Why is the Pg-series NOGO Plug Gage plain instead of threaded?
Question: I have a customer who has just ordered a set of Pg-series gauges for Din 40430. All the Pg-series screw thread plug gauges I have in stock have threaded NOGOs and are made by a thread gage maker. I see your web page says that the NOGO end is plain. I have just bought some new Pg-series screw thread ring gauges and they have the plain NOGOs. I am a little confused by this. Is this a fairly recent change? My customer is just as confused as me, as they have always used threaded NOGOs for the Pg-series screw thread form.
This is not a recent change. DIN 40430 is the standard for the product screw threads. The Pg-series screw thread gages are specified in DIN 40431 which has been unchanged since the 1970s. Having a plain NOGO is not uncommon for a DIN specification. As far as know the Pg-series screw thread form has always been designed with a plain NOGO. If you desire to be true to the standard, you will use the plain NOGO.
Because EVERYONE expects a screw thread gage NOGO to be threaded, gage makers will meet that request if pushed by their customers. Many times it is easier to do what is asked than to do what is correct. After all: The customer is always right.
From a functional perspective, either style NOGO works for the Pg-series screw thread, but if you get into a gage dispute between two gages, one plain and one threaded, the plain gage would have priority over the threaded because the threaded is not specified in DIN 40431.
When purchasing gages which are defined in commercial standards; I find it is usually best practice to go with the design defined in the document. If someone protests, just ask them to produce a copy of the standard and show you where you went wrong. If you take the customs and practice route, you have no support other than the this is what everyone else is doing defense. I think you would agree that is a very weak defense.
How does the Plain Pg-series NOGO work? The Pg-series screw thread has an 80° included angle which is larger than the included angle of most other screw threads. Most other screw threads are designed with a flat at the crest of the screw thread, but the Pg-series is designed to allow a sharp point at the crest of the screw thread. When machining the screw thread assure that there is material at the crest of the screw thread which will be removed by the screw thread cutting tool. The Pg-series screw thread plain NOGO gage actually works because as the screw thread is machined the crest of the screw thread forms a point. As the screw thread size is changed, the position of the point changes and thus can be measured. The Pg-series threaded GO member works in the normal manner by checking the maximum material condition of the screw thread form.
If you read DIN 40431 and find that I am wrong, please show me the logic out of DIN 40431 itself. I always yield to the standard.
Original Posted: 2/27/2012
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