Fixed Limit Gage Considerations

Many of the folks working in metrology labs have strong and broad knowledge base about a wide variety of measuring machines and gages. Others may be new in the field, or have come into metrology with a background more from the manufacturing, or CAD / IT side of things. As a metrologist you may be called upon to make purchasing decisions regarding appropriate gages for use in the lab or on the floor. When that happens it may serve you well to have solid understanding of fixed limit gage tolerances. It is also handy to have a reference that you can provide to a non-metrology type who is tasked with purchasing this type of gage. This article is written Dan Smith in collaboration with Eric Lundquist of AA Jansson Inc., a company that has been calibrating and selling gages and measuring equipment for three generations. Eric provides us here with a very detailed description of tolerancing for some common fixed limit gages.

The fundamental concept of fixed limit gauging is to never accept a bad part. To accomplish this, the tolerance of the plug or ring gage has 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 re-checked with a more accurate method to determine if the part is actually in tolerance. Consider pin gauges,( also called plug gages). Remember 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 them Go /NoGo plug gages.

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 think of the member you are measuring in terms of more or less material.

Plug/Pin Gauge Example
1. Dimension on part that needs to be gauged:
a. The tolerance is taken from the blue print.
b. The nominal hole size on the 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. Determining Plug Gauge:
a. The Go plug would be intended for gauging the smallest acceptable hole size. This size would be 1.0000”
b. The NoGo plug would be intended for gauging the largest acceptable hole size. This size would be 1.0020”
c. We will use the classic 10:1 rule to help us determine the tolerance of the plug that should be used in the ideal world. That rule is that the tolerance for the gage should be ten
times more accurate than the part tolerance. That rules works as follows:
d. The part tolerance spread is .002”, therefore the tolerance of the plug gauge should be approximately 10% of the overall range of the tolerance being measured; 10% of .002”=.0002”
ii. The tolerance is split between the Go and NoGo plug: 5% on the Go and 5% on the NoGo. 5% is .0001”
iii. Go Plugs have a plus Tolerance
iv. NoGo Plugs have a minus Tolerance

EXAMPLE:
Hole Size= 1.0000 +.002/-0.000
Range of Tolerance=.002
10% of Tolerance=.002 x .10= .0002”
Apply ½ of 10% to each member=.0002”/2=.0001”
Low Limit Plug Gage is 1.0000” +.0001” (Go Gage)
High Limit Plug Gage is 1.0020” -.0001” (NoGo Gage)

Here is a graphical depiction of gage tolerances

3. Ideal Gage that is required:
The Go Plug has a nominal size of 1.0000” with a tolerance of +.0001”/-0.0000
The NoGo Plug has a nominal size of 1.002” with a tolerance of +0.0000/-.0001

This is the ideal gage that is required to meet the10:1 gauging concept. However one must consider cost and availability and most importantly one must consider risk.

Obviously there are common sizes and grades of plug gages that are produced. Let’s first look at the standard Gage Maker Tolerance Chart for our size plug gage:

The gage in our example required a .0001” tolerance. There isn’t a grade that has a .0001” tolerance for a 1.0000” nominal size. You can spend the money to have a special gauge manufactured with a .0001” tolerance or more likely you will purchase a standard size that is close to your desired tolerance. If you look at the Gage Maker Tolerance Chart the class Y is the closest grade to our requirement. The class Y has a tolerance of .000090”. By choosing the class Y tolerance gauge we are tightening our tolerance from the ideal gauge tolerance by .000010”. By tightening the gauge tolerance we are effectively creating a situation as the production begins to move away from the midpoint of the tolerance (1.001”) it can deviate further from the midpoint of the tolerance before the fixed limit gauge detects the out of tolerance condition as opposed to the special gauge made to our original .0001” tolerance. If we choose the class Z tolerance gauge we are loosening our tolerance from the ideal gauge tolerance by .000020”. By loosening the gauge tolerance we are effectively creating a situation that allows us to detect an out of tolerance condition sooner as the production begins to move away from the midpoint of the tolerance.

This is an interesting concept, as the fixed limit gage with a lower tolerance can detect a shift in the process trending toward an out of tolerance condition sometimes sooner than the more expensive gage. This implies that corrective action can be taken sooner as well, such as tool changes or making some higher accuracy inspections to make a decision about the process.

Of course this assumes that your process is capable, and runs well within the part tolerance for the feature the gage is checking. If your process is not capable, then the cost of a more expensive gage built to a higher class (class X for example) will allow more parts that are close to the tolerance limits to be approved.

written by: Dan Smith

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