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Left Hand Acme Threads

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Stephen Osborne24/11/2020 09:51:59
28 forum posts
2 photos

I need to fabricate a feednut for my TS milling machine. The thread is 0.75” 5 TPI left hand Acme. Ideally, I would like to buy a carbide threading insert but this item appears to be very rare. So, it may come down to grinding my own tool.

From watching videos from messers Booth & Rucker, they do not grind any relief on the top face of the tool. So in theory the tool could cut both right & left threads.

Is this correct? I would appreciate your comments.

KWIL24/11/2020 11:48:05
3338 forum posts
63 photos

Try SECO

Seco Tools (UK) Ltd
Adams Way, Springfield Business Park
Alcester
Warwickshire B49 6PU

Tel: 01789 764341

Insert 22EL 5 ACME

ega24/11/2020 12:01:55
1945 forum posts
160 photos

One like this might do:

dscn1938.jpg

Adrian R224/11/2020 12:10:57
77 forum posts
8 photos

Posted by KWIL on 24/11/2020 11:48:05:

Insert 22EL 5 ACME

I think that will have the same problem as I found for my lathe - although correct form exists the insert will be too big to fit in the internal diameter. I got some suggestions as to other sources for tooling, see this thread:

https://www.model-engineer.co.uk/forums/postings.asp?th=169205

Martin Connelly24/11/2020 12:59:36
avatar
1624 forum posts
179 photos

RDG advertise a suitable tap. RDG ACME taps

Martin C

DMB24/11/2020 14:58:53
1054 forum posts

3/4" tap? In steel? Good luck Stephen, you'll have nice biceps after! I've seen that sort of size done by motor drive on a rig up outdoors, can't remember whether it was steel gas pipes or what.

John

old mart24/11/2020 15:08:40
2501 forum posts
169 photos

I have had exactly this problem with the Tom Senior LV nut for the Xaxis. 5 ACME is not available in smaller than er/ir 22 and tha minimum size that could be threaded internally would be 1" diameter. I got a tap from Tracy Tools, they do left and right handed taps. When I tried the tap in a piece of aluminium, I had doubts whether I could turn it, even though it was a very long taper. I used an ir 8 ACME insert ground so that only the main tip would cut and produced a partial thread with that at 5tpi. A bit of cheating with the compound widened the thread closer to the proper width, but the depth of thread was obviously lacking. This partly cut thread enabled me to complete the thread with the tap in the gunmetal nuts. The backlash was about 0.005", but that didn't matter, as I used two nuts for an antibacklash setup.

_igp2378.jpg

David George 124/11/2020 22:25:33
avatar
1434 forum posts
459 photos

Hi I made a 1/2 inch square leadscrew nut for my M Type lathe I made a tap to finish the thread from silver steel and hardened it then made a HSS tool from a solid piece and screw cut it then used the tap to finnish it time consuming but I have made a few since.

20190519_091437.jpg

20190520_080901.jpg

David

John Reese24/11/2020 22:27:15
866 forum posts

I recently did a 3/4-4 Acme. I hand ground the tool bit. The bar I used was 1/2". There was insufficient clearance for me to open the half nuts and crank the tool out. I had to leave the half nuts engaged, back the tool out a bit using the crossslide and run the spindle in reverse to get back to my starting point. With the that small a bar and the very coarse pitch it required MANY spring passes to complete the thread.

Be sure you have sufficient clearance on the proper side of the tool. Calculate the helix angle based on the minor diameter of the thread and add a few degrees to that to get the necessary clearance angle for the leading edge of the tool. The trailing edge of the tool need not have any clearance.

Ronald Morrison25/11/2020 11:21:13
59 forum posts
4 photos

Is there a reason you can't use a Delrin (acetal) nut? They would be much easier to machine that metal and can be melt formed to the acme screw you have for a nut with near zero backlash.

Delrin leadscrew nut

Stephen Osborne25/11/2020 18:45:39
28 forum posts
2 photos

Thank you gentlemen for all your useful comments.

I think the $85 insert option from the States is a non-starter, especially as the insert may not fit in the hole!

I like old mart's idea of partially cutting the thread with an approximate form tool on the lathe followed by a tap.

Steve

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