Spec needed for 9" spindle take-up nut

With some sage input from several of the members here, I'm getting to the end of restoring my 9" A model (and she's beautiful, at least to pappa), and have her running well enough to have started making up missing and damaged parts.

I'm down to the last item on the list, which is the spindle take-up nut. The one that came with it was fractured, but will hold well enough with the pinch screw to make cuts toward the headstock without causing problems. But obviously, this needs replacement.

I have the material and the tooling to make a new one, and can get all specs but one. Because of the fracture, which is directly opposite the pinch cut, I am unable to get the minor diameter I need before threading.

I know that I could probably buy a replacement for ten bucks, but what fun would that be?

Anybody out there have the number I need to make my day?

Thanks to all
Mike

If you can measure the major diameter of the thread, you can calculate the minor diameter. The formula to calculate pitch diameter of a 60 degree thread is: .6495 divided by the number of threads per inch, subtracted from the major diameter. .6495 divided by the number of threads per inch, multiplied by 1.5, then subtracted from the major diameter will give you the minor diameter at 75% thread. This formula is correct for metric threads too and represents the height of a thread with it's correct geometry. When taken away from the major diameter, it is the pitch diameter from which all clearance tolerances are taken. I hope that this helps and good luck.

Easy formula for minor diameter of female thread:

Major Diameter - Pitch of thread = Minor Diameter

This formula gives 75% threads for U.S. or metric and works for figuring tap drills as well as bores on larger threads.

This is not all you need however. You need to make a "GO" gage for the thread. This is because you will not be able to try the male thread itself in the female thread as you cut it. This gage is easily made by copying the male thread you intend the female to fit. Chuck a piece of stock and turn it to the nominal major diameter of the male thread. The male thread is probably a few thousanths under a standard but the gage should be turned to that standard. Next, copy the pitch diameter by using thread wires. Don't have any? Buy a set. Pee Dee and other brands are not expensive and it is a tool frequently required in real machine work. Do not cut the gage pitch diameter any smaller than on the male thread. A thousanth or so bigger will give you that much extra clearance. Start trying the gage for a fit in the female thread early. Internal threads are usually made to 75% engagement so they finish on the pitch diameter while still showing a lot of "flat" on the minor diameter. It's easy to cut too much and have an oversize thread before you know it.

Hints on handling thread wires: Take a piece of narrow masking tape, length ~2 times the diameter of the thread you expect to measure. Lay 2 of the wires about 1/4 inch on one end of the tape then lay the remaining wire 1/4" on the other end of the tape. Roll up the tape so the wires are hanging from each end. Now they are easier to handle and if you drop them in the chip pan, you won't lose them. When you're done, leave the tape on and the wires are ready to use next time.

Below is current auction for wires but check around.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=12583&item=3850938990&rd=1&ssPageName=WDVW

Gentlemen,

Thank you both for your input. As it happens, I have just finished the replacement nut, using the same formula that Shelbydog noted (I found it in "How To Run A Lathe").

It took two trials, involving removal of the gear fork and reverse bracket to test the fit, but it turned out beautifully.

Whollymacros, your post is loaded with just the type of stuff that new hands like me need to get on board with. I'll be printing a copy of this thread, and keeping it in my shop notebook, so I can run some trials for familiarization with the techniques.

Again, thanks to both for taking the time to help out.

Mike