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  1. #1
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    Question HELP!! Machining 420 stainless

    I'm making a couple of small gears in this horrible material and I can't seem to get it to behave.

    I was able to get really tight, long pigtails when rough turning with the following:
    Surface speed: 170 m/min
    Depth: 0.5mm (radial)
    Feed: 0.16mm/rev
    Coated carbide insert (with a fancy chipbreaker) with 0.4mm corner radius

    Couldn't get it to actually break the chips though.

    Left .05mm for finish turning and that worked a treat with a ground and coated carbide insert; I forget the feed/speed, but I managed to get 0.37 um Ra surface finish which is fine.

    Now I have to hob them and I'm stuck with HSS because we don't have a 2 Module carbide hob . Also, they specify N4 surface finish (0.2 um Ra) . The parts will be sent out for super-finishing to get there, but I still need to get it pretty good beforehand.

    What I can find online is recommending about 7.5 m/min surface speed for milling, does that sound right? Because it puts me at 60 rpm on a 40mm hob, which seems awfully slow (the change gear chart also stops at 140rpm, so that's gonna be interesting... )

    I suspect this is a prototyping/proof-of-concept exercise, so if it becomes an order for a few hundred we'd (better) get a carbide hob, but I still need to get these done and try to work out the best way to machine this stuff.

    Anyone got ideas?

  2. #2
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    You're younger than I am so likely better with Google, but in case you haven't already found it there's a chart on pg71 of this manual that has speeds and feeds for HSS milling of 420.
    120 sfm, 50 thou doc and 2-4 thou per tooth feed
    So thats around 35m/min - a lot more than the 7.5m/min you're talking of.

    https://www.carpentertechnology.com/...omachining.pdf

    Steve

  3. #3
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    Quote Originally Posted by OxxAndBert View Post
    You're younger than I am so likely better with Google, but in case you haven't already found it there's a chart on pg71 of this manual that has speeds and feeds for HSS milling of 420.
    120 sfm, 50 thou doc and 2-4 thou per tooth feed
    So thats around 35m/min - a lot more than the 7.5m/min you're talking of.

    https://www.carpentertechnology.com/...omachining.pdf

    Steve
    That doesn't sound right i normally work on 30M/min in mild steel and about 12M/min in stainless.

  4. #4
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    I think you would be better off asking this question over on practical machinist, you need information on a very specialized commercial process and i doubt you will get suitable answers here. There are a couple of gearcutters on their who are really helpful, you just have to ignore some of the other idiots.

    At 140rpm you are looking at about 17.5m/min, with a good cutting oil you should get away with a one off. tool life will reduce with the increased cutting speed.

  5. #5
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    I agree with Ash; I would have tried SS at around half the MS speed. Lots of coolant/ oil to avoid heating things up and try to get the feed up to avoid rubbing/ work hardening.

    Good luck.

    Michael

    Just looked up my tables. Module 2 is not a small tooth. Are you able to use another cutter to gash the gears first before finishing with the M2 HSS cutter?
    Last edited by Michael G; 4th Feb 2020 at 08:57 PM. Reason: Added a bit

  6. #6
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    Thanks Steve, I did see that one, but I discounted it because most other places I was seeing similar numbers for carbide tooling, not HSS.

    Thanks snapatap, I'll see if I can drop the speed a bit lower, I just don't want to overload the drivetrain and break something. Hopefully I can get it down, even to 100 rpm, as my chipload is going to be tiny to keep the surface finish good. We use flood oil (Cutmax 226S) for these machines, so hopefully that will be enough to keep the work hardening down.

    We've got 300 series stainless pretty much sorted, but this 400 stuff just doesn't want to play.

    Anyways, it's gonna be happening tomorrow one way or another, so I'll report back with what I find in case some other poor sod needs it.

  7. #7
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    Quote Originally Posted by Michael G View Post
    Just looked up my tables. Module 2 is not a small tooth. Are you able to use another cutter to gash the gears first before finishing with the M2 HSS cutter?
    Gashing with another cutter first would mean setting up an indexing head on the mill and then trying to make sure it's lined up properly when we take it to the hobber; it's doable, but the time cost would be too high.

    The plan is to hob in 4 passes to get to depth; 0.1mm finish cut and the balance divided up.

    Most of our work is in the Mod 0.3 - Mod 1 range so this is positively huge for us Meanwhile, on the other side of the factory, they're cutting up to 1DP and THAT is a big tooth.

  8. #8
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    Well, 140 rpm worked perfectly. I looked up the speed in an old edition of the Machinery's Handbook and it recommended 65 SFM for milling with HSS, which works out to 19.7 m/min or 156 rpm. Not sure where I was getting 7.5 from, must have been looking at the wrong thing

    First pass

    IMG_20200205_084447.jpg

    Starting to look like a gear now

    IMG_20200205_091351.jpg

    Hobbing complete. Everything to the right of the gear was only there so I could hold it for hobbing and will be cut off; can't use a normal arbor because the bore is blind and the collets aren't big enough to hold the actual part (even if they were, it'd be too close to the spindle nose and the hob would crash). Might be able to hold them on an expanding arbor if we're allowed a small centre point in the end (might be too close to the bottom of the bore though), we'll work that out if we have to make a decent quantity.

    IMG_20200205_110424.jpg

    All done

    IMG_20200205_144940.jpg

  9. #9
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    Hi elanjacobs,

    Atlas steels who supply 420 stainless steel can supply machining info about it according to the a statement in the following link.
    Scroll down to the top paragraph of page 3 to read about it.

    http://www.atlassteels.com.au/documents/Atlas420.pdf

    Too late to help but might be useful for someone else.

    Grahame

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