Milling laser cut thin 440C stainless
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    Default Milling laser cut thin 440C stainless

    I've got a job coming up where I need to do some milling of 440C stainless. The material is 0.065" thick or thereabouts and will be laser cut to the appropriate shape before I put in a few features the laser can't do. The features I need to add are just a few chamfers and enlarging a couple of holes (taking off between 0.010" and 0.030").

    My question is this: how much do I need to worry about the heat affected zone from the laser? Would it generate enough heat for long enough to turn the cut edges into woodpecker teeth? Should I expect to be doing hard milling or would the HAZ be small enough that I can just plow through?

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    No specific experience with 440C. So my comments are pretty worthless.

    We use a bunch of laser cut 304SS and it is no big deal to tap the as-lasered holes on a cleanly cut part. We use HSS taps and no issues. Never seen any issues when milling laser cut edges with carbide either.

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    Quote Originally Posted by kb0thn View Post
    No specific experience with 440C. So my comments are pretty worthless.

    We use a bunch of laser cut 304SS and it is no big deal to tap the as-lasered holes on a cleanly cut part. We use HSS taps and no issues. Never seen any issues when milling laser cut edges with carbide either.
    Difference there is that 304 can't be heat treated, 440C can, and it can get stupid hard,
    that's why they make knives out of it.

    I've never dealt with laser cut 440, though if this was my project, I'd be leaning
    towards waterjet before laser, just for the reason you are worried about.

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    Quote Originally Posted by Bobw View Post
    Difference there is that 304 can't be heat treated, 440C can, and it can get stupid hard,
    that's why they make knives out of it.

    I've never dealt with laser cut 440, though if this was my project, I'd be leaning
    towards waterjet before laser, just for the reason you are worried about.
    Waterjet isn't a bad idea. Getting a good laser cut piece relies on the operator to actually be operating the machine correctly to keep the heat out of the part. And whoever nests the parts to do have proper lead ins and outs to not make a hard mess around where they come together. I would say that a well setup and operated laser would have a very minimal HAZ on 1/16" thick material. But a waterjet wouldn't have any HAZ.

    The numbers in the OP also make me wonder a bit on what sort of cut quality he is getting. On 1/16" those holes should be coming in with a few thousandths and there isn't any need to be cleaning 0.03" out of a hole for most applications.

    FWIW

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    Oh, another thing that really helps with cut quality on the stainless is that they are running good slats. We get a lot of shit parts because one of our vendors blasts the shit out of their slats and instead of them being line points they are big ass numbs. So the dross comes out of the cut and bounces up onto the bottom of the part where it becomes one.

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    Quote Originally Posted by kb0thn View Post
    Waterjet isn't a bad idea. Getting a good laser cut piece relies on the operator to actually be operating the machine correctly to keep the heat out of the part. And whoever nests the parts to do have proper lead ins and outs to not make a hard mess around where they come together. I would say that a well setup and operated laser would have a very minimal HAZ on 1/16" thick material. But a waterjet wouldn't have any HAZ.

    The numbers in the OP also make me wonder a bit on what sort of cut quality he is getting. On 1/16" those holes should be coming in with a few thousandths and there isn't any need to be cleaning 0.03" out of a hole for most applications.

    FWIW
    We get very good cuts on our laser, but the hole tolerance is +0.001/-0 and our laser operator isn't quite comfortable with trying to hit that. Those will have 0.010" to take off. The chamfers have about 0.030" extra. The amount of extra material seems like a lot to me, but it is what it is.

    I originally wanted to put this on water but the bossman said laser, so off to the laser it goes.

    Once the material comes in I'll get our laser guy to cut a small test piece so I can see what kind of nightmare I'll be dealing with.

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    Update. Material came in, laser guy cut a test piece, and sure 'nuff, it's stupid hard. Somewhere between 55 and 60 HRC.

    BUT apparently the parts are getting ground and stress relieved prior to my machining so this is all a moot point.

    Either way, I figure someone out there could find this information useful at some point.

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    Quote Originally Posted by dsjansen View Post
    Update. Material came in, laser guy cut a test piece, and sure 'nuff, it's stupid hard. Somewhere between 55 and 60 HRC.

    BUT apparently the parts are getting ground and stress relieved prior to my machining so this is all a moot point.

    Either way, I figure someone out there could find this information useful at some point.
    Thanks for that update. But be aware that "stress relief" itself may not help you unless the HAZ was fully removed during grinding.

    Here's an explanation from Bodycote as to why:

    Stress relieving on metal products - Heat treatment - Bodycote Plc

    "Stress relieving does not change the material’s structure and does not significantly affect its hardness.

    Hardened and tempered parts to be stress relieved must be treated at a temperature around 50°C below the temperature used for previous tempering to avoid an impact on the hardness."

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    Well crap.

    My experience with heat treating, stress relieving, and all that goes with it is extremely limited to say the least.

    Learn something new every day, so thank you for that.

    If it comes back from grinding, etc. still hardened I'll have to figure something out at that point. Though the HAZ didn't appear to be terribly thick - couldn't be more than 5, maybe 10 thou at most. Could try just plowing through or grind it off by hand.

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    Update the second.

    Finally got the parts back from the grinder. Apparently he was having Issues and has been pretty backed up because of it. Something to do with losing employees or losing fingers, not clear on which. Could be both.

    The cut edges are still hard, go figure. Going to conventional mill my way along the to-be-chamfered edges in order to get under that HAZ and hopefully give my poor chamfer mills an easier time. Shouldn't be too horrible.

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    Maybe a gentle reminder to the boss-man that waterjets have their advantages would be in order next time?


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