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John D sharing his spindle encoder speed to leadscrew stepper has me thinking, actually I have been mystified/wondering about this for a long time.

This is a question.
If I have a 8tpi (thread per inch) leadscrew and I want to thread 1" rod at 8tpi with 100 rpm at the spindle the leadscrew has to rotate 100 rpm?

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Yes.? Lead screw pitch is done with the lead screw turning the same speed as the spindle.

What would the leadscrew rpm be at 1,000 rpm spindle speed? What would the SFM (Surface Feet per Minute) be?

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It would be 1000 RPM.? The SFM is dependent on the diameter that is getting the lead screw pitch.? The pitch and the diameter are mutually exclusive.? My ELS will tell you the SFM instead of RPM if you set the parameters.? The math is actually pretty simple.

I am looking at a and getting more confused.
With Tool Steel at 75 -175 SFM? that seems the leadscrew would be in the 1,000s of rpm.

2 x PI x r is the circumference of a circle.? Or PI x d.? So a 1" bar stock has 3.14" circumference or if you divide that value by 12"/Foot you get 0.261799388 feet.?

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If it's turning 100 RPM then it's

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0.261799388 ft * 100 RPM = 26 SFM

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At 1000 RPM it's 262 SFM.

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Run the spindle at 650 RPM and your SFM is 170.? That's for 1" bar stock.

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You have to excuse me I am completely self taught, I flunked out of Machine Shop II 55 years ago
and have not touch a machine tool since.

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There are some basic math operations that you need to be able to do or use some sort of calculator program like this one.

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So let's say you have some 1/4" rod that you want to thread at 20 TPI because it's Sunday evening and the hardware store that sells all-thread rod is closed.

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Diameter is 0.25" Your lead screw is 8 TPI and the spindle is 1:1 for lead screw threads.?

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20/8 = 2.5 so the lead spindle has to turn 2.5x as fast as the lead screw.? Or to put it another way.? The leadscrew pitch is 0.125" and you want 0.05" pitch so for each turn of the spindle you want 0.05" movement of the lead screw which is 0.125/0.05=2.5.

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So if your spindle speed is 1000 RPM then the leadscrew turns 400 RPM.? But what about SFM?? Using the calculator above I select tool diameter 0.25", 75 as the minimum SFM and it tells me 1145 RPM.? And this is where a smaller higher speed lathe is so much handier with the mid range of 125 SFM you get 1910 RPM.

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But if it's a short thread that's really fast for engaging the half nut so most people will turn that more slowly or just use a die.

There are no classes nearby and the only local machinist I know of, are those I go to their yard sales
when they are dead and the shops are being sold off.

This is why we have the forums, which I am so thankful for.
Ralph

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Clausing 8520, Craftsman 12x36 Lathe, 4x12 mini lathe, 14" Delta drill press, 40 watt laser, Consew brushless DC motors and a non working 3D printer