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Thread: Drill a small-diameter pilot hole, THEN a large-diameter finished hole? Or not?

  1. #1

    Drill a small-diameter pilot hole, THEN a large-diameter finished hole? Or not?

    Hi, all.

    I need to drill two 3/8" holes approximately 1" deep into a basswood electric guitar body.

    The holes need to be PERFECTLY(!) located/positioned. They mount the bridge to the guitar body, and any mistake will render the instrument 'firewood.' I'll use a drill press, and clamp the workpiece firmly in place.

    Basswood is considered a hardwood, but a "soft hardwood." I find it to be rigid and dry, and easy to work.

    I read somewhere online that drilling a smaller-diameter pilot hole is actually counterproductive because, "then the 3/8" bit is not cutting wood from the center of the bit" or words to that effect. Apprently that might allow the bit to wander...

    I hope that's not true. In this case I want to drill the smaller pilot hole all the way through. Why? Each 3/8" hole accepts a press-in threaded metal anchor that is splined. A relatively tight interference fit is the result. See attached photo. Drilling a smaller pilot hole all the way through the guitar body will make it possible to press that anchor OUT from below if or when I need to.

    posts.jpg
    Last edited by Tom Burgess; 11-27-2021 at 1:59 PM.

  2. #2
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    It sounds to me like you could drill the larger hole first and then drill a smaller hole the rest of the way through from the bottom of the big hole, and that would solve both problems.

  3. #3
    Agreed, but I'd rather drill that pilot hole first. And I will IF drilling the smaller pilot hole is not likely to cause the 3/8" bit to wander.

    I'm clamping the workpiece to the drill press table, but even so, if a drill can go in some arbitrary "other direction," it will.

    So the question remains:
    Will a pilot hole drilled in wood increase the chance of the larger finished-hole drill wandering?

  4. #4
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    What kind of bits are you proposing to use? Brad point? Forstner?
    Hobbyist

  5. #5
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    I've found my Fisch forstner bits can follow a 1/8" pilot well.
    Timberlight Designs

  6. #6
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    I don't know if the pilot hole would cause the larger bit to wander; I do know a small diameter bit is more likely to be deflected by grain in the workpiece than the larger bit would be, so it might be counterproductive for that reason.

    If I need holes to be perfectly positioned, I make a drill guide out of hardwood or better, something like phenolic that has no grain. Use stock at least 1/2" thick. You can take as many tries at making the guide as necessary to get it perfect. Then you clamp the guide to the workpiece, in this case your guitar, and drill the money holes. You may even be able to make the guide such that it registers on something like the sound hole or edges of the piece so both spacing and absolute position of the holes is set exactly.
    --Certainty is the refuge of a small mind--

  7. #7
    Quote Originally Posted by Stan Calow View Post
    What kind of bits are you proposing to use? Brad point? Forstner?
    Just a standard twist drill for the 3/8" holes, but might consider a Forstner bit per Jonathan Jung's comment #5.

    See my reply to his comment for further reasoning regarding use of a Forstner bit.

  8. #8
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    For the most accurate work, use metal working tools. Start with an optical center punch. Then if you will use a pointed drill, drill with that. If you use a twist drill, use a center drill.

  9. #9
    Thanks for introducing Forstner bits into the discussion.

    Experimenting on pine/spruce 1x4 scrap, I found that a 3/8" twist drill hole drilled near the endgrain edge (simulating placement of the hole in the actual guitar body) was problematic. Insertion of the anchor required too much pressure, so much that the wood cracked. So I moved up in size to a 25/64" twist drill. That delivered what seems like the right resistance when pressing in the metal anchor. And after two days, the wood has not cracked.

    The anchor is 0.88" tall (about 7/8") and is supposed to bottom out in its hole. Of course, the twist drill hole is "somewhat vague" in terms of depth and bottom contour. A Forstner bit would render a nice FLAT-bottomed hole, and it's center point would create a pilot hole that would make it easy for me to drill the second smaller hole for pressing the anchor out later on.

    I always find it difficult to align a Forstner bit with my center mark. So I thought I'd chuck a very small-diameter bit, align that above my mark, and swap in the 25/64" bit for the actual drilling. But my 25/64" bit is so long that I'd have to lower the drill press table...and that would destroy my setup accuracy. So instead, I thought I might mount a transfer punch in my chuck and align its little end point over my drill mark. Then replace the transfer punch with the 25/64" bit and drill the hole.

    I will experiment with my 3/8" Forstner bit. I don't have a 25/64" Forstner bit, but maybe my Forstner 3/8" will cut a slightly larger diameter hole than does my 3/8" twist drill. If the 3/8" Forstner hole creates the correct interference fit, I'm in business(!)

    Fingers crossed. =D
    Last edited by Tom Burgess; 11-27-2021 at 3:53 PM.

  10. #10
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    Make some test holes in the same wood. I usually use a punch to located the hole so the bit doesn’t wander.

  11. #11
    Quote Originally Posted by Paul F Franklin View Post
    I don't know if the pilot hole would cause the larger bit to wander; I do know a small diameter bit is more likely to be deflected by grain in the workpiece than the larger bit would be, so it might be counterproductive for that reason.
    Great point. Thanks for thinking through this.

    Quote Originally Posted by Paul F Franklin View Post
    If I need holes to be perfectly positioned, I make a drill guide out of hardwood or better, something like phenolic that has no grain.
    If I were a luthier or serious guitar technician I would follow that advice. But this is a one-off (actually two guitars) and budget is a constraint. Thanks for the details; I'm sure they'll come to mind at some later point when I'm fully able to implement them.

  12. #12
    Quote Originally Posted by Matt Day View Post
    Make some test holes in the same wood. I usually use a punch to located the hole so the bit doesn’t wander.
    I wish I had some scrap basswood, but no dice.

    There's an area at the middle of the guitar that's hidden front-and-back by plastic covers (pickguard on the front and backplate on the rear). I've been thinking about using that as my test case, but haven't (so far) as I don't want to be known as a 'wood butcher.' Ha! Actually, nobody but me will ever look in there, and these two guitars will have little to no resale value, despite (hopefully) turning out to be far more playable than anything one can buy off-the-rack in the same price range.
    Last edited by Tom Burgess; 11-27-2021 at 4:53 PM.

  13. #13
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    A 10mm Forstner is so close as to be functionally identical to a 25/64" and may be easier to find/cheaper.

  14. #14
    Quote Originally Posted by Richard Coers View Post
    For the most accurate work, use metal working tools. Start with an optical center punch. Then if you will use a pointed drill, drill with that. If you use a twist drill, use a center drill.
    Thanks for good advice. I sold off my small milling machine two years ago along with most accessories. Been kicking myself ever since.

    Never heard of an optical center punch, but found THIS. Interesting and entertaining.

    I found a set of very small center drills at Harbor Freight (yeah, budget constraints apply) but they're very small diameters and I fear the tips will break off, even in wood (ha!). Or maybe if I just look at 'em hard.

    I appreciate the suggestions, but can't justify the cost of buying real versions. =]

  15. #15
    Quote Originally Posted by Zachary Hoyt View Post
    A 10mm Forstner is so close as to be functionally identical to a 25/64" and may be easier to find/cheaper.
    Good to know. Thanks!

    Just did a quick search online, and metric Forstner bits are few and far between. I found a set for $60 that starts with 10mm, and found a single Bosche 10mm for $22. At that rate, first I'll experiment with my 3/8" Forstner bit (it takes special skill to operate a drill press with your fingers crossed). =]

    If the 3/8" Forstner bit drills the same size hole as the 3/8" twist drill, that'll be too small. In that case, maybe I'll drill the hole short, to 5/8" depth using the slightly larger 25/64" bit. Then mark the 7/8" depth on the shaft of my 3/8" Forstner bit and finish the bottom of the hole to a depth of 7/8" with that.

    The bottom 1/8" of the cylindrical metal anchor is a smaller diameter than the rest of it, so the slightly smaller 3/8" hole bottom won't matter. That would give me a flat hole bottom AND would create a pilot for drilling the small diameter hole necessary for pressing the anchor OUT later.

    I really enjoy the way discussion among knowledgeable, enthusiastic people who like to do and make things leads to solutions.

    Fingers crossed...

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