It's a thread Jim, but...

As some of you will know I've been doing battle with my 12x36 lathe to get it to cut a 2mm thread. Having eventually got the wheels set up correctly (thanks to you guys) I had a trial making an M16 x 2mm thread:

Not great is it? Steel is EN3B, tool is carbide insert, speed 65 RMP, radial DOC 0.4mm for the first cut, then 0.2 , then 0.1 down to final depth. All straight feed, no topslide angling.

Should I be using the angled topslide method on this (600kg) machine, or is the problem down to feed and DOC?

Robin

Edited By Robin Graham on 18/11/2018 23:21:55

tool is carbide insert

It's tearing at the metal from the look of it, digging in/ not clearing

A sharp HSS "carves" the metal away

If you're stuck with an insert try finer cuts/ more runs, deffo needs experimentation

Slower speed maybe, so metal gets more chance to clear away

Know diddly about inserts, maybe even faster would be better??

Edited By Ady1 on 18/11/2018 23:47:26

EN3B is unforgiving. Try the same thread in free-cutting mild steel for better results with a manual machine.

Martin.

65 Royal Mounted Police?

Cut away from the end/shoulder and use the manufacturer's recommended speed for Carbide inserts, slower will not be the answer with Carbide.

....not as we know it.........

Great headline to a new topic. Good sense of humour.

Try lighter cuts. Not always good with carbide tips, I know. But worth trying a series of .02mm cuts to finish off. And its common to make several passes at the final depth setting without adding any extra cut depth. This allows any spring in the tool and job to be "worked out" and has a sort of polishing effect sometimes.

Also when you get to the theoretical correct thread depth, you could try advancing the topslide by .02 or so so the tool takes a light clean-up cut down one cheek of the thread. Then reset the topslide back to Minus .02 from the original position (done by winding the handle back past the backlash and bringing the dial back up to just before the original setting). This will clean up the trailing flank of the thread.

Also, while you are cutting the main part of the thread, instead of offsetting the topslide to 29 degrees etc, you can leave it in the normal position and for every movement of the cross slide inwards, advance the topslide half that amount. EG if you take a 0.2mm deep cut, advance the topslide by 0.1mm. This moves the tool bit point in a path almost at 29 degrees and has the same effect as angling the topslide. With the advantage you can perform the previously described clean up cuts along each individual flank at the end of the operation.

Chuck up some scrap bar and have a good play around and practice with it before you go on to make your actual part.

And no matter how good your screwcutting, expect to have to run a file down the tops of the threads to clean up the inevitable burr. (Doubtless there will be magicians out there who can turn a thread without doing this but they are not mere mortals like us.)

I don't know about the rpm for carbide screw cutting tips. I use a brazed carbide 60 degree screw cutting tool on my old Drummond Flagellator at about 100 rpm and it seems to work rather well. Not sure if the inserts can be run that slow? The brazed tool has a sharp ground edge, which not all inserts have.

The last thing I turned (before going on holiday hence the early post) was an M16 thread on some stainless with a carbide tip fed in at 30 degrees and 0.1mm depth of cut with rpm about 70. I lubricated with a paintbrush that had a small amount of Rocol RTD cutting and tapping fluid on it, just enough to see a slight film for each pass. The tip cut fine at this speed and feed and the thread was wire brushed and scotchbrited to finish it. So I would suggest the rpm is not an issue but maybe a combination of material, depth of cut and lubrication.

Martin C

In addition, you are not giving the workpiece the best support if there was no centre support from the tailstock.

Every little helps. It will reduce any possibility of movement from the headstock components, particularly cutting on both faces simultaneously.

The angled topslide technique was not adopted for no good reason.

If tailstock support makes a real difference on such a short thread, there’s something seriously worming with the lathe.....

Angled topslide is fine, but it makes control of the depth difficult so  zero the topslide , touch the tool down,  move saddle past end of work, add the required depth of cut and re- zero x slide, then back off topslide and use to add cuts

At the end of each pass, retract x slide, move to next pass then return it to zero and add next cut with topslide until zero reached    

Don’t forget it’s the trailing flank which carries the load on the thread, so this one should be the pe best finish.

Edited By Zan on 19/11/2018 09:06:37

Most definitely too slow for a carbide tool. I wouldn’t think you’ll get decent results without running 2-300 rpm at that diameter, probably much faster. Might be best to use an external tool on the rear of the work and run in reverse, threading away from the chuck. Then you can run much higher rpm without risking a crash.

I would also use the top slide to feed in on the flank angle. Just because a machine is 600kg doesn’t mean all the gibs are in good adjustment etc.

I wouldn't use EN3B for a thread. Try some good free cutting mild steel with a really sharp HSS tool.

Russell

Throw the carbides away. Carbides with everything is as big a con as the ONLY true muddle ingineerin lathe is a Miffod. Carbon steel cutting tools stay the sharpest unless you're a cleaver, HSS if you cleave a little but both require the use of half a brain cell and a grinder. NO, not a Quorn or Clarkson but the standard bench type and a stone.

Regards Ian.

Use Rocol cutting oil and this helps a lot.

Where are you based ? I could give you a bit of EN1A to try, lovely stuff to thread.

The big CON is anyone telling you ONLY.
HSS, Inserted Tungsten Carbide and even CBN all have their place in the tool box of any well rounded turner, (no pun intended) hobby or professional, as although there is overlap in capabilities each will enable you to do some things which the others will not.

+1

EN3B has a propensity to tear rather than cut, if not treated right. With normal carbide inserts that means higher surface speeds. It is possible to cut decent threads in EN3B with insert tooling. Some points to note are:

Unlike the common CCMT inserts carbide threading inserts have sharp, well defined, edges. What they don't have is top rake. Of course the aforementioned assumes that the inserts are of reputable make.

When I cut threads with insert tooling I plunge straight in, so that the thread depth can be read directly from the cross slide dial. The top slide is left locked.

The DOCs seem reasonable. I'd probably do a pass at 0.4mm and then a couple at 0.2mm. Then one or two "spring" passes. Then a couple of passes at 0.1mm and more spring passes. You should now be within 0.1mm of the theoretical thread depth. So then 0.1mm per pass and a spring pass, measure or try the mating part, and repeat as needed.

For coarser threads I use tailstock support.

Ideally one would run at a higher rpm, but that'll be limited by one's reaction time. I've got a high speed threading unit with auto disengage so I don't need to worry about cutting at higher rpm.

Andrew

Good advice from Andrew and Hopper.

Try another material first, if still no good its the inserts and grade used.
Angling top slide wont cure thats only for diyers.

Circlip how longs the hss gonna last, 1/4 the thread before fetching out and grinding back up constantly, not to mention future jobs some people cant spend al day on a 1 minute job.

Thanks for advice. To summarise, the problem could be:

(a) I'm not going fast enough to let carbide do its thing

(b) I'm using an unforgiving steel

(c) I'm using cr*p tooling.

(b) seems certain, (c) is quite possible (16 quid off Amazon for a holder and ten inserts - seems too good to be true, probably is).

On the principle of eliminating one thing at a time, I'll try (a) first because - well, I had a buttock-clenching incident when I set the dials wrong and a (CCMT insert) tool zipped along at a mad speed. I hit the panic bar in time then looked at a shiny finish... I've also found that parting with carbide goes better at much higher RPM/feed than I thought reasonable. Much to learn!

Robin

Edited By Robin Graham on 19/11/2018 23:54:55

Roughing out the thread like you are doing. Then finish it with a die. If you want a really good finish just screw cutting on a myford, use the travelling steady and do the in feed etc on the compound slide.

Sulphurised cutting oils help as well, like rocol. Carbide threading inserts are just fine on the manual lathe also.

Neil

Is the tool set dead on center height?