So far, most of CNC machining service cases are including inner or external thread machining at APPORO. The reason for CNC machining parts featured with thread is mainly for assembly. However, there are lot’s of different thread specs for various industrial application purpose. For example, ISO Metric Standard, Unified Thread Standard, National pipe thread, British standard pipe thread, Acme thread, etc. Actually, there are various methods for verifying the quality of machined thread, such like three wire method, thread gauge. For fast and instant measurement, we prefer thread gauge which can be generally two types. One is ring gauge for inspecting external thread and the other one is plug gauge for inspecting inner threaded hole. Both these two types are supplied with Go and No Go gauges to check if the thread against its allowed tolerances.

Why we use thread gauge?

The thread gauge can literally ensure interchangeability of precision machined parts between different manufacturing procedures in CNC precision machined components industry.

After drilling a threaded hole or machining an external thread on precision CNC part, we usually use gauges for instant checking if we manufacture the thread properly and precisely. Below shows the two gauges we mainly used at APPORO-CNC:


This type of thread gauge is for inner thread measurement purpose.
Plug thread gauge


Plug gauge is for inner thread measurement purpose. As for ISO metric standard, we generally use both 6H and 6G grade for thread inspection. Each plug gauge is with two threaded ends, Go and No Go. Inner thread situations:

1.) Go doesn’t go: inner threaded hole too small or wrong pitch

Do take notice of this situation! Inner thread too small or with wrong pitch can result in assembly problem or field failures.

2.) No Go but go: inner threaded hole too big

You can take Use As It(UAI) action for this situation if the inner thread still work with mating part without functional issue. Do improve current process to eliminate this issue for long term production.


This type of thread gauge is for external thread verification.
Ring thread gauge


Ring gauge is for external thread measurement purpose. We generally use both 6h and 6g grade for ISO thread inspection. Each set of ring gauge is with two components, Go and No Go. External thread situations as below:

1.) Go doesn’t go: external thread too big or wrong pitch

Do take notice of this situation! Inner thread too big or with wrong pitch can result in assembly problem or field failures.

2.) No Go but go: external thread too small

You can take Use As It(UAI) action for this situation if the inner thread still work with mating part without functional issue. Do improve current process to eliminate this issue for long term production.



How a thread gauge works?

To fit the gauge into or onto the thread of CNC fabricated workpiece. A thread gauge cannot feedback an actual size or measurement from inspection., instead, it feedback a status which is either acceptable or unacceptable. A CNC precision manufacturing part to be acceptable via thread gauge testing means the thread within tolerance, vise versa.


If needed, we also provide customized gauges order for both plug gauge and ring gauge. Contact us to get your own thread gauges, and keep your production lines running smooth without incorrect thread fitting. Let us know about your inquiry including thread spec details for prompt machining service. You can also bound for Wikipedia for more understanding about screw thread.


Reader’s Letter:

Hi Steven,

I am a mechanical engineering student. I was going through some case studies on functional gauge and i came across the article on case study of Thread gauge in your website. I was curious to know how much money can be saved using a thread gauge on an average and some of the companies that use these thread gauge to ensure tolerance limit of the product. So could you please give me some insight on these information?.

Thanks and Regards.


APPORO’s Reply:


Thread gauge is kind of aid for quality inspection, but not for saving production cost from manufacturing. However, to use standard thread gauge for inspection is a must for being a responsible supplier and also the best way to make sure steady thread quality.

For your reference, customized thread gauges which are not commonly used might be at least 3 times to the price of general gauges. The bigger the size of the thread, the more expensive the thread gauge.

Best regards,