How surface hard does plasma cutting make steel?

You will probably say I deserve everything I get when ordering metal over eBay. I needed a chunky steel disc (120mm by 15mm thick) and I took a punt by ordering a plasma cut disc on eBay.

It was cheap (and free postage, of course) but had a really rough, uneven edge and even a notch on the periphery. An awkward size for my chinese lathe with 10mm chuck and I had to make up a plate to hold it. All good fun and drilling the disc for fixing screws went well and very accurate for me!!!

I started to radius it but the indexed turning tool just seemed to bounce off it without really cutting the surface. I did make a bit of progress, though. I then tried a HSS tool and that actually cut quite impressively but the edge didn't last for just one pass; It was just worn down. Eventually, I dug out a brazed carbide tipped tool and that finished the job. Cut depth needed to be about 0.05mm in order not to stall the motor but I can accept that.

Once I cut below the heated level, the steel was more manageable. I realise I was asking the lathe to punch above its weight but this workpiece was so tough. I have to conclude that the plasma cutting had put in some surface hardness.

A Google search does tell me is can be a problem but it doesn't;t seem to be a common forum topic. What's the general opinion (not about using eBay) I only paid £7 for the blank). Metals For U is great value when you start to buy a lot but a single piece does cost you.

My feedback to eBay won't be gushy but I know that was my fault and there are some good deals there/

It's usually a good idea to grind off any slag on the edges of plasma or flame cut or laser cut steel disks before trying to turn them on a lathe. Even if the slag is not too hard, the interrupted cut along the rough serrated edges is hard on the lathe.

The hardness of the slag depends on the carbon content of the steel. Plain carbon or high carbon steels can have very hard slag, ie 60 Rc or higher. Mild steel (low carbon steel) slag is usually only a little harder than the parent steel as it is not hardenable except by case hardening.

High heat always does things to metal

Welds can be a nightmare to drill through, very hard at times

Slow speed on the backgear if it is available is one route on a lathe

Good evening, I've played this game too, and I think you have several difficulties compounding the problem.

Firstly the disc you bought had a tag in it on the periphery. This tells us it's likely the offcut from cutting a hole - so that's why it's cheap. Cheap it may have been, but you would have been much happier with a clean disc,

The plasma cutting action of itself (assuming it's just mild steel) won't harden the disc. That's not the problem, the issue is that the oxide layer (and the snotty grotty bits) left on the outside is (b$$$$y) awful stuff, and your poor little lathe did well to tackle it. Buying a blank which is clean cut on the outside will make your life infinitely easier.

That said, cleaning up the oxide layer with an HSS tool is a recipe for going nowhere. You need a carbide cutting edge of some kind, and inserted tools are good but there ain't no substitute for horsepower. Sorry, but that's cold comfort in your case. For some reason brazed tools are often better at the brutal business of getting back to clean metal, it's all in the composition of the carbide edge. That is as much as anything about the cost per edge of an inserted cutter against the re-sharpenability of a brazed cutter.

You can do an awful lot to make it easier by tackling the periphery with a grinder of some description - a bench grinder is good, an angle grinder with a linishing disc (sanding disc - but coarse and aggressive and a hard abrasive) - is better if only because it saves ruining the shape of the grinding wheel. A belt sander will do a good job.

Essentially the moral of the story is - as ever - you gets what you pays for. If you can find a supplier who DOESN'T stick you with a in-cut on the edge (where the plasma pierced the sheet metal before setting off on the circular path) you will get on better, a few minutes work with some sort of grinder will save you a whole bag of grief, and then get in there with a tool which won't burn out (even HSS will) if you approach the edge at a ridiculously high surface speed. If I buy a cut blank I expect it to be clean edges and would get very chopsy if it had a cut in the edge I had to machine away.

Good luck,

All the best Simon

I've recently bought some 145/180 10mm rings, possibly same supplier. Happy with cost and material, which is structural steel. Machined nicely on my Clarke CL500 with a mid range insert; the somewhat rough tapered edge cleaned up much better than I expected. Drilled and tapped a few 3/8 bsf and 8mm holes just fine. I have the discs out of the centre for projects, and the same tool cleaned up the peripheral notch, without damaging the cutting edge. Parts now on a vintage 1926 car, but pic attached.

I've recently bought some 145/180 10mm rings, possibly same supplier. Happy with cost and material, which is structural steel. Machined nicely on my Clarke CL500 with a mid range insert; the somewhat rough tapered edge cleaned up much better than I expected. Drilled and tapped a few 3/8 bsf and 8mm holes just fine. I have the discs out of the centre for projects, and the same tool cleaned up the peripheral notch, without damaging the cutting edge. Parts now on a vintage 1926 car, but pic attached.

They probably used compressed air for the plasma gas, its the cheapest option. The surface hardening is nitriding caused by nitrogen in the gas. A few passes with a flap disc usually gets rid of it. If you need a softer edge on plasma cut parts then it would require pure oxygen for the plasma gas. The notch in the edge of the disc is probably the pierce point, sound like they didnt use a lead in the start the cut properly.

At the end of the day, plasma cutting and flame cutting etc are industrial processes, presumably expected to be machining with industrial sized machinery, which of course wouldn’t bat an eylid at a bit of a hardened skin. So not the suppliers fault in any way. Best way to get through it for a hobbyist would be to hit it with an angle grinder before attempting turning. As for the blow hole, just order a slightly oversized disc.

You can see 'my' start in the centre disc. £10 extra for bespoke cut, regardless of number.

I find this sort of clean up is an ideal job for the obtuse "unused" corners on CCMT inserts. I've always got a stash of inserts with the proper corners blunted from normal use so I don't care if a hard edge or similar eats the obtuse corners in fairly short order.

Even the pros can ahve problems with plasma cut material.

Clive

@Simon Williams. You said it!!. A double / triple and quadruple whammy. But a lesson learned. Thanks a lot for the reply. This brazed carbide tools haven't been a lot of use since I bought them with the lathe but this time, although slow, it did the job. AS you say, with a bit of power behind them, they could shift material in a brute force sort of way

I did actually wonder about that hole and thought "If I was cutting out a disc, I'd put the hole on the other side of the circle." haha. I will put that in my review!! But I need slots in the edge so That hole can locate one of them.

I was wondering about how to grind the edge - just to get it more circular with no conical distortion - but I couldn't think how to do it. I considered my little dremmel style tool, clamped to the cross slide but flying grit into the lathe scares me and the burrs are so fragile. Freehand would be hard but I could have cleaned up the burned looking bits. I should have taken a photo of it before.

And thanks for the other answers chaps.

Edited By andrew lyner on 16/12/2020 22:54:13