Indicating an Internal Thread

I need to indicate an internal thread. It is a 2 1/4-8 thread and it is on a backplate. Someone bored the register off center and I need to bore and resleeve the backplate but the register bore has got to be on the same centerline as the thread to make this work. I have the back plate set up in the 4 jaw now I just need to figure out how to dial it in so the bore and the thread centerline line up. Any suggestions greatly appreciated. Thanks, Steve

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Dial in the pitch diameter of the thread while the feed train is set up for

8tpi and engaged, so your indicator follows the thread as the chuck is turned over by hand.

Ideally, your ball on the point of the indicator should be close to same dia as a thread measuring wire would be for measuring an 8 tpi external.

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I'm not sure exactly how accurate you need to be, but one time I needed to do something like this, I turned a small piece of threaded rod down smooth on one end in the lathe. I screwed this rod into the threaded hole and indicated off the turned end.

To mount the threaded rod for turning, I placed a piece of bar stock in the

3-jaw chuck, and then bored and threaded it to the same thread. I then tightened the threaded rod into this simple jig to hold it centered in the chuck....or course doing all of this without unchucking anything.

I'm sure this isn't an exact method...but it worked very well in my case...especially since the threaded hole I was working on was kinda small. Not sure how well it would work for a larger hole like yours.


Reply to
Todd Rearick

One way is to make a tight fitting plug gage threaded the same as the backplate. Leave a section sticking out that you can indicate on.

Another approach (easier) would be to a) make a short ring spacer to go over the register diameter on the spindle. b) re-sleeve the backplate undersized. c) put the backplate on the spindle, wrong-way-around, with the ring spacer in place to allow it to bottom on the spindle.

Then bore the register diameter.

The first way would be more accurate, I think.


Reply to
jim rozen

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