Hardened Steel Speeds/Feeds Sanity Check

[JJ0704]

I'm an amateur garage hack with a 1hp mill that normally just plays with aluminum. Well, now I find myself with a project to machine some parts that are made out of 8620 steel that is case hardened. I've never cut steel before, much less a hardened steel like this.

I have a 1/4" carbide 4 flute end mill that I plan to use for this task. The depth of cut is about 1/2" and I plant to cut from just one side using conventional cut rotation with about 0.030 per pass. I don't have coolant, so it will be dry cutting. According to the Whitney Tool Company Speeds and Feeds Calculator online with a suggested 100 surface feet per minute and 0.002 chip load per tooth, I should be running my mill at about 1528 rpm and use 12.22 inches per minute feed rate.

Does this seem reasonable at all? I thought that seems pretty fast and aggressive for my little mill. I'm okay going slower, but I also have heard that too slow or not aggressive enough can be bad for carbide end mills. Any advice for a steel newbie would be greatly appreciated.

Thanks in advance!
JJ

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[awerby]

Can you anneal the parts and re-harden them after milling? It might be possible to cut off the case-hardened layer with your carbide tool, but it might not work at all, depending on your mill and the depth of the surface hardening. Those feeds and speeds sound unrealistically high for a miniature mill and hardened steel. Try about a quarter that fast, and get a few more of those carbide tools because you might break a few before finding the sweet spot.

[JJ0704]

No, I can't anneal them. I was hoping someone with a smaller size mill like mine had some experience with hardened steel and could share some pointers for me to start from.

[Jim Dawson]

You should be able to cut it OK if you can get below the case hardening in the first pass. Not sure about the 1/4 end mill, it's a bit skinny to try to load too much on a small machine. Carbide likes a rigid machine. You should have enough horsepower, no problem there.

[JJ0704]

I have some larger carbide end mills, but they are used (I don't know how many hours are on them). Should I go as large of a diameter as possible?

[Jim Dawson]

You might give them a try.

[JJ0704]

Jim, based on your advice I bought a new 1/2" TiAlN coated carbide end mill. I used the Whitney Tool calculator's suggested ~650rpm and 6ipm feed rate for a 0.002 chip load on a 100 sfpm material. My cuts were side cuts at about 1/4" depth and 0.040" per pass and it worked great with my little 1hp mill. The chips were curled tight and some were starting to turn straw and the odd one was blue.

Thank you!
JJ
p.s. - I forgot how well a brand new end mill cuts!