What are Your Thread Gages Really Telling You?

It's easy to underestimate the importance of the humble thread gage, given the relatively simple nature of this tool. Most workshops that create parts with threaded elements use thread gages daily, but it may not always be clear to the workers who use these tools precisely what it is that they are measuring. Since basic go/no-go thread gages are such simple devices to operate, it can pay off in the long run to understand a bit more about how threaded surfaces work and why a thread gage test might fail.

The Difference Between Go and No-Go Gages

Both types of thread gages are used to measure an internal or external thread, but their purposes are drastically different. A go thread gage is used to ensure that a threaded fastener or component can properly mate to the threaded surface on the workpiece. This type of thread gage is also known as an acceptance gage, and inspecting the threads of any workpiece using one is a vital part of even small-scale manufacturing operations. A no-go gage, on the other hand, is used to reject parts that exceed maximum tolerances. While a go gage must be able to thread into (or onto) a threaded workpiece, a no-go must not be able to thread into or onto the same piece.

How Thread Gages Check Thread Profile

Although thread gages are commonly said to measure aspects of the thread profile, these are not truly measuring tools. Instead, thread gages are inspection tools that can provide some simple functional information. An acceptance gage checks that a thread has the proper pitch diameter to accommodate the part that is intended to mate with. For internal threads, this means that the threaded part can accept bolts of the correct pitch, while this confirms that an externally threaded part can accept nuts of the right size.

Regardless of whether you are inspecting an internal or external thread, the gage tool works by simply fitting or not fitting. If a 'go' gage can be smoothly threaded into or onto the threaded section of a work piece, then the thread sizing is correct. Likewise, if a 'no-go' gage cannot be threaded without being forced, then the threaded portion of the workpiece does not exceed the expected maximum pitch diameter. Taken together, both types of thread gages are sufficient to determine that the threaded part of a workpiece was manufactured to the correct specifications. Contact a company like WESTport to learn more.