Indexable tool holders

Andrew, re turning aluminium...

I've not done much with Al, only recently done the first serious job, turning and facing an 8" disc of the stuff using Sandvik inserts from Greenwood Tools, and did have a similar problem initially.

Examination of the tip revealed the presence dreaded BUE phenomena. When this builds up, it has several bad effects, listed in no particular order:

1) If it stays in place, it changes the geometry of the cutting edge and the chip forming surfaces of the tip. Chips don't form properly, or in the right direction, or both. Bits of metal stick to the turned surface, giving poor finish.

2) It can break off cleanly, the tip returns to normal functionallity.

3) It can break off and take some of the carbide with it - the tip is wrecked!

In my case, and possibly yours, it was 1) above. I tried the usual paraffin lubricant, but the fumes were most unpleasant, so that experiment didn't last long. The answer that I eventually found was to run very fast - 2500 rpm (the VFD is set to 60Hz) at a high feed rate (the machine has power cross feed, thank God!) whilst standing well back and allowing it to shut the feed off of it's own accord! Excellent finish.

If it happens again, I suggest that you take the tip out and put it under a lens. If you see something like this:

then BUE is the problem. The photographs are actually steel on a Sumitomo titanium carbide tip, NSC geometry, which identifies the complex forms of the chip forming bits on tip.

Andrew, re pedantry... feel free! I use the 'correct' equations when calculating things with Mathcad, which tracks units and ensures consistency. But I think the crude 'torque x RPM' version gives a better sense of things to many people who might not be quite so familiar with mathematical rigour. As long as they don't try and do any sums with it, of course!

Russell: the height tolerances are for turning, facing is not so critical, but they're manufacturers recommendations, not mine! Obviously, you're right, as diameter reduces, the tool angles change if not exactly on centre height - rake, as well as clearance.

As diameter reduces it does, theoretically at least, become more important to get closer to centre height, but in my experience it doesn't seem to matter very much, and I can turn very small diameters without difficulty, without getting over-fussy about centre height. As long as it's not above centre height, of course.

However, you need the right tip to do it - have a look at the edges of the Sumitomo NSC tip above, you'll see that, by carbide tip standards, they're quite sharp. Some (many?) tips have rounded edges, which limit their minimum depth of cut and minimum diameter.

Oh yes, whilst looking at that tip - set the tip of the tip to centre height, keep the gauge well away from the chipbreaker parts - the Sumitomo NSC shapes are significantly HIGHER than the very tip of the insert, so sitting the gauge thereon would result in setting the tip far too low.

Ah, could that be Martin's problem with that spirit level height gauge that he uses?

I agree with JasonB, when facing across a large diameter I keep one hand on the speed control of the VFD, (I have a geared potentiometer) which allow a variable rate to be applied. Some DROs (and I did not choose this option ) will do this for you as it is measuring radius and hence can calculate the necessary rpm.

As regards far eastern inserts, the Korloy AK inserts are available as CCGT 060202-AK etc for aluminium. Grade NC5330 works very well on stainless.

Edited By KWIL on 12/09/2013 13:52:05

JasonB:

Would it be possible for Caneng to suggest a few suffixes that we should look out for that would be suitable on hobby lathes

Blunt answer - no! Each manufacturer has his own designations, new ones are forever appearing, and some are for very specific cutting operations in specific materials.

I can, however, give some guidance based on experience. But before I start, I do not work for, and never have worked for, Sumitomo! I came across their tips umpty-one years ago, and the recommendation to try them out has been so succesful that I've never had any urge to use anything different for most of my work. They've cut steel, hardened steel, HSS, cast iron, stainless steel, copper, phosphor bronze, Tufnol, Floursint, even wood - just about everything except aluminium. All sorts of diameters and shapes, hexagon bar, square bar, rough and hard castings. So they do provide a good basis for identifying what a model engineer might want to write down as a purchasing specification for his tips. Here's the Sumitomo diagram that identifies the cutting region for the NSC geometry:

From that we get that the NSC geometry is intended to operate with depth of cuts in the range 0.5 - 2mm (0.020" - 0.080" ) and feed rates of 0.1 - 0.3mm/rev (0.004" - 0.012" ). Within those ranges they will cut and CHIP FORM exactly as their designer intended. However, they will work outside those ranges, albeit without 'correct' chip forming - but acceptable in a model engineering, non-production environment. I've used them down to 0.002"/rev feed (slow feed tumbler gear on the S7) with depth of cut from say half a thou up to 0.125" - that photograph of the swarf was 3mm d.o.c., can't remember the feed, probably 0.004"/rev., but definitely not chip forming correctly!

So a tip that will do d.o.c. 0.5 - 2 mm at feed 0.1 - 0.3mm/rev would be a good starting point. These tips would not be suitable:

ap is the d.o.c., fn is the feed range, Vc is the cutting speed. Don't worry about cutting speed, not significant for model engineers, and they'll all work outside those ranges. The speed ranges are for 15 minute tool life - cut faster and they'll wear out quicker, cut slower and they'll last longer. We're not worried about optimising tool life, and they'll all go faster than we would need anyway!

The coloured letters are the material classifications for which the tip is designed: P is virtually all 'normal' steels - unalloyed, low alloy, high alloy, cast steels. M are the stainless steels, K are the cast irons, plus aluminium alloys. The others are not relevant to model engineering activities. Well, maybe if you're building gas turbines...

The presence of cutting data for P and M materials shows that the tool is suited to cutting steels and stainless steels. But it would probably cut K as well, perfectly satisfactorily for our purposes, but not optimised for high performance on cast iron in industry.

Carbide grade - not something to get hung up on in any detail, we're not in need of optimising everything for production, so we can use less than optimum materials. I would suggest going for titanium carbides (cermets) or coated carbide grades, as these are good for minimising (but unfortunately not eliminating) BUE problems. The gold clour on tips is titanium nitride, so it's obvious that they're coated. But be aware that some coatings (e.g. Al2O3) are not gold, so not being gold coloured doesn't necessarily mean that it's not coated.

Coatings are also present to reduce things like flank wear on the tips, it's not all about BUE

Tip radius 0.2mm. Not as strong as 0.4mm, but a lot less trouble on a small machine.

But one thing you should NOT do is to buy el cheapo tips from a second hand tool dealer at an exhibition. And NEVER buy tips that are just piled up in a box where you rummage about for something that looks right Only buy tips in original boxes, and only in sealed boxes at that - if the seal is broken, don't touch 'em. I did that once, having dropped on a full box of Sumis at a good price - they wouldn't cut properly, and a trip to the microscope showed that every one was worn! Fortunately they were from a decent source and I got my money back. But I've never bought from them since that experience. Plenty of industrial grade suppliers around, better to stick to them as the source for your tips.

That's about the best I can do on advice, but for recommendations:

Sumitomo tips in T1200A titanium carbide, NSC chip breaker for turning and facing. For Martin's tool holders, CCGT060202-NSC/T1200A.

Sumitomo T1200A again for boring tools, but with the W chip breaker, it's brilliant. Beware - the W chip former is assymetrical, you will need the L (left) version for normal going right-to-left boring operations.

Parting - only ever used Iscar tips and blades. Tried several variants of carbide, coating, chip former, never found much difference between them. Problem with parting tools is that blades and tips are manufacturer specific - an Iscar blade will only take Iscar tips, you can't mix manufacturers.

Hope that helps!

Edited By Cabeng on 12/09/2013 15:03:18

Edited By JasonB on 12/09/2013 15:59:24

Who or what put those ~@^&^^%$ smileys in after I'd posted that message? Not me! I deleted them, and somthing or other has put them back.

Cabeng

I've had the same thing done to my posts. Its the stoopid editor!

It likes you to put a space before a closing bracket, OK its not the correct way of doing things but its just something we have to put up with on this forum.

Onec again I must compliment you on your really good advice, You have an easy to read, style of writing and give good explanations and straightforward advice, thanks.

Ian P

As this thread is about indexabletools, has anyone tried the HSS indexable tips from A R Warner in the US? Any good and do they have a UK distributor? It seems to me that foramatuer use these could be ideal, at least until one had made a decent T&C grinder.

BW

Andries

Cabeng: I turn a lot of aluminium (mostly 6082), generally have few problems, and can get an excellent finish of about 1µm Ra. I have found that inserts specifically for aluminium seem to behave better. Here's a picture of the type of inserts I use:

Note the polished finish, positive rake and sharp edge. In general facing is not a problem, but, as I mentioned, when turning the swarf normally comes off cleanly, albeit in a long ribbon. But sometimes, seemly at random, part way through a cut the swarf will 'birdsnest', and if it gets trapped between the tool and work it obviously scratches the finish. From the information on the box I seem be to pretty much in the recommended areas for doc, feedrate and speed, although at the lower end of the feedrates. For the tip shown, 0.8mm radius, the maximum feedrate is 0.5mm/rev. So may be I should try pushing it a bit and see what happens. I very rarely get any obvious BUE with these tips. However, that is not to say that the swarf going doolally isn't caused by BUE if only temporary.

On another matter I need to get my act together on setting tool heights. A lot of my turning is on largish diameters up to about 16", so I've been a bit cavalier about setting tool heights in the past. That's one of the little lightbulbs your posts have switched on; I need to look at height setting more carefully. I plan to buy an el cheapo digital height gauge, measure the centre height, and then make a cylindrical square of that height. I can then use the square to set a 'zero' on the height gauge and use the height gauge as a comparator on the insert. That way I'm not relying on the absolute accuracy of the cheapo height gauge. I've got an Etalon height gauge that I can use to make the initial measurement. I need to buy some 1.5" silver steel to make a hob for the worm gear for my traction engine steering, so I can use some of that to make the cylindrical square, and harden it.

Regards,

Andrew

Watch the el cheapo digital ones, Someone bought me one from one of the regular advertisers here, it rocked on the base, the scribe pointed upwards and its the opposite hand to most height gauges which makes it hard to us eif right handed.

I also use the same type of tip for Aluminium but with a much smaller 0.2 corner, works very well on bronze as well.

Personally, I've tried some of these height setting gadgets and find the easiest way is to set the tool height by eye against the tailstock centre and then take a trial facing cut across some scrap and make fine adjustments to eliminate any central pip.

Russell.

That's basically what I've been doing, except that I find that the pip breaks off at some point before one is exactly on centre height. For larger diameters the method is fine, but it is certainly not going to let me achieve the accuracies quoted by Cabeng.

Jason: Thanks for the heads up; fortunately I'm left handed. I've been looking at two height gauges; a 4" one from Machine-DRO, and a 6" one from Arc. My lathe swings more than 8" over the cross slide, so if I bought the 4" one I'd have to hack it about anyway. I may well have to modify the 6" one as well, but time will tell. The purpose of making a cylindrical square is to allow the height gauge to be used a comparator, rather than an absolute measuring gauge.

Regards,

Andrew

PS: Forgot to add, I've also got the aluminium specific inserts in 0.4mm and 0.2mm radius. I try and rough with the 0.8mm radius and use the smaller radii inserts for finishing, if I need a sharpish internal corner.

Edited By Andrew Johnston on 14/09/2013 11:11:48

I'll just say you may be better off with the 4"

Looks like it'll be the 4" then, and I'll throw the base away and make a new one - Andrew

Perhaps Cabeng could discuss more re tip geometry. It seems many users are not quite aware of the various top rake angles available with carbide inserts. I designed lathes for 12 years and I know the top rake angle was carefull selected for each customer runoff (to the likes of Caterpillar, John Deere, Ford, New Departure,etc). Most steel and iron jobs were run with negative rake insetrs due to it being a more robust insert. Another concern was the BUE. Another big concern was life of the insert.

I do not have first hand experience as I still use hand-ground HSS tools in my hobbyshop.

Bruce

Some VERY informative posts, worthyn of being seen by the world at large, and kept!.

Setting the tool to centre heightis, in my view, very important.

If you don't all the clearance and rake angles go to pot.

Mostly, I use replaceable tips, and have a holder to use the unused corners.

HSS for chamfering, and mostly for parting off.

Being as careless as I am, there are plenty of damaged tips available for roughing work!

And the posts have explained, why, possibly some of my tips don't last very long.

My tools are set to simple centre height gauge, (invented by someone else) by adjusting until the tip corner just touches the underside of the blade.

The gauge is just three pieces of metal and one grubscrew. (Four and two grubscrews if you use it to set tools in a rear toolpost).

The gauge takes very little time and effort to make and saves time and tools, and improves finishes.

(On this subject, using a replaceable tip at low speed, on steel, with hand applied neat cutting oil, a really good finish results).

You can set a tool to centre height by trial and error, and then set the gauge to the tool, and use that setting for all subsequent tools.

If anyone wants to know the complicated /pedantic way that I set the height of the blade on the gauge, I can go into detail. But it does give the exact centre height for a particular lathe, despite any eccentricities.

Howard

After cogitating about buying a height gauge I've decided against it. The 4" one I was looking at didn't have very good resolution, as it was intended for woodworking. Also, the manufacturers' datasheet was confused about resolution and accuracy. If they can't get that right it doesn't give you warm and fuzzies about the product. In due course I'll make myself a simple mechanical height gauge to set tool heights. At least I'll save on batteries.

This afternoon I did some turning experiments with a bar of aluminium (6082) and the 0.8mm and 0.4mm radius inserts. The bar was 2" diameter and was running at 800rpm, so about 400sfpm. I started with the 0.8mm radius insert, a doc of 50 thou and a feedrate of 12 thou/rev, higher than I'd normally use. The swarf came off to the left as a nice tight spiral. Increasing the doc to 0.1" caused a bit of a birdsnest. Increasing doc again to 0.2" curled the swarf to the left, but it still got tangled up. Reducing feedrate to 4 thou/rev caused a birdsnest.

With the 0.4mm radius insert a doc of 50 thou and 4 thou/rev caused a birdsnest. A doc of 5 thou and feedrate of 2 thou/rev had the swarf coming off in thin spirals, a bit messy, but the swarf was too fine to affect the finish.

All in all very interesting. I guess I need to get organised and write the down feeds/speeds and rpm that work so that I can refer back in the future, rather than 'winging' it. As a general point I could probably do to increase feedrates when roughing, and possibly decrease when finishing. I didn't pay that much attention, but I got the impression that the tool was not cutting when returning the saddle. That could make it easier to get parts on size. Ideally if I put on a doc of say 10 thou I expect the part to be 20 thou smaller. But life isn't always like that!

I've dug out my copy of 'Modern Metal Cutting' for re-read.

Regards,

Andrew

After returning late from a long weekend away, I did a reply to postings that had come in during my absence, only to have this bloody system loose it! Too late to re-do it now at 02:50 hrs, I'll get back to you tomorrow. Or later today!

Harking back to my days in education as a materials scientist I recall that metal is not cut but spit apart rather like a wedge splitting a tree trunk. After the initial penetration the wedge tip does not contact the wood but forces it apart such that a split runs well ahead of the wdge itself.
A lathe tool causes a crack to form ahead of the tool and no metal is in contact with the actual tip. The point of the crack propogates at the speed of sound (in the metal) and creates shock wave (sonic boom) ahead of it which helps break the atomic bonds between the metal atoms and significantly reduces the energy required. If the tool advances at the speed of the crack it can be kept rolling but if slower the shock wave dissipates and the advantage is lost. Since the crack and shock wave cannot move faster than the speed of sound trying to speed up means enough stress has to be created to produce a new crack ahead of the primary crack in virgin metal. This substantially increases the energy requiement.

So for mass production the target is a specific speed through the metal.

Ah, is that why I sometimes get something like a sonic boom when I push things too hard?

Seriously though, what is the speed of sound in, say, mild steel?

Russell.

It varies according to the type of steel and wave, but it's normally in the range 3-6000m/s. There's no way my lathe can get anywhere close to those speeds.

Andrew

Sounds like an urban myth to me.

Neil