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Drilling precise, repeatable holes.

Discussion in 'Luthier's Corner' started by HaMMerHeD, Jun 30, 2020 at 10:00 AM.

  1. HaMMerHeD


    May 20, 2005
    Good Morning, LC.

    I'm having a problem, and I'm hoping some of you you might have some wisdom you could share. I am working on a multi-scale headless baritone guitar, and I'm having some trouble drilling bridge mounting holes.

    Here's the hole layout: 20200628_192341-01.jpeg

    The bridges are milled from 3/8" stock and painted or powder coated, so their outer diameter is right at 10mm. They mount via small machined aluminum plates, which match the holes above. There are six of them. The crossed holes are what need to be drilled in the body, and the bridge bodies are locked down by screws that go through the bridge body, into the base plate where the uncrossed circles are.

    Right, so the problem is, this is an F-spaced guitar, so string spacing is right at 10.5mm. That leaves 0.5mm of space between the individual bridges. And they're about 75mm long, so any precision error is magnified.

    My drill press was old and abused when I got it 5 years ago. It has about 1mm of runout in the quill, so it's not reliable. I've been racking my brain to come up with a precise jig for repeatably drilling 12 precise holes, and coming up empty.

    The problem is compounded by the fact that the holes in question are fairly small at 5/64", which is not as far as I have been able to find, available with a brad point.

    The only option I have come up with is to mount the bridges to a plate, and then mount that plate to the guitar, like this:

    That helps, in that I would only have to precisely locate 6 holes in the plate (the six set screws), but I'd also have to machine grooves in the plate to preserve alignment. I just don't have the tools for this.

    I got a quote from an online machine shop, but it's $232, which I think is a fair price for all the setup and tooling required, but it's still priced out of reach for me on this project.

    Before I save up my shekels, I thought i'd see if any of you guys had some brilliant solution. So..thoughts?
  2. Bruce Johnson

    Bruce Johnson Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    There's a simple trick to drilling holes accurately in wood: Start with a very tiny pilot hole.

    You've done the right thing with that Cad-generated layout printed on paper. Tape it down on the body, check the location twice. Use a sharp-pointed center punch to make a cone-shaped indent in the wood, right through the cross hairs of your paper template.

    Now here's the trick: Use the smallest drill bit you have, like 1/16" to drill a pilot hole right in the cone. You want a drill bit that's smaller diameter than the cone indent, so the entire drill bit goes down into the cone. It's best to drill the tiny hole with a drill press, so it stays perpendicular, and you can feed it gently. Let it cut by itself as it drills, don't force it down.

    Then, drill the hole with the larger size bit to the final size. The web of the larger bit will follow the little hole down, because it's the path of least resistance.

    If you are drilling a larger hole, say 1/4", then do an intermediate step like: 1/16", 1/8", then 1/4". You want the web thickness of the larger bit to be about the size of the prior pilot hole.
  3. HaMMerHeD


    May 20, 2005
    Yeah, I always drill a pilot. And doing this got me almost there, but some of the holes were still slightly off, by less than half a millimeter. But like I said, I need better precision than that. I think the problem there is more with my eyeballs than my tools.

    I think the best solution is that machined plate. I could keep re-filling and re-drilling, but the common denominator with that will always be my old-ass eyes, which weren't great even when I was a kid.
  4. dwizum


    Dec 21, 2018
    What are you doing prior to drilling the pilot? If you punch the holes prior to drilling pilots you should be able to get them dead accurate. Clamp the bridge to the body and use a transfer punch that exactly matches the diameter of the hole in the bridge and you'll get a precise punch at the center of the hole.

    If you're doing your punching with a center punch instead of a transfer punch, the machine shop instructor I had back in college taught me a neat trick - punch with three light taps instead of just one blow. This allows you to adjust for accuracy. First, make a very light tap to barely mark the surface. Then check that it's accurately located. Most of the time it won't be, since none of us have good eyes! If it is not dead accurate, tilt the punch for the second tap - in hardwood, even tilting at a small angle can help relocate the punched hole by half a mm or more. Then once it's accurate after the second tap, give it a third light tap with the punch held perpendicular to make a clean accurate cone shaped punch.
  5. Gilmourisgod


    Jun 23, 2014
    Cape Cod MA
    I’ve been doing the pilot hole trick for a long time. It works to start a screw, for big holes like a tuner post, you can’t always drill right through with a small pilot bit, they wander. I use hinge drills that self-center on stuff like bridge plates, my eyes and hands aren’t good enough to center holes every time.
    grimjim likes this.
  6. Paulabass


    Sep 18, 2017
    I like my small drills VERY short. Snap one off, and regrind. Use that to start a 1/4" deep hole. Far more precise than 2" of drill sticking out of the chuck.
    Do you still have the offcuts from bandsawing the body? You may be able to tape them on, then build a fence that you can slide the body blank along in a straight line.
  7. Arie X

    Arie X

    Oct 19, 2015
    here's my 2 cents:
    -get a cross slide table for your drill press. a little pricey for one job now i know, but it'll pay for itself in future work and capability.

    -get some optics if you aren't using any now. it's time for a visionvisor i'm afraid :)

    -mount your center punch in the drill press. much more perpendicular than your hands will ever be or you can get one with a centering fixture. they even make an optical one. frankly i don't bother with a centerpunch, i'll just use a centerdrill.

    -a 5/64" brad point probably doesn't exist and isn't needed for that small of a hole.

    -1mm runout on a beat up drill press isn't unusual, but it can be annoying for sure. you might want to repair or upgrade.

    -that modeled drilling template is more complicated than it needs to be. what's with the scallops, angles, and corner radii? eliminate those features and it's a 50 buck part.
    Last edited: Jun 30, 2020 at 12:54 PM
    mikewalker and Obese Chess like this.
  8. HaMMerHeD


    May 20, 2005
    I have a punch. It is not precisely sized for the bridge screw holes, because I don't have a set if center punches for every size hole in the world.

    I have a hard time even getting the punch on center. Most of the time it doesn't matter if its a fraction of a millimeter off, because most bass and guitar parts have some tolerance. But this system seems to have none.

    Like I said, my eyesight is terrible. Even with glasses i have some trouble with depth perception on very near objects so i have some difficulty determining if the point of the punch is dead centered over the mark.

    I think I'll just get the machined plate.
  9. HaMMerHeD


    May 20, 2005
    I don't have the budget for a table for the drill press, or to replace the drill press. The rendering wasn't for a drilling template, but as a replacement for the 6 small bridge mounting plates. This is one of the bridges:

    The small silver piece is what needs to be mounted to the guitar body. The thing I modeled would replace that for all six bridges. The groove is required to properly align the bridges.
    kodiakblair likes this.
  10. Arie X

    Arie X

    Oct 19, 2015
    ok i see. use your paper template for the mounting holes, a drill bit sized for the clearance hole diameter in the base plate as a starting pilot "point". use bruce's sizing up drill bit method to get the hole where it needs to be and install one bridgelet onto the body right over the paper template. position the next one as needed but put a suitabley sized shim next to it to preserve the side to side spacing and repeat.
    Last edited: Jun 30, 2020 at 2:26 PM
  11. HaMMerHeD


    May 20, 2005
    ^^ Seems like a good plan. I'll give it a wack.
  12. swink


    Jan 10, 2019
    Line out your mounting pieces on Double faced tape with correct spacing blocks in between the bridges (hardwood or metal)
    stick it to a test piece of wood and dill pilot holes before you drill the correct holes.
    remove everything from test piece. insert metal tubes with a smaller drill sized inner-tube in the new holes. Cut the tube at same length as the test piece. Use the test piece as guide for the real deal.
  13. Dirk Diggler

    Dirk Diggler Supporting Member

    Mar 3, 2004
    Anytown USA
    It's really all about marking accuracy, if you can't punch it centered, then you'll never drill it centered.

    And absolutely pilot always after a properly located punch. If your punch is too large consider a metal scribe like for inlays.

    Oh yeah do them 1 string at a time, that might help. Only 2 screws at a time that way.

    By the way let us know how those tuners/bridges work out, as headless and fan fret guy, I've been eyeing that variety for a while.

    Good luck and happy consistent drilling.
  14. HaMMerHeD


    May 20, 2005
    I think this is principally what I'm struggling with. I started wearing glasses when I was eight, and by the time I was 18, my vision had deteriorated to 20/440 and 20/480. Glasses make it generally good enough to get around, but I'm having a really hard time telling if the punch is actually centered over the crosshairs of the drilling diagram I've taped to the thing. I have been able to do this in the past, but for this build, it feels nearly impossible. At least I think I have. I don't know that I've ever used a bridge that with as little margin for error as this.
  15. grimjim


    Jan 26, 2014
    Chicago, Illinois
    Endorsing artist;DNA Amplification, GHS strings
    When doing something like this in a cabinet shop, I would make a drilling jig out of 1/4" plexiglass. lay out your holes like you did on the paper peelcoat on the plexiglass, drill the holes by hand with a 1/16" bit, countersink the holes but only halfway through, drill your 1/16" pilot hole to 3/32", clamp your jig onto the body and use a vixbit to lay out the holes. This will give you a centered pilot hole every time and the jig is reusesable. You can also test it out on a piece of scrap to verify it's correct. If not all you've wasted is a little time and some scrap wood, just start over. Once it's dialed in, it really should be on the first try, you'll have a valuable shop jig.
  16. grimjim


    Jan 26, 2014
    Chicago, Illinois
    Endorsing artist;DNA Amplification, GHS strings
  17. WardEarth

    WardEarth Supporting Member

    Feb 18, 2010
    Anchormanville, CA
    I’d recommend starting with a punch. Then a tiny drill. And move up to the size you need.
  18. HaMMerHeD


    May 20, 2005
    Alright. I gave it three tries, each one as imprecise as the last. Seems like .25mm shouldn't be that big a deal, but it's a deal-killer on this project. I feel like I'm making my already s&!t eyesight worse by trying to line this stuff up. I have a luthier friend who builds violins, so I'm going to give him a shot at it. If he can't do better, I'll just buy the damn bridge plate.
  19. BobKos


    Apr 13, 2007
    Not that this has to do with the OP, but I noticed something. The strings all run in a tapered path towards the nut. It is much narrower at the nut than at the bridge. I am thinking there should be a similar offset with the individual bridge pieces unless there is compensation built into the actual string saddles. The photo of the template looks as if the bases are all shooting parallel straight down the body. Just a thought.
  20. tr4252


    May 27, 2013
    I like to use templates. Especially when working with wood, and all its density variations. One thing about a template or jig, if it's not quite right, you can make it over and over until it's perfect without putting a mark on your expensive blank through experimenting. Since this project is (probably) a one-of, you could get away with brass or aluminum; you won't be wearing it out with years of constant use.

    Your CAD pattern is a good idea; I do this myself frequently. If you stuck one to a piece of 1/8" or so aluminum, punched and drilled it, saw that it was good enough (tried it out on scrap wood, in other words), and attached it to your blank, you could be assured that the precision would be more than enough for this type of project. And it would be "repeatable" on other instruments should you decide to use the same bridge layout. (At least until you wore out that cheap and dirty drill jig.)

    I've only been designing, building, and using jigs and fixtures 50 years or so, but I still hear "You can't make a cheap drill jig out of aluminum, the first pass will ruin it". I remind people who say that, that a drill cuts with its point, not its flanks. But it's the sharp sides (think flutes) that tear up a jig, so I knock down the side cutting edges with a few passes of a stone.

    Also, you will find that your paper pattern stuck to your brass, aluminum, whatever, will make it easier to position your center punch; it helps keep the punch point from skidding around while you're trying to line up a hammer blow. (try an automatic center punch some day, you won't regret it).

    I'm getting long winded, but if any of this sounds like it might be useful to you, I have all these years of dirty tricks for making stuff work.

    mikewalker likes this.

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