Lathe spindle spider internal threads calculation

[Rcdizy]

The O.D of the threads on the spindle are 1.794". and the threads are 16 TPI

I would like to try and make a female part to thread on there. to use as a stop to make repeatable long length parts, and also use as a spider

How do I calculate the best bored i.d prior to threading, and the best depth of thread cut?

my cross slide moves the actual dial measurement. (in .001. takes .002 of the diameter of the work.)

Most of the formulas I found are for external screw thread cutting.

I will not be able to test the part for fit, as I will lose my threading start place if I remove it from the chuck at any point.

 

[Dave Paine]

This would be a time when I would prefer to measure rather than attempt to calculate since I am not sure if this is a standard pitch as in depth not tpi.

Thread pitch diameter measuring wires are not expensive, just can be a bit of a hassle for us folks who do not use them often.

A video by Stan (Shadon HKW) on how to use these.

Using the 3 wire method for measuring thread pitch

 

[Rcdizy]

So, while I don't have a thread wire set, I have some .035 wire. over the wires I carefully measured 1.8132" I know its 16tpi

forgive my ignorance I have never done this..... found this in a hand book

 

[Dave Paine]

Looks like you have some wire of the right size so you should now be able to calculate the Pitch Diameter from the measurement and the formula in the handbook. This is not the ID but you do not need to do any calculations.

You can now use the wires on the thread you machine and look to get the same 1.8132 in measurement when you machine the threads.

You will likely have to measure a few times and take light cuts to carefully get to the desired measurement.

Good luck and let us know how this works out. Fingers crossed "Murphy" does not pay a visit with the project.

 

[Rcdizy]

You could use the three wire method on internal threading?

 

[P. Waller]

Assuming a 60 Degree V thread, (everyone knows what happens when you ASSume)

The theoretical sharp "V" depth would be the cos of 30 degrees X the lead, .866 X .062 = .054 per side, 1.795 - .108 in your case.
In practice this will not work well as you can not produce a sharp "V" with common tooling.
As this part does not hold the wings onto the aircraft a useful and simple minor diameter calculation is the major diameter minus the lead.

Measuring the external thread over wires will not help you because you can not easily or at all measure internal threads this way. If you have a good deal of time to spend on this measure the external thread over wires then make a plug gauge using the same dimensions, when it fits in the part you are good to go.
Measuring over wires is tedious at best and once the thread lead exceeds the spindle diameter of the micrometer a gauge block must be used, this is one more thing to drop.
1 1/4-4 Acme thread using a .100 block over wires.

 

[Jimsehr]

If I made a cap like that I would finish the outside and the length of cap . Then I would insert cap in chuck jaws and use a marker to mark part location on chuck and cap. Then you should be able to remove and reload by matching the marks
On both pieces. Mark both parts where they meet with a small dot.

 

[Rcdizy]

I guess that would work if it were bottomed out against the chuck, or spaced repeatably the in out way on the chuck as well as the degree index mark?

 

[ttabbal]

I think I would use the measurement to turn a matching outside thread. Then use that as a go/no-go gauge for the internal thread you are trying to make. Get close to the calculated measurement and test for fit taking light cuts till you get the fit you want.

 

[bigloudvtwin]

I think I would use the measurement to turn a matching outside thread. Then use that as a go/no-go gauge for the internal thread you are trying to make. Get close to the calculated measurement and test for fit taking light cuts till you get the fit you want.

Bingo! You would essentially clone the end of your spindle out of some scrap pipe, round stock etc. Part it off, then you'll be able to keep your spider/stop chucked up while threading it. Go slowly in small increments using your clone to test fit.
Best idea I've seen on this thread yet IMHO.