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mike4
10-16-2014, 02:01 AM
I dont want to hijack any other posts , I have to cut a LH acme type thread in a cylinder to provide a locking nut for a chain tensioner.

I saw a similar thread cutting exercise earlier where a member made some steel threads for his chuck as the originals were trashed or similar.

I was taught how to do this about thirty or more years ago , and I was wondering if I could get some pointers on setting the compound , material is 1018 steel bolt size around 1" .

I remember that for these threads that the angle is somewhere about 22 degrees and to take light cuts .

Thanks in advance

Michael

big job
10-16-2014, 03:26 AM
If I'm reading this correct Mike ; change the feed to "feed toward the tailstock" swing the compound to the left 14 1/2 degrees- put lathe in back gear - set the
cutting tool with the proper acme pitch gage... the acme form is 29 degrees. half of that is 14 1/2 degrees and that depends how your lathe is marked.
I think thats what you want to know.
sam

PStechPaul
10-16-2014, 03:50 AM
I was the one who made 3/4-8 LH SQ threads from 4140 alloy steel for my 4 jaw chuck. But those were external threads. You should be able to use a boring bar with a specially ground tip to cut the internal Acme threads. You should carefully measure the existing thread to see if it is a standard Acme or some other trapezoidal thread form. I found some discussions about this:

http://www.practicalmachinist.com/vb/general-archive/cutting-internal-acme-threads-88497/
http://www.practicalmachinist.com/vb/south-bend-lathes/cutting-internal-acme-thread-229074/
http://bbs.homeshopmachinist.net/threads/811-Internal-ACME-thread-cutting
http://bbs.homeshopmachinist.net/archive/index.php/t-17454.html

http://en.wikipedia.org/wiki/Trapezoidal_thread_forms

http://upload.wikimedia.org/wikipedia/commons/thumb/b/bd/Acme_thread.svg/240px-Acme_thread.svg.png

I would suggest making a tool with a 29 degree V-shaped tip, with a small squared-off or rounded nose, set the compound at 14 degrees, and advance until you are close to the required depth. Then you might be able to set the compound at 90 degrees and take cuts that advance along the axis until you get the width needed. Finally perhaps you can make an accurately ground bit with the exact thread form needed and use it for a finishing cut.

Another way may be to cut square threads with an internal grooving tool, and then use the Acme form tool straight in, which will cut at an angle on both sides and create two strands of swarf, which will clear much better than cutting all three facets at once.

Another possibility is to cut the external threads on some tool steel, mill several flutes, cut a taper, harden, and make a tap.

These are only ideas, so use at your own risk. I've never cut internal threads, much less Acme, so perhaps others who have can comment. I might try it just for the experience, while my lathe is still set up for the 3/4-8 LH. I can make a nut for the old chuck screws.

[edit] Found a video:


https://www.youtube.com/watch?v=4yZa9D3GK84

rcaffin
10-16-2014, 05:02 AM
> Finally perhaps you can make an accurately ground bit with the exact thread form needed and use it for a finishing cut.
Um. Maybe not.
Unless you have a VERY heavy lathe, such a wide cutting face on a lathe tool is very likely to chatter.

Cheers
Roger

J Tiers
10-16-2014, 08:19 AM
that's going to depend on the pitch, or more accurately, on the depth of thread. And probably the boring bar is going to limit things more than the lathe itself.

I have a 10" Logan, and had no trouble cutting 10 tpi when making a feedscrew, using a full depth final cut and a full form cutter that I made. Set to the angle just under the thread half angle, as with a v-thread, and cut. This one needed a follower rest, naturally. No chatter, and the cut width was getting up there on the final cuts, close to 0.250".

You can also do the cuts in sections, one side at a time, if the full depth cut is likely to be too much. Rough out the thread, and then clean up one side at a time. Inside a nut, that might be pretty hard to do, and a full form cutter may be best.

If you need max accuracy, cutting away from the headstock is likely to use a less-worn side of the leadscrew and nut. Probably for the described part that isn't a problem.

Here's the finished screw, sorry I don't have pics or video of cutting any part but the pinion, which isn't relevant.
http://img.photobucket.com/albums/0803/jstanley/Rivett%20608/Rivettnewcrossfeed4_zps7dde0d05.jpg (http://smg.photobucket.com/user/jstanley/media/Rivett%20608/Rivettnewcrossfeed4_zps7dde0d05.jpg.html)

oldtiffie
10-16-2014, 09:11 AM
Her is the OP's (mike4) post.


I dont want to hijack any other posts , I have to cut a LH acme type thread in a cylinder to provide a locking nut for a chain tensioner.

I saw a similar thread cutting exercise earlier where a member made some steel threads for his chuck as the originals were trashed or similar.

I was taught how to do this about thirty or more years ago , and I was wondering if I could get some pointers on setting the compound , material is 1018 steel bolt size around 1" .

I remember that for these threads that the angle is somewhere about 22 degrees and to take light cuts .

Thanks in advance

Michael

Note that the job is a left-handed internal thread so it likely that the tail-stock will not be used.

The nominal size of the thread and its bore and OD are not given - neither is the pitch of it.

I'd guess that a larger thread will accommodate a larger stiffer boring bar and that the job will be held in a 3- or 4-jaw chuck.

mike4
10-16-2014, 06:52 PM
Thank you for the replies , this is going to be fun as I need to purchase a smaller boring bar to fit inside the nut , my 1/2 is too big with the correct cutter .

I will explore the option of regrinding some old carbide grooving inserts to the thread size , that will make mounting inserts easier as I have several grooving tools which could be ground to fit inside the nut.

This is a one off job and its possibly the only time I should be doing this , so a special holder and new inserts is out of the question , just to sit on a shelf for years.

I'll have a go next weekend and if it works possibly post some photos.

Michael