Thread Plug Gauge - NO-GO Rejection Criteria

SATHYABALARAMAN

Involved In Discussions
#1
I have sent a thread plug gauge for calibration (1/4"x19 BSP). As per standard IS 2643,

GO PCD =12.304 to 12.314 with wear limit 12.295

NO GO PCD 12.426 to 12.436 with wear limit 12.421

Measured values using length measuring machine is

GO PCD = 12.298 to 12.302

NOGO PCD = 12.428 to 12.450

Nogo is +0.014 in plus side.

Do I have to reject the thread plug gauge?
 
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kgriff

#3
Sathya,
I'm not too familiar with BSP threads, and I don't own the standard you referenced but, based on the tolerances and measured dimensions in your post...
I would reject this thread plug gage. Since the No Go is oversized, you have the potential to call bad parts good.
For example, you have a threaded hole that is oversized. The hole allows the Go gage to go, since it is not only big enough, but too big. The No Go gage, which if it were in tolerance, would also go in this hole, thus telling you the part is bad. Since your No Go is oversize, it does not go in the hole leading you to, mistakenly, believe you have a good part.

Using hard gaging of this sort, whether it be thread plugs or plain plugs, the intent of the gage is to "bracket" the part tolerance such that, if properly sized gages are used, the part MUST fall between the gage sizes. (i.e. Go gage goes, so the hole is big enough, No Go does not go, so the hole is not too big.)
 
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