[SQLGuy]

Hi all,

I am rather new to this. I recently bought a Harbor Freight Sieg 7x12 and have done one small project on it. I would now like to do some threading, specifically for a hard-to-find watchmaking specific thread size (40tpi x .275).

I have read the Mini-lathe manual from Little Machine Shop, and some online documentation, but I am still a bit confused with threading procedures.

Here's how I have the cross slide and tool bit set up for threading. The tool bit is HSS.



And here are samples of a couple of threading attempts I made.



The bottom one is on aluminum, which came out OK, though still a bit rough. The top one is on annealed silver steel (I am attempting to make a tap) and it's just ugly (right end... don't worry about the left end). The threads end up very rough and subsequent passes at .005 still seem to strip off way too much metal.

I would have thought 40tpi would be an easy one to start with since the half-nuts can supposedly be engaged anywhere, but I am clearly doing at least something (if not everything) wrong.

FWIW, I went through today and adjusted the saddle thinking play there (which did exist and I did fix) might have been part of the problem. The attempts above, though, were after this adjustment. On a semi-related note, I noticed that there are two set screws visible from the top of the saddle (chuck end). The back one seems to be non-functional, as it goes into a blind hole. The front one I would have thought would adjust for front to back play on the saddle, but it doesn't really... I fixed this by making a brass plug with a 45 degree face to it and setting it under the front set screw. This allows me to put pressure on the 45 degree face of the ways and really cleans up the front/back lash of the saddle.

Any suggestions on the threading?

Thanks,
Paul

[lavrgs]

How many passes did you make? Is the tool sharp and exactly at 60 degrees. How do you center the tool? The threads look offset, probably because the angle relative to the stock is off. There are gauges you can use to grind the tool but a carbide single point is probably going to be good out of the box. Do you have a gauge to set the tool height? Being on center will help clean things up and take lighter cuts. So I'm not an expert on this stuff but maybe some food for thought.
One more thing, did you use any cutting fluid?

[SQLGuy]

I made probably five passes

The tool is LMS's 5/16 threading tool and came pre-sharpened. It's new.

I've got the cross slide set at 29.5 degrees per the instructions I've read. I'm just not clear whether this is what they meant by 29.5 degrees.

I set the tool at a right angle to the stock by eye, so it may be off by a degree or two, but it looks pretty right-angle in person. The wide angle lens of the camera distorts things a bit - also, the tool has offset cutting edges so that it can be used in tight areas... i.e. the relief on one side of the tip is not symmetrical with the relief on the other side.

The tool is the correct height for this lathe and lines up with the center of the chuck/stock.

The instructions in the LMS manual say to take cuts of 5 to 10 thousandths. Initially I was trying with 3 thousandths and got similar results to those seen here. These were done with passes of 4 to 5 thousandths. How much lighter should they be?

Thanks,
Paul

[lavrgs]

4 - 5 thou sounds ok but that's 8-10 on the diameter. I could recommend getting the tool closer in - not so far out. But it sounds like you're pretty close. Set up is the key - just double check heights and angles. At work there is a tool that is a cylinder with a cap that is used the set the height. A few thousand off can make a difference. Cutting fluid will help too. Check the tool to make sure there is no build up or chips. You could also try spinning the aluminum a bit faster. What RPM are you running?

[SQLGuy]

I used a bit of Mobil 1 10W30 for cooling/lubrication. Should I be squeezing something in there that will do more to wash the chips away?

Chips are building up with each pass, but each pass also leaves swarf that's still attached to the threads (bristles sticking out). Sounds like I need to clear this while cutting, but how?

I ran these at about 250 - 300 RPM. I'm not really concerned about the aluminum, as that was just for practice and to see how the results would compare to the steel. It's the steel I'd like to get a clean cut on.

By the way, the procedure I've been following is to cut, disengage the lead screw, back off .08, move the saddle back, move in .08, add .004 or so to the cross slide, then re-engage. Why couldn't I just run the lathe in reverse from the end of the cut to bring the saddle back for another pass?

Thanks,
Paul

[lavrgs]

Try running a bit faster. 800-1000 for aluminum 4-600 for steel. Your procedure is good. the answer to your question is backlash. Try some wd-40, I think the Mobil 1 is a bit too heavy.

[alexccmeister]

I think your compound slide angle is too much, please check the pdf on this page (Post #17) for details. It details step by step how to cut thread with and without a threading dial. I learnt alot from this.

I have a 7x14 which is technically the same as your 7x12. Good luck.

Alex

[sdmaker]

yep, set your compound to 60 degrees and try again. your compound angle should be coming in at the same angle as the grind on the tool.

[djmurray]

I agree with the gentleman that says your compound is set to the wrong angle. It APPEARS from you picture, that the 29-1/2 degrees is based on a parallel axis to the work. It SHOULD be 29-1/2 degrees from the VERTICAL (i.e. 90 degree from the work) axis.

The theory for using 29-1/2 degrees is as follows:your threads have an included angle of 60 deg., so if you came STRAIGHT in, you'd cut the same amount from each side of the threads. In order to keep this post relatively short, I won't get into why this doesn't always work as well as we'd like. The solution is to cut only one side of the threads, which would be setting the compound at 30 degrees from the vertical to the work. The problem with 30 degrees is that, if you're not perfect in your setup, the side of threads you're NOT cutting can end up being a bit rough (I.e. little steps in the surface). Setting up for 29-1/2 degrees still primarily cuts only one side of threads, BUT it also shaves a very, very small amount of material off the other side as well, leaving you with clean surfaces on both sides of the thread.

Some machines may have enough flex that they don't shave that very small amount off BUT, the cutter does ride firmly against that edge, more or less ensuring a cleaner edge.

In my experience, carbide doesn't leave as nice a finish as a very sharp, properly ground tool steel bit. Also, use REAL cutting fluid - it does make a difference! If I still end up with a bit of swarf on threads due to a dull tool, etc. a small modeler's triangle file allowed to lightly ride down the threads a time or two usually cleans things up.

Speaking of setup, you can get a thread gage from Enco or MSC real cheaply that makes verifying your bit's profile when sharpening and setting up the bit to the proper angle before cutting threads a snap. When I bought mine, I bought two because I really depend on it and if I misplaced my only one, I'd be stuck for the day because I can't find them locally! You GOTTA get one if you're serious about cutting threads!

Just my personal opinion. Worth every penny it cost you!

Good luck!
Dennis

[teamus]

are you picking up at the same line when you engage feed?

[r44astro]

There is an old book you can buy I think the name is "how to Run a Lathe" it's 40+ years old written buy southbend I think. This book and a Machinist Hand book can teach almost anything you would want to do.

[chuckels]

Paul,
in your post you state your moving in on your cross slide.
you should be moving in on the compound which is set at 29.5 if you move in on the cross slide your wiping off the thread you just cut which is what i see in your pics.
Chuck

the 29.5 degrees is from perpendicular, 0 degrees would be straight into the work. I have successfully cut nice thread with any where from 10 to 29.5 degrees. just remember if you use the cross slide you are still going straight in no matter what angle the compound is set at which will ruin the previous cut.
I usually zero the dial back out with the cross slide run the carriage right to clear the end of the piece than go back in with the cross slide to zero and take 3 to 4 thou with the compound.
also you say you are making a tap. keep in mind that taps are not truly round but cam ground to give back clearance at the cutting edge. i have made taps in the lathe but only for cleaning out holes or forming in very soft materials.
hope this babble helps,
Chuck

[smiley_b1]

Hi Paul
You are missing one inportant thing with your tool. It has no rake. It is pushing the metal, this is why you have a rough threed. You need to add a rake to the tool so it can slice the metal. It is a long time since I was a fitter and machinist so I cant remember the angle, I think it was 8deg. It is the same as a regular cutting tool. New tool usaly anly have the threed angle pre shaped yo need to add the rake as you have differant rakes for differant materials.

I hope this helps

Cheers

Smiley.

[alexccmeister]

BTW, if you do get the book suggested by r44astro, please don't get it from 'One Horse Outfit' (Username OHOJIM) at ebay if they still exist. I bought the book from him and lost money cos it never came. Not just me, lots of people got cheated by him.

[PRADEEP R.KHARE]

Hi all,

I am rather new to this. I recently bought a Harbor Freight Sieg 7x12 and have done one small project on it. I would now like to do some threading, specifically for a hard-to-find watchmaking specific thread size (40tpi x .275).

I have read the Mini-lathe manual from Little Machine Shop, and some online documentation, but I am still a bit confused with threading procedures.

Here's how I have the cross slide and tool bit set up for threading. The tool bit is HSS.



And here are samples of a couple of threading attempts I made.



The bottom one is on aluminum, which came out OK, though still a bit rough. The top one is on annealed silver steel (I am attempting to make a tap) and it's just ugly (right end... don't worry about the left end). The threads end up very rough and subsequent passes at .005 still seem to strip off way too much metal.

I would have thought 40tpi would be an easy one to start with since the half-nuts can supposedly be engaged anywhere, but I am clearly doing at least something (if not everything) wrong.

FWIW, I went through today and adjusted the saddle thinking play there (which did exist and I did fix) might have been part of the problem. The attempts above, though, were after this adjustment. On a semi-related note, I noticed that there are two set screws visible from the top of the saddle (chuck end). The back one seems to be non-functional, as it goes into a blind hole. The front one I would have thought would adjust for front to back play on the saddle, but it doesn't really... I fixed this by making a brass plug with a 45 degree face to it and setting it under the front set screw. This allows me to put pressure on the 45 degree face of the ways and really cleans up the front/back lash of the saddle.

Any suggestions on the threading?

Thanks,
Paul

dear mr.paul
rotate your tool holder slide(compound slide) by27 1/2 degree
set 0 on main slide after touching on Ø of component ,set 0 on compound slide by rotating dial.
set tpi for pitch.
keeping main slide always on 0 feed(depth of cut) with compound slide,
cut first pass of thread, retract main slide & reverse while comming back give next depth on compound slide,after return of slide to starting position bring back the main slide to 0 on dial.& repeat this way till you reach to full depth of thread (this is equal to tan 27 1/2 deg xpitch)
with this method you cut only one flank of thread & you get best finish. try & give me feed-back.
thanking you
pradeep r. khare
[email protected]

[Vern48]

Hi SQLGuy:
Two books I have, both from LMS, the afore mentioned South Bend book,
How to Run A Lathe (#1596), and Running an Engine Lathe (#1594).
Get both they're cheap.
Something not mentioned about the 29 1/2 degree is that the Sine of 30 degrees is .5.
When you advance the compound 1 thou it is actually only going in a half thou.
I found the coumpound on my mini to be somewhat less rigid than I liked.
I replaced it with a solid block. I still cut threads with success using just the cross slide.
Not trusting myself with the threading dial, I just turned the cross slide out and reversed
the lathe.Then ran the cross slide in again.
I have cut threads at a rather slow RMP, maybe a 100 or so. No problems. No anxiety.
I use Rapid Tap fluid for thread cutting, works good. Also the small triangular file works
well to chase the thread. Remember to hold it at an angle to the Z axis.

Cheers.

[alexccmeister]

Hi Vern,

Great tip on the triangular file. Will try that sometime.

Alex

[machinist_1]

The reason for the 29.5 is so the tool cuts on the leading edge and cleans on the trailing edge. you will get great results if your angle is right. looking at your pic. you are setting your angle at 60.5. Also, try using the same number or line on your thread dial.

Hope this helps: Rick

[fourxblz]

40tpi is pretty fine, so take lighter cuts and it will turn out better. Since the mini lathe has reverse you can just reverse it instead of disengaging the half nuts. That's what I've done even without retracting the cross slide.
Everyone here already covered everything else. The first time I tried threading, I did the same thing, I had the "wrong" 29 1/2 degrees. Also do not rely on the degree scale that comes with the mini. Just get the angle right, take lighter cuts for finer threads and you'll see that threading isn't that hard.
Also, a scotch pad works good to deburr afterwards.
Show some new pics when you do it again.

[kawazuki]

Since the mini lathe has reverse you can just reverse it instead of disengaging the half nuts. That's what I've done even without retracting the cross slide.

That works fine if you can guarantee you have absolutely zero backlash between spindle and leadscrew! Much safer to set the cross slide to zero, stop at the end of the cut, withdraw the cross slide & tool 1 full rotation, reverse leadscrew (whilst leaving the nut engaged) past the start of the cut, feed in the cross slide back to zero, move the compound in by whatever you want to cut and hit go! As long as the tool is always moved forward or in to take up slack and backlash before it cuts metal the thread should be good.