Getting Answers on Thread Measurement
We can't have too much information in dimensional metrology.
If you’re having problems with thread measurements but the answers you’re getting from your usual sources aren’t solving them, it’s natural that you would try and get some unbiased advice from a specialist. If you take this path, be prepared to answer a lot of questions before any worthwhile answers are forthcoming. This column includes a lot of those questions so you can be prepared and save everyone a lot of time when you talk with a specialist in this area of metrology.
The first question is the obvious one, but the answers are often incomplete: What thread are you concerned about? 1/4-20 UNC might be your answer, but if you neglect to mention that it is a two start or double-lead thread or perhaps a pre-plate thread for example, the recommended solution may not solve your problem. Before I forget, make sure you mention whether it’s an internal or external thread. If you can provide a part drawing showing the thread you can save a lot of discussion time but often even they are not properly specified.
Many folks take thread details off a component drawing but fail to mention that the threads are on a series of studs in a five hundred pound block of steel so bringing the studs to a measuring system is not an option. Also, the studs may be spaced so close together that traditional gages can’t be used to check them.
What is the function of the thread? The answer to this general question about the product thread may be more important than you think. For example, if its bolting door hinges on a wood burning stove, the gaging can be quite different than if it is to be used in a similar function on aircraft.
The game changes if it will function as a leadscrew in a precision mechanism. If it has to seal against leakage of fluids it changes again. The answer to this question will be kept in mind as the techie stuff is discussed.
How are you producing the thread? The reason for this question is because some methods of manufacture are notorious for producing certain problems. The answer you give enables the specialist to know extra elements any checking system recommended will have to be able to detect.
The material the thread is made from can be the source of a lot of measurement problems such as some non-ferrous metals that tend to load up your gages or react differently to temperature fluctuations.
At what point in manufacture are you having problems? In other words, are the in-process checks saying one thing while the final acceptance indicates something else? “What methods are you using for each?” is the next question for you to answer. If one of those methods is a variable gage, specify if full form or PD type contacts are being used. The make and model number of such devices should be at hand as performance can vary between types and makers.
Most threads are produced without significant problems. What leads to hair pulling and grinding of teeth is a rejection notice from a customer. If this is the case, let the specialist you’re dealing with know upfront. Get the customer’s answers to the above questions, especially when the rejection is due to your components not working with theirs. It could turn out that no one is measuring or gaging the threads properly. If you get lucky, it could be that you are doing things properly but your customer is not.
If your gages are accepting the parts but your customer is rejecting them and both are the same type of gage, send them all to a calibration laboratory. The reports on them may show that while they appear to be the same, dimensionally they are not. Problem solved.
Should your foray into thread measurement be all about finding a different method from what you are now using, the question to be answered is: Why? If you are trying to reduce the cost of gages you may find that the alternate method you are considering won’t do the job properly. ‘Savings’ can disappear if rejects show up. If your goal is to speed up the process, the same situation can arise.
You can’t have too much information when asking specialists for help in any type of dimensional metrology. Finding the specialist is usually the most frustrating part of the situation.