Screw Cutting an ACME thread

[NickG]

Hello,

I need some advice on cutting an internal 8tpi ACME thread on the lathe. This is to repair my milling vice which I recently found out had a part broken as some of you may have seen in my Poppin flame licker thread.

My lathe has a screw cuttting gear box with full range of feeds and speeds so that isn't an issue, it's also easily man enough for the job. The real problem is, I don't really know how to grind the tool. I guess I could work it out from the thread geometry, probably on CAD but it would take a while. I need to know what clearances I need on the tool and what size cuts I should be taking etc, also whether it is critical to feed in and an angle etc.

Please don't suggest any other options for repairing the vice, I know there are other ways to skin a cat and I have a few options but I want to have a go at repairing it the right way first.

Thanks,

Nick

[sbwhart]

Nick

You can use the bolt as a grinding gauge for the tool, don't worry too much about clearance angles as long as you've got some side clearance and some top rake it will be OK. From my Machinery hand book acme screw threads have an inclusive angle of 29 deg so I would rotate the compound over 14.5 deg and feed in at that, you may find it easyer to turn the tool upside down and turn on the far side of the job that way you can see what's happening better.

Hope this helps and good luck

Stew

[Bernd]

Nick,

Might need one of these to grind the cutting tool with. It's an Acme thread gage. Lower left notch is for grinding the  proper angle. All the notches with numbers gages how wide the tip of the tool should be for your given tpi. So you should grind the nose of the tool to the 3 notch from the right on top, #8.



Good luck.

Bernd

[No1_sonuk]

If you're not averse to getting a book:
http://www.amazon.co.uk/Screw-cutting-Workshop-Practice-Martin-Cleeve/dp/0852428383

That has a lot of info, including tool geometry for "normal" and trapezoidal thread cutting.

Is it actually Acme, or is it square?

[Jasonb]

And if its a new machine it may have an ISO trapizoidal thread which has a 30deg angle not the 29deg of ACME but would likely be 3mm pitch.

Jason

PS you can buy ACME taps

[Stormin]

I had a problem with the nut on the vice that came with my milling machine. Measured up and thought it was 5/8 acme and bought an appropriate tap. Turned out the thread on the screw was 16mm trapezoidal so ended up cutting a new acme screw as well to repair the vice.

[No1_sonuk]

Oh, and is it multi-start?

[NickG]

Thanks guys,

No1 / Stormin, this is a very old vice that's why I thought 8TPI acme from my measurements and inspection of the thread, think it's just single start.

Thanks Stew, that's what I found on the internet too teh 29 deg included angle. I think because it's quite course, and deep it maybe needs quite a bit of side clearance - maybe that's where I'm going wrong.

Bernd, yes I could do with a thread gauge like that. I was trying to grind it using the screw but it may be a bit worn. Whatever I did when I tried to grind my tool - it didn't work. Think I may do a couple of practice runs on something soft like nylon.

Jason, cheers, I looked for a tap and you can buy it from Tracy tools but £22 each which sort of defeats the point a bit! Having said that, it, if need be and I can't screw cut one, it could put an otherwise good vice back into service and still save me a few quid.

Even if I can't get it to work, it'll still be an interesting exercise for me, something new and if it does work - another string to my bow!

Thanks for the help.

Nick

[NickG]

No1 - might get that book actually, it's pretty cheap I thought it was going to be a fortune when you mentioned a book! 

Cheers

[Stormin]

Thanks guys,

No1 / Stormin, this is a very old vice that's why I thought 8TPI acme from my measurements and inspection of the thread, think it's just single start.

Jason, cheers, I looked for a tap and you can buy it from Tracy tools but £22 each which sort of defeats the point a bit! Having said that, it, if need be and I can't screw cut one, it could put an otherwise good vice back into service and still save me a few quid.

Nick

Mines a very old vice too but still had an unexpected thread.

Acme taps are a little cheaper here: http://www.rdgtools.co.uk/acatalog/ACME_TAPS__RIGHT_HAND_.html

It was enough work to cut the external thread never mind the internal for the nut as well.