Plasma cutter feed rate

I have a contoured part I cut out of 1/2 inch
thick 1018 cold rolled steel. The circumference
of the part is 18 inches and it's taking about
13 minutes on a CNC Mill with no oil, that's the
tool friendly speed making 3 .175 deep cuts and
one finishing pass. I was wonder how long it would take a CNC plasma cutter to rough one out
so all I would need to do is a 2 minute clean
up run on a mill. Also how much would such a
unit cost? The part has 6 radii on it from
1" to 3". I have never even seen a plasma cutter
but since this is my own product a $5,000 or
less capital investment will yield dividends.
Right now there is no room for a machining
center with flood coolant. Any advice or
thoughts will be appreciated, thanks,.....Bob

A decent CNC plasma cutter is not cheap- one that will cut these parts out is probably gonna run a minimum of 30k, more like 50K if you plan on mostly running 1/2" thick material, as the $30,000 machines are mostly for sheet metal, used by HVAC companies to do ductwork.

So if you were thinking of spending $5000 to do it in house, another plan might be in order.
Speed of plasma cutting is dependant on the machine, which has two parts- the power supply, and the actual cutting machine. With my low end machine, and a cheap ($2500) plasma cutter (thats the power supply and torch) I run 1/2" at around 15-20 inches per minute. A bigger, high definition power supply would probably let you up the speed, but then you are talking $10,000 and up just for the power supply.
Hand cutting parts like this is theoretically possible, but I dont see how it would save any time- the edge would be rough enough, and would have to be so much oversize, that you would have to do the entire CNC machining step anyway, only because the heat affected zone is always hardened, you would probably chew thru more end mills, and have to go slower.

So in short, unless you send it out and have someone else do it, plasma is not going to solve your problem in your budget.
You could buy a nice $100,000 waterjet machine that would do this, but my guess is for that kind of money you would rather just buy a bigger, badder VMC.

One down and dirty solution is to buy a hand plasma cutter, and mount the cutting head firmly to a bar that is bolted on the mill table. The torch is on a bar hanging out in space in front of the mill.

Now put a table under the cutting head and put your 1/2 plate under it....

Ignite the torch, hit 'cycle start' and stand back. You can use a M2 or M3 code to pierce a hole, then start a short path to your circle.

That would work- there is a guy over on the HSM forum who did that. But make sure you build some kind of a barrier between the plasma and the machine- plasma makes really nasty grit- the sparks which come off the bottom are actually grit, when they cool down, and they are very hard and very abrasive, and fly everywhere. Not a good thing to get near delicate machine ways.

You still have the problem of the heat affected zone, and the fact that due to the conical shape of the plasma flame, the cut is not straight up and down, but angled a few degrees.
So you would have to chop off maybe .10 with the mill, all around. You would want to pierce the start hole outside of the line of cut, as well, as it is bigger around than the cut kerf.

To cut 1/2" plate like that, you want at least a 60 or 75 amp plasma cutter.
This one, for example, is rated at only 50% duty cycle on 1/2" plate-
http://www.welders-direct.com/merchant2/merchant.mv?Screen=PROD&Store_Code=WD&Product_Code=083178
or this one-
http://www.welders-direct.com/merchant2/merchant.mv?Screen=PROD&Store_Code=WD&Product_Code=1-1320-1

I would get prices for having somebody else either plasma or waterjet cut the blanks for you- waterjet has the advantage of no HAZ, so no hardening of the area you need to mill.
Plasma runs at 45,000 degrees, so anything that can harden, does. Plasma kerf is HARD stuff, eats right thru grinding wheels and sandpaper. I have only milled it a time or two, but it is hard- I try to grind it first before milling. With carbide on a CNC mill, though, it should be ok.

Can you sub out the cutting? Just for reference, the 200 amp plasma that I operate runs at 60 to 65 ipm. I spend way more time unloading the table or fishing small parts out of the water table than actually cutting sheet. I am supposed to get 2 new helpers to unload while I keep the torch running. I am not holding my breath though. I am running a Linde CNC running Centricut software.

The steel doesn't have to be directly hardenable for the plasma cutter to harden the edge. I don't know for sure whether the edge ends up nitrided from the exposure to the nitrogen in air, or what the process is, but the edge will be hard even in 1010, and its not slag on the edge, but rather the steel itself is hardened several thousandths deep. It will eat endmills like there's no tomorrow. I spent good money to find this out

You might want to look around for a flame cutter. They won't operate quite as cheap as a plasma cutter, but will easily burn your contoured part in a couple minutes or less, and the edge won't be hardened in the process. Optical tracer type cutters are widely available and cheap these days. The safest ones to buy are the ones with a Westinghouse tracer on them. Koike-Aaronson bought the rights to continue building the tracer from Westinghouse-Canada a few years ago, so they're still in production and parts are available. Mine is an Airco, 20-25 yrs old, and its as trouble free as any tool I have. Its set up to run on oxy and MAPP instead of acetylene. Like any burner, it will get right on thru some oxy on heavy cuts, but the fuel gas cost with the MAPP is dirt cheap as compared to acetylene. Lightest thing I've cut was 11ga, and the heaviest was 5" plate.

A flame cutter, run by someone who gives a crap how the parts turn out (like the guy who's paying for them) will produce some nice parts. OTOH, blanks from a steel warehouse tend to be somewhat hit or miss, mostly miss, as far as being able to have them cut close to final size. I had two burnouts a couple weeks ago for a lathe job, burned from 3" plate. Had a center hole blanked out at 3 1/4" to finish at 4". The damn holes were so far off center that one of them cleaned up with .030 to spare, while the second one was probably 3/16 shy of a clean bore when I quit on it at 4". Next time the job comes up, I'll just get the circles and pop a hole in with a 3.5" spade drill since the first op on these is done in a W&S 4A before going to a turning center for the bulk of turning.

Added: Most burning machines have at least two heads, so you can burn two blanks per cycle.

First of all a tip of the cap to Damonfg as I
took his advice and bought a Lincoln buzzbox
for welding and it fit the bill perfectly.
Would have thanked him with an e-mail over his
advice a month ago but it wasn't listed. Thanks
for the sound advice, since this is a new product
of mine, they sell for $50 each and I "hope"
to sell 40 or so a month (I have sold only two
so far), it appears I should just keep making
them the same way, I could cut the time in 1/2
with a flood coolant VMC and throw 4 vises on it
to boot. Don't want to farm anything out as I sell to a cut throat industry and have had
cheap copies of my products kill sales before
when I thought I was selling at a fair price.
I also seem to have no luck when subbing out
work, too many people in Southern Cali talk
the talk but don't walk the walk. Thanks for
the education guys,....................Bob

Dualkit,
Good luck and thank you!

insert mill running dry (if your machine has the stones to turn it)