Minor repairs on lathe spindle threads on my American Beauty

[Eric Gourieux]

I have an American Beauty with spindle threads that have some minor damage. I'm not sure why/how this happened, but that could be a topic for a different thread (pun not intended). Brent from Robust recommended a die from McMaster-Carr, but I'm having trouble determining which die. I purchased a die that has the specifications: 1 1/4-8 UN. The threads on my AB are 1 1/4 - 8 threads. This die is too small. On the McMaster website, they list dies as UNC and UNF, and from what I can determine, these stand for "coarse" and "fine". I purchased this particular die, since it was the only one I could find that was 1 1/4 - 8. I don't know anything about machining, and my only experience with using a tap was to make my own vacuum chucks with the Beall tap. Can anybody help me with this nomenclature on the dies? Has anybody else had experience with repairing the threads on their lathe spindle?

[Steve Schlumpf]

Eric, the UN means unified threads and a 1.25 x 8 should fit. A photo of the die would help. In my experience, one side of the die has a slight cone shape to it and allows you to start the die onto the bolt or in this case, the spindle. I would also expect it to be really tight and most likely end up cutting ever so slight as you thread it onto the spindle.

Not knowing how much damage you have to your threads - any chance you can use fine files to fix things?

[Leo Van Der Loo]

If the damage is small and not over the whole length of the treaded end, I would use a Thread file, they are designed to do these repairs.

It shows the different files here.

A local machine shop should have them, maybe even lend you one

https://www.amazon.ca/Nicholson-Thre.../dp/B001T4KRHC

[Leo Van Der Loo]

Leo, the thread files you linked to are for external threads only. There is a repair file made for internal threads made by JAWCO but they are difficult to use if the thread damage is anything more than minor.

That is what the OP needs it for, as he says, "I have an American Beauty with spindle threads that have some minor damage."

[Bruce Page]

Leo, you are right, I misread his problem. A small triangular file would also work to clean up the thread. Everyone should have at least one in their tool box.

Eric, does the die you purchased have an adjustment screw on the side? You can use this screw to open up the die so it will take less of a cut. First mark the screw slot with a pencil or Sharpie so you can adjust it back to its original setting.

[William C Rogers]

Eric
a picture of the damage would be helpful. Also HF carries a small file set called needle file.

[Steve Schlumpf]

You know, the spindle is hardened, so a die might be really hard to use. One other option would be to use a 1.25 x 8 nut and just thread it on to straighten the threads.

[Dale Miner]

Just to clarify, 1-1/4" x 8 tpi is neithe NC (UNC) or NF (UNF).

1-1/4" x 8 tpi is a NS (UNS) thread.

Make sure your die is clearly marked 1-1/4 x 8 tpi. If it is marked 1-1/4 x 7 tpi it is the wrong die.

A 1-1/4" NC is 7 tpi. Trying to use a 7 tpi die on a 8 tpi spindle will make it appear the spindle is to large and destroy the spindle in the process.

If the damage of the thread is confined to a small area, a 3 corner file would be your best bet with trying the chucks frequently during filing until they run on without any noticeable difference from before the damage occurred.

UNS chart. http://www.gewinde-normen.de/en/unif...al-thread.html

UNC chart. http://www.carbidedepot.com/formulas-tap-standard.htm

[Sid Matheny]

If this is the die you got I think it should work.

https://www.mcmaster.com/#25585a12/=15j05xu

[John K Jordan]

I would also far prefer to use a thread file for restoring damaged threads. A thread restoring die, tap, or file does a better job than a die or tap made for cutting threads.

Many times I have used this kit for metric and SAE threads:
https://www.amazon.com/gp/product/B000P6UNHEged threads.

This kit doesn't have files for an 8 tpi UN8 thread found on lathe spindles, but if I could find one I'd use it rather than a thread restorer die unless the threads were SERIOUSLY damaged. It would probably be a lot cheaper, too. But I agree - a file should make quick work of minor thread damage.

JKJ

[Eric Gourieux]

Thanks for all of your replies. I ended up running a 3 corner file over the problem area and it works great. There was only some minor damage, so it didn't take much. I'm all up and running again.

[Tom Albrecht]

Great.

Now I would like to hear what you think caused the problem. I have my suspicions, but I'll wait for now.

[Leo Van Der Loo]

Great.

Now I would like to hear what you think caused the problem. I have my suspicions, but I'll wait for now.

You don’t think it was chuck grub screw only tightened lightly, do you ??

[Doug Rasmussen]

It's probably very good that you didn't use the die on your spindle. In fact, as you say it appeared too small, I doubt you could have turned the die. It takes a horrendous amount of torque to turn a die that size if it's removing even a small amount of material, especially on a heat treated spindle.

There are tools specifically designed for thread repair. My favorite for situations like a spindle repair would be this one from McMaster #2630A12. @ $105.

[Perry Hilbert Jr]

I also get leery about using a die on such threads. There is so very much more than just the outer diameter and threads per inch. Had a rifle permanently ruined by a jerk gunsmith who thought all 1x14 threads are the same. Turns out there are different angles for the peaks and valleys of the threads. A British 1x14 is not compatible with an SAE 1x14 even though both are "v" threads and the same number of threads per inch. Different angles of the "v"