ISO METRIC THREAD GAUGES

 

BS 3643 comprises two parts. Part 1 provides information and data on the basic principles, and is based on ISO 965/1 and ISO 965/3. Part 2 tabulates diameters of coarse, constant and fine pitch series threads, and is based on ISO 965/3.

 

BS 919: Pt3: 1968 contains the recommended gauging system for checking ISO metric threads in accordance with BS 3643. It is based on ISO 1502. The basic profile of is threads is shown below.

 

BS 3643Pt1 and Pt2 specifies the fundamental deviations and tolerances for e.g. 5H, 6H, 6G AND 7H for internal, and 4h, 6g, 6e, 8g for external threads. Generally 6H/6g are regarded as "medium fit". The nut and bolt deviations and tolerance zones are illustrated here.

 

BS 919Pt3: 1968 specifies the limits for gauges for testing the product thread. The basis for determining the gauge limit is the magnitude of the product tolerance, i.e. the larger the gauge limit regardless of diameter.

 

The recommended gauges to be used are as follows:-
For testing internal product threads a double ended screw plug gauge (Style E) may be used. This gauge comprises a Go element which tests that the Major and Effective diameters are not to small (maximum material limit). It also tests for pitch and flank errors in the product thread. The Not Go element tests only that the Effective diameter is not too large (minimum material limit). The Minor diameter of the internal thread can be checked separately using a double ended plain plug gauge.

 

For testing external product threads Go and Not Go ring gauges or caliper gauges having Go and Not Go anvils may be used. Wherever tractable, external product threads should be tested by means of ring gauges as these provide a full functional test of all thread features: pitch, angle, thread form and size.

 

Gauges for testing products in the pre-paring condition can be supplied. For internal threads class 6G may be used as a pre-plating condition for 6H after plating. For external threads class 6E may be used as a pre-plating condition for 6g after plating. Class 6g may also be used as a pre-plating condition for 4h after plating. When ordering gauges of other plating conditions it is helpful to advise either the effective diameters, or the amount of plating to be applied.

 

The regular checking of gauges in constant use is important. Plug gauges should be checked by direct measurement of the major and effective diameters, using a Floating Carriage Measuring Machine or similar specialized equipment. Ring gauges may be checked by direct measurement, or preferably by using Go Check and Not Go Wear Check plugs, as specified in BS 919 Pt3: 1968