Parting off help!

hi, i have some experience with operating lathesso am not a complete beginer and am competent with turning/facing/boring operations but parting off has got me really stuck at the moment.

i am using a 1/2" HSS parting blade, 15 degrees clearence on the front and a slight backrake ground into the top (litrally just a few degrees, under 5).

it goes very well with aluminium or brass its a joy and leaves a nice finish amazingly, but when i try to part steel as soon as ive gotten through the surface it starts howling at me and i can feel a slight vibration through the whole bench!

i have double checked that everything is as tight as it can be with next to no backlash and i lock the saddle to the bed when in position, im going into a peice of 20mm mild steel bar that it supported with a centre in the tailstock, the cut is about 10mm from the chuck and i have about 18mm of blade from holder to tip.

i can only assume my tool geometry is wrong now as it cuts aluminium so nicely, does anyone have any advice??

oh and im running the slowest pulley speed on a Myford ML10 so about 280rpm.

cheers

Increase the feedrate; I never part off at less than 4 thou per rev. Below that the tool rubs, cuts and repeat, leading to howling and vibration.

Andrew

My first question is what speed are you parting off at? Are you using any kind of coolant or cutting oil?

Parting off is generally undertaken at lower speeds than normal turning, and the important thing is to keep the tool cutting and not rubbing. Some kind of cutting oil should help too.

i have tryd increasing feed rate, im feeding by hand and as soon as i hear the squeal start i push on and feed faster and it just gets louder and louder untill i (and the neibors) cant stand it anymore lol :/

i have slowed it down as much as i can, about 280rpm would you recomend slower? i dont have an oil gun so cannot engage the backgear until i get one :/

i am using cutting oil (cant remember the brand/type) just with brush and bowl, with aluminium i used plenty of WD40.

I was parting off some steel yesterday that was 19mm diameter and I was going at 0.15mm/rev so about 0.006" per rev. on some scrap steel that was once the morse taper on a large drill so not an easy machine steel. The thing is I was doing it in a collet not a chuck and this makes a very big difference to how well parting goes. I think "This Old Tony" on Youtube did a comparison of two chucks, one with a small overhang and one with a large overhang and found the small overhang was a lot better.

If you haven't got collets as an option have you got a fixed or movable steady that can be mounted close to the parting point on the headstock side? I wonder if such additional support would make the job easier if you have to use a chuck.

Martin C

I'm an absolute beginner and expect to make mistakes and get things wrong which is o.k as long as I don't damage myself or the lathe - like feeding a drill from the tailstock too far out and have the thread come out - ,dugh, no end stop on the screw! Members here are wizards and very helpful.

I practiced parting on some 40mm ali stock. I learned you can get a lot of heat which eventually softens the ali and churns up the parting cut. I take it slower with cooling tea breaks now. I learned that when you get the feed in and spindle speed right you get a nice continuous ribbon coming away from the tool. I managed a continuous ali ribbon of 5 ft before stopping the cut to let the stock cool. I think my parting tool has some cobalt/chromium in it and is only 3mm? I've learned (as others here will tell you) it's not the spindle rpm that's important but the linear cutting speed. Small diameter machining needs higher rpm and larger diameters a lower rpm to keep to an optimum linear speed for the metal. I tried some mild steel with lube. I got squealing as the tool got deeper into the cut which thought was't coming from the cutting tip but the sides of the tool?

I found some online calculators on the internet - input metal type, diameter and it works out the optimum rpm for linear speed of cutting. But TBH you can see, hear and feel when the spindle speed is too high or low and with a variable speed lathe it's easy to change.

I've read engaging lathe auto feed is good to get a smooth turning finish, but I'm nervous mine won't disengage so easily when I want it to stop feeding and crash the tool. My 280V seems very clunky engaging the screw feeds and I haven't yet investigated.

Hi,

I'm also a relative newby and have just benefitted from a discussion on this site, about holding the work for cutting off (see to ER is human - about ER collets).

The gist of this is that holding the workpiece firmly and as close to the collet/chuck as possible is critical to parting. Like you, I got all sorts of unnerving noises and even had pieces coming free of the chuck. I'm now sourcing a small set of ER collets which I hope will alleviate the problem.

You might think that acetal or machining nylon would be easier; I machine quite a lot of these plastics and find heat generation a major problem with parting off, because they need much less heat to melt than any metal. I find that with these materials taking a 'firm' cut at a slower rotational speed helps, combined with withdrawing the tool 2/3 times to clear the debris and allow some cooling.

Stuart

i have now had some success ! i ground a bit more of a rake on the top of the tool and tried with a shorter peice of stock and it went through nicely. before i had about 150mm out of the stock (i did have it supported with a centre and was only 10mm away from the chuck) i assume if i set up the fixed steady it would have helped ?

i put a piece of 20mm bar about 30mm out of the chuck and cut 10mm away and it parted fine

i am definatly considering getting a chuck + set of ER collets in the near future

Well, (obviously?) don't have have the tool (or work) sticking out any more than necessary.

But with a HSS parting tool, side clearance is often the real issue and the simple solution is to go in a short distance and then pull back, step over a very small amount and make the cut again but deeper (thereby making a wider cut that ensures good tool clearance). Keep cutting in, stepping back, then over and then back in again - going in by stages. This is a simple solution to parting off and works well for the 'ordinary' kind of HSS parting tool.

For deep cuts on larger diameters, I generally use an 'insert' parting tool that has a 'v' groove moulded into the top surface, which folds the swarf inwards (making it narrower). I can then go straight in - however, it's about 3mm wide, so can be quite wasteful in materiel. A much narrower HSS blade can part quite deeply and the 'step-over' method only needs a few extra thou to work well and so can still be economical on anything under 2" or so (e.g. a parting tool stick-out of 1" or less).

Try it, it takes the anxiety out of parting and just costs a little in extra waste material.

Regards,

IanT

Can you cut from the rear with the cutter inverted?

I almost always do it like that because my rear tool post sits on my cross slide all the time, if possible. I just set the power feed to a reasonably thin cut, but sufficient to avoid rubbing, and set it going - with me concentrating on keeping the cut lubed with cutting oil.

Cutter is likely a cobalt HSS, sharpened occasionally freehand on a belt sander. The strip will last years - it came with the lathe - and I have a spare blade somewhere.

I have to make sure that the cutter is extended sufficiently to cut just past the centre line without the cross slide travel running out! That means the cutter is usually extended more than ideally necessary, but it’s good enough (and better than shifting the tool post closer to the centre line).  I just have to be careful with that.

Edited By not done it yet on 26/06/2021 21:03:37

Possibly a resonance issue, a tweak faster or slower on a variable speed lathe may well have stopped it.

Neil

As already advised, if possible, part off using the tool, inverted in a rear toolpost, and keep up a constant feed.

Don't let the tool rub..

Sometimes, with a fine feed, cutting oil can be disadvantageous. The tool cuts intermittently,,because with oil, it needs more pressure to break through the oil film and cut. A greater fed rate can reduce this effect

It is taken as read that the tool is sharp, as rigid as possible and mounted with the cutting edge on the centreline of the lathe.

Howard

I now use a QCTP and parting tool holder on my Super7 with a tool marked HSS 3/32 x 1/2 x 4 M2. Just fit holder to QCTP, loosen post, run carriage towards the chuck and gently press side of toolholder against chuck face to align it and tighten the toolpost. I run lathe at half normal turning speed for that diameter and metal, slow, steady feed and cutting oil. Mild steel comes off in concertina shaped ribbon swarf with a frying bacon sound. Perfection!

I do have a GHT casting for his rear parting toolpost. When I get a roundtoit, I will add a small steel plate to the LH side at the bottom to butt against the LH side of the cross slide for quick alignment. It's an idea I've seen somewhere. I plan to use the same parting blade horizontally as it is now, since it works OK, rather than 7° tilt. If I fit the rear t - p, it can stay on board, saving constant tool and holder changing.

Edited By DMB on 27/06/2021 22:27:31

Might be worth being careful how much rake you add - the more rake the more inwards force exerted by the cut there is, and the more prone to jamming.

4 degrees positive seems ideal, though the commercial tips tend to use a small amount of negative rake - negative rake generates an outward force of course, so they are very unprone to jamming or chatter. They also recommend mounting the little GTN series tips slightly above the c/l.(Largely stopped using them since they offer no advantage over the Eclipse blades at 4 deg positive)

I gave up using a rear toolpost - just took off the top rake, and used Eclipse 3/32 blades at the normal turning speed in the Myford turning 220M07 and haven't had any chatter or jamming problem since.

Edited By mgj on 27/06/2021 23:51:08

mgi,

The very reason why I am planning to use the small steel plate on the side of the GHT toolpost, is so it can be taken off as necessary but refitted quickly.

I have an original Myford rear toolpost and a couple of small toolholders with very thin, bendy blades that create convex and concave surfaces, so all that lot put on one side. Have looked at websites for Arceuro, RDG, Chronos and Rotagrip (most likely of suppliers) but cannot find identical to what I now use, which has proved to be superior. It uses a clamp block with large tapers to match the main block, held by 3 Allen keys.

John

Edited By DMB on 28/06/2021 08:08:13

Further to my previous post, there is a photo of my QCTP parting toolholder in my album. 3/32" x 1/2" blade. Setting of parting or any other tool is easy enough, just take light skim facing cut. Any pip will reveal just how off centre height. Always done that and not messed around with various devices available.

Unlike mgj, I always use the 3/32 HSS parting tool, inverted in the rear toolpost, with no Top Rake.

Of late, i have become so confident that i engage power cross feed, and watch the swarf falling down!

BUT with the tool only just clear of the chuck jaws.

Howard

It is the lack of rake that makes life easy. I only use 4 deg (in the main) because I have toolholders that hold the blade at the optimal 4deg, so the top is ground flat and the toolholder does the rest. Grooving and everything else is done with zero rake.

I don't like having a rear toolpost fitted because on a small lathe it gets in the way, for me anyway. The only reason I tried it, was because of parting problems, and it offers no advantage - and you wouldn't expect it to. If one resolves the forces the diagram is exactly the same, front or back, and in particular the backlash space is on the work side of the feed nut either way round. (One day I will get round to making a backlash eliminator of the Myford! It has been on the to do list for 10 years now!)

So the concern is a component of the force on the cutting tip acting to drag the tool tip forwards into the work: if you have positive rake such a force exists by definition, and if you have negative rake such a force doesn't exist. But clearly some positive rake is helpful in promoting a nice clean cut and a good finish. But not too much. Hence limiting it to around 4deg.

That's for general purpose steel, but more grabby materials like bronze and copper need to be treated differently and the recommended angles are different. I normally use a commercial tip for them, because of the negative rake: that works without getting too involved with specialist tools.

DMB - the decent tapered Eclipse H5 blades are available - at a price. MSC do them. Interstate seems to be pretty good too. Seems to be a lot of "HSS" around much more cheaply, but it is I find, rather useless.

Edited By mgj on 28/06/2021 11:00:54

I am completely sold on T shaped blades with a hollow top. Similar to the ones Eccentric Engineering sell, although I have a SOBA 1/2" from Chronos. The swarf is folded slightly which makes it narrower than the slot so less chance of jams. No worries about grinding the top rake, and the tool is held at 0 degrees so no height issues.

I never rely entirely on the pip on a light skimming cut. It may achieve no pip and indicate centre height to the nearest tenth of some measurement, but is meaningless, other than as a starting point, unless the whole lathe cutting system is totally rigid - which it is not.

I’ve never checked the cutter centre height on my rear toolpost parting cutter. It works, so why bother? Fiddling will, IMO, not make it work any better. I’ve seen considerable tool deflections on some youtube videos as the cutter engages with the workpiece. Particularly with lathes fitted with a QCTP with loads of cutter overhang (in both axes) and operating way off the centre line of the cross slide.