Parting Off Blues

Until today I thought I had parting off cracked.

I normally use an NCIH19-3 blade holder with a 3mm carbide insert. It's mounted the right way up on a hefty rear tool-post and the lathe is run in reverse. I lock the saddle.

The blade works very well on mild-steel and brass but Aluminium is more difficult. The slot tends to choke with swarf and sometimes welds to the insert even when lubricated generously with cutting fluid or paraffin. Nonetheless I've been parting 1½" diameter Ali rod successfully for months by clearing the swarf.

This afternoon I tried to cut a deep slot into a 5" diameter Aluminium rod. (It's too big to fit in my bandsaw.) The lathe was rotating at 450rpm.

Very strange. Initially I got chatter and dig-ins bad enough for the VFD to cut-out. Trying again, I got a shower of bits with less chatter, but the lathe still sounded unhappy and tended to stall.

Going in a little deeper the insert suddenly started cutting a thick ribbon; the chatter disappeared and the lathe purred happily. Deeper still and the chatter and stalling returned until the cutter suddenly bit and spewed nice ribbon again. It seems that the insert cuts properly at some diameters and not others.

The insert tip is set a smidge below centre height. Could my problem be due to the insert shifting in the holder, perhaps because I didn't push it in hard enough in the first place? In the photo you can see the lower front edge of the insert aligns with the outer edge of the holder.

But after a stall I noticed that the insert had shifted inwards:

That would drop the height of the cutting edge and might explain my problem. If so:

• Am I failing to mount the insert properly - not firmly enough or even upside down?
• Has the blade failed? (There isn't much metal in it and it didn't cost much!)
• Am I bending a decent blade by overstressing it, or
• As it's an inexpensive blade, should I invest in a quality make in the hope that it will be tougher?

Any other suggestions welcome.

Thanks,

Dave

SillyOldDuffer:

Sorry to hear about your setback.

Did you try putting in a new insert to see if the problem lies with the existing? I have no experience of this blade but is there an aluminium-specific insert for it?

That said, it does look as though the blade has yielded under the dig-in. What about the other end of the blade, assuming it is double-ended? I guess they are made this way because the blade pocket does not last indefinitely.

It would probably help to run the tool upside down in forward, the idea being that the chips then fall away from the cut (I don't think you should be aiming for a continuous ribbon, by the way).

Finally, can you mount the tool with minimum overhang in the Z axis?

Edited By ega on 13/12/2017 17:41:00

The holders do wear/distort gradually.

Your insert looks a tad blunt, but it may be the photo.

It it was set a bit high and was rubbing, that might have caused the poor cutting, a minor dig in, shift insert and now on height and cuts OK?

I run mine upside down in forward no problem plus i cannot under stand why you do it that way the reason for running upside down is to push the main spindle down to reduce pressure on the bearings regard norm

You should see a convex/concave sidewall to your slot if the blade's materially deviating from the straight and narrow. I've had that happen even on delrin when I've set a parting tool carelessly, and irrespective of whether I was using HSS or carbide.

There's a thing on p.25-6 of Chapman's Workshop Technology Vol.III called a 'Whitaker ring' where a tool (facing in the case illustrated, but possibly valid for parting too) hits a 'sweet spot' of cutting speed and contact temperature. Assuming a constant RPM, are you passing through, and then leaving behind, 'ideal' cutting speeds for the diameters you've reached?

The worst similar case I had was some thick-walled bushes made of (allegedly) EN8, that seemed to work-harden and gall horribly in turning, boring and parting. I never knew quite why, and resolved it without knowing quite how...

Are you sure that the insert is the correct one for your blade? the lower front edge is not in line with the blade holder, else the slot has been opened up due to poor blade quality. Similarly when (upper photo) the insert does align with the lower edge the top edge does not line up with insert. The extractor tool needs to be able to touch at rear so the upper photo is more correct.

Whitaker's ring....... in Chapman's !!!!!!! first time I heard/read that was in college and that was over fifty years ago, Chapman's was a great book, what was the blue text book called? maybe first year engineering or basic engineering? I remember it wasn't very good.

Sorry I went off topic.

Edited By colin brannigan on 13/12/2017 20:18:41

Hi Dave

Just looked at my part of tool (looks identical to yours) and my insert has also been pushed back also. I removed the insert and put it in the reverse end which has never used and it dosn't go in so far and is flush with the end. If the tip is pushed back, because of the angle of the slot the centre height will drop and probably chatter. I have reset the tip height and parted of a piece of 25mm silver steel with no problem but the tip is pushed right back and resting on the bottom of the slot and has a lip like photo of yours. It cut well and so I will continue to use my part of as is.

David

Dave, I was told recently by machining researchers at major engineering research centre (in other words it's on very good authority!) that indexable tip tools are actually overkill for virtually all amateur lathes (irrespective of the quality e.g. Myford, Colchester...)

The materials going into indexable tips cannot tolerate the inevitable chatter on our size lathes. You actually want decent HSS tools instead that can flex and counter these vibrations far better than indexable tools.

Just a quick response to Norm while I think about the other answers. Mark Frampton raised the same 'why not upside down' point in a PM. Yes, I think that would be better. The reason I'm not doing that way is because my rear toolpost is slightly too small to allow an upside down blade to be centred. I'm going to make a bigger toolpost when I can get a suitable lump of steel.

That said I've not had issues parting off in reverse until today, hence the query. One advantage of running in reverse is that it tends to push the tool and saddle down, which is what they're designed for. Running forward against an upside down tool at the back tends to lift the saddle, which might not be so rigid. It may not make much difference - I suspect the big advantage of my rear toolpost is simply that it's much less bendy than the 4-way toolpost at the front.

Ta,

Dave

A Colchester an amateurs lathe! The world turns on a Colchester! 😉

And how many amateur size lathes have they tested with indexable tips to come to that conclusion I wonder?

On this subject I prefer the empirical evidence of my own experience...

Is that bumble bee flying past?

Neil

Dave you have answered your own question, I too have found that turning or parting off with Alu a bit indifferent. What I have found is that the Alu bar is not homogoneuos, that is it has layers that are soft and also harder. The fact that you found a sweet piece in the middle that cut well is indicative of what I am describing. I am assuming that it is HE30 or equivalent?. I changed supplier and had no more problems. Springs to mind, 'End of Bar' or aged metal. Not much sale for 5" dia.

Well they got that wrong then

https://youtu.be/35mr0K6GWKk

The video is of 30mm Aluminium but I use the same tool where appropriate on silver steel, stainless steel, brass and bronze, feeds and speeds are more the key than rear tool posts or extremely expensive QCTPs.

In the video I'm using an RDG Dickson copy with a home made tool holder in a QCTP holder, the sideways overhang is far more than would seem ideal but it simply works on a well set up Myford Super 7

Edited By Nick Hulme on 13/12/2017 22:41:06

I think many on here would actually agree with this but that’s not to say insert tooling can’t be used to good effect on small Lathes.

Whilst industrial inserts really don’t work very well on small machines there are lots of inserts now available that are aimed at the home user. I particularly like using the polished carbide inserts on my modest 8” Lathe. Time and time again though new users asking for advice are here are directed to using HSS tooling before trying anything else.

Sadly some have chosen to miss read what you’ve said and even resorted to making stupid petty comments as usual.

You can get uncoated inserts for parting and grooving aluminium, although you have to look for them. I have some Korloy GTN type uncoated inserts (grade H01) that I got from Cutwel (see the last page of this catalog extract).

And the shape of the chipbreaker in these GTN inserts is designed to reduce the width of the swarf to clear the slot so it doesn't bind. It is then intended to coil up and periodically break off / drop out of the groove. Gravity is not required.

The only issue I've had recently was when the work slipped in the chuck and I had a major jam due to the power feed continuing with the work almost stationary. Shattered the insert but my pants retained their integrity on this occasion.

The MGT and KGT type parting / grooving system has a bigger choice of uncoated (and coated) inserts in various widths and profiles. They are available from people like APT Tools, Cutwel etc. Again, the uncoated inserts are "H01" grade in the Korloy system.

Here's a vid of the Korloy system ("Sawman" 3mm width - using the "GTN" type inserts) in my amateur Colchester Bantam. As you say, they seem entirely unsuited to our little machines.

Murray

Edited By Muzzer on 13/12/2017 23:13:38

When i got my first insert type parting blade the tip was factory fitted and pushed all the way in so when i replace a tip i tap it in with a plastic hammer or a chunk of wood and an engineers hammer .

I had a similar experience with some 4 inch alloy it didn't seem to matter how much cutting fluid i brushed on it would still weld to the insert , i ender up using wd40 with the nozzle stuck down in the groove . I pretty much had to spray constantly to stop the damn thing from jamming up .

I wonder if the heat generated was causing the alloy to expand and clamp up on the sides of the insert ?

Up until that point the rear mounted upside down inset blade had cured any problems i had with parting on a lightly built chinese machine !

Kerosene or WD-40, and at 5", a bit slower, by the time your in about an inch then the 450rpm would be ok. How close to the chuck is the cut, you want to be as close as you can. I only have a very limited experience of TC parting tools, enough to know that the one I got for a dollar isn't much use to me, it's 4 mm wide, and I only have 1.5hp, so HSS does all I need.

Ian S C

I use the same type of parting blade but with a 2mm tip. Home made rear tool post with the parting blade upside down. I can part 1 1/2" EN1A at 800 RPM and around 0.002" per rev power feed. Aluminium however is hand fed and lots of suds and regular backing out of the slot to prevent the material clinging to the tool. Only the higher spec grades of aluminium seem slightly better to part off. I could set up flood coolant one day but to much cleaning down for the occasional job. Has anyone found any tips for these parting blades specifically for aluminium ?

Many thanks for the pointers and advice - I think I understand what's going on now.

Firstly, confirmation that Aluminium is difficult helped. In addition to being sticky and with a low melting point, I hadn't realised that it might also be age hardened and/or stratified. Also there's a strong tendency for aluminium swarf to jam in the slot, for the metal to melt on to carbide, and - being relatively soft - to dig it when cutting conditions are poor. Lesson learned: take extra care with Aluminium.

I've taken a close look at my set-up and found a number of issues:

The insert I was using isn't blunt or chipped. However, it has collected a layer of aluminium on the tip. It's very firmly attached: I think welded. I can imagine this dramatically reducing the effectiveness of the edge. It might also explain why the tip flips between cutting and not cutting. Possibly after the layer of Aluminium has built up and stopped the insert cutting, the tip chatters and rubs. In doing so the heat and vibration may eventually be enough to dislodge the unwanted layer causing the insert to suddenly start cutting again. Any rubbing thereafter would put another layer of aluminium back on the edge causing the cycle to repeat. The original layer of aluminium on the tip was very likely caused by rubbing. Rubbing is caused by insufficient speed, feed-rate or lubrication, and by swarf catching under the edge. Possibly using the wrong insert for Aluminium. All operator errors.

The next problem is that the insert is pushed too far back into the holder. I think this was due to a dig-in. Although the insert survived, the holder has stretched slightly. Compared with the unused end of the blade, the insert in the used end is distinctly looser.

The pushed back insert still works on small diameter rod, but there's a problem with the same tool with a large diameter. Note that in this photo using the insert has pushed it even further back, which also drops the edge away from the centre line.

Worse, when the tip is pushed back, the lower edge of the blade holder contacts the work first. There's enough clearance when parting small diameters for fouling not to happen, but it matters at 5". Lesson learned: keep an eye on this type of holder, especially if there's been a dig-in. The blade I'm using is inexpensive; brand-name equivalents are about 4 times as expensive. Possible the steel is better.

Conventional advice with chatter is to back off. This is what I did, reducing speed to 480rpm. Armed with your new advice I tried the same set-up with a new insert in the 'good' end of the blade. I also ran the lathe at the industry recommended speed for cutting Aluminium with carbide - 2200rpm at 5" diameter. This is moderately scary.

What a difference. Lathe whirred happily and chips came off in a shower. No sign of chatter and I cut the groove shown in a few seconds.

May I congratulate everyone on their answers. You chaps excel at this sort of question. I got loads of good advice, solid expertise, and valuable pointers I might never ever have thought of. Many thanks.

Dave