Methods to increase tuner bore holes in headstock

Tom0Blam0

Tom0Blam0

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Mar 16, 2014
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#1
I have a bass with irregularly sized bore holes. I also have some tuning machines that fit 11/16" bore holes. So my plan is to enlarge the existing holes, but I want to make sure it goes as smoothly as possible. I have a drill press, but I want to make sure I use an effective process. I was thinking of using a step bit to increase the size to 11/16" from the back of the headstock and then increase the entire size of the hole to 11/16" with an 11/16" spade bit.

My question is, will this strategy work? Is this the most effective method of going about increasing the size? Alternative strategies that would work better/be safer. I want to avoid damaging the neck as much as possible.

Any advice is much appreciated.
 
#2
Other than the spade bit it sounds like a good approach. I would use the step bit to enlarge the hole from both sides of the headstock, then finish it with a spur bit. Regular twist drill would also work rather than the spur bit. I generally avoid spade bits wherever possible - they are a pretty crude device.
 
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#3
The neatest and safest way to enlarge tuner holes like that is with a router and a small (like 3/8" dia) top-bearing bit. Take a piece of MDF or hardwood, and make a neat 11/16" hole in it with your spade bit or whatever you have. Tape and/or clamp that onto your headstock, lining it up over the old hole just as you want it. Use the router, gently, to enlarge the hole, working the depth down in 1/8" steps.

The faster alternative is a special counterbore bit. I have ones that I've machined up myself, and supplied to other Luthiers. But you probably don't need to go there for a one-off repair like this.
 
#8
Being a machinist I'd use my trades approach. Definitely secure the headstock as best as possible, angularity and all. Indicate each hole individually with a dial indicator and REAM or possibly endmill each one. Might be time consuming but I'd be confident it was done properly.
 
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#9
saltydude said:
Being a machinist I'd use my trades approach. Definitely secure the headstock as best as possible, angularity and all. Indicate each hole individually with a dial indicator and REAM or possibly endmill each one. Might be time consuming but I'd be confident it was done properly.
Click to expand...

certainly my preferred choice (i own a mill) but not everybody has access to one. :) tool and die work ruined my life -now i can't even whittle a stick without some level of precision over-kill involved. i have a severe quality problem.
 
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#10
If you decide to drill thru, I would clamp a scrap piece of wood over the exit side of the hole, and let the drill go into the scrap wood. Otherwise the headstock wood can tear out. Even better, predrill the new hole thru another piece of scrap and clamp that on the entry side as well, using it as a guide.

What I know as a spade bit won't work unless the hole hasn't been drilled already, but a small pilot hole might be ok.
 
#12
The problem with a tapered reamer is that it makes a tapered hole. The bushing is designed to go into a straight hole. In a tapered hole, the bushing will be too loose at the top and too tight at the bottom.

You can use a tapered reamer to bring the top edge of the hole to 11/16", then use that to guide-start a straight 11/16" drill bit, turning it gently by hand. Riskier than the other methods we've talked about, but it will work.
 
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#14
If you try to use a Forstner bit to enlarge an existing hole, it will jump and kick and chatter as it tries to get started. Just like a brad point, it needs to locate its point in something solid.

But there are two ways that you can use a Forstner bit to enlarge a tuner hole:

One way is to plug the hole first. Make up a wood plug from a piece of dowel or scrap wood and lightly glue it in place. Locate the center carefully and drill a little hole, like 3/32" down through it. Now you can drill the hole out with the Forstner bit (or a brad point bit). The point of the Forstner bit will follow the little hole.

The other way is to pilot on the OD of the Forstner bit. If you have an 11/16" Forstner bit, use it to drill a hole in a piece of 3/4" MDF. Then clamp that piece of MDF tightly down onto the headstock, lining it up over the existing hole. With the drill press shut off, lower the Forstner bit down partway into the hole in the MDF. Holding it in there, start the drill press and start drilling.
 
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#15
I've never done this on a bass but to enlarge 1/4" guitar tuner holes to 10mm I have a 1/2" reamer that I ground off at approximately the 10mm point so I run that through the existing hole then clean up (carefully) with a 10mm drill bit. You might have to look a little to find a reamer big enough for bass tuners. A machine shop could do the grinding. Don't know if the cost is worth it for a one time job, however.
 
#16
Well yeah, the neatest, fastest way to enlarge a tuner hole from one size to another is with a custom pilot reamer. Like this:

IMG_1826B.jpg
IMG_1827B.jpg


That's exactly what this is: A custom made reamer which fits in one size hole and enlarges it to another size. A couple of seconds in a drill press. I make them up from tool steel or stainless steel. With a lathe, a mill and a spin index fixture, they are really quite simple to make. I have maybe 30 of them I've made up in various sizes for different jobs. This one is actually for tuner post bushings. I've made custom reamers like this for many pro Luthiers.
 
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#19
Bruce Johnson said:
If you try to use a Forstner bit to enlarge an existing hole, it will jump and kick and chatter as it tries to get started. Just like a brad point, it needs to locate its point in something solid.

But there are two ways that you can use a Forstner bit to enlarge a tuner hole:

One way is to plug the hole first. Make up a wood plug from a piece of dowel or scrap wood and lightly glue it in place. Locate the center carefully and drill a little hole, like 3/32" down through it. Now you can drill the hole out with the Forstner bit (or a brad point bit). The point of the Forstner bit will follow the little hole.

The other way is to pilot on the OD of the Forstner bit. If you have an 11/16" Forstner bit, use it to drill a hole in a piece of 3/4" MDF. Then clamp that piece of MDF tightly down onto the headstock, lining it up over the existing hole. With the drill press shut off, lower the Forstner bit down partway into the hole in the MDF. Holding it in there, start the drill press and start drilling.
Click to expand...
I did so with excellent results by using a drill press and clamping the bass solidly to the table.
 
#20
Bruce Johnson said:
Well yeah, the neatest, fastest way to enlarge a tuner hole from one size to another is with a custom pilot reamer. Like this:

View attachment 2743928 View attachment 2743929

That's exactly what this is: A custom made reamer which fits in one size hole and enlarges it to another size. A couple of seconds in a drill press. I make them up from tool steel or stainless steel. With a lathe, a mill and a spin index fixture, they are really quite simple to make. I have maybe 30 of them I've made up in various sizes for different jobs. This one is actually for tuner post bushings. I've made custom reamers like this for many pro Luthiers.
Click to expand...


Yes sir. And if I was in "official" business I'd be asking how big of a check you want for a couple.:thumbsup:

For now, I'm muddling through with what I have.

Always a joy to see your work though.
 
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