Insert life turning 4140 HT

I'm currently turning some 4140 HT from 22 mm to 10 mm with live centre support at around 100 m/min, DOC 1 mm, can't tell you the feed rate. I've tried various inserts including cermet but am getting terrible insert life, indeed the cermet seems to be worst of all with chipping damage. At 10 mm it is then gets threaded, and the threading insert seems to be holding up ok, but that's at quite low speed as this is all on a manual lathe.

I wonder what sort of insert life I should be getting, as this is getting ridiculous. I'm thinking of trying HSS as at least I can regrind the edge when it's trashed. I'm going through about 1 insert per piece at the moment!

The chips depend on the insert used, but are generally coming off ok, though I'm having issues keeping heat out of the work. The finish is good, though oddly enough the cermet didn't do as well. I've settled on a coated insert, I'd have to check the grade. The chips are definitely affecting the coating however.

did you try faster with cermet?

edit: and then there is cbn.

are you sure about the HT? is it just RC28-32, or... something way harder?

Inspect the worn-out inserts under a loupe and figure out what the wear characteristics are, then adjust cutting condition accordingly:
Wear types

Are you running coolant ? You should if possible.
Increase depth of cut and feedrate if possible.
100m/min is not fast for carbide, but if you don't run coolant it probably is fast enough.

Most of the inserts are being chipped. With the coated inserts sometimes they are being chipped by the chips themselves as they are coming off the chip breaker, so back from the cutting edge, although the cutting edge itself is also taking a beating.

I'm not running coolant, and it would be tricky to do on that machine but I'm increasingly thinking I should switch over to it.

I don't know the hardness, just that it's tough. The chips will form ok, then suddenly start stringing. If I feed faster the finish starts to suffer, but could do that on roughing cuts I guess.

I'm taking this down to 10 mm so run out of RPM, so couldn't increase the sfm. I can feed faster but 1.5 mm doc seemed about all that lathe is capable of in that material. It's a light lathe. It seems happy enough however, and it's not an interrupted cut or anything, just that I look at the insert and see it's being damaged. It's not always evident until I inspect it under the loupe, although other times it's definitely toast.

Here's a couple of photos of the coated insert. I can't make out the writing of the grade so I'll need to confirm that tomorrow. I don't have my loupe with me, but looking at the wear I thing the camera has reversed the image.

Just taking another look at this now, I'm definitely no insert expert, far from it in fact, but it looks to me like it's been running real hot. I'm having a lot of issues with heat with this turning, so that would certainly correspond with what else is happening. I think I'll try reducing the speed a lot and see what effect that has on things. At least while I'd doing the roughing passes anyway. That insert hasn't gone through a lot of 4140 BTW!

At what angle is that insert presented to the work (relative to the axis of the work)? Looks like notch wear on the left side of the first picture. I would lay that puppy down to chip thin while roughing if you can.

I'd say you're using the wrong style chipbreaker for roughing with. Roughing inserts have a flat land at the cutting edge: the heavier cut they are designed for, the wider that land should be. I'd call what you were using, an old style finisher. A roughing or medium roughing insert won't have any trouble taking a cut 3mm deep, that is what I normally expect, and on the manual lathe. Feed should be about .3mm/rev or more for proper performance of the chipbreaker on the proper insert.

I'm seldom running at theoretical SFM for small diameters, but still get good tool life and performance.

Yes, the part is going to get hot if machined dry for several minutes at a time, can't help that.

What brand are the inserts?.

Just to reiterate a couple of points, this is a lightweight 10" lathe, and I'm pretty much always restricted to what would be considered "finishing" cuts and insert profiles on heavier machines. Nothing wrong with that, it just takes longer! A true roughing approach will just overwhelm the machine, it's as simple as that and is what it is. Over the years I've tried various approaches and have had to settle on finishing inserts and as high a rake as I can get away with. The issue on the very high rake tools is normally stringy chips (and a complete non-starter on this material, unremarkably), and sometimes fragile edges. A high rake insert wasn't suitable, no surprises there.

As mentioned I've tried many different inserts, all from reputable manufacturers. The cermets for sure are Kyocera, the others I can't recall. By the same token, my name isn't Pete Sandvik, and I can't just go out to the insert vending machine and push a button to have the ideal insert pushed out. The insert I showed the photo of would be one of my more robust IIRC, but I'll have another scratch around and see what others I may have. I just remembered I have some other holders and various inserts so can give them a run. I don't think the insert per se is the issue to be honest as they're all doing similar things.

The approach depends on the holder, one was diamond insert presented at 90 degrees, so the nose lagged, the other the shorter diamonds and that holder is a turning/facing. I prefer that holder as I need to face with it too.

Yes I know the work will get hot, but it seemed to me that the heat wasn't going in to the chips, not sure why not. Maybe that's just the way 4140 behaves? Don't get me wrong, they weren't exactly causing a local snow alert when they fell, but with that speed and DOC on 1020 for example I'd have a steaming pile of blue poo sitting in the bottom of my pan. This one seemed to be more like a light sabre sitting between the chuck and centre! The pieces aren't exactly glowing white hot, but let's just say I wouldn't want one dropping down my shorts either!

Well I tell you what the problem is and you won't listen but offer up a bunch of lame excuses. I know wtf works, and you don't, obviously. Get a real machine and live a little.

HuFlungDung said:

Well I tell you what the problem is and you won't listen but offer up a bunch of lame excuses. I know wtf works, and you don't, obviously. Get a real machine and live a little.

Click to expand...

So your "suggestion", is for me to trade in this machine and get a larger one so I can run your roughing inserts because you've declared yourself the sole custodian of all knowledge related to machining? Great, I shall do so come the break of dawn if it's ok by you to wait that long.

4140pht isn't anything special.
We run it 800sfpm, .150-.200 d.o.c. and feed at .016 ipm here with Sandvik insets.
Of course this is on a cnc lathe.
What brand and style of inserts are you using?
My guess is that if the insert is chipping, you're going too slow on the rpm.

The flank wear shows you have reached the life of the insert. A slower spindle or different insert may lengthen the time to worn out.

Aside from the insert issues, there is also the possibility the spindle is just turning to fast. Chatter may be what is chipping your cermets, even though you can't hear it or see it in the finish. A skinny diameter at your speeds can often cause trouble. For smaller work you often have to forget about reaching recommended surface speeds and go with where the lathe and workpiece are happy.

I'll have to get back to on the brand as it looks like the inserts aren't in their original boxes. I can say they're KC850 carbide, and I suspect they'd be Kennametal as I like them and went through a run of buying pretty much just Kennametal.

I just tried a few cuts with a slower speed and increased feed. Unfortunately I don't have time to do any more tonight as I need to get busy ordering a flatbed truck to haul this lathe out by dawn, to keep some tosser in Canada happy

I asked for opinions and can assure everyone that I value and appreciate their opinion. But it's already clear that opinions are like assholes, everyone has one and (apparently) they're all a bit different. Or so my doctor tells me. The grown-ups here appreciate that what works for their lathe may not work for my POS. But it's a POS that I have and I've spent many hundreds of hours standing in front of it, so I guess it's here to stay for a while.

I didn't get enough cuts in to really test the longevity of the insert with a slower speed and increased feed, but the interesting thing was the coating was getting whacked off the chip breaker just like in the photos above. The edge looked ok under the loupe however. Gbent I think you're on to something with your suggestions.

As others have pointed out already it's not a difficult material to run. Mtndew's numbers are very close to mine. You won't get anywhere close to that in your lathe but I doubt it will matter particularly.

The biggest killer in your case is the lack of coolant. Turning 4xxx steels dry is a fools errand.

KC850 is an old grade with poor wear resistance by modern standards - soft but strong. Probably a reasonably good choice, given your limited speed and rigidity, but will suffer more than others without coolant.

Put up some screens, flood the cutting edge and go for it.

It's hard to judge the scale of your photo, but the radius of the tip might be a bit too big. If anything my first lathe (ancient, made in Australia "Advance") was even smaller than the Hercus and I ended up with TCMT 110204 finishing inserts which required a very fine feed to get any finish but didn't load up the 1/2 Hp machine too much.

Sorry to bump the thread, but sometimes edits get lost of the thread is already read.

I wondered what causes the coating erosion on the chip breaker? 95% of the turning I do is 1020, some aluminium, and occasionally other "stuff". I agree with the comment above 4140 shouldn't be any biggie, and I don't recall having any great issues in the past. Is the erosion just a factor of the 4140 toughness as it hits the chip breaker? I can't say I've ever noticed this on other materials and especially on brand new inserts after such a short run. like I said, tonight I turned maybe 500 lineal millimetres, so SFA, and I could clearly see the chip breaker was taking a beating. The chips were breaking fine BTW, probably about 1 - 1.5" long, still with no real indication of heat in them. I tried a reduced DOC also of 0.5 mm and it seemed everything was happier. Not real flash, but it is what it is I guess.

Somebody also asked if I was sure it was 4140 HT, and that's definitely possible that it's not. I didn't buy that as a full bar length, and it's possible (but not likely) it's something else. It's definitely tough though and you don't want stringers that's for sure! I need to buy some full lengths (6 metre lengths here in Oz), so that will be the definitive answer on that one, but at this stage I think it is 4140 HT. 99% sure anyway.

We crossed there, so thanks for the advice on the coolant. Yes I'm increasingly thinking I need to get it on that machine. It's set up for it but currently has oil in the tank. I rarely, if ever, use oil as it's real messy. Normally I'm using cermets on 1020 and don't really miss the coolant. It turns spectacularly with them and all the heat goes in the chips.

Incidentally to put this in perspective, as most members here are either from North America or are familiar with US machines, think of a South Bend light 10, and you're in the machine POS category. But I bought it new, and it's accurate, very accurate in fact as I spent a lot of time making it so. So it's what I have to contend with. It gets the job done, but as SAG alluded to, I need to be patient with my cuts

I have a similar problem with the lathe built-in tank/pump. Use a 5 gallon bucket, small submersible pump and divert the return drain back to the bucket.

Check : Make sure you have a way to get coolant cleaned out from your cross slide. I've seen some horrible corrosion of the feed screws etc from water based old coolant getting to where it wasn't supposed to be.