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Rookie Mistake- broken drill bit

Discussion in 'A BEGINNER'S FORUM (Learn How To Machine Here!)' started by dulltool17, Sep 11, 2017.

  1. dulltool17

    dulltool17 United States Active User Active Member

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    I'll put this here because I should have seen it coming....

    Drilling two cross holes 1/16" Dia in 4140 HT. broke the bit as it crossed the first hole.

    Drill is/was HSS

    ? will a solid carbide end-mill remove the remnants?

    Broken piece of the bit appears to be ~1/8" long
     
  2. Karl_T

    Karl_T United States Active User Active Member

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    That should work. be careful not to break the carbide in there too.

    In general, if its not a valuable large number of hours part, i just start over when breaking a cutter in the work.
     
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  3. RJSakowski

    RJSakowski H-M Supporter - Premium Content H-M Supporter-Premium

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    There is a good chance that you will break the carbide end mill. Usually, when a drill breaks it leaves one of the flutes sticking up and the end mill will grab the flute and jam, breaking the end mill.
     
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  4. whitmore

    whitmore United States Active Member Active Member

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    Hate when that happens. Could you drill hole #1, plug it with steel wire, and then drill hole #2
    through the plug? It'd soften the cut interruption that probably caused the break.
     
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  5. tq60

    tq60 United States Active Member Active Member

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    Flip it and drill from opposite side.

    Be careful to greatly reduce feed when you get to near chunk.

    You should be able to determine that be placing part in vice and using broken bit in chuck.


    Flip then peck drill when close and when it just starts to break through get music wire to punch it.

    Sent from my SAMSUNG-SGH-I337Z using Tapatalk
     
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  6. 12bolts

    12bolts Global Moderator Staff Member Active Member

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    Find/make a punch of suitable size and get in there and mash the carp out of the broken drill bit before approaching it with the carbide. Knock all the shards down.
    Depending on the part and as Karl pointed out, I would be inclined to fill it with lead before completing the cross drilling.

    Cheers Phil
     
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  7. dulltool17

    dulltool17 United States Active User Active Member

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    Thanks for the replies, guys-

    - did drill from opp side. Now I have exposed flutes from both sides. At least I know where the tip is.
    - as for the idea of breaking the end mill.....thought that might be the case. Thanks, RJ, for confirming that thought.
    - I tried to use the remaining drill shank as a punch, but broke that too.

    I think if I can get a stout enough punch, I'll try to punch it back through the way it went in initially.

    Again, thanks, guys. Hope I can give some decent advice in return.
     
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  8. whitmore

    whitmore United States Active Member Active Member

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    (about drill broken in hole)
    One kind of drill that WON'T break, is a soft (brass works) rod with a little abrasive grit. Make
    a glycerine/water/grit slurry, paint it into the hole, and gently stir it into the hard bits with the
    brass 'drill'. Keep backing out and adding grit, and don't push too hard, it'll go through. Eventually.

    Ultrasound drilling (similar grinding technique) and EDM will also work, but it's unlikely those
    are available...
     
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  9. dulltool17

    dulltool17 United States Active User Active Member

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    If the punch doesn't work, I'll try some valve grinding compound on a brass rod.

    No EDM, as you guessed, but this would be a great case for having one..
     
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  10. Ulma Doctor

    Ulma Doctor Infinitely Curious Active Member

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    that is a very cool trick!
    i'm going to put that in the back of my head!
    thanks ;)
     
  11. tweinke

    tweinke United States Active User H-M Supporter-Premium

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    This may be a stupid thought, what about freezing it with an upside down can of compressed air and breaking it up with a punch. might take a few cycles of freezing and punching. I did this with a broken tap and it did work for me. Or was I just lucky?
     
  12. tq60

    tq60 United States Active Member Active Member

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    Use a small nail with the tip ground smaller to punch it from opposite side.

    The nail will give and bend into the hole.

    Make length of nail just about 1/8 taller than tge depth and place it on something firm like a partial open vice then smack it gently with the biggest happen you have.

    Gentle tap with huge mass much more energy transfered than harder hits with small one.

    Sent from my SAMSUNG-SGH-I337Z using Tapatalk
     
  13. dulltool17

    dulltool17 United States Active User Active Member

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    Got it done- used a stout punch (actually a center punch) to beat it in, then drilled a bit larger to give the punch clearance.
    Repeated a couple of times until I could simply push it through.

    Part is 100% salvaged, as the side I enlarged is going to be drilled on a taper for a gas regulator valve. All's good! thanks to everyone for the ideas. I have a feeling that, rookie mistake or not, this will happen again.........
     

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