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  1. #1
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    Default deep hole drilling in Acetal

    Hello all
    Any help that will make drilling deep holes in Acetal easier. Drill sizes 5 and 7mm diam and 300mm long.
    Should I use coolant or not?

    Regards

  2. #2
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    Oooh boy that's deep, 60x diameter. I probably wouldn't use coolant, you are just going to have to peck a lot. I wouldn't bolt my tailstock down, stand at the rear and just slide the whole shebang backwards and forwards.

    I think acetal can absorb water (probably takes a long time) so another reason to skip coolant. Having anything stick the chips together is bad news it won't take much to get enough heat to remelt them.

    Best of luck! If you have more than a few I'd consider buying a gundrill and garrying together a through coolant setup.

  3. #3
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    The main issue that causes problems with machining plastic is heat. Some form of cooling is vital.
    I don't think it is Acetal that absorbs water - probably thinking of Nylon as that definately does.

    Whether you use water or compressed air, you will need to peck drill and clear the chips often so that they do not remelt.

    Michael

  4. #4
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    I can't actually find anywhere that even sells 60*diameter spiral or parabolic drills, what tool are you using?

  5. #5
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    Given that it's acetal, you may get away with extending a long or extra-long series drill bit with a smaller diameter rod - a good straight piece of course, and carefully fitted into a hole bored into the soft part of the shank.
    That said, you may be able to find something long enough - somewhere along the road I acquired this long 1/4" diameter drill bit, made by P&N:

    IMG_3365.JPG
    Has about 220mm of flutes, which have an interesting profile designed to assist in chip removal I expect.
    IMG_3363.jpg
    Sorry about the low res photos.

    Cheers,
    Bill
    Last edited by WCD; 29th Nov 2019 at 11:51 PM. Reason: correction

  6. #6
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    Hi Guys,

    Many years ago Dormer made me a couple of special drills to drill a 5/64" diameter hole through 8" inches of PTFE, (Teflon) rod. They had a split point and a very slow spiral flute. Very tedious work, slow revs and silicon oil as a lubricant. If you tried to push it the hole wandered off and the drill came out the other end way off centre.

    Michael is right Acetel does not absorb water ! However heat is your enemy here, a water based coolant might help. The problem is going to be getting that water down to the cutting edge.
    Best Regards:
    Baron J.

  7. #7
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    south coast, nsw
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    I did a job some time ago that called for a 5mm hole in a similar length of delrin. I broke the job into several shorter lengths and rejoined after drilling with mortise and tenon joints. It helped that the outside profile was not straight so I could arrange the joints to be at less noticeable 'change' points on the profile. Any wandering was corrected by turning the tenons between the new centers after drilling.

  8. #8
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    Default deep hole drilling

    Thanks to all that have replied, I will post a photo of the drill bits and item I am drilling.
    Yes, heat, as I have found to be an issue, but the biggest issue, I have found, is keeping the hole central. Just wondering what the difference would be between Acetal-H and Acetal-C, as H is one base where C consists of 2 materials, one may have a "softer" centre.
    20191130_081357.jpg20191130_081538.jpg
    Regards
    Last edited by mechdraw; 30th Nov 2019 at 10:18 AM. Reason: add pics

  9. #9
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    Hi Mechdraw,Guys,

    Very little I would have thought !

    As far as keeping the hole straight is concerned, since you need 5 mm and 7 mm hole diameters, I would use a stub drill to start and create the first couple of inches, then change to a longer drill. That will at least ensure that the hole is in the right place and straight to start off with.

    But as I said previously, you cannot push too hard or the drill will gradually run off, particularly if the grind on the drill does not have 100% equal length cutting edges. A four facet grind is your friend here.

    There is also the issue of abrasion causing wear on the cutting edges ! This will increase the heat generated as the cutting edge is worn away. Most plastics are quite abrasive, Acetal a little less than most. But you are going to have to do a lot of pecking.
    Best Regards:
    Baron J.

  10. #10
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    How much deviation do you have at the end of the hole? what is your tolerance? I think with a twist drill that long you will struggle to get a dead straight hole. I assume you are drilling this in a lathe? If your not you should be, this will help keep your hole straighter. I would drill and bore the start of the hole +.02-0mm of the drill diameter this will help the long drill start as straight as possible.

  11. #11
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    Default deep hole drilling

    Hi, this is being done in a lathe, the runout adds to about .040"

  12. #12
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    I think your only hope of getting it any straighter than that is a gundrill, or rotate both the drill and the workpiece. Is it important that the hole be straight or just that the ends run true in relation to the OD? If the latter just turn between centres after drilling.

  13. #13
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    Default deep hole drilling

    Quote Originally Posted by caskwarrior View Post
    I think your only hope of getting it any straighter than that is a gundrill, or rotate both the drill and the workpiece. Is it important that the hole be straight or just that the ends run true in relation to the OD? If the latter just turn between centres after drilling.
    I'm turning between centres at the moment, but it means I have to use larger diam material, just extra waste.

  14. #14
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    Is this the same sort of Acetal that people use for making replacement cross slide nuts etc? I.e. It's a thermoplastic that can be joined by melting?

    If so, it might be possible to make that piece out of say 3x 100mm lengths.
    Drill the holes, heat the ends and drop them on a bit of threaded rod to hold them together while they set before turning the outside diameter to size.

    Just a thought....

    Steve

  15. #15
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    Hi Mechdraw,

    Quote Originally Posted by mechdraw View Post
    Hi, this is being done in a lathe, the runout adds to about .040"
    Ouch ! You need some soft jaws to get around that amount of runout !

    This is what I did to ensure concentricity when I machined some gears.

    DSC04.jpg DSCF-06.jpg
    Ignore the teeth on the outside, I used slices from an old toothed drive shaft. I drilled holes so that they just fit over the chuck jaws, then drilled and threaded them for a retaining screw. Closed the jaws with them in place and then turned a recess to suit the diameter I wanted.
    Best Regards:
    Baron J.

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