Parting with cardide tools

[loply]

Hi folks,

I bought a Glanze indexable carbide parting tool, the kind with a "blade" and a weird shaped insert that locks in place by use of The Force.

Tried it the other day - made sure the tool was totally square to the workpiece, 20mm mild steel, tightened everything up and locked the sadle, and fed it in at 150rpm using a constant flow of engine oil to keep things cool.

It cut like butter with no noise or chatter and just silently peeled the metal off.

Got about 2/3rds through when all of a sudden the insert snapped in half and chipped the pocket that it sits in... 

Just wondering what I did wrong? I have ordered a new insert, luckily the "blade" of the tool is double ended so I can use the other pocket.

If this happens again I will need a new blade and a new insert... this could be expensive if it happens frequently!

Cheers for any tips.

Rich

[Nemt]

Your number of revs is not interesting. Your cuttingspeed is important. When using carbide tooling, in general your cuttingspeed has to be high enough to achieve good cutting results. When parting of, cuttingspeed decreases when getting closer to the centerline of the workpiece. The lower the cutting speed the higher the forces on the tool! That is why your tool is breaking comming closer to the end!
Nemt

[Bernd]

You didn't mention if you had set the tool right on center.

Bernd

[loply]

Tool was centered first and judging by the way it was cutting it was not a problem, neutral feeling to the handle (not sucking in or pushing out) and seemed to cut well.

How much force should I be putting on the cross slide handle? I tried to "keep the chip coming" and just kept it at a rate which made no noise and kept a continuous chip peeling off. Is this too fast? Not fast enough?

I could probably have cranked it harder but I feel it would have caused chatter.

[DaveH]

To maximise the rigidity especially on small lathes you need to part off as close to the chuck as possible.

Just my 2p's worth.

DaveH

[PekkaNF]

uuh...

It's hard to tell, but here is my experience: I used to use no-brand carbide bits before. Quite a few broke at first contact. I pretty much concluded that it's recyled mystery carbide that is to be blamed. Switched over a brand name and didn't had this happen again. Most carbide inserts don't like "rubbing" or chatter. I have also heard that you eitehr have to use them dry or to flood with coolan, anything between (or hobby standard squirt when coolant has smoked off...) and they became brittle. Believed that, but I haven't conducted any long term test on this subject.

Pekka

[mike os]

will follow wih inerest... i have he same problem...with the same tool

with speeds of 500-1200rpm...... just seems to impart more energy into the break rther than make any difference... power & manual feed, coolant or no coolant....my thinking is crap holder, crap bits/

[lordedmond]

As others have stated your sfm /revs are to low  luck to start at around 600 rpm for that size , I would go to about 1000 and go in under power cranking the speed up on the way in

it need to crackle a bit when cutting

BTW  good speed and a good chip load in required along with a increase in speed as the dia. goes down

Stuart

[No1_sonuk]

I have the same tool, and broke a couple of tips.

Now I don't run the machine below 1200rpm, with a decent amount of force, and it works fine.

[mgj]

I have the same blade and tip I think  Uses a GTN xx clone.

I part off reasonably slowly, because thats the way I have always done it, and I am not about to change since I get perfect results and a good finish. I have, in the past bust a couple of tips. And like you, when getting near centre, but in both cases the cause was a relatively slender job, which climbed.  The answer was tailstock support, and I suspect that that climb has more to do with it than revs - well I think one can be certain of it, because the steel doesn't know what is cutting it, and carbide has few feelings. All that mtters is flex and the rake, and the cutting forces generated,  whatever the tool material..

Lathe size?

The supplied tips are GTN 2 clones, and they are negative rake tips, which means that relatively they are "harder" to push in which is why its actually very difficult to get them to dig in, at any speed - even the 300rpm I use on the Myford. If you want a smoother feel to the cut, then use a GFN tip, which is dimensionally interchangeable, but is a positive rake tip. Alternatively, increase support/rigidity, and  decrease the feed rate as you get near centre. Obviously.

[loply]

This is a small lathe, a 7x14 thing.

I have ordered a new replacement Glanze tip, which is the negative rake type.

Incidentally I also noted that the Glanze "blade" which goes in it's own tool holder and accepts the insert was 20mm, so I found and oredered an HSS 3/4" parting blade too. I checked the holder carefully and it will hold the 3/4" piece.

Once the new insert arrives I'll try again with a higher speed, and if it breaks I'll use the HSS  

Incidentally the reason I ran such so speed was because of a video I found on YouTube wherein the author proclaimed the benefits of parting at slow speeds on small lathes. Alas I can't find the exact video now.

It did seem to work, a MUCH smoother cut than I've previously had, just with the obvious downfall at the end  

[John Hill]

That style of parting blade is so expensive I have pretty much put mine aside.  That was after having broken both ends of one blade and buying another blade which by then cost me more than the original blade and holder.

BTW, I ground the end of the broken blade to make a regular parting blade, it works so well it makes me wonder if I will ever buy any more chips!

[loply]

John, I did look at the end of the blade that I've broken and wonder about grinding it into a parting tool, it's almost the right shape already, and is probably decent steel.

Having read around a bit it seems the consensus is that carbide parting inserts are an expensive route, I'll see how the new insert goes when it arrives and if it ever breaks I'll just forget about it... Not worth the money for what it's worth!

[mgj]

I don't know Loply. You buy a decent Eclipse blade and they cost a fortune. - plus you need grinding kit to sharpen - and to sharpen really well you need control too.

I bought 10 GTN tips, and I am still on my first (after I learned how to use them).

The problem is that most of us graduating onto indexable tips, are not used to this negative rake bit. They do cut (very) well, but the reason they don't dig in is because the negative rake creates an outward vector(unlike positive rake which is pulling the tool in). However that does place a premium on rigidity because one does have to push a bit harder than with a positive rake tool - as one would expect, they are designed to make one do that.

So, one has to learn how to do it right, rather than blame the tool, I reckon. Though, having said that and realised where I was going wrong, I did regrind all my HSS blades and dropped the rake angle right off to a couple of degrees or so, and all the dig ins, which is what made me go to indexable tips, just disappeared.

Got rather a lot of parting gear, me, and took quite along itme to get it all working right! (Like 20 years, mainly because IMO, the world of parting, with back toolposts etc is full of old wives tales)

[loply]

mgj, interesting points, I think as much as anything as a casual hobby user I'm drawn towards the HSS route now because it makes me more independent. I'm fearful of depending on the indexable tool now because if it breaks again I will have to order a £30 blade to fix it, which is the opposite of what I want when I'm halfway through parting off an item on a Sunday afternoon! I can't justify keeping many spare inserts and blades because of the prohibitive cost.

With HSS I guess I would just regrind if it chips.

Having said that, I have one new insert coming in the post and will do my best to make it last!

How do you grind and sharpen your HSS blades? Roughly how much front clearance angle? Can you not run them with a totally neutral top rake?

[No1_sonuk]

I'm fearful of depending on the indexable tool now because if it breaks again I will have to order a £30 blade to fix it, which is the opposite of what I want when I'm halfway through parting off an item on a Sunday afternoon!

This is what a hacksaw is for...

[AdeV]

I ended up going with Mircona parting off tools; they're available in a wide variety of shapes & sizes and - if you keep an eye on eBay - the holders usually come up for a reasonable price (not that there have been any for a while, ironically). The Mircona hold is much more rigid than the insert-in-blade-in-holder type, and whilst i've lunched a few inserts (mostly on harder stuff, mostly on approaching the centre), I've found them both extremely good at parting off; and also quite useful for other operations (even standard turning!) They cut well on 3 sides, where a blade-type holder would simply bend away.

[Ned Ludd]

Hi Ade,
It's not that difficult to make your own holder for the Mircona style inserts, saves waiting for one to come up on  feepay, cheaper too.
Ned

[mgj]

Can you run HHS with a totally neutral top rake. I should think so if you want to. A lot of the indexable tips are negative rake. I've never ground mine at zero - I use about 2-3 degrees positive in HSS. Just enough to get the chip curling out nicely, but not enough (so far) to create any significant inwards pull. That's disaster, because if the feed one applies isn't greater than the tools inwards pull generated by the rake, then the feed nut will/can pull forwards into the backlash of the feedscrew and bingo, one jam up, front or back toolpost. So my solution is very little rake and a good bold feed.

I do now use HSS more than indexable - simply because my HSS is ground left and right handed by a couple of degrees, and mostly one doesn't want a neutral front face - I always want to separate a bit from something else, and I want the pip on the bit left behind. Like washers and things, where removing burrs is a real pain.

How I grind them - mine are all Eclipse blades. On a Quorn - I doubt you need that precision as long as one has a few card templates - better to be approximatley right than exactly wrong IMO - but I have a Quorn so I use it.

Front clearance. Not much. For strength one wants as much as possible under the cutting point, without letting it rub. About 5 deg seems to work. You 'll soon know if it rubs, as you increase the feed. You either have to back off the feed (seriously bad idea see comments above) or add a touch more clearance. (A decently polished .090" Quorned blade is so sharp it will cut pretty much as fast as one can feed in 220M07. I learned that after I spent lots on  indexable!)

[Bernd]

I've always just ground the front clearence and left the top at O°.

Worked both on brass and mild steel.

Bernd

[Stilldrillin]

I've always just ground the front clearence and left the top at O°.

Worked both on brass and mild steel.

Bernd

That's always worked for me...... 

David D

[mgj]

Yessssss - and I bet you two have never suffered from jam ups either?

One has to watch some tolholders that tip the blade up at an agle. You'd have to grind a few degrees negative to end up with zero true.

[Bernd]

mgj,

I've suffered more jam ups than I care to remember. I totaly wiped out one blade with automatic feed. By the time I got it stoped the chuck had stopped turning. In other words I had stalled the lathe, but not before there was a teribble crunching noise and parts of the blade went flying around. I've even seen it happen to proffesionals were I used to work.

Bernd

[mgj]

Oh - I thought that only happened to me. 

Well I have got over it - at least for the last 6 years!!!!

[loply]

Well the HSS blade arrived but unfortunately it is tapered on both sides and so won't work in my Glanze holder!

I presumed it would be a straight blade as it wasn't described as tapered. Dunno if it's worth sending back for £5. Anybody want a 3/4" tall 6" long HSS blade?

I think I'll just grind a tool from a 12mm blank tonight instead... Bah.