Suppose you are new to the world of inspection and metrology of threaded parts. At the first sight you might be flabbergasted by the myriad of metric thread gauges used to inspect screws and nuts. The various terminologies and technicalities might confuse you to the core. But, no need to panic as yet! This article will provide you with a run through of the primary types of gauges and their application field areas.

1. UN thread gauges:-

Thread series which have uniform or invariable thread size per inch are measured using Unified National thread gauges. This series includes 8 constant variations namely – 4TPI, 6TPI, 8TPI, 12TPI, 16TPI, 20TPI, 28TPI and 32TPI. On co-inciding a fixed series with C, F, or EF these thread gauges get distinguished as follows:
  • UNC – Unified National Course Series
  • UNF – Unified National Fine Series
  • UNEF – Unified National Extra Fine Series
  • UNS – Unified national Special
2. ACME thread gauges:

This type of metric thread gauges are primarily used for thin walled applications. They are strong and durable. They are especially significant where coarse pitch is needed similar to tubes used in oil field equipment.

3. Trapezoidal thread gauges:

They are mainly used for translational motion. Some popular application areas for such thread gauges include lead screws or screw jacks. To be more precise, trapezoidal thread gauges finds use where rapid movement is needed or where a large amount of load is required to be shared.

Choosing your thread gauge:-

Now, since you have got to terms with some of the basic thread gauges it will be worthy to take a look at the proper technique of choosing
a thread gauge.

The primary criterion for a metric thread gauge is to meet a country’s thread standards. For instance, in US they are ASME/ANSI B1.2 in case of unified screw threads while ANSI B1.16M in case of metric screw threads. Moreover reference temperature for the gauge should be around 68F to make sure of its inspection accuracy.

However, when purchasing a thread it is imperative to keep track of the following points:-
  • The hardness of the gauge should be considerably above the standard minimum requirements.
  • Standard and special tolerance models should be available.
  • Metric thread gauges should be true to its dimension.
  • Gauges should be manufactured by companies that are ISO 9001 certified.
  • One should provide you with inspection certificates if needed.
  • Higher wear and tear resistance.
  • Gauges should ensure improved gripping and safe handling through the use of Knurled handles.
  • Gauge handles should preferably have double surfaces which leave enough space for specific marking requirements.
In addition to the above points, reliable threads should provide accurate measurements with efficient readings. Moreover, a thread head often gets worn out due to over use. Therefore care should be taken to check for wear and tear and such gauges should be replaced to maintain accuracy.

Parting words:-

Therefore one can say that selection of the accurate threads and gauges will help you in accurate measurements. Moreover one should also lay emphasis on the standards of the metric thread gauges to ensure efficient working and accurate results.

Frank Xu is the author of this article on Metric thread gauges.
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