards the indicator isconstrained by a spring. (The set-up in Fig. 13.18 does not show the ball tips).The instrument is set to a reference standard, with the dial pointer at zero. To use thegauge, the floating head is retracted to insert the ball tips in the internal threads of thework, and released to allow the tips to engage the flanks of the thread under thepressure of the spring. The dial indicator then shows the deviation from the nominal sizeto which the gaugeis set. The instrument may be used on work in the machine, or on the working bench.The fixed head (A) carrying the left hand ball tip is adjusted by a fine screw to set thegauge to the reference standard. The reference standard is built up from slip gauges asshown in Fig. 13.19. The two end pieces have V-jaws of an angle of Vee corresponding
http://lh4.ggpht.com/_X6JnoL0U4BY/S02UwAWmJYI/AAAAAAAAFO8/OoqGE4J7RQk/s1600-h/tmp594_thumb1.pnghttp://lh3.ggpht.com/_X6JnoL0U4BY/S02Uu-Fq36I/AAAAAAAAFO0/qY-O5zjSuy8/s1600-h/tmp593_thumb1.pnghttp://lh4.ggpht.com/_X6JnoL0U4BY/S02UwAWmJYI/AAAAAAAAFO8/OoqGE4J7RQk/s1600-h/tmp594_thumb1.pnghttp://lh3.ggpht.com/_X6JnoL0U4BY/S02Uu-Fq36I/AAAAAAAAFO0/qY-O5zjSuy8/s1600-h/tmp593_thumb1.png


10/16

to the thread, i.e. 60 degree or 55 degree.

Fig. 13.18

Fig. 13.19 The dimensions J1 and J2 are marked on the pieces, and are the depths from the faceto the apex points of the Vees. Assuming the effective diameter and pitch of the threadto be known, the distance S is found from the formulaS=X+y-Zwhere X = Mean effective diameter.y = Depth of the thread from apex to the apex of the V-form.

The value of y (Fig. 13.20) depends on the included angle of the thread, and is equal to0.9605p for 55 threads and 0.866p for 60 threads.Z = Ji + J2 ie., constants for the end gauge pieces.The assembled slips are set in a holder with a slip equal to half the pitch, beneath oneend piece to compensate for the helix angle.The reference gauge thus assembled is ready for setting the comparator. Ball tips mustbe of a suitable size for thethread. The size is not critical provided the ball point fist the thread so as to bear on theflack near the mean pitch line.For threads from 4 to 7 t.p.i., a ball of 0.095 inch dia is used, from 7 to 12 t.p.i., 0.060
http://lh5.ggpht.com/_X6JnoL0U4BY/S02UytFdp2I/AAAAAAAAFPM/8Fl_8JkhOwE/s1600-h/tmp596_thumb1.pnghttp://lh3.ggpht.com/_X6JnoL0U4BY/S02UxduouJI/AAAAAAAAFPE/KFNG-05dw7k/s1600-h/tmp595_thumb1.pnghttp://lh5.ggpht.com/_X6JnoL0U4BY/S02UytFdp2I/AAAAAAAAFPM/8Fl_8JkhOwE/s1600-h/tmp596_thumb1.pnghttp://lh3.ggpht.com/_X6JnoL0U4BY/S02UxduouJI/AAAAAAAAFPE/KFNG-05dw7k/s1600-h/tmp595_thumb1.png


11/16

Fig. 13.20 inch diameter and from 12 to 20 t.p.i., 0.035 inch diameter balls are used. A pair of V-

jaws, 55 or 60 covers all pitches from 4 to 20 t.p.i. The method of calculating the valueof S from the effective diameter excluded the radius OY at the crest and root of thethread, (Fig. 13.21) as the form is considered to extend to the apex of the Vee. In some

cases it may be necessary to accept the major diameter as it may be the basicdimension of the thread, and the form at the crest and root of the thread must then betaken into account.

For metric threads, S = D + 0.2165p Z; For Whitworth threads, S = D + 0.3202/j Z.(Hi) Laymans method of finding the effective diameter (internal thread) is by taking theimpression of threads with the help of wax or any other material, say sulphur. Sulphur ismostly used because it can be used many times.13.4.9. Checking the Thread Form and Angle by Optical Projection

of Thread. This method is applicable only to external threads because internal threads cannot beprojected.The standard type of projector is used, consisting of a projector lamp, a condenser lensor collimator, the projection lens and the screen.The screw thread to be examined is placed in the parallel beam of light between the
http://lh6.ggpht.com/_X6JnoL0U4BY/S02U1Jvk1RI/AAAAAAAAFPc/280eZVj4Uwo/s1600-h/tmp598_thumb1.pnghttp://lh3.ggpht.com/_X6JnoL0U4BY/S02Uz_PSU1I/AAAAAAAAFPU/OkeL5mieKik/s1600-h/tmp597_thumb1.pnghttp://lh6.ggpht.com/_X6JnoL0U4BY/S02U1Jvk1RI/AAAAAAAAFPc/280eZVj4Uwo/s1600-h/tmp598_thumb1.pnghttp://lh3.ggpht.com/_X6JnoL0U4BY/S02Uz_PSU1I/AAAAAAAAFPU/OkeL5mieKik/s1600-h/tmp597_thumb1.png


12/16

condenser lens and the projector lens.The modern projectors are equipped with work holding fixtures, the projection lamp andthe lenses situated on top of the cabinet, and the screen at the front. The light rays fromthe lens are directed downwards into the cabinet, and hence to the screen by a system

of prisms and mirrors, bringing every thing within the reach of the operator.The enlarged image of the thread form appears on the ground-glass screen on which ismounted the temnlate or drawine of the form made to scale eaual to the magnification ofthelens. This way the two forms (i.e. ideal and projected) are compared.One of the difficulties in projecting screw thread is the fact that form is specified on anaxial plane. So we must consider the correction for it.Referring to Fig. 13.22 (i), the normal pitchp is less than the axial pitch P and is given by

the relation :p = P cos $ ; where is the helix angle.Referring to Fig. 13.22, (ii)..

Values of A and are known and 2Xis the included angle of the thread

projected on the screen. On the projected thread form we find the included angle ZX and then compare it to thetheoretically calculated value. 2X = 2 tan-1 (tan A cos $).The included angle can also be determined by two ball method (Refer Art. 9.8.3 andProb. 13.7).13.4.10. Measurement of Pitch. The accuracy of pitch in any form of thread is very important. Therefore, it is veryimportant to be able to measure this element of thread to high degree of accuracy, at

least double that of the effective diameter measurement. The measurement must bemade in such a way that other features or dimensions e.g., diameter and thread angledo not influence the result.13.4.10.1. External Threads.
http://lh3.ggpht.com/_X6JnoL0U4BY/S02U4l_3H8I/AAAAAAAAFPs/EqbbC8824UU/s1600-h/tmp5100_thumb1.pnghttp://lh3.ggpht.com/_X6JnoL0U4BY/S02U2h4Dg8I/AAAAAAAAFPk/zjvfy8tGJiE/s1600-h/tmp599_thumb1.pnghttp://lh3.ggpht.com/_X6JnoL0U4BY/S02U4l_3H8I/AAAAAAAAFPs/EqbbC8824UU/s1600-h/tmp5100_thumb1.pnghttp://lh3.ggpht.com/_X6JnoL0U4BY/S02U2h4Dg8I/AAAAAAAAFPk/zjvfy8tGJiE/s1600-h/tmp599_thumb1.png


13/16

1. For less accurate methods, the Zeiss pitch or lead measuring instrument may beused. It utilises contact members having two ball points which are applied to theeffective surface of the thread. These points are aligned parallel to the thread axis eitherby a thread pin at the back or a special back rest having a plane face parallel to the

thread axis. The instrument is adjusted to zero before making a measurement, with theaid of a special micrometer gauge supplied for the purpose, or by using a standard pluggauge. Upon applying the instrument to the thread it registers the pitch deviation fromthe standard measurement. The scale of the indicator has a range of 0.1 mm andeach division reads to 0.01 mm. The measuring accuracy of the indicator is 0.003mm.2. The pitch of external threads can be measured by using screw pitch or profile gauge.Such a gauge consists of series of thread forms with varying pitch. The one which

coincides perfectly with the thread under test gives the pitch. The accuracy ofmeasurement depends on the method of sighting used to judge the perfectness.3. A more accurate method is the microscope method. Screw threads can be inspectedand their profile angles and linear pitches checked with the aid of a goniometricmicroscope. The parts to be gauged are usually held between centres and illuminatedfrom below, their silhouettes appearing in the field of the viewing eyepiece. Effectivepitch diameters can also be measured bv this method.The method of measuring pitch is shown in Fig. 13.23. The microscope has tworaticules that can be oriented to the slopes of the thread and the point of intersection of

these is used as the measuring reference. The movement of the longitudinal carriage isread off the linear scale, the micrometer microscope being employed for this purpose.The linear measuring accuracy is within 0.001 mm and for angles, it is 10 sec. of arc.

A comparatively simple method of testing the pitch of a screw thread with the Cooketool room microscope fitted with its projection screen is as follows :The screw to be checked is mounted in a cradle under the microscope objective and thenecessary adjustments made to project the sharp enlarged image of the thread on thescreen. The appropriate thread form on the microscope thread template is then broughtinto coincidence with the projected image, as shown in (Fig. 13.24) and a reading of the
http://lh3.ggpht.com/_X6JnoL0U4BY/S02U56sGeEI/AAAAAAAAFP0/GrpYJSL-bWw/s1600-h/tmp5101_thumb1.png


14/16

longitudinal table micrometer screw taken ; this can be done to an accuracy of 0.0025mm. The table is then moved by means of the micrometer screw until the image of thenext thread on the screen under inspection fills the template profile and the reading ofthe micrometer again taken. The difference between the readings gives the measured

pitch of the screw. The procedure is repeated for each individual thread in order to findthe separate pitch error, if any. Finally, the difference between the

initial and last readings of the micrometer when divided by the number of threads thathave been measured enables the mean pitch of the screw to b e estimated.For still more accurate purposes it is necessary to employ a special screw pitchmeasuring machine by which the actual pitch error of individual threads can bemeasured. The Pitter and Matrix are typical examples of pitch measuring machines.

The Pitter screw measuring machine employs various stylus points to suit screw threadsthat are to be checked. The screw under measurement is held stationary betweencentres on the machine. The indicator unit, carrying the stylus which bears on the flanksof each thread successively, is carried on a slide which is mounted on balls. The slide isactuated by means of a micrometer. The act of rotating the micrometer spindle causesthe slide to move in relation to the fixed centres, i.e., causes the indicator to move inrelation to the work being measured. The stylus which is mounted on a leaf spring, fallsin and out of each thread ; the pointer of the indicator reads zero (it is adjusted to read

zero in the first groove) when this stylus is in a central position in each successivethread. The micrometer reading is taken each time the indicator reads zero ; thesereadings then shown the pitch error of each thread of the screw that is being measured.Special graduated discs are provided to fit the micrometer to suit all ordinary pitcheswhilst special can be provided for.It may be mentioned that a small handwheel below the micrometer actuates screw forthe purpose of moving the indicator in relation to the slide so as to bring the stylus
http://lh3.ggpht.com/_X6JnoL0U4BY/S02U7TgAtXI/AAAAAAAAFP8/xotFPQPDDyI/s1600-h/tmp5102_thumb1.png


15/16

opposite to the screw to be tested in any position between the centres. The total travelof the micrometer is 25 mm.

Illustrating that for best results the stylus point should make contact on or nearthe effective diameter.

As the pitch of the micrometer screw is checked accurately when the machine isinspected and a curve of errors is provided, it is possible to attain a high standard ofprecisior in measuring screws. The pitch errors are extremely small, being of the orderof 0.002 mm fo] a thread. A test screw is also supplied with the machine and a chart of

pitch error for this screw.The Matrix pitch measuring machine operates on a similar principle to the Pittermachine. It is robust in construction and sensitive in measurement, revealing pitchaccuracies of 0.0025 mm over a distance of 50 mm for all thread forms. In this machine(Refer Fig. 13.26) a micrometer head is provided on the headstock which is fixed to thebase. The rotation of micrometer head produces movement of the longitudinal carriagealong the bed of the base.

-Fig-13.26 Another carriage carrying the indicating and amplifying units comprising a radiusedstylus and visual scale allowing a zero reading to be taken, and also capable of movingat 90 to the longitudinal carriage ; is mounted on the former carriage, and can betraversed longitudinally and locked in any position. A weight ensures a unidirectionalthrust at all times. The micrometer screw of 40 t.p.i. has a 50 mm traverse and also hasa compensator for any small residual pitch errors. In operation, the screw thread to bechecked is placed between centres and the correct stylus mounted in the indicatinghead.When the test screw is in position between the centres, and the correct stylus chosen
http://lh3.ggpht.com/_X6JnoL0U4BY/S02VBhlfjBI/AAAAAAAAFQM/SbFm96iG5eU/s1600-h/tmp5104_thumb1.pnghttp://lh3.ggpht.com/_X6JnoL0U4BY/S02VAYGcxGI/AAAAAAAAFQE/zH7Ph4jAkK0/s1600-h/tmp5103_thumb1.pnghttp://lh3.ggpht.com/_X6JnoL0U4BY/S02VBhlfjBI/AAAAAAAAFQM/SbFm96iG5eU/s1600-h/tmp5104_thumb1.pnghttp://lh3.ggpht.com/_X6JnoL0U4BY/S02VAYGcxGI/AAAAAAAAFQE/zH7Ph4jAkK0/s1600-h/tmp5103_thumb1.png


16/16

i.e., the one which makes contact at or near the effective diameter, the carriage carryingthe indicating unit is traversed until the stylus is located in the first thread of the testscrew and the indicator is coincident with the fiducial line ; the second carriage is thenlocked. The stylus, by virtue of an ingenious mounting device, is capable of free

movement riding up and down the thread flanks on linear movement of the screw threadby rotation of micrometer head. The stylus is now traversed along the thread, pitch bypitch, reading being taken each time the indicator is set at zero. The micrometer can befitted with a series of graduated dials that can be changed quickly. With the proper dialfor the pitch that is to be measured the readings of the errors are obtained from thedisplace-ment of the lines on the disc which is graduated in (0.002 mm) divisions. It is aftermaking this test, to turn to the first thread and repeat the readings, and the micrometer

should read zero again.13.4.10.2. For Internal Threads. The pitch of an internal thread can be measured on any of the standard pitch machinesby using an adaptor. This adaptor carries a bar which can be inserted into the ring, thestylus being fitted to the bar end engaging with the thread in the usual manner. The ringgauge is mounted on a face plate or on the head stock of the machine, which willaccommodate rings upto several cm. in diameter. For very large rings, a special set upon a surface plate is necessary, utilising an indicator and slip gauges.
http://lh6.ggpht.com/_X6JnoL0U4BY/S02VDI60EfI/AAAAAAAAFQU/7R5PaidbJ7U/s1600-h/tmp5105_thumb1.png

Documents

Thread Micrometer Method