Not a bad idea Goztrider, but the prob is having to cut where it's been hardened. Woulda probably ate up the brake lathe cutters as well. Fwiw - I have three grades of carbide TPG-321 inserts. C2, C6 and some that I don't know what they were, found them in the bottom of an old machinist's toolbox I bought at a yard sale. The C-6 went away in a hurry on the interupted cuts and I found that for regular cuts it works ok, but the mystery carbide inserts did pretty good when using the edge and not the point when cutting the same stainless on the face of the plate. Even so, they'd probably go away cutting the edge in a normal manner. One place I lucked out was I can get away with a 5.750 OD instead of the 6.000 I thought I needed.
The crusty brown and black "dross" leftover from a plasma cut on thick stainless is really hard and brittle and abrasive, and it really chews up cutting tools. I found that using a grinder with a 4-1/2" zirconia abrasive flap wheel works pretty well to grind the rough discolored edge back to clean stainless. Zirconia has sharper grains that cut faster and don't heat up the surface as much as the usual aluminum oxide abrasive, and the flap wheels also help to keep the stainless cool. A normal grinder can tend to heat up the stainless and turn it blue, but zirconia just cuts it away clean without heating it up too much. If you have to machine or drill or saw through the edge of something that's been plasma cut, it's worth it to grind all that black **** away first until you get to clean stainless. It's like granite and just destroys the cutting edge of some tools.
It's hard to say how it would work the cutters though, again not knowing the hardness of the stainless - and not understanding either - lol. I know the hardest part of cutting on the brake lathes is the deflection of using just the drum cutting tool. Theoretically, the cutters should hold up to a certain degree as warped brake rotors tend to be hardened - right? If you're only cutting 1/4", then you are looking at having the disc rotor cutters cutting in 1/8" each. Again, the thing would depend on the cutting tips on the brake lathe and how they would react to the stainless.
Cast iron is very easy to cut but very abrasive, so the inserts used to cut it are very hard and abrasion resistant, but not shock resistant at all. If you try to use a cast iron grade tool on work hardened stainless, only sadness and wasted cutting edges will result. The tool will just shatter on the first trip through the interruption in the cut. It'll be hard enough to cut it, but not tough enough to survive the set-up, plus, the edge prep will be all wrong (med-wide land honed onto the edge with a negative rake for cast iron, versus a very keen edge with a high positive rake for the stainless). It's the old "Don't ask a hammer to do a screw-driver's job." thing.
you can call MSC to get a recomendation on a particular tool for your application, I was going to write down the insert numbers of the Seco carbide inserts i was using,but i forgot. These are ductile iron castings that are through hardened, I had the same problem with inserts breaking,until i tried these, the castings are sand cast and rough surfaced,plus the outer layer seems to have some sand in it. I have inserts that will cut the hard abrasive material, but are flexible enough to not break in the process. took some experimenting. the castings hammer the piss out of the tool and insert on the initial first cut. I have also cut stainless with these same inserts.
If you are using stainless plate, it is probably a 301, 302 or 304 alloy. Possibly a 316, which is the mother effer of all stainless steels. (There is a magnetic 400 series, which is readily heat treatable, but not as common in bar stock.) Toss in the interrupted cut and some slag to cut through and you have a job to do. I always tried to get as close to a "near net" shape before I tossed it into a machine tool. I've found that cutoff saws with a flood coolant and a slow speed arer good to cut the corners off first. I did like the suggestion of a rotary table (a whole lotta crankin' would be going on) and using a Bridgeport, but you gots to use what you have on hand. The MSC catalog has a pretty good tutorial on inserts, also the major manufacturers (kennametal, Sandvik, etc) all have help lines and these guys are happy to shoot the **** with you all day. Bob
If you have a DRO on your mill with a bolt circle macro, you could lay out a circle with large ID holes (like 3/4"-1") such that the holes overlap one another (called "chain drilling") along the outside of your plate to hog down the work-hardened area. The larger the OD of the tool you use to do this chain drilling (I would actually use an endmill myself), the smaller the serration cusps left behind on the OD of the part. That would at least remove the hardness part of the challenge, and only leave you with one cut that is interrupted.
you can try using a 3/8 wide parting tool with very lite cuts. take your time and keep the part cool. also try using like a micro 100 cutting tool and try cutting from the side.
Nearly every indexable tooling supplier makes a toolholder that uses parting inserts to do just what you suggest. Some call it face grooving, some call it trepanning, but the operation is the same. The trick is that their tool holders have a radius formed into the blade so it can be taller than the insert and not drag in the kerf. Observe: http://www.kaisertool.com/2004Catalog/cat-2-13_2-14.htm No affiliation with the company, just found them in a Google search.
And stainless work hardens as you cut it ,so each cut needs to be deep enough to get under the hardened skin. Usualy at least .010 better .015. Make life tuff when you need to get another .002 -.004 off a piece.
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