Other Dimensions Thread Speak

March 8, 2012
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Are you speaking the same language?



Like all industries, the gage makers of the world have terminology and understandings peculiar to their field, some of which can leave the uninitiated wondering just what we’re really talking about. I would be remiss if I didn’t note that some of the people you think should understand the language-gage makers and calibration labs-don’t.

This applies to other types of gages and instruments to a certain extent, but with the exception of gear metrology, thread gaging is on its own planet in the universe as we know it. Those gear people are worse of course, but they’re in a different galaxy altogether so I won’t go there.

Pitch diameter is the feature most folks are concerned with when it comes to screw threads. Since it is an invisible feature compared to a normal diameter, it is understandable that there can be variations in how people define it. Sure, most folks know that it’s a diameter, the presence of which is detected through measurement, but what causes confusion are different names that are applied to it and what those differences mean in practical terms.

Simplistically speaking, it is a diameter that is physically located by the use of thread measuring wires of specified size over which a measurement is made in the usual fashion. In North America it is treated as a single, stand-alone feature in most cases. Thread standards give details of the form of a thread and show the pitch diameter graphically. However, from a metrology point of view, pitch diameter, or PD, can mean different things. Some of these differences are now being indicated on calibration reports for gages in the form of different labels for this feature.

There are two primary categories of pitch diameter, one referring to simple pitch diameter as a stand-alone feature, while the other relates to its performance, which includes all related elements of the thread. The physical sizes between the two can be quite different.

Simple pitch diameter as an independent feature may be referred to by that name or it may be called vee-groove diameter or simple groove diameter. It means that the declared size does not include any other thread elements that could affect it.

Functional pitch diameter includes adjustments or corrections to the measured simple pitch diameter size for other features that directly affect its operational size such as linear pitch. In England, the term “effective” is often used for the latter situation indicating that such influencing factors are included in the reported size. This is standard practice for many countries in Europe and Asia.

Standard thread combinations in North American standards usually ignore these modifying factors if they are within specified limits and for component parts little or no difference will be apparent. But when it comes to thread gages, major differences will be evident.

You can see from this rather lengthy explanation that disputes over a pitch diameter measurement can be never-ending if both parties are not speaking the same language. And if you intend on having NIST settle a dispute, you should know their standard practice is to provide a functional pitch diameter reading. Specify what type of PD you want them to provide to avoid problems.

To make matters more interesting, the measurement uncertainty involved in either method of determining a pitch diameter size also will vary considerably. All factors being equal, the larger uncertainty will accrue to the laboratory providing a functional pitch diameter reading due to the influencing factors it includes.

In North America, if not specified otherwise, calibration labs and gage makers mean “simple” or “vee-groove” diameter when they speak of pitch diameter. Standard thread diameter/pitch combinations listed in the standards usually fall into this definition. This changes when special combinations are involved and/or pitch variations exceed a stipulated amount or, in some cases, the corrections have to be applied irrespective of their magnitude.

I won’t go into those other thread speak definitions for things such as linear pitch and lead at this time. These are fraught with problems where defining what each really is can be argued forever, and the battlefield extended to cover the measurement of them, something few people have equipment to do.

Now that I’ve upset just about everyone with this column, I’m going to hide out in the hills until they stop looking for me.



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