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Full Form vs. Truncated/Modified

Full Form vs. Truncated or Modified Whitworth Screw Thread Style

This is easy. The Whitworth Screw Thread Form found in the various published standards for: BSP; BSPP; BSPT; BSW; BSF; G-series; R-series; all require a radius screw thread root and screw thread crest for both the internal screw thread and the external screw thread.

Truncated is a very flexible word. The definitions range from very complicated to quite simple and its use covers a wide range of topics. Look it up on Google and see what I mean. In this situation I believe that the two definitions that make the most since are: 1) Geometrically speaking; truncation is an operation that cuts a shape creating a new facet. 2) The most basic definition; truncated means shortened. The Whitworth screw thread form is sometimes modified by cutting off the radius and replacing it with a flat, basically shortening the screw thread form.

Thus, when the Whitworth screw thread is called: Truncated, or alternately: Modified; it means that it has been modified from the requirements of the screw thread standard. The drawing above shows the basic modification made to the Whitworth screw thread form. At the screw thread crest the radius is removed and replaced with a flat positioned at the tangent point between the radius and the flank angles. The shape of the screw thread root is more or less undefined (since the standard is not being adhered to, who really cares anyway) and monitored only by the GO screw thread gage. What may be found at the screw thread root is anything from full-sharp to flatted to some-radius. The screw thread root shape depends on the initial shape of the screw thread cutting tool and to a large extent on how much screw thread cutting tool wear has occurred.

Wait a minute! Did I say: It has been modified from the requirements of the screw thread standard? Doesn't that mean that it no longer meets the requirements of the standard? Why would one want to make a thread which does not conform to the standard? Why is deviation from the standard so prevalent that it demands discussion on this web page? Time for a little history.

Back in World War I and World War II era, when the USA and UK were working together with war machines, the USA machinists needed to make parts to fit British equipment. The parts that fit British equipment had Whitworth form screw threads. Because the British screw thread was the Whitworth screw thread form, the British gage makers had developed different thread grinding equipment which could easily reproduced the Whitworth screw thread form. In the USA the Sellers screw thread form; the National Thread: NC; NF; NEF; NS; was in common usage. The Sellers screw thread form does not require radii, it has a flat screw thread crest and root. Because the USA gage makers work with the flat root and crest they had not developed a good method to make the radius root and crest. To get a thread gage that was good enough to get the job done the gage makers decided to truncate the BSP gages similar to how they make gages for the J-series which also requires a radius on the part. The Sellers screw thread form is now the primary screw thread form used around the world in the: UN-series; M-series; J-series;and NP-series.

With globalization being pervasive, much of the product made in USA is being shipped elsewhere in the world. The rest of the world makes the product as full form and uses full form gages. We do have better cutting tools today. Full-form Whitworth screw thread cutting tools are readily available at reasonable prices. The gages are a different matter. The USA gage makers have tried on several occasions to make the gages to meet the requirements of the Whitworth Screw Thread Form standards, but have consistently found the effort to be non-profitable, and have reverted to the quick-fix of making the gages in the truncated or modified form. Some have gotten so used to making modified from the standard gages that they no longer advise their customer that the gages do not meet the requirements of the standard and they no longer mark the gages: MOD or Truncated.

We need to encourage USA manufacturers to produce product to the standard and measure it to the standard so that it will fit with other goods made elsewhere in the world and not be rejected. If you are in the market for a gage, then you are interested in doing the job correctly and meeting the requirements of the standard. Since that is the case I suggest that you specify Full-Form when Whitworth screw thread form when purchasing cutting tools and gages. The Whitworth screw thread form gages that we sell will fully meet the requirements of the appropriate standard (unless you direct us to violate the standard).

What if you already own gages? We are a green thinking company. From our beginning we tried to recycle thread gages and continue to do that even today. While truncated gages do not meet the full requirements of the standard, we suggest that you can continue to use them as long as you are using Full-Form cutting tools and monitoring the radius by some other inspection method. If you are in doubt about gages already in your inventory there are several ways that you can ascertain if they meet the standard: 1) If the gage was made in the USA, it does not meet the requirements of the standard. 2) If the ring gage is the American Gage Design (AGD) style, it does not meet the requirements of the standard. 3) View the GO work plug gage in your optical comparator to see if radii are present, if not it does not meet the requirements of the standard (the NOGO gage is not required to have radii).

If you choose to use Full-Form gages, be sure that all your cutting tools are also full form. In tolerance parts made with Truncated or Modified cutting tools will be rejected by Full-Form gages, as they should be. Also, if the part is being manufactured and size is being checked at the machine, internal screw threads will end-up oversized, and external screw threads will end-up undersized because of major and minor diameter conflicts. Parts made with Full form cutting tools will measure fine with Truncated or Modified screw thread gages, but an additional inspection step should be included to assure that e radii remain in tolerance.

This data is accurate to the best of my knowledge as of 7/13/2009.
If you find error or desire to make comment contact Wayne at:office@gagecrib.com

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