Thread gauges for reliable thread inspection
Thread gauges check whether a thread is within its tolerance, without a measuring instrument and without special knowledge. VÖLKEL makes more than 1,600 precision thread gauges to DIN ISO 1502, ANSI B1.2 and DIN EN ISO 228-2: plug gauges for internal threads, ring gauges for external threads and STI thread gauges for the tapped hole of V-COIL wire thread inserts. They cover M, MF, UNC, UNF, UN, G (BSP) and NPT in the common tolerance classes.
Go, no-go, limit: how thread gauges check
Every check needs two ends. The go end must screw in over the full thread length, otherwise the thread is too small or the profile is faulty. The no-go end may enter at most two turns, otherwise the thread is too large. A limit plug gauge combines both ends on one handle, with a red ring at the no-go end. Ring gauges come separately as go ring gauge and no-go ring gauge. Step-by-step handling is in Check internal threads with thread plug gauges.
Choosing the tolerance class
The gauge must match the tolerance of the thread. For metric internal threads 6H is the standard, 6G and 7G apply to coated workpieces or larger clearance, 4H to a closer fit. For metric external threads 6g and 6e are usual. American UN threads are checked with 2B and 3B (internal) or 2A (external), pipe threads G to DIN EN ISO 228-2 with classes A and B. The relationships are explained in Thread tap tolerances at a glance.
STI threads need their own gauges
The tapped hole for a wire thread insert is larger than the nominal thread. A standard gauge does not fit there; for checking after tapping with the STI tap there are the STI thread gauges in tolerance ISO 1 (4H). Why that is so is in Checking STI threads correctly.
Longer life with DLC
For high inspection counts and abrasive materials there are plug gauges with DLC coating, which are considerably more wear-resistant than bright gauges. More in Thread plug gauges with DLC coating.
