# Checking An Internal Thread Pitch



## 2volts (Jan 30, 2016)

I needed to check the pitch of an internal thread that was easy to insert the thread pitch gauge into but awkward to see if what felt like the right pitch actually was right.

 I got some bluetak (the stuff you stick paper to walls with) and pressed that into the hole/onto the thread then took it out and was easily able to confirm the thread pitch was 20tpi as I suspected.

pete


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## Wreck™Wreck (Jan 30, 2016)

You modeled an external thread by molding an internal thread?


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## Paul in OKC (Jan 30, 2016)

Wreck™Wreck said:


> You modeled an external thread by molding an internal thread?



Sounds like it. Play-Doh is usable for things like that, too!


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## 2volts (Jan 31, 2016)

I measured the minor diameter of the internal thread and looked it up and was fairly sure it was a 7/16 thread then had to confirm the thread pitch that it was 7/16 x 20 UNF.
It felt like 20tpi using the gauge I just wanted to make sure. 

pete


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## turnitupper (Jan 31, 2016)

If you have used up all your Blue Tak mounting your Farrah Fawcett posters, a method I use is to wrap some masking tape on the shank of a drill so it is a bit larger than the hole and screw that in the thread.
It won't tell you if it is 55 or 60 degree but you can generally work that out by the pitch and diameter.
Soft wood of any shape large enough to screw in the hole, rolled up cardboard or even paper will work in a pinch.
The dirtier the hole the better in this instance.
John.
P.S. That Blue Tack must be very old, it has turned yellow with age.


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## Tony Wells (Jan 31, 2016)

If I need to cast a positive replica of a feature, be it a groove or thread or whatever, I have a 25# box of dental plaster. It sets up in about 5 minutes. I have then sawed cross sections for optical comparator viewing or measurement. It doesn't shrink much if any that I have been able to catch. A spoonful in a Styrofoam coffee cup and that's it. It gets pretty hard, harder than normal plaster of paris. You can build dams from modeling clay for OD work that you need it for, or any inside. Won't work for lathe cut dovetail grooves though. It captures details close enough to run a profilometer on. As long as you don't fill an ID more than halfway, it comes out easily enough.


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## gi_984 (Feb 2, 2016)

Tony, that sounds interesting.
     I've used Brownells' Acraglas resin for glass bedding rifle actions in stocks.  Depending on the action you need to make a dam for the resin so it doesn't run everywhere.  Problem is you need to coat the surface with a release agent so you can remove the action from the stock after the resin cures.  So I wouldn't trust it to determine a fine measurement in a machining application.
    Could you describe in a little more detail how you would get the impression of a internal thread in a blind hole?  Trying to visualize how you keep the dental plaster from running down into the blind hole.


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## Billh50 (Feb 2, 2016)

Tony,
Many factory inspection rooms use that method. They use tape for any dams needed to keep the plaster in place.


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## kingmt01 (Feb 2, 2016)

Melt sulphur is another method of casting. There is some shrinkage to account for depending on how close you need to measure.


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## David S (Feb 2, 2016)

Of course if you happen to have some two part silicone moulding compound, you can make a dam and use that to make a plug of the internal thread.  It is quick release and essentially zero shrinkage.  So should be be good.

David


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## Tony Wells (Feb 2, 2016)

If the hole is vertical, naturally it will want to run down and fill the hole. I have sprayed cooking non-stick stuff in the hole and as the plaster is setting, inserted a u shaped loop of wire, sometimes a paperclip, depending on size, and it will simply unscrew. I make sure the hole is very clean, and use a little air to make sure the spray is pretty dry before doing this. And of course, there can be no features that will entrap the plaster. Ideally, the part can be oriented to put the feature horizontal.

Bill, I worked inspection for a few years, and that's where I picked this method up. A QC guy with Dresser Industries put me onto it and got me the box of plaster. It goes a long way.

MSC and several others sell a 2 part replica rubber compound just for this if you want to get official.


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