Turning 17-4 (A)

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Matt in TN
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Re: Turning 17-4 (A)

Post by Matt in TN » Thu Sep 19, 2019 11:03 pm

Thanks - good advice. The heat is definitely something I'm trying to avoid and something I'd classify as a mistake. But mistakes are where I learn the most. :?

I ran out of 17-4 so I switched to 416 for the last 3 baffles as I didn't need the strength that far out. It's been awhile since I cut 416, but I don't remember having this "gummy" issue with HSS that it seems to be having with the inserts.

Or maybe the surface finish is so nice with 17-4 I've been spoiled and 416 cuts like it always has.

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yondering
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Re: Turning 17-4 (A)

Post by yondering » Thu Sep 19, 2019 11:34 pm

Matt in TN wrote:
Thu Sep 19, 2019 9:50 pm

The pic doesn't do it justice - it was a beautiful golden straw color with black streaks and bright magenta swarf! And the dang thing kept cutting just fine. Those inserts sure do put up with more abuse than my HSS tooling.

Image
That's not a good thing. If the part is heating up that much as you're turning it, you're doing something wrong. I cut a lot of 17-4 too, and even without coolant it stays pretty cool with any sort of turning (it does get hot drilling, but stop way before it changes color!!).

Heat is the enemy in machining, and you'll need to figure out what you're doing wrong and how to keep the part from heating up like that. Making sure the tool is on center and sharp are a couple to check for starters; if your tool is rubbing or smearing instead of cutting cleanly it'll build up a lot of heat like that.
You may be trying to take too much depth of cut as well, judging from the pic, especially with a relatively unsupported part like that.

ECCO Machine
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Re: Turning 17-4 (A)

Post by ECCO Machine » Fri Sep 20, 2019 1:53 am

Matt in TN wrote:
Thu Sep 19, 2019 11:03 pm
Or maybe the surface finish is so nice with 17-4 I've been spoiled and 416 cuts like it always has.
Pretty much.

If you want a nice as-machined finish on annealed 400 series, you need to hone & polish inserts like you would HSS. Or you can just deal with it after the fact using files and sand paper. That's what I usually do with 410, 416, 422, 440, etc.

416 hard turns nicely with inserts, though. Not a great plan for baffles, but doing things like muzzle brakes, that can be a good approach, since hardened 416 isn't that hard. Don't try this with 440c, though!

Personally, I find the higher cost of material and faster tool wear on 17-4 worth it over using 416 since there's less time-consuming finish work and a simpler, faster heat treat. That's even ignoring the vastly superior mechanical properties and corrosion resistance of 17-4.
FFL07/02SOT Gunsmith & Machinist

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