Go/No-Go and Fixed Gauges

A micrometer tells you a number. A go/no-go gauge tells you only one thing: is this feature inside its limits or not? That simple answer is what you want when you are checking hundreds of parts, because it is fast and takes very little judgment. Fixed gauges are made for one size, so the skill lies in using them gently and knowing what they cannot tell you.

What You Will Be Able to Do

• Explain how go and no-go ends relate to a part's limits.
• Use plug, ring, snap, and thread gauges correctly.
• Find the size of a hole using a set of pin gauges.
• Pick the right gauge for a plain bore or a tapped hole.
• Know when a fixed gauge is not enough.

How a Go/No-Go Gauge Works

A gauge has two sides, each made to one limit of the part.

• The go side is made to the size with the most material on the part. It must fit. If it does not, the feature is too tight.
• The no-go side is made to the other limit. It must not fit. If it does, the feature has gone too far.

A good feature accepts the go side and rejects the no-go side. A hole of 0.500 +0.002 / -0.000 would get a go plug near 0.500 and a no-go plug near 0.502.

Fixed gauges are attribute gauges. They sort good from bad but do not record a number, so they cannot show that a job is slowly drifting toward a limit. For that you need a tool that reads a value.

Types of Fixed Gauges

On a double-ended plug gauge, the no-go end is usually shorter and often marked red. A plain drilled or reamed hole gets a plain plug gauge. A tapped hole gets a thread plug gauge. A plain plug cannot tell you about threads.

Using Plug and Ring Gauges

1. Check that the gauge is the right one for the part number and revision, and that its calibration is current.
2. Wipe the gauge and the feature clean. Deburr the part. A burr at the mouth of a hole can stop a good go gauge.
3. Line the gauge up square with the feature.
4. Try the go side using only light hand pressure. It should enter its full length under its own weight or a very light push.
5. Try the no-go side the same way. It should not enter. A very slight start at the edge can be normal on a chamfer, but your procedure sets the rule.
6. Sort the part, and record the result the way your plan says.

Never force a gauge. Forcing wears the gauge and can jam it in the part, and it can also pass a part that should have failed. Do not twist a plug in a tight hole or hammer it free. If a plug seizes, tag the part and gauge and get help.

Gauges wear with use. The go side wears most because it enters every good part, which is why it needs regular checks.

Pin Gauges

Pin gauges are precision pins sold in sets with one size per pin, often in steps of 0.001 inch. They let you find a hole's size without a bore gauge.

1. Clean the hole and pick a pin that goes in easily.
2. Step up one size at a time until a pin will not enter.
3. The hole's size lies between the last pin that entered and the first pin that would not.

Worked example: a 0.3120 pin slides in and a 0.3130 pin will not. The hole measures between 0.3120 and 0.3130 inch.

A pin tells you the largest round shape that fits, so an oval hole may read tight. Pins have a class that sets how close they are to size, and a very common shop class is ZZ, about +/- 0.0002 inch. Handle pins with care, and never use them as punches.

Thread Gauges

Thread gauges test whether a thread will assemble and whether its pitch diameter is within limits.

1. Clean the threads and start the go gauge by hand. Do not use a wrench.
2. The go gauge should thread on for the full length of the thread with light finger force.
3. Try the no-go gauge by hand. It may start, but it must stop within a few turns. Inch threads under ASME B1.2 allow up to three full turns, and metric threads under ISO 1502 allow up to two. Use the limit on your inspection plan.

If the go gauge passes and the no-go stops as it should, the thread's pitch diameter is within tolerance. If the go gauge will not enter, look for chips, burrs, or an oversize external thread or undersize internal thread. If the no-go gauge runs in, the thread is out of tolerance.

Thread gauges tell you about fit. They do not tell you about thread depth in a blind hole unless you check that too, so watch how far the gauge goes before it bottoms out.

Reading the Results

A rising share of go gauges that feel tight, or no-go gauges that feel near fit, is an early warning that the process is moving. Tell the operator or the inspector who can look at it with a variable tool.

When to Use Something Else

Use a variable tool instead of a fixed gauge when:

• You need to chart a process and see trends.
• You are setting up or adjusting a machine and need to know how far to move it.
• The tolerance is under review, or a customer is asking for numbers.
• The print calls for a form control, such as roundness, that a simple plug cannot show.

Common Mistakes

Check Your Understanding

1. A go plug enters a hole easily and the no-go end also enters. What do you know?

The hole is too large. Reject it after you confirm the right gauge was used.

2. A go thread plug stops halfway down a tapped hole. What do you check before you reject the part?

Check for chips or burrs in the thread, and whether you have reached the bottom of a blind hole. Then check that the gauge is the right size.

3. You need to learn a hole's size and do not have a bore gauge. What can you use?

A pin gauge set. Step up by size. The hole is between the last pin that fit and the first that did not.

4. Why would a shop use a micrometer on a job that a snap gauge could check faster?

A snap gauge only passes or fails. A micrometer gives a number that shows drift and tells the operator how much to adjust.

Key Takeaways

• The go side must fit. The no-go side must not.
• Use a light touch. Forcing a gauge ruins it and hides the truth.
• A plain hole takes a plug gauge, and a tapped hole takes a thread plug.
• Pin gauges bracket a hole between two sizes.
• Fixed gauges sort. They do not show drift. Use a variable tool to see trends.