Thread plug gauges for internal threads
With a thread plug gauge you check an internal thread in seconds: the go end must fit, the no-go end must not. VÖLKEL makes more than 600 precision thread plug gauges as limit plug gauges with go and no-go end on one handle and as single go plug gauges and no-go plug gauges, to DIN ISO 1502 for M and MF, to ANSI B1.2 for UNC and UNF, to DIN EN ISO 228-2 for G (BSP) and for tapered NPT threads.
Limit plug gauge or single gauges?
The limit plug gauge is the practical choice for production and incoming goods: go and no-go end sit on one handle, the no-go end is recognisable by the red ring and the shorter thread. Single go and no-go plug gauges make sense when only one end is used, when weight matters for large diameters, or when a worn end is to be replaced on its own.
The right tolerance
For metric internal threads 6H is the rule. 6G and 7G check threads with a fundamental deviation, as cut before galvanising or coating, +0.1 mm the oversize version, 4H the closer fit. UN threads are checked with 2B (standard) or 3B (close). The gauge must match the tolerance the thread was cut to; see Thread tap tolerances at a glance.
How to check
The go end must screw in without force over the entire thread length. The no-go end may engage at most two turns. Both without tools, with a clean and deburred thread. The full instructions are in Check internal threads with thread plug gauges.
DLC coating and STI
For high inspection counts there are plug gauges with DLC coating. For the tapped hole of wire thread inserts you need STI thread plug gauges; external threads are checked with ring gauges.
