Help with faceplate threads

[schemer]

A while back I bought a used 2 1/4 - 8 TPI slotted faceplate for/from a southbend lathe to use in a project. I also bought a new threaded backplate adapter for a chuck with the same 2 1/4 - 8 TPI thread. Fast forward to getting around to project and now I have a problem. I made an adapter for the project and cut a 2 1/4 - 8 TPI thread and it works fine on the new adapter but will not go into either side of the South Bend used faceplate. I am guessing the threads got banged on both ends but I need to fix it. It looks like the best bet (although expensive) is to buy a tap and run it through the threads. I know you can "chase" a thread and I have done it before on external threads, but never even cut an internal thread. Is it possible to repair this with a trick I don't know of like using a threading tool as a scraper or some wierd thing?
Thoughts?
Thanks,
schemer

 

[Technical Ted]

It's really hard telling not seeing the thing in person, but I would be suspicious that it was truly 2 1/4 - TPI if it doesn't fit at all.... Does it really look like both ends are that pounded up? Usually, the threads are recessed so they don't get damaged.

Do you have any way to check the major diameter of those internal threads? Like a pair of old fashion internal calipers or something? You should be able to at least check the TPI with a scale.

Good luck,
Ted

 

[Dave Paine]

As Technical Ted mentioned, confirm the threads on the faceplate are 8 tpi. I am presuming the diameter is 2 1/4in. You should be able to easily confirm the pitch with a thread gauge.

You can make your own tap to chase the internal thread. Machine a piece of stock to 2 1/4in x 8 tpi, then grind hollows so you have 2, 3 or 4 flutes. This should work for fixing any damaged thread. Machine a taper on the front as you would see on a tap.

 

[Zathros]

It helps If you would own à threading file.


Grts the other Ted.



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[schemer]

Thanks for the replies guys. I have a thread file so I will check and see if it has an 8 tpi section and check it. The adapter I made starts to screw in for approximately 1/8 of a turn. I guess I could make a thread chaser tap but I need to see if I have a chunk of 2 1/4 laying around or get one. I have some cheap screw thread gauges but no 8 tpi on them. I need to get on ebay and buy some Starrett ones. With a 6 inch scale I measure approximately 2 1/4 on the faceplate and the threads really look ok to me. I am guessing when I bought it I assumed the ebay seller knew what he had and it looked good to me when it arrived but I had no way of checking it. I will try to get more info with measurements and see what else I can offer for specs.
Thanks,
schemer

 

[Technical Ted]

I suggest you use your scale and count how many threads there are per inch in your faceplate. You've got a pretty big bore and if it is 8 TPI you should easily be able to count 8 threads per inch on your scale or, in other words, one thread per 1/8" division on your scale. From what you are saying, it sounds like it's not actually 8 TPI. Thread gauges are very nice, but using a scale as I've described, especially on a very large, coarse thread like this, should be fairly easy and accurate.

Good luck,
Ted

 

[schemer]

I suggest you use your scale and count how many threads there are per inch in your faceplate. You've got a pretty big bore and if it is 8 TPI you should easily be able to count 8 threads per inch on your scale or, in other words, one thread per 1/8" division on your scale. From what you are saying, it sounds like it's not actually 8 TPI. Thread gauges are very nice, but using a scale as I've described, especially on a very large, coarse thread like this, should be fairly easy and accurate.

Good luck,
Ted

I will do that tomorrow when I get back out in the workshop. I did try to measure yesterday with the scale when I first ran into this problem on my first attempt on making the adapter (didn't look as good as I would have liked with rough thread) and it seemed close, but I will check more carefully and accurately. Will report back tomorrow.
Thanks,
schemer

 

[Silverbullet]

Many are either 2 1/4 " -6 or 2 3/8" - 6 . One of the two south bend used for years in there lathes. Ck the threads with a thread pitch gage. Or count the tops of the threads per inch.

 

[schemer]

Many are either 2 1/4 " -6 or 2 3/8" - 6 . One of the two south bend used for years in there lathes. Ck the threads with a thread pitch gage. Or count the tops of the threads per inch.

Man I was hoping you were right so I could sell this and buy another one. But I checked this morning and with a calipers the ID's are the same and with a 6 inch scale likewise. I put the 6 inch scale across the threads and they are right on at 1/8" apart on both. I will take some pics and post them here in case a sharper eye than mine can spot some obvious issue. Seems like a tap is going to be the best approach because if I sell it on ebay the buyer is going to have the same problem.

 

[Wreck™Wreck]

If I understand your post correctly you have a lathe with a 2 1/4-8 threaded spindle nose.
You then turned a 2 1/4-8 plug gauge which works with a new internally threaded part yet not an old internally threaded face plate of the same thread?

Am I on the right track so far?