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Thread: accurately enlarging a hole.....

  1. #1
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    accurately enlarging a hole.....

    I'm interested in hearing how people would reliably concentrically enlarge an existing 1/2" through hole in something like the face of BB plywood or MDF to 3/4" for about half its depth , preferably with only the types of tools the average serious amateur woodworker would own. Tolerance of about .005 -.01"?

  2. #2
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    I might put a snug fitting forstner bit in the existing hole. Then tighten on the drill quill. Then clamp the piece in place snug to the table. Then remove the bit and replace with the larger bit.

  3. #3
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    Counterbore with pilot that can be removed and exchanged. Common for metal work.
    Two days ago I needed a 1/4" pilot so I took a 1/4" bolt and drilled it axially for a 3/16 hole. Cut off the head and cut the short length I needed. Cut and soft soldered a bit of 3/16 rod for a shank and used it. Worked fine. Should really have silver solder or brazed it.
    Bill D

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  4. #4
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    Probably with a rabbiting bit on a router.oops edited because the bearing needed would probably not fit in a1/2” hole
    Last edited by Steve Jenkins; 01-10-2022 at 3:22 PM.
    Steve Jenkins, McKinney, TX. 469 742-9694
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  5. #5
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    Quote Originally Posted by Carl Beckett;[URL="tel:3167386"
    3167386[/URL]]I might put a snug fitting forstner bit in the existing hole. Then tighten on the drill quill. Then clamp the piece in place snug to the table. Then remove the bit and replace with the larger bit.
    This is what I commonly do for alignment in preparation for a secondary operation.
    Take me to the hotel - Baggage gone, oh well . . .

  6. #6
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    HF step bit. Self centers in 1/2 hole, them bore to desired depth.
    Jerry

    "It is better to fail in originality than succeed in imitation" - Herman Melville

  7. #7
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    Another way that I was taught is to cover your existing hole with masking tape (painters tape), then take your 1/2" bit,align with your hole, and using the point, make a dimple in the tape. Remove the bit and insert your 3/4" bit. You now have a dimple or small hole to align your larger bit.
    My Dad always told me "Can't Never Could".

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  8. #8
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    Quote Originally Posted by Carl Beckett View Post
    I might put a snug fitting forstner bit in the existing hole. Then tighten on the drill quill. Then clamp the piece in place snug to the table. Then remove the bit and replace with the larger bit.
    +1, that’s how I would do it and I’m a retired machinist with a milling machine.
    The only thing I would do differently would be to chuck the alignment tool first and then bump the plywood into position, then clamp it down. Either way will work If the plywood piece is small and lightweight.
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  9. #9
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    Cut a short piece of 1/2" dowel that fits in the hole. Mark the center of the dowel on one end. Place the dowel in the hole so the marked end is up. Drill the hole larger to the desired depth with the larger forstner bit.
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  10. #10
    Quote Originally Posted by al ladd View Post
    Tolerance of about .005 -.01"?
    Your wood can change that much overnight due to humidity changes. It's woodworking, not machining.

  11. #11
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    There is no chance a 1/2” hole is changing by .005-.010” overnight unless it has been cut in green wood. OP is cutting ply and MDF, both materials are reasonably stable.
    Bumbling forward into the unknown.

  12. #12
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    Tolerance of about .005 -.01"? Tolerance for what? Diameter? Depth? Concentricity? I prefer a router with a guide bushing and template. If that works with your extreme tolerance of .005" is another question. Curious of the application now with that kind of metal working tolerances.
    Last edited by Richard Coers; 01-10-2022 at 9:07 PM.

  13. #13
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    Thanks for all these replies.

    I was curious about step drills-- I have one coming, never used one before.

    Sounds like the consensus is for a forstner bit in existing hole aligned on drill press and clamped in place -then switch bits. I guess I was unsure about the reliable tracking of a forstner bit guided by its rim like that. The dowel (or plug)suggestion is interesting too, and maybe used in conjunction with the forstner bit trick would be better yet . The other possibility I've considered is router with template guide, either changing bit, or template guide size with template left clamped in place. The job will utilize router with template guides already. But I know unaligned template guides can create problems.

    The need for accuracy is in concentricity, but that will require comparable control of diameter -depth not so much . The application is for a threaded fastener, guided through fairly tight tolerance bushing, to align with a fixed nut below, securing a fence to a table. Maybe I'm exaggerating the need for precision. I know how annoying it is trying to thread a knob into a misaligned pair of parts. I suppose I can give more room in ID of bushing for the fastener to wander in, but then it might be harder for the fastener to find its threads. It's something that will be assembled and disassembled fairly frequently in use.

    The item is a high precision jig that I hope to offer in a kit form, built with a router bushing guided template and unusual hardware I'll supply. So I'm trying to figure what will be doable for average woodworkers with average tools. It will be important to keep the fabrication by the builder not-too-fussy, and hopefully fun, and the end product needs to be accurate and easy to use.

  14. #14
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    Quote Originally Posted by Steve Jenkins View Post
    Probably with a rabbiting bit on a router.oops edited because the bearing needed would probably not fit in a1/2” hole
    This is exactly what I was thinking. I have a flush cut bit that would easily fit in a half inch hole and a set of all difference size bearings I can use with it.

  15. #15
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    Quote Originally Posted by Jerry Wright View Post
    HF step bit. Self centers in 1/2 hole, them bore to desired depth.
    Irwin makes a single flute step bit that does an excellent job keeping itself centered. Better than the two flute step bits. I have enlagred holes in saw blades with one without throwing the blade off center.

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