Enlarging Headstock Holes For New Tuners

[renoyd]

I am changing out my Golden Age tuners with a set of of very smooth (and attractive) Rubners. The stem center spacing is spot on but I need to enlarge the hole diameters a bit. I was going to utilize the old bushings which have a litte smaller OD, but I need to enlarge the back of the holes a bit anyway because for clearance of bearing rings at the base of the stems, so, time to redrill the holes all the way though to accommodate both those rings at the back and the new bushings (wich are nicer anyway) at the front of the headstock. I have a drill press and am somewhat OK with woodworking, (though not a first rate woodworker or luthier). So the question: with a good drillbit, can I rely on simple self centering when enlarging or do I need to do something more involved? Start from the front or the back, use tape to preven chipping, etc. etc.? I think I can handle the filling and redrilling of the screw holes which are slightly off. --- OR, should I just leave it all to a real instument repair person/luthier? It is a nice mandolin. Thanks.

[Nevin]

Others who know a lot more than I do will chime in but I would use a reamer. It is nearly impossible to center on an existing hole that large.

[Marty Jacobson]

I use a #6 straight flute taper pin reamer. Make a 1/4" hole in scrap wood, then enlarge the hole with the reamer until the bushing presses in nearly all the way (i.e, leaving about .010" gap) with just the pressure of your thumb and index finger. Then you can tape off the reamer as a depth stop at a little less than that. Using the reamer on a hand drill is safe if you keep the reamer perpendicular to the peghead surface and stop a little early and work in to final size slowly. Don't forget to clear the chips from the reamer's flutes periodically.

Using a taper pin reamer is preferable for this application because you have a relatively large amount of material to remove (.040" or so) and the taper pin reamer will stay centered much better than the typical repairman's reamer (which have a more extreme taper). A violin endpin or peg reamer is similar to the #6 I mentioned, so you could use one of those, too. But you don't usually chuck those up into a drill.

Also, I have found the Rubner bushings to vary by about .005" in diameter on the knurled bit, so make sure you are using THE bushing for each hole to gauge progress, not a representative one for the whole set. Other than that, they are superb machines and the value they represent is almost absurd.
Reamers: McMaster http://www.mcmaster.com/#2990A24
Ebay: http://www.ebay.com/itm/Taper-Pin-Re...item19f6d67760

[EdHanrahan]

Disclaimer: I'm a strictly amateur fix-it guy!

... can I rely on simple self centering when enlarging or do I need to do something more involved?

The "self-centering" of the drill won't do it. When the sizes of the pilot & final holes are close, the differences between end-grain and side-grain tend to push the bit around; maybe bend it slightly, even in a drill press.

When I was in your position and the price of real luthiers' tools seemed astronomical, I found a conical stepped drill bit that worked (works!) well as a reamer. Unlike most that I find on-line now, it has a narrow taper, sort of like this:
http://www.mscdirect.com/product/details/06560619

The downside is that mine was about $4 in 1990 or so!

[John Kelly]

Go with the reamer rather than a drill bit, as the others have said.

[Jim Hilburn]

This is my collection of reamers I've gathered over the years. The one on the left is what Stew Mac sold to work with waverlys and they are just a tad too big for Grover's but by the time the finish is on it requires a re-ream and I use the "P" for that.
Assuming the post holes are already 1/4" then I only ream for the bushings and raised part on the plate, not all the way through. But that's just me. I don't see using a tapered reamer for this job. Those bushings should have a good tight fit top to bottom.