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Old 07-28-2019, 07:30 AM
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Anybody here cut threads with a lathe?

Hi All,

I've got the time to set up my old pre-war Atlas to cut threads. I've read the manual, but can't get my head around one thing. The manual states "the crossfeed head must be set at 29". Since the tool is set perpendicular to the work and the carriage does the actual feeding (threading), what difference would it make?

TIA!

Russ

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Old 07-28-2019, 07:57 AM
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Traditionally, the compound is set to 29* because that is slightly less than half of the included angle of 60* threads. I've seen it set to 30* also.

The cross slide is set to "0" with the point of the tool just touching the work for new threads. The cut is fed in with the compound on the 29* setting. For each additional cut, the cross slide is returned to zero and the next cut is fed with the compound until the threads are complete. In theory, the 29* setting allows the back side of the cutting tool to just ride down the other side without doing much work.

This is better for most tools because the cutting is done on the leading face of the threading tool. If the cut was fed straight in, both sides of the tool would have to cut and this adds a lot of load the cut. Most light duty machines don't do well with a straight in feed.
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Old 07-28-2019, 05:48 PM
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Agreed.
In single point threading, if you plunge straight in, both sides of the tool make the cut and the loads are hard on stuff. Especially if the threads are small so cutting it at 29 makes just one side cut. Threads are cleaner, straighter, less chatter and flex in the tool post.
I have a small 20” lathe and i use it a lot because it’s handy
for most of my turning projects. It cuts threads but I have to custom grind a tool at less than 30 and set the tool post at 28 to get clean threads because it flexes at the compound and headstock bearings.
It’s actually a POS but I’ve figured out how to make it usable just like all machine have their quarks. I’m also not using this one for close tolerances but for roughing out stuff. This is has a reverse so I’m able to back out and reverse it, run it back in .005 and make another pass without worry of getting the pitch messed up.

Last edited by johnsongrass1; 07-28-2019 at 05:53 PM.
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Old 07-29-2019, 07:11 AM
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OK, thanks! I was thinking it easier use the crossfeed to move the tool in/out, so the 29 makes sense if you use the compound instead. My manual is not real clear on this and I still don't see what difference it would make but I'll do as stated. You're only getting the tool out of the way in order to traverse it back to the starting point, right? Anyhow, I want to "relearn" this the right way. It's been 45 years since I cut threads and I've "slept a few times" since then.

Russ
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Old 07-29-2019, 08:02 AM
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tool bit holder

I have some angled tool bit holders so I can start cutting closer to the chuck
like these
https://www.mscdirect.com/browse/tnp...+-+Exclusive+B
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Old 07-29-2019, 08:13 AM
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This guy is a good instructor and explains why cutting on the right edge works better on 60' threads.


Actually, all his videos are good information.
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Old 07-30-2019, 09:51 AM
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Thanks, that and the follow-up were were great vids! Now I understand about the compound feed, only the left side of the tool cuts. I was misthinking that driving it straight in, both sides would cut. I have some 60 threading bits but haven't looked at them closely. One more Q that wasn't covered: The start position once the carriage is returned from a cut. I'm thinking it doesn't matter as long as you hit the correct mark on the dial?

Now there's another problem. My lathe is an 80 year old Atlas that came with no extra back gears and, of course, the ones I have are not right. And I'm not even sure which model it is (plate shows: H48, 029050). I was under the impression it's a 6", but the limited info I can find says the 10" models came with a 3/4" lead screw which mine has. Plus the gear quadrant is different from anything else I'm seeing. The 6" gear sets are numbered "M6-101-xx", while the 9" thru 12" sets are "9-101-xx". Both appear to be the same, but I don't know if the pitches match up or not. Mine are 3/4" bore, double-keyed, 1/2" thick at the hub, and are marked with the tooth count followed by the letter "A".

The "bracket only" pic is the one I have, the other is from the 6" lathe manual with gearing set up for 18T. The bracket shape prolly won't matter as long as the gear relationship is maintained, but it may help identify the part numbers for the proper gears?

Thanks!

Russ



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Old 07-30-2019, 12:20 PM
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You are correct about the position of the carriage as long as you hit the proper spot on the chasing dial and nothing else moves.

Be careful about slack in your machine. Nearly all older machines will have some and some will have a bunch due to wear. Once you have returned the carriage to the start of the threads and returned the cross slide to zero, pull back on the tool post to make sure the slack is out. Be careful of the spinning work piece. Failure to do this may result in the tool digging in much deeper than you expected.

Also, on a light machine like an Atlas, take a "dry" cut or two every now and then. This is a cut with no additional feed on the compound. The machine will have some spring in it and the dry cut will allow the spring to relax. Failure to do this can also cause unexpected digging in or undersize threads.

Can't help you with the change gears. Check EBAY. Change gear sets come up on there every so often. You will need some combination of gears for the thread pitch you need. There are formulas to calculate the gear ratio needed based on lead screw pitch. You don't always have to use the combinations listed on the machine. Those are just for your convenience. The machine doesn't know as long as the ratio is correct. In the "old days" it wasn't unusual for machinist to have to calculate a special ratio for some odd ball thread. I've done it, but it's been at least 40 years. Here is a page I turned up with a quick search:

http://www.lathes.co.uk/latheparts/page14.html

Here's the quick and very simplified explanation. If the lathe has an 8 thread pitch leadscrew and you wanted 8 threads per inch, you would need a 1 to 1 ratio. 16 TPI would require a 2 to 1 ratio. 4TPI would be .5 to 1 ratio. The actual ratio needed is essentially what percentage of 1 to1 do you need based on lead screw pitch. The complication is when compound gearing is needed to get what you want.

Last edited by Hotrod46; 07-30-2019 at 12:49 PM.
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Old 08-12-2019, 09:40 AM
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Well, that went good! I found a complete gear set and was able to cut "custom" 1 1/16-18 TPI threads for what is prolly a 26mm x .75 female opening. Threads in perfectly...

Russ





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Old 08-12-2019, 10:27 AM
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Nice to hear it worked out for you. You can often cut short sections of inch pitch threads that will mate satisfactorily with metric before the pitch gets too far off. Depends on the application as to whether it's acceptable. Looks like that plug worked well. Congrats. Most folks are scared to attempt threading on a lathe.
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Old 08-12-2019, 06:44 PM
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Machinist will hate me for this but.... If you're cutting threads on parts that are safety related I would strongly suggest trying to find the part with rolled threads. There is a great deal of difference in the tensile strength of a rolled thread over a cut thread. l won't go into detail on this because there is plenty of info re this matter on the net. Don't misunderstand me about the use of a lathe. There are countless uses for one in the hot rodding hobby and there is nothing more satisfying than making a part yourself.
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Old 08-13-2019, 07:00 AM
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Quote:
Originally Posted by Hotrod46 View Post
Nice to hear it worked out for you. You can often cut short sections of inch pitch threads that will mate satisfactorily with metric before the pitch gets too far off. Depends on the application as to whether it's acceptable. Looks like that plug worked well. Congrats. Most folks are scared to attempt threading on a lathe.
In fact, I used a center-drilled 1" pipe plug to stabilize the outboard end while I spun it at 30 RPM to clean up the shell. It was pretty crusty when I got it. The plug threaded in about 3 turns, enough to hold it tight w/o buggaring up the threads...

Quote:
Originally Posted by Stump34 View Post
Machinist will hate me for this but.... If you're cutting threads on parts that are safety related I would strongly suggest trying to find the part with rolled threads. There is a great deal of difference in the tensile strength of a rolled thread over a cut thread. l won't go into detail on this because there is plenty of info re this matter on the net. Don't misunderstand me about the use of a lathe. There are countless uses for one in the hot rodding hobby and there is nothing more satisfying than making a part yourself.
Yeah, as you can see this strictly for looks only. I was gonna get one 3-D printed in plastic but decided to try doing it myself. Saved $$$ in the process if you overlook the cost of the gearset (I never count the cost of tooling).

Russ

BTW, this a 105mm WWII Kreigsmarine naval artillery shell. I had someone 3-D print me a projectile for it, should be here later thi week...

Last edited by S10xGN; 08-13-2019 at 07:06 AM. Reason: What is that?
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