Help with Plug for 1/2-20 UNF

I'm at a complete loss to understand exactly what you're looking for. I'm not familiar with anything similar to that which you've described. One normally inspects such things with an optical comparator, although there may very well be a mechanical inspection device similar to that which you described. Can you better describe what you're trying to accomplish? Typically, ring or snap gages, in a round about way, check all the features, although it may not be apparent that they do. Only problem with snap or ring gages is they tell you if a part is good or not, but provide no report on variations. Good or bad, that's about it.

Harold

You are looking for a thread gage. I would first investigate MSC corporation, I think they sell them.

From the term 'plug' it sounds like you are trying to gage a female thread. If it is a male thread then you could do it by the wire method.

Jim

================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ==================================================

You can see what I am talking about on page 8 of this catalog:

What I want to check is whether I have enough crest (I believe it is also called the Minor Diameter) of a female 1/2-20 UNF thread.

Yes, I an trying to check whether a female 1/2-20 UNF thread has enough crest (I believe it is also called Minor Diameter). I have some tie-rod nuts (with a female 1/2-20 UNF thread) that customers are reporting that they become loose too easily. I believe the problem is that the threads of the nuts do not have sufficient crest to engage well with the male threads of the tie rods...

First, my apology for getting your question twisted in my head. Once I started thinking external threads, apparently nothing was going to turn me around.

You do not need any type gage unless you don't have other means of measuring. You are right, it's the minor diameter of the thread of which you speak that you are concerned with. That is an absolute, with a tolerance, which can be measured by various means including drill blanks, various plug gages, or any piece of material that will fit the hole snugly. If you have a lathe, it would be easy enough to turn the two diameters referenced below for a quick check. It should accept the one, but not the other. Otherwise you are out of bounds. No need to buy a precision gage unless you are trying to sell the threads to a government agency, at which time gages may be required. If you want to determine an exact size, turn a piece until it is a snug fit in the existing threaded hole, measure the diameter of the gage you just turned, and that will give you a very good idea of the actual size of the hole, certainly close enough to make a decision about the minor diameter of a disputed thread under the circumstances of which you speak. For a 1/2"-20 UNF 2B thread, the minor diameter should be .4459"/.4537".

Hope this helps.

Harold

I think I saw something like this in a recent Fowler flier . You might try them or maybe Fred will pop up here and respond .

Excellent tips! Thanks, Harold.