Thread: Threading...Aaarrghh!!!

Originally Posted by Techo1

There's something wrong with your depth calculation there, the actual single depth of thread is around 1.28mm, if using angular approach a little Trig. will give the depth at 30 degrees.

What do you get at 60 degrees.

The only people who say they have never had trouble threading on a lathe when starting out are the ones that have never threaded on a lathe...

What I might suggest is practising on some free cutting steel... That takes away one variable of crappy sticky material and lets you concentrate more on the nitty gritty..

I did some absolute shocker threads when I started out... They made yours look good... Now threading is just routine..

Hi,

Thanks for all the replies. As soon as the first post said compound I knew where I went wrong. I moved the compound around another 30 degrees and tried again with a successful result (well good enough for now).

My OD is a bit small - 15.90mm - and I took another small cut when I should of done a spring cut so the nut is a wee bit sloppy. But I've got plenty of round stock to practice on.

I should mention that I did try plunging in but I had the wrong gear on the lead screw (should of had the 60 tooth gear) so that explains why my feeds have been out as well and cut the wrong pitch.

I'm using the reversing method as find it more accurate than using my thread dial - will practice with it on.

Thanks for the replies

Ben

ps the photo's are out of order, one shows my original set up at 30 degrees and another at 60 degrees - which really confuses me

Originally Posted by cba_melbourne

This is different in Europe, where the other threading method whereby the topslide stays set parallel to the ways but is offset for every pass is mostly taught and used. In practice, both methods do exactly the same thing, eg cutting the thread on one shoulder only.

Originally Posted by Pete F

Sorry could you run that one past me again? I couldn't follow that sentence at all.

The compound slide stays put, parallel to the spindle axis. As per normal, the form tool is perpendicular to the work. The depth of thread is achieved by feeding in with the cross slide. That's generally the way that I thread. "Properly" you should feed the tool back and forward a little with the compound so that only one side of the form tool is cutting, but that relies on a compound with no backlash. I do a variation of this when I cut ACME threads, in that I grind the tool narrower than required and proceed to cut the thread to full depth. Once there I measure the root of the thread* and using the compound widen it the necessary amount (both flanks) to get a nicer finish on the thread.

Michael

*I use gauge pins on end for this - the size is the pin that doesn't show daylight underneath the pin end when both corners are in contact with the thread flanks

I get the impression that some people in this group look down upon plunge thread cutting, but if my memory serves me correctly, that is the method included in the Hercus book "Text book of turning".

If I'm wrong, please correct me.

You do need a lathe with a bit of guts, but it works fine for me.

Rob

I got caught by 30°/60° thing the first time I tried single pointing a thread, but I've successfully cut a lot of threads since then.

A lot of what I do is in 1020 which means I often end up with torn looking threads. To get around that, if I have the right die I single point to 90% and finish off with the die.

At the end of this download is something I find useful, a chart of infeeds for a range of pitches and TPIs.

http://www.sandvik.coromant.com/site...c-2920-031.pdf

Originally Posted by nearnexus

I get the impression that some people in this group look down upon plunge thread cutting.

If I'm wrong, please correct me.

I think threading is one of those things that once you learn a method you tend to stay with it, especially if you are not exposed to alternatives. Skewing the compound around is usually the method recommended for smaller lathes and I suspect that is what a lot of people use because it suits their machines. With larger (more powerful/ more rigid) machines there is more room for experimentation, but until you have a machine that is capable there is little reason to change. I only started plunge threading because I read about it and thought I'd try it but with the lathe I had at the time it was perhaps not the optimal choice.
If you subscribe to the view that with a QCTP you change the tool angle not swivel the toolpost (another controversial topic) then plunging then becomes the logical way to single point by that doctrine.

Michael

Originally Posted by nearnexus

I get the impression that some people in this group look down upon plunge thread cutting, but if my memory serves me correctly, that is the method included in the Hercus book "Text book of turning".

If I'm wrong, please correct me.

You do need a lathe with a bit of guts, but it works fine for me.

Rob

In the hercus textbook they do describe plunge cutting. But then add "when roughing out larger threads, 12 TPI (2mm) or coarser, it is advisable to advance the tool sideways slightly by means of the compound rest on each successive cut....." They leave it open by which method this sideways advance of the tool is to be achieved.

My experience is that light hobbylathes are not rigid enough for brutal plunge cutting - offsetting the cut sideways (by whatever means) delivers a better surface finish, especially in some tougher steels. I also like to think that plunge cutting creates greater cutting forces. On long and slender items this may cause excessive workpiece deflection. Then you may need to use that travelling rest sooner than otherwise necessary. But yes, if one sees an industrial CNC lathe plunge cutting a large thread in a single pass under flood oil coolant, whilst leaving a perfect mirror finish behind, one can see how this method can work perfectly on a very rigid production machine .

so umm............ if you advance the compound 0.020" at 29 deg there is less cutting forces than if you advance the cross slide 0.020" ?

hmmm........... the forces would be the same if all you are doing is advancing the tool forward the same
measurement, in other words the same depth of cut, the tool is 90 degrees to the work in either situation and cuts exactly the same when it enters the metal

so why does plunge cutting require a more robust machine ?

On my machine, plunge cutting or indeed taking too much off on each pass leaves a damaged/terrible thread. The reason being (in my case) Loading up the leadscrew too much tends to flex the roll pin where the leadscrew attaches to the QCGB. This has the consequence of allowing the tool to cut in a slightly different spot on each pass, creating a situation where the crests of the thread end up being paper thin. I discovered this when doing spring cuts and wondering why material was still being removed after 2 or 3 passes. It was the roll pin relaxing back to it's usual spot after being loaded up with a too heavy cut in the first pass. Actually, wrt plunge cutting, when I'm up to the final pass and only have say 0.05mm left, I plunge the last pass and make the form tool cut on both sides. This tends to "tidy" up the shoulders. Simon

Originally Posted by shedhappens

so umm............ if you advance the compound 0.020" at 29 deg there is less cutting forces than if you advance the cross slide 0.020" ?

hmmm........... the forces would be the same if all you are doing is advancing the tool forward the same
measurement, in other words the same depth of cut, the tool is 90 degrees to the work in either situation and cuts exactly the same when it enters the metal

so why does plunge cutting require a more robust machine ?

Hi shed, It's because when you advance with the compound (set at 30 or 29.5 deg) only one side of the form tool cuts. Whereas when you feed with the cross slide, both sides of the tool is cutting. Don't forget, plunge cutting to 0.020" also removes more metal in a given pass than moving the compound by 0.020" Edit: used compound when it should have been cross slide! Cheers, Simon

Originally Posted by simonl

Hi shed, It's because when you advance with the compound (set at 30 or 29.5 deg) only one side of the form tool cuts. Whereas when you feed with the compound, both sides of the tool is cutting. Don't forget, plunge cutting to 0.020" also removes more metal in a given pass than moving the compound by 0.020" Cheers, Simon

g/day simon, the tool enters the cut exactly the same way, I think the word "plunge" throws everybody off.
You don't actually don't plunge the tool into the job, the tool enters the job exactly the same way and only cuts on the leading edge with both methods.

shed

Originally Posted by shedhappens

g/day simon, the tool enters the cut exactly the same way, I think the word "plunge" throws everybody off.
You don't actually don't plunge the tool into the job, the tool enters the job exactly the same way and only cuts on the leading edge with both methods.

shed

Not sure on that. I think I'm going to disagree but I'll have a think about it. Cheers, Simon

Originally Posted by shedhappens

g/day simon, the tool enters the cut exactly the same way, I think the word "plunge" throws everybody off.
You don't actually don't plunge the tool into the job, the tool enters the job exactly the same way and only cuts on the leading edge with both methods.

shed

shed, that is not the way it is described in the Hercus book. Unless I understand it wrong. They really mean cutting on both sides, without mentioning the word "plunge". Cutting like that is like turning with a form tool. The larger the tool contact area, the greater the cutting forces, and the greater deflection and chatter problems.