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Understanding Thread Gauges

In manufacturing, component production is governed by an international standard.

Each standard has set dimensional limits and tolerances that a manufactured component must meet to be accepted. These dimensions need to be checked and are often recorded.

A range of metrology equipment is readily available to measure diameters, lengths, surface finish etc. You can click here to view Cutwel’s full range of metrology products.

When producing a threaded component there are several methods / tools available to check thread dimensions.

Measuring a thread’s effective pitch diameter is done using a thread micrometer, which is fitted with a vee and a conical anvil. This can also be done using thread wire sets and a standard flat face micrometer.  

Thread gauges are not used for measuring a thread – they are used to check that the thread dimensions fall within an upper and lower dimensional limit.

Thread gauges are hardened and ground to accurate dimensions that meet the required standards specification.

What is a Thread Gauge and what is each type of gauge used for?

A thread gauge is used to check the dimensions of a specific thread form angle, pitch and diameter.

Plug gauges are used to check internal threads, whereas ring gauges are used to check external threads.

Each gauge comprises of two parts: a GO gauge and a NO GO gauge. Both parts should be used to check the thread.

If you are machining a component with an external thread – you would use both a GO and a NO GO ring gauge. The GO gauge will screw on to the part. The NO GO gauge should not screw on to the part.

For a thread to be within its permissible dimensions, a GO plug gauge should screw into an internal thread and a NO GO should not screw into it.

Gauges are available for all thread forms and sizes including 60 degree form – ISO Metric / Metric Fine, ANSI UNC / UNF / UNJ / NPT - to 55 degree form BSP, BSF, BSW and Trapezoidal and Acme form.

Cutwel can offer any thread gauge you require. If not from our standard range, we can offer special gauges made to your specification / requirement.

Tapered Threads

Tapered threads (NPT and BSPT) need to be checked with a stepped plug / ring gauge.

These gauges check the effective diameter of the tapered thread at a standard length from the component face. When mating components are fitted together, a seal is formed on the taper at a given position.

 

The thread gauge is screwed into or onto the component. The start of the thread / component face should stop between the upper and lower limit on the gauge (the flat section).

 

Pre-plated gauges

This style of gauge is used to check the thread size before a coating is applied to the thread.

Different components will require different coating thickness, so the importance of the information provided is critical.

To ensure the appropriate gauge is chosen, the finished thread size (i.e. M16 x 2) and thickness of the coating (i.e. 0.1mm) is required. We can then make a reliable calculation as to the new major and minor diameters of the gauge required.

 

Tolerances

There are various tolerances of thread gauge available depending on how loose or tight your thread needs to be:

 

DIN standard

4H – Tight fit

6H – Standard fit

6G – Fit with some movement

7G – Loose fit for treatment or coating

 

ANSI

3B – Tight fit

2B – Standard fit

1B – Fit with some movement

 

 

 

Tips for correct usage

 

A GO thread plug gauge is used to check the finished size of an internal thread by means of checking the major diameter as well as the pitch. It will NOT check the minor diameter. The GO thread plug gauge is screwed in by hand without excessive force and will screw all the way to bottom of the thread. If the gauge does not go all the way down the thread will not be to size.

 

A NO GO thread plug gauge is used to check that the finished size of an internal thread has not exceeded the specification by means of checking the major diameter as well as the pitch. It will NOT check the minor diameter. The NO GO thread plug gauge is screwed in by hand without excessive force and will screw in no more than 2 turns. If the gauge screws in after more than 2 turns then the thread is incorrect.

 

A GO thread ring gauge is used to check the finished size of an external thread by means of checking the minor diameter as well as the pitch. It will NOT check the major diameter. The GO thread ring gauge is screwed on by hand without excessive force and will screw all the way to end of the thread. If the gauge does not go all the way to the end the thread will not be to size.

 

A NO GO thread ring gauge is used to check that the finished size of an external thread has not exceeded the specification by means of checking the minor diameter as well as the pitch. It will NOT check the major diameter. The NO GO thread ring gauge is screwed on by hand without excessive force and will screw on no more than 2 turns. If the gauge screws in more than 2 turns then the thread is incorrect.

 

 

Different types of Thread Gauge Calibration

We offer 3 types of calibration dependant on your requirements.

 

Level 1

Standard calibration - measures pitch and diameter on one level.

 

Level 2 (JBO-CALIB)

As per level 1 plus major diameter on plug gauge and minor diameter on ring gauge.

 

Advanced (UKAS)

Calibration of diameter, thread angle, flank angle and gauging of pitch.

 

If you have any further questions or enquiries about the thread gauges, please get in touch with us on 01924 869610 or email sales@cutwel.net