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Thread: Bone Screws

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  1. #1
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    Question Bone Screws

    Hey yall,

    I work for an orthopedic implant manufacturer. We make countless bone screws, many of which are cannulated (thru hole that's gun drilled).

    Has anyone ever gun drilled while simultaneously whirling OD threads? The gun drill would have to be live and the material rotation controlled in C axis for the whirling. I was thinking about pecking the gun drill the per turn / whirl segments either during the pinch turning (if my endworking drills clear the whirler below) or during the whirling itself.

    Anyone have experience with this or suggestions?

    FYI material is Ti 6Al-4V, thread major is 6.5mm and machine is a Tornos Deco 20a.

    Thanks for any suggestions.

  2. #2
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    Re: Bone Screws

    Wow !
    I doubt many on this board can offer advice...

    Afaik, gundrilling does not need pecking.
    Just need right surface speed.
    If end-of-work is clear, it should work, in theory.

    Not practical experiece.

    Coolest thing we ever made was ballnuts (reverse broached on C axis, HAAS ST30Y) in (brass or) bronze.

  3. #3
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    Re: Bone Screws

    Quote Originally Posted by hanermo View Post
    Wow !
    I doubt many on this board can offer advice...

    Afaik, gundrilling does not need pecking.
    Just need right surface speed.
    If end-of-work is clear, it should work, in theory.

    Not practical experiece.

    Coolest thing we ever made was ballnuts (reverse broached on C axis, HAAS ST30Y) in (brass or) bronze.
    Thank you for the input. I agree, it should work in theory. I didn't mean peck drilling exactly in the traditional sense. Only that I can only turn and whirl in .500" length segments so the drilling depth per operation will be restricted to this. Our part's OAL is generally 2.5-4" long. My concern is during turning the main spindle is moving at 1800 RPM, during whirling about 20 RPM. The gun drill is 8000 RPM! I'm not sure how a live gun drill would do at this speed.

    Does anyone know any conversion factors if I'm turning at 1800 RPM and the gun drill is normally 8000 RPM, how fast would a live drill have to go if the main spindle is at 1800 RPM?

  4. #4
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    Re: Bone Screws

    6200 RPM?

  5. #5
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    Re: Bone Screws

    Quote Originally Posted by dharmic View Post
    6200 RPM?
    Yes, from what I'm finding you're correct. If the spindles are opposing directions (two spindles facing each other, both going clockwise or counter clockwise) the speed RPM is subtracted, making it 6200 as you've said. If they're going the same direction (one clockwise the other counter clockwise) the live tool would be moving at only 4400 RPM.

  6. #6
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    Re: Bone Screws

    Think of it like this. You're driving your car at 50km/h and pass a tree on the roadside. Relative speed is 50km/h. This is like running your 8000RPM tool into a static workpiece.

    You're driving your car at 50km/h and pass a car doing 20km/h coming the other way. Relative speed is 70km/h. This is running your 8000RPM tool counterclockwise whilst your workpiece is spinning at 1800RPM - counterclockwise on its own, but relative to the tool it's the opposite direction. So they add up. Your relative RPM is 9800 and yes, to get the relative speed down one of them has to drop by 1800 giving you 6200RPM on the tool (or back to a stationary workpiece).

    You're driving your car at 50km/h and overtake a car heading in the same direction doing 20km/h. Relative speed is 30km/h. This is running your 8000RPM tool in one direction and workpiece in the opposite spin but again, because they're facing each other, the relative speed is 6200RPM. To get that 8000RPM relative speed you'd have to crank your tooling up to 9600RPM because the tool cutting edge effectively has to catch up to the work.

    Think about it - the 4400 RPM never happens, it's 8000RPM desired relative RPM ± whatever the workpiece is spinning at, the plus or minus being determined by the direction of the workpiece.

  7. #7
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    Re: Bone Screws

    55thSwiss, I may have some suggestions, I have about 10 years experience on Deco machines. Attach a drawing or email it to me: dcogs1@msn.com

    -David

  8. #8
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    Re: Bone Screws

    Quote Originally Posted by DCogswell View Post
    55thSwiss, I may have some suggestions, I have about 10 years experience on Deco machines. Attach a drawing or email it to me: dcogs1@msn.com

    -David
    Thanks David. I left my laptop at work this weekend but I'll shoot you a print tomorrow

    Sent from my SCH-I535 using Tapatalk

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