Measurement of Screw Threads

8/18/2019 Measurement of Screw Threads

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Metrology ofScrew threads


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Introduction• Screw threads have 2 functions to perform

viz. transmission of power and motion andto act as a fastener

•

The objective of dimensional control in thecase of plain shaft and hole is to ensure acertain consistency of t.

• In case of threaded work, the objective isto ensure mechanical stren th which isdependent on the amount of !ank contactand not upon the t.


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Screw Thread Terminology


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Errors in Threads• In case of plain shafts and holes , there is only one

dimension which has to be considered.

• "rrors on this dimension if e#ceed the permissibletolerance, will justify the rejection of part.

• In screw threads $ important elements re%uireconsideration and error in any & can cause rejection.

• 'll these $ elements ()ajor diameter, )inordiameter, "*ective diameter, pitch and an le of thethread form+ must be checked and au in mustcovere these elements.


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Measurement of VariousElements of a Thread

1) Measurement of Major Diameteri. sin hand micrometer•. -i ht pressure must be used in order to

reduce the error due to compression.

•. esirable to check the micrometer readinon a cylindrical standard of appro#imately

the same size to eliminate zero error.

•. )icrometer anvils should touch at least twocrests at a time


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ii. sin /ench )icrometer• sed for reater accuracy of measurement.

• ' ducial indicator is used in place of the #edanvil.

• )achine is used as a comparator in order toavoid any pitch errors of micrometers , zerosettin etc.

• 0alibrated settin cylinder is used as settinstandard whose diameter is same as the major

diameter.


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• The cylinder is held and the micrometerreadin is noted down.

• The cylinder is then replaced by thethreaded workpiece and a ain themicrometer readin is noted down for samereadin of the ducial (reference+ indicator.

• The followin data is noted down.

r

•


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ii. )easurement of "*ective diameter( itch diameter+

•. Thread micrometer method•. 3ne wire method•. Two wire method•. Three wire method


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3ne wire method


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• In this method one wire is placed between2 threads at one side and on the other sidethe anvils of the measurin instrumentcontacts the crests as shown in the ure.

• 4irst the micrometer readin is noted on astandard au e whose dimension is nearlysame as to be obtained by this method.

• Actual measurement over wire oneside and threads on the other side si!e of the gauge di"erence in the

two micrometer readings#

•


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Two wire method


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Three 5ire method


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/est Size 5ire• /est size wire is one in which case the wire

makes contact with the thread !anks i.e. thecontact points of the wire should be , on thepitch line or e*ective diameter.


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Measurement of Minor diameter of Internal Threads

i. sin taper parallels• 4or diameter less than 266 mm taper parallels and

micrometer is used.

• Taper parallels are pairs of wed es havin parallelouter ed es.

• iameter across their ed es can be chan ed byslidin them over each other as shown in ure.

• Taper parallels are inserted inside the thread andadjusted until a rm contact is established.

• The diameter over the outer ed es is measured

with a micrometer.


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ii. sin 7ollers•. )ethod used for diameters reater

than &6mm

•. recision rollers are inserted insidethe thread and proper slip au esinserted between the rollers as showin the ure.

•. )inor diameter is the len th of theslip au es plus the diameter of the

rollers.


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Measurement of Screw Threads