In order to overcome disadvantages of solid gauge wear, adjustable pattern screw thread gauge was evolved which is widely employed for precision gauging purposes. Although wear can be compensated for but B.S. allowance of 0.002? (0.005mm) for solid gauges is reduced by about 0.0005?(0.0125mm).

The value of se constants will depend upon prevailing accuracy of tools and machines used to produce screw threads. Increase in accuracy will reduce value of constants. The specifications provide for three grades of fits, ( a ) close, ( b ) medium, ( c ) free.

Multiple start screw threads is produced by forming two or more than two helical grooves, equally spaced and similarly formed in an axial section on a cylinder. This gives a ?quick traverse? without sacrificing core strength.

To prevent binding at root of tap wide tolerance on core diameter of internal threads is given. It permits a tapping drill being used of ample size. This follows a general practice and results in a flat crest to root of thread.

On ISO metric threads, tolerances are defined by tolerance position and tolerance grades. There are 7 tolerance grades and se are 3, 4, 5, 6, 7, 8, and 9. 3, 4 and 5 are for precision threads and 6 for medium quality and for less precision methods are 7, 8 and 9.

In major diameter, tolerance is given by expression Effective Diameter tolerance = +0.01?p. It is applicable for all grades of fits for external threads. In major diameter, no such tolerance is laid down for internal threads.

Errors on minor and major diameters will cause interference in mating thread. Due to errors in minor and major diameter, wall thickness and root section will be less, also flank contact will be decreases and component will be weak in strength.

In case of plain holes and shafts, re is one dimension only which has to be considered i.e. diameter. Errors in diameter if exceed permissible tolerance, will result in rejection of part. But in case of screw threads re are minimum 5 important elements which are Major diameter, minor diameter, effective diameter, angle of thread and pitch and error in any one of se will result rejection of thread.

Effective diameter tolerance depends on Diameter, length of thread, pitch. It depends on m non linearly. Equation of tolerance is a?D +b?L +c?p where D = major diameter, L = length of engagement, p = pitch, a, b, c are constants.

The compensations for pitch and angle errors are also provided for by application of corrections for each of se quantities. Effect of pitch error upon effective diameter of a screw thread is
D1 = E x K where D1=Effective dia. equivalent of pitch error, E = max. cumulative pitch error along thread, K = constant for shape of thread.