Drilling case hardened 8620
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  1. #1
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    Default Drilling case hardened 8620

    Do to my own stupidity, I forgot to drill a 1/8" diameter hole 1/2" deep in a 8620 shaft. Well now that I case hardened it, what are my options?

    Case is about .020" thick and Rockwell 60.

    Grinding through the case is not possible in the location of the hole.

    So what kind of drill is recommended to get through the case?

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    Drill it with a carbide spade drill , commonly called "Hi ROC" drills or spot aneal it. To do so chuck a piece of drill rod the diameter of the desired hole up and at very high speed feed it hard against the spot until it is bright red. Back off and let it cool slowly. Should be drillable with a decent Hi speed drill when cool or fr that matter while still hot.

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    We use these for drilling our tool holders when balancing. Also 8620 but 56-58rc not 60.

    High Performance Solid Carbide Drills - MariTool

    Thank you.

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    Oh gee... that is sort'a hard.
    I'd guess set your SFPM arounf 50 fpm with carbide tooling, and make your chip-load stupid light, say...0015"

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    Blow thru the case with a stubby 1/8" carbide end mill. Run it slow. Heck- probably do the entire hole with it if luck is on your side, or just switch to a HS 1/8" drill- the core should be no more than 30 Rc or so. 1/8 carbide endmills are pretty inexpensive so. . . .

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    Quote Originally Posted by snowshooze View Post
    Oh gee... that is sort'a hard.
    I'd guess set your SFPM arounf 50 fpm with carbide tooling, and make your chip-load stupid light, say...0015"
    I wouldn't call .0015 a light chip load for a 1/8" drill in hard steel but if you cut that number way down I would say you should go right through it.

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    HiRoc drill will definitely get through the case. Once through, I'd switch to a more conventional drill.

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    Drilling it with a standard carbide drill will be pretty straightforward, especially given the thin case. I have used AITiN coated carbide drills from Maritool to drill many many 1/8" diameter holes in 63HRC A2. I got about 90 holes per drill running them in a VMC, drilling 0.156" deep. Small pecks were key for me.

    Rigid setup is very important, you're not going to be able to do this in a drill press. But a nice manual milling machine or a VMC and you'll be fine.

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    When you switch drills make your hss drill smaller, say a #31 (.120) to make sure it doesn't rub on the case.

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    I had a sort of similar job once when I was a first year apprentice, and no carbide drills. We had some burnouts that had just been burned from 4140 but they had forgotten to anneal them after burning, so you can just imagine what the skin on the outside perimeter was like.

    Had to drill some ¼" holes from the outside to a bore in the interior, about a ¾" depth. I thought I'd just slow the spindle right down to a crawl and munch through with a HSS drill. This worked for a while, but I had something like 20 pieces to do. I was swapping drills every 2nd piece. RPM was something like 5 or 10, using cutting oil on a 5" spindle HBM. The drills were squeaking like nobody's business as they went through that skin. Thrust pressure was through the roof for a ¼" drill.

    Finally one took more of a bite than it should and snapped, grenading the drill. Several pieces hit me in the face, drawing a good bit of blood. Glad I was wearing safety glasses. After that I did what I should have done in the first place and asked the foreman what to do...the pieces got spot annealed. No more problems.

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    Quote Originally Posted by eKretz View Post
    Finally one took more of a bite than it should and snapped, grenading the drill.
    Occasionally, when faced with this problem and no suitable carbide tool, I have used a center drill. Those are much shorter and stiffer and work better in hard material than a conventional drill. (For a short distance, anyhow - like getting through the case.)

    Sometimes you just gotta get the damned thing out the door this afternoon, ya know ?

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    Yep. If I absolutely HAD to do this now with a HSS drill I would use a spotting drill. Hindsight being what it is and all...

    However, carbide drills are the way I'd do it if given the choice for sure!

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    I do quite a few cuts at .0007 IPT, yes.
    Clear down to .0004 at times with small cutters in hard stuff.
    Seems crazy, but also seems to be required.
    But since it is only a case harden, you need only to breech the surface prior to drilling as normal.


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