Hard turning, slightly interrupted cut, no CBN?

[Flynth]

I'm looking for an alternative to CBN (too pricey) to turn down annealed chrome coatings off from ex-hydraulic rods. I estimate the hardness (based on feeling it with a file) at between 50~60hrc (more towards the low likely). The cut is interrupted, because there are often gouges on the rods, usually ground a bit to make tool re-engagement ramp in easier.

All of this is with CNMG 12404 inserts.

What I tried do far:
1.
2 normal steel turning inserts coated, running them at low sfpm (45sfpm, 10 thou depth of cut, 2 thou per rev feed). Both inserts turned fairly well giving blue curly chips, edge failure was due to chipping. First insert fairly quickly (within 2 minutes) the other lasted good 15 min or so. Chipping wasn't catastrophic so I kept turning with the second chipped insert and I did manage finish the section I wanted to cut off.

Unfortunately, in the process I used up 3 edges (already dull, but not chipped). This is too expensive to be useful.
2.
Special PVD coated carbide "for up to 65hrc" described as "best toughness for interrupted cuts" etc. I tried VP15TF by Mitsubishi and IC908 by ISCAR. Why those two? Because I could buy individual inserts, not a whole pack. Both manufacturers specify roughly the same parameters 120sfpm, 10 thou deep, 2 thou per rev should be pretty gentle.

Mitsubishi insert chipped within the first minute. I changed parameters slightly (raised speed, lowered depth of cut), the second edge chipped as fast.

Iscar's insert completed 2 passes (of an inch long section) successfully, but looking at it with magnification it has a tiny chip on end cutting edge! (not on the leading edge). As it is already chipped I don't think it'll last long.

So I'm looking for suggestions that don't involve CBN. I saw online someone using ceramic grade Ab30 from Ingersol successfully, but those are unavailable here in Europe unless one wants to buy a whole pack.

Looking at insert catalogs it seems all manufacturers try to push their expensive CBN inserts for hard turning hiding other options fairly well. It took me a lot of digging to find that those two carbide grades I tried so far are for hard turning.

So does anyone know any other insert grade that can help? Some ceramics, perhaps?

I haven't tried hss, I assumed it would get obliterated instantly, but maybe someone knows some way?

 

[Nutfarmer]

Not sure what you are working on. I have had good luck with turning chrome plated hydraulic rods by turning deep enough to cut under the chrome. The chrome will peal off and allow the base metal to be machined as normal.

 

[rodm1]

It starts at 9:00 mark ceramic at 240RPM with coolent depth of cut 1/4 of normel roughing pass maybe.

 

[Flynth]

It starts at 9:00 mark ceramic at 240RPM with coolent depth of cut 1/4 of normel roughing pass maybe.

He's using the ab30 material I can't get around here unfortunately. You can clearly see it written on his insert. Also I think he mentioned it. BTW, that's one of my favourite machining channels on YouTube.

Not sure what you are working on. I have had good luck with turning chrome plated hydraulic rods by turning deep enough to cut under the chrome. The chrome will peal off and allow the base metal to be machined as normal.

It looks like a piece of a hydraulic rod slightly under 4in in diameter. I got it from a local scrapyard. I originally thought it was case hardened as it was rusty, I thought chrome doesn't rust, so I heated it up to red hot(980 deg C) in a furnace and I let it cool down slowly with the furnace. It did get softer, but the top 40~60 thou is still pretty hard. A normal file marrs it, but not a lot. I assumed it is chrome, but I really don't know. The fact it has rust seems to be a counter argument, but what else could it be that remains hard after heating red hot? So I've been treating it like chrome. I managed to make a number of things from the inside of that rod. It machines very nicely once that hard layer is removed.

There is no way I can get under that layer in one pass with the hardness and all. I can literally see my lathe moving when I take an interrupted cut bigger than about 20 thou. Mind that I can take a cut 10 times that in mild steel.

My lathe is an old machine(made in 1960s), worn appropriately to it's age, but I can still machine effieciently unhardened steels, annealed silver steel and anything else I tried except this. I had a theory that perhaps the wear of the bed, the support and cross slide all add together and it causes the work to push the tool below center then grab and crush it, but that's not it. When I see it flexing I observed it very closely and the tool moves back, not down. This is using a 20mm tool holder with a cnmg 12404 insert, the lathe is a Polish make approximately 16in swing, bed is about 4ft. It weights slightly under 2 tons.

If this rod was nice and smooth I probably wouldn't have that much of a problem (I'm yet to find out, I have another one that is mostly smooth). It is the interrupted nature of the cut that messes it up.

How thick was the chrome layer on the rods you machined? I estimate mine to be 40 to 60 thou.

I have to say after I got under one of the gouges so there was no interrupted cut anymore the iscar ic908 was cutting well. I upped the speed to max recommended 150sfpm and the chip was red hot. I kept the depth of cut at 20 thou and feed at 2 thou. This is with the edge already chipped from the interrupted cut before. Perhaps I'm asking too much of carbide?

That's why I'm asking for alternatives.

Edit: I did buy a cheap(er) Chinese CBN insert some time ago, but I keep it for an occasion I have to turn down a tiny bit of some really hard stuff. Does anyone have any experience with cheap Chinese CBN? Is it possible to use them on an interrupted cut?

 

[Jake M]

You're fighting two different things with the material. The chrome isn't thick, it's just a couple of thousandths of an inch. Below that will be an induction hardened layer. That layer is actually visible as a discolored ring around the outside of a clean cut in the rod.

Interrupted cuts in this stuff require incredible rigidity (at least in my world it qualifies as that). The chrome is hard, and is just about murder on most cutting tools. And the hardened part really wants a different tool. The folks making or repairing these cylinders use lathes with horsepower that will allow a carbide insert to get under the chrome, AND under the hardened layer (usually in the neighborhood of 0.040 inches deep), and at a very significant feed rate, will get rid of both in one pass. You won't find them cutting damaged rod that way though.

Cylinder rods are mystery metal. Unless it's high end stuff, it's kind of safe to guess that it's a 10XX steel, although that's a pretty big range. The steel is not typically specified, and they're built to a performance point. That means they're made out of whatever suitable material can be bought by the train car load, made into the finished rod, plated, hardened to spec, etc, for the best price on any given day.

In general, I find that if you're going to try to "anneal" the induction hardened layer, it's a crap shoot. You have to heat the rod ALL THE WAY THROUGH, otherwise it acts as it's own heat sink, and you're kind of likely to make the problem worse by cooling it too quickly. On the best of days, it's not really annealed, as the outside never gets to be quite the same as the inside. No rules are fast here, as you truly don't know exactly what it is, except in rare circumstances. I gave up trying, and just take it as it is, and it seems to make the result at the cutting tool "more" consistent. I didn't say consistent...... I said "more" consistent.

When you say you can see the lathe moving under the cut... Well, so can I.... And that's a problem. Nothing hard enough to cut up cylinder rods is really going to appreciate a lack of rigidity that one can actually see. That puts the onus on you to get it to cut freely enough that it doesn't push the lathe around. Starting with the insert. And as you're cutting, keep in mind that this material will work harden. You need to cut below that with the cutting edge. If I were able to get a 20 thousandths depth of cut at 0.002 inches per rev, I'd probably want to switch that around to see if i couldn't get ten or fifteen thousandths at 0.003 or 0.004 per rev. I'm not sure what you've got, but it might take a different edge geometry or nose radius than what you're using. That stuff (especially the outer hardened layer) really doesn't like fine cuts. Inside of the hardened layer, life gets better.

As of right now, I've got a system that pretty much works for me. It's still a chore, but with the cost of "new metal" these days... I can afford a chore. I take the chrome off with the most adorable little ceramic insert holder you've ever seen, from the Sherline lathe people, and I usually go about 0.005 inches deep on a well centered rod. That removes the chrome. I take it off whatever chunk of material I can get in the lathe, before it's cut to length if that's possible. Once it''s set up, running and cutting good..... It'll cut all day without issue. the only "hard" part there is it has to be clocked in so that that insert NEVER comes out of the chrome, over the whole length of the piece. After that, I'll cut off what I'm using currently, and leave the partially peeled piece in the scrap bin inventory, so I won't have to do that setup eighteen times. The next "underlayer" comes off fairly well, even hardened, with standard non-descript, generic import carbide inserts. You've just got to play with the depth of cut, to make sure you're feeding fast enough, even at the expense of depth of cut. Once you get both the X and the Z component of the cut to happen underneath the work hardened surface, it goes WAY better.

And the last part, the part you're not going to want to hear.... If scrap rods were anything easy to work with, the damaged ones would be all over eBay, with all the other cuts and drops that are up there for sale as useful material to "somebody". Anything that's damaged to the point of causing an interrupted cut, I cut that part out, and toss it. I have found, that for me, I just don't have a rigid enough lathe to handle that kind of hammering, no matter what kind of tools I throw at it. The tools and tool damage add up to the point that I could buy new material and come out ahead. I get great results (after an admittedly prolonged and expensive learning curve) when I "save" the straight and undamaged portions, but the damaged stuff, not so much so. Peeled, broken chrome? No problem. (Well, maybe "not much" problem). Gouges, scratches, or other damage that's deep enough I can't get below it on the first cut? The dollars and cents just don't add up, it's gonna cost me an insert or two. Or five. I will say this though- If we start with the premise that the lathe in question is at the limits of it's rigidity doing this, which is NOT how it's done in industry...- If you've already fought through the chrome and the hardened layer on a damaged rod, a good grade of high speed steel cutting tool, with very modest back rake, and a fairly high side rake will take an interrupted cut pretty OK in the parts of the rod below the outer hardened part of almost any hydraulic cylinder rod. And once it's cylindrical again, life gets a lot easier regardless of what cutter you throw at it.

 

[Flynth]

Unless someone comes up with some miracle process I think I'll have to agree with you rods damaged yo the point of causing interrupted cut are not worth it.

It is very likely in my attempts to anneal that rod I haven't heated it to a temperature high enough. 930C looks orange. I kept it at this temp for 2 hours (it took about 3h to get to this temperature). Then I left it to cool in the furnace with door closed. I think it was well heated through, but evidently I didn't have enough of a temperature.

My other findings seem to be very similar to your experience.

Now I wonder about that Chinese CBN stuff. Perhaps not for interrupted cuts though. Maybe that requires it's own thread. If anyone has any experience with it please let me know.

 

[Jake M]

Now I wonder about that Chinese CBN stuff. Perhaps not for interrupted cuts though. Maybe that requires it's own thread. If anyone has any experience with it please let me know.

I wish I had something to add to that thread. I am very curious. These cylinder rods are absolutely not ideal raw stock, but it's very stout material once you peel 'em, and they are readily available to me often enough that I can not over look them.

 

[Jake M]

Aww.... See what you did? You just gave me the perfect opportunity to sidestep any blame for what's arguably bound to be another bad decision. I'm calling this your fault.

Screw it... I'm not sure what might be prudent in terms of nose radius, so I just went with what usuallly works well, and ordered a couple each of (literally) the cheapest ones on eBay that clearly state they are for steel, AND will fit in a tool holder I already own. (There are cheaper ones by a whisker, but I need that crutch...) They come from "overseas". That's literally where they say they originate. (All seas lead to where?) Two weeks to a month for shipping. Horray..... But they were CHEAP..... DCMT070204 and DCMT070202. For what I paid, I half way expect those inserts to be painted bright red, with braise blobs hanging out so far that I'll have to grind it off to get the tip to contact the workpiece.... But we'll see. Whenever they feel like mailing them.

 

[Flynth]

Aww.... See what you did? You just gave me the perfect opportunity to sidestep any blame for what's arguably bound to be another bad decision. I'm calling this your fault.

Screw it... I'm not sure what might be prudent in terms of nose radius, so I just went with what usuallly works well, and ordered a couple each of (literally) the cheapest ones on eBay that clearly state they are for steel, AND will fit in a tool holder I already own. (There are cheaper ones by a whisker, but I need that crutch...) They come from "overseas". That's literally where they say they originate. (All seas lead to where?) Two weeks to a month for shipping. Horray..... But they were CHEAP..... DCMT070204 and DCMT070202. For what I paid, I half way expect those inserts to be painted bright red, with braise blobs hanging out so far that I'll have to grind it off to get the tip to contact the workpiece.... But we'll see. Whenever they feel like mailing them.

I hope they work. I have two of cnmg 12408.

One thing to watch for in very cheap inserts like this(I just noticed it when I went to check prices now) is that they come with varying number of edges. The very cheapest have only one cutting edge. More expensive ones have 2, even 4. When looked at up close the edges look like the insert is a tool holder, and the edge is a tiny insert with a padding insert beneath. I wonder if it's brased in place.

One thing that is a bit peculiar is that (presumably) 45 degree chamfer on the cutting edge. I heard CBN removes material by friction, but such positive rake? It seems insane. Well, I can't test them too soon as my lathe is otherwise occupied, but I will at some point.

One observation I can add to this thread is before I got those special grades of carbide to test I used dull steel inserts. One was a cheapest chinesium grade (up to P20 steel) with a simplest chip breaker in a form of half cylinder depression (positive rake). Another one was much better (for P20-P40 steels, M20-M40 stainless) made by mid-tier reputable brand (Polish Baildon). When I say those edges were dull, they were already used to turn steel and wouldn't leave a nice finish, but there was no visible chipping etc. The better carbide chipped a lot sooner than the cheaper one. Those 3 edges I used were 2 of the better carbide and 1 of the cheaper and that final edge lasted way longer. In fact I finished to peel what J wanted with it.

So the lessons learned are, use the cheapest steel carbide for interrupted cuts.

Also when the edge chipped it continued to cut for a good 5 minutes till the end. Based on the shape of the insert ended up with I suspect an insert with a larger nose radius would be better for such stuff.

Regarding viability, I think the rest of that damaged rod will go into my "large steel scrap barrel". I'd rather leave my dull carbide inserts for regrinding than chip them. Once they're chipped a lot not much can be done with them.

 

[682bear]

I've used CBN inserts a lot on hard materials... they do not do well at all on interrupted cuts. I would almost guarantee HSS would last longer than the CBN inserts...

That is my experience, anyway... about the second time it hits the interruption, the tip will break off. They are great on a smooth cut...

I'll second the suggestion to cut deep enough to get under the chrome layer.

I once watched my father machine an Enfield .303 barrel... it had a hard (chrome?) coating... he machined the coating off with HSS by getting 'under' the coating on the first pass... it worked, but didn't leave a good finish. His second pass cleaned the finish up pretty nicely.

-Bear