cutting a left handed thread

[Nathan Gould]

This isnt strictly for a knife, but its still quite an interesting project i suppose.

I'm making some engine mounts for a formula student car at the moment, and i need to make an external left handed thread, and we havent got enough money left for a die.

I've set the lathe up as in this video:

http://www.youtube.com/watch?v=FRa5An0i_sM

Now, for the first few cuts, it seems to be working fine, but then normally on the third or fourth pass it strips off all of the previously formed teeth. Is this because ive done something wrong or is it due to the backlash in the crosslide making it impossible to return to zero properly a repeated number of times? Also, im using a tool designed to cut a right handed thread, will this mak a difference?

any help gratefully received!

 

[12345678910]

http://www.youtube.com/watch?v=FRa5An0i_sM

Now, for the first few cuts, it seems to be working fine,

but then normally on the third or fourth pass it strips off all of the previously formed teeth.

Is this because ive done something wrong or is it due to the backlash in the crosslide making it impossible to return to zero properly a repeated number of times?

Also, im using a tool designed to cut a right handed thread, will this mak a difference?

any help gratefully received!

I may be FOS and it's hard to tell over the internet without pics or anything.



I believe backlash won't suddenly wipe off your thread, it will just mean the next cut is too shallow, or too deep


I'm guessing you are hitting the same split nut intervel a few times, then engaging on the wrong one.
In the video 4:02 he mentions that since he has 10 tpi, he can engage at any line.

But he should have explained that much more.
That changes for different pitches. There is math involved I don't know off the top of my head.
What pitch are you cutting?

Engage on the same mark every time.
Or

Try engaging that split nut and leaving it engaged until you are done.
Then just stop the lathe and run it reverse to back it up and make the next pass.

Same with the handwheels, don't back them out, just reverse over the path they just cut.


Have you considered buying a left hand screw and turning a shoulder, welding it into place somehow ?

 

[Nathan Gould]

Thanks for the reply, i'll try and get some pictures tomorrow.

I had thought about getting a left handed thread and welding it on, but as the part is going to be under alot of pressure i wouldnt be too sure of my welds, plus i would be worried that it wouldnt be properly concentric so the two adjusters either side wouldnt offer even movement.

Thanks for the advice about the split nut, that seems like a really good idea i'll try it first thing tomorrow and see if it works!

heres a few pics of the car at the moment, it still needs lots of work before july the 13th when we go to silverstone! Though tomorrow, we get out uprights, hubs, driveshafts, wheels and chassis back from the cnc place and the powdercoaters so assembly can begin.

 

[Dmitry.M]

is it due to the backlash in the crosslide making it impossible to return to zero properly a repeated number of times? Also, im using a tool designed to cut a right handed thread, will this mak a difference?

Yes and Yes. Use a numbered dial on your lathe (if you have one) that will allow you to start the feed at the same spot. If the thread is deep and wide you may want to "work it out" starting one notch before and after at first few cuts. Hope you understand what i mean... Than advance deeper by a thousand. Use lubrication. I recommend carbide insert. It will make nice cut and proper profile.
Make sure you have nice groove to start and 60 degree angle at the start and at the END.

 

[Nathan the Machinist]

I won't go into how to cut a thread on a lathe, that information has got to be available all over the internet. I will point out that if you're doing it properly, backlash is irrelevant.

I'd set my compound rest at 29 deg for feeding in, as is customary, but on the opposite side as usual because you'll be cutting away from the spindle. (unless you flip or spin your tool and cut it in reverse). I'll point out this is 29 deg to axis of the thread, which is often not the 29 deg on some compounds because they are zeroed with the compound aligned with the spindle. In that case it would be 61 deg. You infeed with your compound this way to prevent backlash from causing a drunken thread.

I think that "you can start at any time" thing sometimes works for some whole numbers, but not the halfway between numbers. On the lathes I've fooled with, the only thread that you could engage the halfnut at anytime was 16 TPI.

Be sure if this is an english/metric lathe the metric gear isn't in place.


All that said, the advice of just leave the nut engaged is probably the most foolproof approach.

 

[A C Richards]

I too have worked on lathes that had a half way point on the thread gear that would really play havoc if you did not catch the nut in the right place. I'm with Nathan, leave the half nut engaged and just reverse the machine. Especially if you are running from the spindle out. You can run right off the end of the workpiece. Good luck.