Other Dimensions: What Spec?
October 29, 2007
Like many of my colleagues in the gage making business, I am astounded at how many manufacturers blaze away making parts to iffy or unknown specifications. Often the parts are made to a drawing with fuzzy details, and this lack of information is only brought to attention when a truckload of rejected parts hits the receiving dock.
This also occurs on drawings for gages where one would think optimum detail would be provided. Alas, this is not always the case and the gagemaker is left guessing as to what is required. This problem is prevalent when it comes to thread gages, so let’s take a look at them.
The most detailed drawings complete with all thread details can be the most misleading and are usually double-checked by gagemakers. Why? Because the data used may have been taken from a privately published handbook and contain errors. Because folks tend to hang onto such books for several years, some of the data in the source document may have corrections or other changes since the handbook was published.
There are conventions followed when you don’t specify details properly. For example, if you specify a class 2 fit on a thread, it is obsolete from most gagemakers’ point of view. They will supply it of course, but you’ll pay for a special gage. If you specify class 2B then you’ll get a gage to the current standard but it will be different than the class 2 fit. The difference between the two is 0.0013-inch on the no-go members-several times the gagemaker’s tolerance.
If the letters UN enter into your description, unified ANSI standards are implied. If you say UNC or UNF but don’t specify pitch diameters, you’ll get a gage to the standard. But if you want special pitch diameters-say for pre-plate inspection-you should use the UNS designation-meaning special-so the gage supplier knows. And it would be wise to state pre-plate in your designation so there is no chance of error, assuming your calculations are correct.
Documents published by national standards writing organizations are usually reviewed on a five-year cycle so that they remain up-to-date. The only way to ensure you don’t get caught with obsolete information is to have the source document from the folks who produced it rather than a copy from a friend or a second-hand source. And if it’s five-years-old, check with the organization to see if it is current, withdrawn or reaffirmed, meaning there are no changes as of the date indicated.
In the metric world they have published standards for everything under the sun including thread and plain gages. And, because the metric system is supposed to be universal in nature, you might consider yourself safe with metric gages from any source. But, like everything else, the devil’s in the details and there are a lot of devils in the metric thread gaging world.
It all boils down to North American practice compared to European practice. In Europe, gage dimensions are outside of those of the product, while here they are inside the product dimensions. You can see that simply specifying M5 x 1 for a thread could generate that truckload of rejects I mentioned earlier.
Another convention that prevails in the world implies that if an American company specified the above thread, ANSI standards would be used. If a company in Europe specified it, ISO would be the standard unless otherwise stipulated. So the battles start when you don’t say what standard the thread should be made to when the product is being imported or exported.
And when it comes to metric threads and gages, there are several systems in use around the world. The nominal sizes look to be the same but often, that is all that is “universal” about them. Protect yourself by stating whether it’s ANSI or ISO standards you require and make sure you have a copy of the standard from the source.
To make matters more interesting, unlike North American standards, there are no measuring forces stipulated for measuring products or gages with threads to European and other metric standards. This, plus some of the information mentioned earlier, may be the reason you and a supplier do not agree: you’re singing the same song from different song sheets.