Screw Thread Gauges

The simplest method of checking threads in the production shop is to use screw thread limit gauges.  These gauges possess the same thread form as the mating thread.  The gauges are screwed on to the thread being checked.

Thread gauges make sure that screw threads are of the correct size as specified in the applicable standard.  A GO and NOT GO screw plug gauge will make sure that an internal thread is correct.

Plain GO and NOT GO gauges will check the diameter of an internal thread.

GO and NOT GO screw ring gauges  are used to check that an external thread is correct.  Plain ring or calliper gauges are used to check the diameter of an external thread.

RING SCREW GAUGES

For the gauging of bolts or external threads the equivalent mating gauges are known as ring screw gauges.  As in the case of plug screw gauges a limit system can be provided by a full-form GO and NOT GO effective ring gauge.  As the factors involved are exact counterparts of the gauging of internal threads, the GO ring gauge has a full-form thread.  The NOT GO gauge is truncated on the minor diameter, and cleared on the major diameter at the root of thread.

GO and NOT GO ring screw gauges are used to check that an external thread is correct.  Plain ring or caliper gauges are used to check the diameter of an external thread.

PLUG SCREW GAUGES

When gauging nuts or internal threads of full-form a GO plug gauge is used.  The GO gauge is accurately made to the minimum dimensions of the required thread.  It will assemble with the component and will make sure that the major, minor, and effective diameters are not below the minimum dimensions.  The GO gauge also makes sure that any errors in pitch, angle and thread form are within limits.

The GO end of a plug screw gauge is longer than the NO GO end.  This will indicate a defect deeper into the hole.