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Question about overheated drill bit...

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  • #16
    Originally posted by Edwin Dirnbeck View Post
    ... I have found time and again when you are drilling nasty material ,once it squeels ,it is best to stop and touch up your drill. Edwin
    And that goes for ANY cutting tool, in the mill or lathe, or drill press, etc.

    Normally from going too fast, and/or from allowing the tool to "rub", which nearly always causes trouble with dulling the tool. After it's dull, it will rub even more, and squeal.
    CNC machines only go through the motions.

    Ideas expressed may be mine, or from anyone else in the universe.
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    • #17
      Thanks for all the replies guys. Loads of good advice and ideas here.

      I have not gotten back to this yet, but the story is this:

      Part is just a 2" diameter disk, 1/2" thick with a 0.375" hole in it. I had the disk in the lathe, faced it off flat and spotted the center with a spotting drill. Slid the tailstock back and swapped the drill. I turned the speed down, but was surely spinning at least somewhat too fast. The real killer was that when I slid the tailstock back up to start drilling, I locked it down but apparently did not do so securely enough. I started drilling, keeping the pressure up to keep it cutting, but did not realize that in addition to sinking the drill into the part, I was also pushing the tailstock back. As the bit was not cutting as it should, it must have dragged enough to work harden the piece and you know the rest.

      I'm going to try regrinding the bit an see what happens. I can always cut it back and re-point and thus have a nice new stub bit.
      we'll see how this goes...

      Thanks again!


      • #18
        If it's an HSS drill then stick it in the Darex and clean it up.

        It may help to break through the work hardened stainless by re-starting with a 1/4" drill for 1/8" depth and then going back to the newly sharpened 3/8" drill.

        Lock the tailstock up this time
        Location- Rugby, Warwickshire. UK


        • #19
          OK, post mortem:
          so I got a few minutes to sneak back out to the shop last night. Took my discolored bit with the slightly malformed edges and had a closer look. It was not smoking hot anymore, so I could do so without burning my fingers.

          Turns out the end was not as bad as it first looked, most of what appeared to be mashed edges was actually chips that had sort of fused to the end of the bit. Most of it chipped off, it was sort of melded/stuck to the bit. The cutting edges were clearly ruined by the episode.

          So I made enough passes on the Darex to make the drill point look like a drill again. I did not remove all that much of the end, maybe 1/16" I'd guess. I put another piece of SS in the lathe, slowed WAY down and made sure the tailstock was locked in place(!) and proceeded to drill a nice 3.8 hole in it. So it looks like, aside from being a bit shorter with a bit of discoloration still near the end, the bit is not really any worse for wear. So I did get the thing stinking hot, but apparently not hot enough to unduly louse up the heat treat.

          I used Mr. Dirnbeck's idea of opening the original work piece up with an old masonry bit or two (thanks, good idea!) to get past any work hardened stuff and then finished the piece off.

          Thanks for all of the comments. There is quite a lot to digest just reading through these replies.
          Much appreciated, guys!