Turning, CBN, Induction Hardened Parts...

I'm hoping to mine others' experience here, specifically to see if there's anyone out there with success in turning softer steels by using CBN inserts.

The Background...

We have a long-running production job, where parts are induction-hardened, then brought in for finish-turning.

Trouble is, the parts are not hardened along their entire length, so there are hard & soft zones to turn through. Soft zones is 20-26 HRC. Hard zones are 50-55 HRC.

So far, our tried & true workhorse has been a carbide insert, which is designed for turning in situations such as these. Seco TH1000, MF2 chip-breaker. These inserts require a slower cutting speed (SFM) due to being carbide, and the wear-resistant coating being relatively thin. If we could cut these parts faster, obviously that's a win...


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Today's challenge...

Today however, we're running a similar part where the soft material zones are 30-35hrc, and again, hard zones are 50-55hrc. Tool life with the TH1000 insert is drastically lower, while using an even slower cutting speed.

Today, I dug through some old sample inserts, and found a Sumitomo chip-breaking CBN insert. Tossed it in, 750sfm across the hard & soft zones, dry cutting.

Interestingly, the tool cut well. It did not crumble the edge in the soft material, as we've seen with other CBN trials in the long-running part. Surface sheen/finish looked great. And despite dry-turning, the part stayed cool - never getting above 'just barely above ambient temperature to the the touch.'

But, sizing was a mess... Diameters were all over the place across the different part features. Adjusting the offsets to bring the part into spec, showed even more variability, and un-even stock removal compared to the previous cut. The next part was just the same. Great finish, good insert performance, terrible size control.

Total stock for finish turning was .020"/.5mm per side.




We get excellent consistency with the TH1000 grade, so this un-predictability with the CBN tool was unexpected.





Question #1 - Has anyone on here had luck using CBN tools on softer material?

Question #2 - For those with experience turning hardened steels with CBN - How much stock do you leave for finishing?

Same nose rad as the carbide? I will assume same shape and holder rakes.
Did the hard sections hold size as moved or did all go nuts? IE: soft vs hard size control or everything all over. How much?
Edge prep (t-land) on the Sumi? Once you went CBN you went a lot negative top on that soft stuff with a catalog top edge.

Bob

That is weird that the sizes are moving around. As CarbideBob said, the CBN with the T-land is very negative. Any chance the machine being used is sloppy or significantly lacking rigidity? I'm thinking the negative cutting action could be pushing the turret away from the part.

I've never cut soft steel with CBN. When cutting hard steel, I think we leave only around .005"-.010" per side.

Would you consider grinding these parts instead of turning them?

The inconsistency in size holding could be from the part prep prior to this operation meaning inconsistent amount of stock removal or hardness ranging from part to part. With CBN tooling the cutting forces are higher so tool defection can become an issue. I would suggest trying it again with a 2 cut strategy , 1st cut to semi-finish the diameters and then 2nd cut to finish. Rule of thumb is to leave the 1/2 the T-land leg distance as the finish DOC (IE: .01" x 25° T-Land leave .005" per side)as a starting point. Usually for CBN to be effective the material should be over 45Rc however I have seen some instances where it will perform ok in softer material. You may want to consider looking into a Cermet material type of insert

Single pass on this part or a cut and then second to size?
CBN should cut dead soft just fine, the problem here is carbide is so much cheaper and more cost effective.
I admit to being very confused on the size control problem. ....
Bob

CarbideBob said:

Same nose rad as the carbide? I will assume same shape and holder rakes.
Did the hard sections hold size as moved or did all go nuts? IE: soft vs hard size control or everything all over. How much?
Edge prep (t-land) on the Sumi? Once you went CBN you went a lot negative top on that soft stuff with a catalog top edge.

Bob

Click to expand...

Bob, same nose radius, same holder, just swapped inserts.

The TH1000-MF2 Carbide insert, has a .002" x 5* negative land on it's cutting edge. Cutting depth is .020" per side.

I looked up the Sumi CBN insert in the catalog this morning. Has a .12mm/.005" x 35* negative land, and a .015" max cutting depth. So in hind sight, I was trying to cut too deep with it, (although the actual cutting edge measures around .085" long, so go figure...) It also has a MASSIVE negative angle too, which is slightly surprising to me.

FWIW - I picked THIS particular insert, because out of my stack of test tools, it had (4) unused corners, and a chip-breaker. My thought was that that chip breaker may perform better in the softer material, giving a *little bit* of an up-sharp edge to actually 'cut' the softer metal. I guess I was half-right at least. The edge didn't crumble in the softer material.




In reference to the machine - It's a Mazak QTS350, A2-11 spindle nose, part is held in beautifully cut pie jaws, with a center in the end. Doesn't get much more rigid than this, for a part this size.

When we use the carbide inserts, tool pressure is very predictable. There are no major jumps in size as the tool enters/exits hard material. Nothing more than 3-5 microns. Taper across the length of the part is consistent as well.

When I went to the CBN, I did not notice changes in size across hard/soft zones, but I DID have to adjust for taper across the length of the part. When I offset the tool to approach final size, I had to re-adjust taper again - This is not the case when we use the carbide insert.

The inconsistent sizing was enough for me to stop the test.





In the past, this inconsistent sizing was also an issue with our CBN tests in the softer, long-running part. We went to two-pass cutting, but could never make tool life.

I could go to two-pass cutting now. It might not yield a major cycle time improvement, but if the tool life is longer, that means fewer adjustments, which is a win. (Hopefully.)





I'm thinking however that I need to find a chip-breaking CBN tool, with a less-negative land. My thoughts being that the chip-breaker will still offer a somewhat up-sharp edge for cutting the softer material, and the not-so-negative land will offer more predictable tool-pressure related cutting conditions. Perhaps I should look at a positive/single-sided holder too, for less tool-pressure.

^ Let me know your feedback. Thanks all.

You went 30 degrees more negative on the shear plane and it then pushes???
You know better than this..... edge hone?
Look at the produced chips under a stereoscope from the carbide and CBN insert. They talk and tell.
Bob

CarbideBob said:

You went 30 degrees more negative on the shear plane and it then pushes???
You know better than this..... edge hone?
Look at the chips under a scope from the carbide and CBN insert. They talk.
Bob

Click to expand...

Haha

Fair, Fair...

This was a last minute thought, so perhaps some grace may be given. I had to change the machine over, and thought, let's see what we have that we could try out...


Next time, I'll do my homework FIRST.

Jashley73 said:

I looked up the Sumi CBN insert in the catalog this morning. Has a .12mm/.005" x 35* negative land, and a .015" max cutting depth. So in hind sight, I was trying to cut too deep with it, (although the actual cutting edge measures around .085" long, so go figure...)

Click to expand...

I program CBN at 0.004" DOC per side. There's some variability due to parts not being perfectly round due to heat treat distortion, but that's the ballpark. Insert is Sumitomo grade BN600.

I don't think DOC matching the nose radius applies to CBN. It would be a nonstarter anyway in machining case hardened parts where the case depth is only 0.020ish to begin with.

CarbideBob said:

I admit to being very confused on the size control problem. ....
Bob

Click to expand...

Tool pressure maybe? More pressure in the harder areas?

Jashley73 said:

This was a last minute thought, so perhaps some grace may be given.

Click to expand...

No, I will not cut you any slack. You know better.
The cut metal removed. Look at this.
Many think it weird that the first thing i want to see is a bag of of chips or strings.
Bob