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Thread Gages Q&As

Typical questions encountered when using Thread Gages to inspect parts in the machine shop.

Q.  We have a thread ring gage that allows the part to go in on one side but not the other side. How can this happen?

A. Two possible reasons and they depend on how much use the rings gage has had.

First is the most logical reason for both old and new rings is damage from dropping the ring or shipping damage. If the ring is dropped just right, on a corner, it can be sprung. This might be correctable if it is not permanently distorted by resetting the ring to your setting plug.

Second, which happens on used rings, is that the parts are screwed into one side predominantly, thus you will see a higher amount of wear on one side versus the other. In these cases, even using a setting plug will not correct this. The set plug will always come in contact with the least worn pitch diameter when set.  There is always the remote chance that the ring was not held uniformly in the chuck when the tap was run through it or there was heat treat distortion but normally this is checked before shipment.

Q. I have just purchased a new thread ring but no setting plug and tried to use my old setting plug to check it out and it does not fit why?


A. We have this question quite often and it relates to the setting plug tolerances along with the tolerance stack up on the ring as well as the setting force used when the ring is set at the factory. Of course shipping damage is always a possibility but you need to understand that when the new ring is set up at the factory they use their production setting plug which may ( will) usually have a different PD than your current  setting plug which is allowable with a typical tolerance. When the setting plug is made it has its own tolerance so you can find two setting plugs at either end of their allowable values so that a ring set with one will not allow entry by the other set plug.  A word of advice is to either send in your own set plug to have a new ring set to or make sure you purchase a new set plug and specify the new ring is adjusted to your new plug at the factory.


Q.  I have a situation where my thread plug G end will not go in my part but my No Go will go in. Can you tell me how this can happen?


A. This is usually associated with a single point turning operation but not always. It can happen for the following reasons. A NoGo gage plug is actually truncated slightly to reduce the major diameter or thread crests. This is due to the premise that the NoGo gage is checking the PD primarily and not the minor diameter of your internal threads. That is checked with the Go gage.  What we have seen in the past is two issues happening at the same time. First the tip of the tool cutting the thread is wearing faster than the rest of the cutting tool so the root of the product thread becomes shallow. Second the tool wear is adjusted to the point that the pitch diameter is oversized as the tool is compensated.

The result of an oversized PD which allows the NoGo gage to enter but with a corresponding shallow thread root that stops the Go gage form going in the product. This is not always the issue of course but it is a good place to start based on the way the thread form is created and if it makes sense. A damaged Go gage is also a good possibility not to mention a non round bore which can at times create this kind of unusual issue.


Q. We have been using our thread ring now for a while and we need to get it recertified and is is a special what do I need to do?

A.  A really common question. First of all, do you have the special set plug that was used to originally set and certify your ring?

Q. No, but we purchased the original ring from The Gage Store, don't you have the set plug?

A. No, we do not, in almost all cases, keep a special set plug we make. If  we make a special ring we make a set plug that is intended for that one use and nothing more unless you order a standard set plug with your thread ring. The temporary set plug is  intended to be used one time and then it is usually repurposed by regrinding for another special ring until it is no longer usable.

It you have a special ring and did not purchase a set plug with it then a new set plug must be produced in order to reset and certify your used ring. Always consider purchasing the set plug when you purchase the ring if there is any thought that this application has any possibility of a medium to long run.


Q. I have parts that require checking before plating how do I order these gages?

A. Two methods to get it correct, first if you have the specific PDs of the pre-plated threads you are cutting we will make the gages to inspect them based on your min and max PDs. A second method that is used to calculate the PDs of the gage in accordance with ASME is to take the finished PD sizes like on a 1/4 20 3A UNC male thread.

The PDs of 3A thread rings are .2175 Go and .2147 NoGo. In order to make a gage to check the undersized make threads you cut that allow for the plating thickness we take the plating you will apply, .0005" as an example to make it easier for this exercise.  The formula is .0005 x4 equals the offset to the standard PDs. We will correct the final PDs by .002", subtracting this to to the ring nominal PDs making the new gage PDs .2155 Go and .2127 no go. These smaller PDs are required to check your pre-plated male thread. Of course we subtract 4 times the plating thickness from the threaded plug checking internal threads. Again this is the standard calculation referenced in ASME but there are always exceptions.

    Q. The below statement is taken off one of your pages on the website. The section concerning “should not thread over more than 3 times” is     this statement taken from a industry standard, like a ASTM spec or other??  We are currently being audited and this situation came up. In all     my 40 years of measurement, I always used the 3 turn statement. The auditors would like to see the statement in some formal standard?


    A. The statement is taken directly from ASME B1.2 in a description of the Not Go gage        function paragraph 4.2.1.

 Q. Dear Gagestore,

    I have another question:

    For thread plug gages, we can check the pitch dia by three wire method. But how to check pitch dia for thread ring gage? Please advice (including catalog of the tool too).

     A. The most recognized method is the use of a certified setting plug for the gage in question. The industry methods takes a male setting plug gage and    certifies that gage. The setting plug is then inserted in the thread ring and the ring is adjusted to the plug as described on our web site in accordance with industry practices. The ring is first adjusted till a slight friction is felt on the truncated portion of the set plug then threaded further over the full form portion. If the gage binds when threading over the full form it is worn out. If it threads over the full form with a similar feel you can assume the gage is still within the useable limits.

Q. What is the difference between a 3/8-24 UNJF-3B & 3/8-24 UNJS Thread plug. Please explain.


A. We see a lot of call outs that may or may not be important. Normally the convention for this thread is 3/8 24 UNJF. When we see the "S" on any call out it sends up a flag to review the thread form called out on the drawing to see what is different about the thread. Sometimes it can call out a different pitch diameter, major diameter or minor diameter. Nothing is guaranteed except we need to look at the drawing to make sure there are no special notes on the thread that require a change to the gage we would supply to check it.

Q. Technical Department,

My name is Dan and I work for an aerospace firm producing space structures. We have been using your flexible hole location gages for some time. One of our customers has inquired as to what class of thread is on the gages since there is a bit of an interference fit. Apparently there is some concern as to thread deformation in the use of these gages. So can you tell me what class of thread these gages are machined to?

 Hello Dan,

These gages are designed specifically to cover the entire tolerance range of the thread. The split versions are made in such a way that the split allows them to conform to the lower limit of the thread PD and the body actually has a small taper to it so that when you screw it in the entry end conforms to achieve a firm contact on the PDs of multiple threads.

This means, as you thread the plug into your hole, you want to thread it in only enough to create a firm datum with no wobble. The smaller solid gages are just tapered so as you thread them in the same till they lock up on the part PDs to establish the centerline of the PD. Because you are not checking the thread PDs only locating to them, there is no specified class in ASME  for these plugs. When they are made the gage's PDs, upper measuring face and pin are all ground concentric to each other within +/- .0001"

You do not want to thread either design in any further than snug to create no wobble. If the split plug is threaded in too far you can actually damage the split to the point it will no longer function properly.

The key is to hand thread them in till you have resistance and you can feel no movement of the pin. Never thread them in to the shoulder.




If you have questions feel free to send your questions to Technical@thegagestore.com

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NOTICE: All information provided on this reference sheet is intended to be for general guidelines and may not represent the full specifications or instructions as outlined within ASME requirements or other technical specifications which govern the manufacturing and use of thread measuring gages. thegagestore.com/LEECO INC, does not represent this information as anything except a common sense general set of guidelines. You must always follow the technical specifications relative to the gage, thread design or other related gage attributes and usage that apply. When in doubt always acquire the current specification from http://www.asme.org/ codes & standards.


IF YOU HAVE QUESTIONS OR COMMENTS, E-MAIL Technical@thegagestore.com OR FAX US AT 860-404-8903