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Thread: Drilling 1/2" Bolt Clearance Holes

  1. #1
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    Default Drilling 1/2" Bolt Clearance Holes

    I have several hundred 17/32" holes to drill into some A36 1/4" to 1/2" steel plates with a bridgeport clone. Normally I centerdrill first, followed by a 1/4" drill to clear the web of the larger drill and finally the larger drill(118*). If I do this, I will be here until Christmas. Would it be practical to drill with a 135* split point without centerdrilling first? Would a good quality HSS (or cobalt) drill survive 500 holes without sharpening (I can't sharpen split points)? Any recommendations would be appreciated.
    Doug

  2. #2
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    Demokratik Republik of Washington
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  3. #3
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    With that many holes, I'd drill an 1/8" dimple and then find somebody with a punch.

  4. #4
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    Cobalt split point at 600 to 640 rpm with coolant should work just fine, messy though with coolant and cuttings flying around the shop. peter
    The difficult done right away. the impossible takes a little time.

  5. #5
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    I would geta rotabroach infuser and the proper sized rotabroach. If you decide to go this route make sure to have a good swarf handling system. The infuser turns on the coolant when the center pin touches the part and turn off the coolant when it ejects the plug. The down side is the swarf, tThe upside is nice round on size holes that requires little power.

    Or as Jim says , punch them.

    Pete

  6. #6
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    The drill may not last 500 holes, depending on your technique. You do not need to centerdrill or pilot drill. Split point drills are not prone to walking.

  7. #7
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    If you can get a stubby drill it will have even less tendency to wander. I'm solidly in the punching camp, though. After 40 years of doing this we just bought our first ironworker and I can't believe how fast you can make holes with it...
    Keith
    __________________________
    Just one project too many--that's what finally got him...

  8. #8
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    It's too bad you don't have access to a CNC.
    An Iscar Chamdrill could drill a couple hundred holes in less than an hour.
    Sorry.

  9. #9
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    Dec 2010
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    Your normal practice is way off and that is the reason you have to ask for a faster way. You don't center drill your holes, center drills are for making centers (it is like starting a hole with a chamfer). Second, buy a 120...135 degree stub hole starter drill to make the first divot, just 1 mm deep and follow that with your 1/2" drill (118 degree angle). Because the divot has a larger angle, the drill has no other place to go than the divots center before it starts cutting.

    Other option is to buy a couple of 1/2" split point drills and just go with them, nothing else required for the hole. This size is still cheap, even in quality bits.

    And last bit of advice: if you need to chamfer the holes, do it by hand. Battery drill, chamfering bit in it, slow gear on and have at it. Way faster than fiddling with the tools in a mill or some other way (without a CNC).

  10. #10
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    Feb 2010
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    You said Bridgeport clone, what are the tolerances on the center distances?

    How many holes per plate?

    Could you stack the plates? A 17/32" hole through 1" thickness is readily doable without pecking if you have coolant or cutting oil. Clamp, or better yet, tack weld together to prevent movement.

    If tolerances allow, you could drill a "master" plate to use as a jig and drill directly, or use the master with a transfer punch and lay out the pattern and avoid all that cranking, or better yet, ditch the clone and use an honest to god drill press and dispense with the pilot holes.

    Jaakko is right on about using a drill motor and countersink for de-burring.

    Dave
    Last edited by becksmachine; 03-09-2013 at 01:58 AM.

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