Page 128 - CITS - Fitter Trade Pactical - Volume - 2

FITTER - CITS

While measuring the screw thread, the holder with the one wire is placed on the spindle of the micrometer and
the other holder with two wires is fixed on the anvil.
Selection of ‘best wire’ (Fig 6): The best wire is the one which, when placed in the thread groove, will make
contact at the nearest to the effective diameter. The selection of the wire is based on the type of thread and
pitch to be measured. The selection of the wire can be calculated and determined but readymade charts are
available from which the selection can be made.

Fig 6

Table 1
Measurement with measuring wires. Metric threads with coarse pitch (M)

Thread Pitch Basic measurement Measuring wire Dimension over
designation P (mm) mean d (mm) dia. W (mm) wire M (mm)
2 1 1
M1 0.25 0.838 0.15 1.072
M 1.2 0.25 1.038 0.15 1.272
M 1.4 0.3 1.205 0.17 1.456
M 1.6 0.35 1.373 0.2 1.671
M 1.8 0.35 1.573 0.2 1.870
M 2 0.4 1.740 0.22 2.055
M 2.2 0.45 1.908 0.25 2.270
M 2.5 0.45 2.208 0.25 2.569
M 3 0.5 2.675 0.3 3.143
M 3.5 0.6 3.110 0.35 3.642
M 4 0.7 3.545 0.4 4.140
M 4.5 0.75 4.013 0.45 4.715
M 5 0.8 4.480 0.45 5.139
M 6 1 5.350 0.6 6.285
M 8 1.25 7.188 0.7 8.207
M 10 1.5 9.026 0.85 10.279
M 12 1.75 10.863 1.0 12.350
M 14 2 12.701 1.15 14.421
M 16 2 14.701 1.15 16.420
M 18 2.5 16.376 1.45 18.464
M 20 2.5 18.376 1.45 20.563
M 22 2.5 20.376 1.45 22.563
M 24 3 22.051 1.75 24.706
M 27 3 25.051 1.75 27.705
M 30 3.5 27.727 2.05 30.848

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CITS : CG & M - Fitter - Exercise 36 CITS : CG & M - Fitter - Exercise 36