# Imperial thread cutting on a metric lathe



## bucketboy (Mar 30, 2010)

Hi guy, I am new to this turning malarkey and have just taken delivery of a Warco WM250. I have done some simple turning in the past but never any screw cutting so I thought I'd have a go : First off I had to grind up a 55 degree tool :-\ as it turns out a lot easier than I thought  I turned the job down to the required dia and undercut at the end to the minor dia, so far so good. Changed all the gears for the 19tpi then started to do the thread cutting but what number on the thread dial, indicator ??? a quick look in the book showed me metric thread numbers but no imperial :-\ so I just left the halfnut engaged and used the reverse on the lead screw, it worked OK but took ages. 

Forgive me if this is a stupid question but can I use the indicator for imperial threads on a metric lathe? 

I think it turned out alright for a first attempt :-\












Bb


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## Blogwitch (Mar 30, 2010)

Bb,

You've got it spot on, you can't cut imperial using the drop in dial on your metric machine, unless it is an all singinig all dancing lathe. In fact cutting metric threads can be a real PITA as well, because you usually have to change a gear on the bottom of the drop in dial depending on what pitch you are cutting.

You might find that doing both types the way you are using now will be just as easy by the time you have done a few.

The results you have got look just fine, maybe a little longer and deeper thread runout required.


Bogs


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## kvom (Mar 30, 2010)

Assuming it has a metric leadscrew, you can't do it the normal way. There was a similar discussion on the PM Monarch forum recently:



> You don't actually have to leave the halfnuts engaged. You just have to use the same index mark on the threading dial for each cut, and you can't ever let the threading dial "lap" you. If you keep track of what "lap" you're on, you can even use the handwheel to move the carriage back to the beginning of the cut (instead of the halfnuts). But either way you get back there (halfnut-powered or manually), the spindle will have to turn the same number of revolutions in the reverse direction to regain your original position on the threading dial.



It's a lot easier on a variable-speed lathe that you can speed up in the reverse direction.


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## bucketboy (Mar 31, 2010)

kvom  said:
			
		

> Assuming it has a metric leadscrew, you can't do it the normal way. There was a similar discussion on the PM Monarch forum recently:
> 
> It's a lot easier on a variable-speed lathe that you can speed up in the reverse direction.



DOH, I never thought of that :-[

Bb


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## fcheslop (May 18, 2010)

I might be telling my granny how to suck eggs but as a rough guide when cutting imp threads on an imp machine. 1280 divided by tpi then divide by 2 equals depth of cut.
regards Frazer


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