What's a good endmill for cutting stainless plate? (encountering problems)

Post up the sfpm you were running on the tool, and the feed rate. Either your idea of 'slow' for the rpm is incorrect, or you were running the feed rate so slow as to allow the material to work harden under the tool.

Just a guess, there will be others here that will be more accurate.

Jim

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Besides rpm, endmill diameter, depth of cut, and conventional or climb milling should be considered. I like big roughers.

wws

321 is a horse of a different color. When you think you're slow enough, cut the speed in half, and don't even think of letting that cutter idle for even one turn. We made a lot of stuff for aircraft exhaust systems, and 321 is the material of choice. Doesn't make you like it, but does make you slow down. Try different coolants/cutting compounds, sometimes that helps. Carbide cutter may help, just as likely to be more of the same. Might try a cobalt alloy cutter. Rotsa ruck.

Marshall, I'm not familiar with 321, but with 316 the best cutter I have used is from M42. This is an chrome vanadium ISO alloy and it is high in cobalt (5%). Use a very slow speed, a high feed rate, lotsa coolant and small cuts. Take your time. Do not use a dull tool. As soon as the edge starts to fail, change immediately to prevent excessive work hardening. Steve

Sorry for the late reply.

The spindle speed was 200 rpm. Originally the feed rate was 1.5" per minute, but that was when we wrote the program for the mild steel. By the time we were taking a good cut into the metal, we had the feed rate slowed down to about 1/10" an inch per minute.

Keep in mind that we were trying to cut through this 1/2" piece in one pass. It looks like we're going to have to bump up the feed rate and only go half way through the piece on each pass, or maybe even less.

Ok, that's only *part* of the number you need. How large is the end mill diameter? From that you can calculate the *surface* feet per minute that the cutting tool is running at. That's what counts.

Jim

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The feed rate, if it was right originally, should not be that much different. The spindle RPM ought to be much less. But, as Jim said, what's the cutter diameter of your 4-flute end mill?

RPM ~= SFM * 4/(cutter diameter) Feedrate = RPM * feed per tooth * 4 for a 4-flute end-mill

(the recommended feed per tooth also depends on the cutter diameter)

Best regards, Spehro Pefhany

Well, there's your problem! With any material that work hardens (SS is notorious for this, but many other harder materials also do it) you reduce the depth of cut, and may need to INCREASE the feedrate! There's no way you can make this cut in a single pass except with monster machines and very large diameter cutters.

(A horizontal mill could do it in one pass if rigid enough, but due to the nature of the cutting action, a vertical mill ends up producing very thin chips at the sides when plowing at full width, and this is where the work hardening begins to develop.)

Jon

sadly you will have to cut a few inches and replace cutter because the 321 stainless is to tough for it to work

Bullshit . They make solid carbide end mills just for such tough materials . Cobalt EM's too might do the job , I haven't tried mine on that particular alloy but it worked well on some 4140 I cut recently - I'd go with at least

3/8 or 1/2 inch , low rotation speed and fairly heavy feed with plenty of coolant .

Aaaand the fundamental difference (which leads to Snag correctly recommending slow rotation and heavy feed) is that unlike mild steel, stainless work hardens very significantly. If you "take it easy" and cut fine shavings you will fail 10 times out of 10. You have to cut serious chips or not at all. It can be hard to wrap your head around, since the instinctive response is to back off and go easy since it cuts hard - that just happens to be exactly wrong for stainless.

Each new cut needs to get "beneath/beyond" the work-hardened "crust" of the previous cut.

Lubrication was not mentioned - use some.

Just out of curiosity, now that the OP has work hardened the part, how does he get going again? Will the solid carbide cutter do it?

Pete Stanaitis

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If it does like 4140 that hard area will only be a skin a few thou thick . If he can get under that skin it should be OK - and yes , carbide will cut it . So will diamond ... I have a handful of diamond router bits , they were too dull to cut particle board/MDF at 35,000 RPM but they do a swell job for light cuts at lower speeds in very hard materials . They don't like interrupted cuts ...