understanding fixed limit gages.pdf - Understanding GO/NO...

Understanding GO/NO GO Gauges
(Fixed Limit Gauging)
One of the most frequent questions that we are asked at A.A. Jansson is, “how do I choose a plug
gauge for my measurement application.” Therefore I put together this document to help you understand the
concept of fixed limit gauging.
Keep in mind the fundamental concept of fixed limit gauging is to never accept a bad part. In order
to accomplish this the tolerance of the plug or ring will be designed to actually have the potential to reject
good parts.
When this method rejects good parts that are near the extreme limits of the part tolerance the part
can be rechecked with a more accurate method to determine if the part is actually in tolerance.
In the following examples I don’t want to get hung up about semantics. Some people call pin
gauges, plug gauges, and others call plug gauges, pin gauges. Either way is acceptable.
Keep in mind that a Go plug has a plus tolerance and is designed to gauge the smallest acceptable
hole size, and a NoGo plug has a negative tolerance, designed to gauge the largest acceptable hole size.
Subsequently a Go gage should be able to pass through the hole and a NoGo plug gauge should not. This
is why they call it Go NoGo gauging. Others call it Fixed Limit Gauging.
The opposite is true for ring gages. A Go ring has a negative tolerance and is designed to gauge the
largest acceptable diameter and a NoGo ring has a positive tolerance and is designed to gauge the
smallest acceptable diameter. The reasons for this will become clearer as we go through a couple of
examples.
It is easy to get confused between how the tolerance of the ring and plug is applied in relationship to
the Go and NoGo member. Sometimes it is easier to thing of the member you are measuring in terms of
more or less material. The reason is that a Plus tolerance applied to a plug makes the actual plug size
larger. The plus Tolerance applied to a ring makes the ring gauge diameter small. Please keep this in mind
when you follow the examples.
Plug Gauge Example
1. Dimension on part to gauge :
a. The tolerance is taken from the blue print.
b. The nominal hole size on part to gauge is 1.0000”
c. Tolerance of the hole is +.002”/-.000”
d. This means the hole must be manufactured somewhere between 1.0000” and 1.0020” in size.
2. Go Plug:
a. Go plug would be designed for the smallest hole size. This size would be 1.0000” with a plus
tolerance
b. We will use the 10:1 rule to help us determine the tolerance of the plug that should be used.
i. The part tolerance spread is .002”, therefore the tolerance of the plug gauge should be
approximately 10% of the overall tolerance being measured. 10% of .002”=.0002”
ii. Go plugs have a plus Tolerance
iii. The tolerance of a class ZZ plug gauge with a nominal size of 1.0000 is +.00024. This
is derived from the Gagemaker’s Tolerance Chart.