Screw Thread Gauge Tolerance (Metrology)

N.P.L. London during First World War laid down tolerance for plug and ring screw gauges which remained practically unchanged till Second World War. These were published in ‘Notes on Screw Gauges’ and covered in three classes, i.e. Reference, Inspection and Production ; workshop gauges being within the work tolerance, and inspection and reference being outside.
The tolerances were graded solely in terms of diameter. The inspection and workshop tolerance on gauges upto 1.5 inches being 0.0006 inch on effective diameter and 0.002″ for 6″. Reference gauge tolerance was half of these values. This tolerance included virtual effect of pitch and angle errors.
In 1940, B.S.S. No. 919 was issued on screw gauge tolerance as was emergency. Specifications widened tolerance somewhat, rather more on smaller size than larger. At the same time fresh conception of effective diameter was introduced. In order to discourage manufacture of gauges with simple effective dia. on high limits so as to allow maximum pitch and angle error, the tolerance was divided into two separate elements :
For gauges upto 1.5″ dia., Tolerance on effective diameter = 0.0004″ Equivalent of pitch and angle tolerance on effective diameter = 0.0005″ The simple effective diameter tolerance on NO GO gauge was same but virtual tolerance was halved. This is quite logical because short length of thread is not liable to much pitch error.
The manufacturing tolerances on screw gauge are laid down in B.S.S. 919—’Screw Thread Gauge Tolerances’ and in the earlier notes on screw gauges of N.P.L.
In connection with GO screw gauges, the latter publication contains tables of limits for both plug and ring gauges grouped under the headings of ‘Reference’, ‘Inspection’ and ‘Workshop’.
The compensations for pitch and angle errors are also provided for by the application of corrections for each of these quantities. Effect of pitch error upon effective dia. of a screw thread is : Z>! = E x K
where D± = Effective dia. equivalent of pitch error, E = max. cumulative pitch error along the thread, K = constant for the shape of thread = 1.92 for Whitworth = 2.27 for B.A. and = 1.73 for International metric threads.
The compensation for pitch error AEh used in this connection is the maximum error which is found between any two threads along the gauge.
The compensation for angle error, AE2 is related to the pitch p as follows : AE2 = p x AG/100 where A8 is the angle error in degrees.
It is necessary that A£2 andp should be expressed in the same measure, i.e. English or metric. The angle error in this formula is the arithmetical sum of the errors of the two flank angles irrespective of their signs.
If the general form of thread and the major and minor diameter are satisfactory, there still remains the following requirements on effective dia which must be met before the gauge can be accepted.
The items defined under (a) and (b) below must fall within the prescribed tolerances : Plug gauges : (a) The measured effective diameter along the pitch line.
(b) The measured effective diameter + AEi + AE2 Ring gauges : (a) The measured effective diameter along the pitch line.
(b) The measured effective diameter – AEi – AE2. It should be noted that the max. allowable pitch error, if no angle error is present, or the max. allowable angle error, if no pitch error is present, can readily be found from the formulae given, by evaluating AEi or AE2 to the full effective diameter tolerance allowed on any given size of gauge.

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