Drilling concentricity
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  1. #1
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    Can you clever people people solve a problem for me. I had to drill a 3/4" hole 24" long down 4 aluminium bars 1" diameter. No problem - do it in the lathe with a long series drill from both ends.
    This got me thinking, why did this drill true and concentric (with the drill still and the work rotating) but if I had done the job in a drillilg machine with the work still and the tool rotating the hole would have wanderd off centre?

  2. #2
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    Some things, Grasshopper, must not be questioned.

  3. #3
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    I've wondered the same thing, and I think it has to do with the axis of the hole being clearly defined by the rotation of the work: if the piece is solidly held in the chuck, then the axis of rotation is fixed.

    However, in a rotating spindle like a drill press, you've got both the inaccuracy of the spindle and tool misalignment with the intended path to contend with, as well as the flex and wobble of the tool as it penetrates. Small errors add up, but the errors are inherently smaller on the lathe setup, than the drill.

    That said, there can still be some drill wander even in lathe drilling.

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    The usual way to ensure straightness and concentricity is to rotate the job and the drill. I've done 1/16" holes 3" long with perfect concentricity this way. In sticky pure aluminium even .

    - Mike -

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    Though I am sure there are more technical and "correct" explinations...

    It is less work.

    With the part rotating and the drill still if the drill starts to wander off center it cuts an oversize hole. Cutting out more metal takes more energy. The flute cutting the larger hole is pushed back towards the axis of rotation. In this situation having some flex in the drill helps it drill true to the axis. A short stiff drill will act like a boring bar. The hole will be concentric to the axis- but will be oversize if the drill was not true to the axis or rotation.

  6. #6
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    As Forrest so elinquently stated

    """
    Some things, Grasshopper, must not be questioned.
    """

    Count your blessings, as using another drill from the same batch could easily have made you ask exactly the opposite question.
    If the drill wants to walk, it will walk without asking your permission.
    It is also true that you will generally get better results if both parts are rotated.
    Now for the future though in cases like this one, I would always figure in the fact that the drill will walk, and I would first drill it and then qualify the OD onto it between centers.

  7. #7
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    Who's having problems drilling a hole on mills? I drill holes like that on mills all the time (even in aluminum). All depends on what you know I guess ( not a slam to you !! )... and what you're using as a "drilling machine"...

    Point is, it can be done,.... I am doing it.


  8. #8
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    psychomill, what kind of mill?
    If a vertical mill, are you feeding just the knee or knee and quill to go the 12"+ depth? Using powerfeeds?
    Please enlighten us.

    Les

  9. #9
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    I have been putting off a job which requires .375 holes through 18-inch pieces of 52100. What do you mean by "series drill"? Maybe I'm missing something about this....

    Dennis

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    I like Thiele's intuitive explanation. A more complete technical explanation would probably be less understandable. His makes immediate sense. Nicely done. Kim Steiner

  11. #11
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    Could someone comment on how you would rotate both the workpiece and the drill bit, using commonly available machines? By commonly available, I am thinking of a lathe, or milling machine.

    I think using the term "series drill" referred to longer drill bits, such as taper length, or the 12" style.

  12. #12
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    Could someone comment on how you would rotate both the workpiece and the drill bit, using commonly available machines?
    Some years ago, my employer needed to drill 1/2 diameter holes through 24 inch long aluminum rods. His solution was simple and effective.

    He removed the quill assembly from the tailstock of a Clausing engine lathe and replaced it with a shop-made pilot assembly consisting of a sleeve that fit tightly into the tailstock. At each end of the sleeve was a 1/2 drill bushing.

    With the aluminum workpiece secured in a collet and aligned with a catshead and steady rest, the lathe was powered up. The tailstock, adjacent to the face of the workpiece, guided a long 1/2 twist drill (like electricians use) which was powered by a standard 1/2 electric hand drill. The operator stood at the tailstock end of the lathe and pecked away with the hand drill.

    This was about 35 years ago but IIRC, drilling the hole took about eight or ten minutes and it was concentric. I don't recall losing any parts out of a run of thirty or forty ...

    Cheers,
    Randy


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