Cut a recess for a speed knob, a la PRS?

[dwizum]

Some of you may be familiar with the controls on PRS guitars. There are little recesses cut into the carved guitar top so the speed knobs sit down below the surface.

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I'm building a guitar with a style for the controls, and I am hoping that someone here has cut recesses like this, because I really can't think of a reasonable way to do it without some sort of odd proprietary router or drill bit. Any ideas?

 

[Jisch]

I bought a router bit to do that, I was going to just lower it into the wood, it's a half round bit. I never tried it, I'm not sure why, probably something I planned to do and forgot about it by the time I got the end of the build.

 

[Bruce Johnson]

That's not as hard to do as it looks. First, the pockets are routed before the whole top is shaped out. That makes it a lot easier. Before the top contouring, those pockets are probably approximately round, with a flat area in the middle. The contouring is what makes them oddly shaped.

Use a routing template, mounted on the flat top surface, and a Cove type bit (rounded shape). Carefully rout down to a depth, watching the thickness left between the pocket and the control cavity coming up from underneath.

 

[dwizum]

@Jisch - what size and profile was the bit you had bought?

@Bruce Johnson - that's about what I had imagined. I guess it just becomes a game of getting a cove bit with the right "shape" - to get the sides as steep as you want them. Just looking at these photos it's hard to tell, but the sides of the pockets look pretty gentle, it must have been a pretty high diameter cove bit.

My top stock is about 5/16 so I don't have a ton to play with. The carve and the pockets will be more subtle than on a PRS. If I leave 1/8" under the control pockets then that means my pockets can only be 3/16" deep. Even at 1/8" I wonder if I should back the top with an aluminum plate or something to keep the controls from getting punched through if they get hit.

 

[Gary_M]

I've seen videos of builders modifying a standard spade or paddle drill bit to do this. (grind a new profile, re-sharpen, but leave the point intact.) Then it's a matter of drilling a pilot hole, then using a drill press and some hand sanding to bring it to that smooth final shape.

 

[Ripthorn]

PRS just use a 1 1/2" ball nose end mill,I believe. Could be 1 1/4". I've done it a couple times, don't be afraid to tweak by hand.

 

[dwizum]

I wanted to close the loop on this in case others find this thread. I got these cut about 6 weeks ago and the guitars have since been finished and turned out wonderfully. I ended up using a cove bit with an oversized template and a bushing on the router. It worked beautifully. I did a few test cuts to get the depth and bushing size correct to get the "slope" I wanted on the sides.

Here's what the template ended up looking like:

Fresh cut recesses:

I just used the cove bit for the entire recess, so you can see the fuzz on the flat center portion, but that cleaned up quickly with sanding. The original plan for these only had three controls, but we switched to a piezo bridge and I went back and added another knob, so the final result was 4 recesses cut.

The top carve ended up being much more subtle than I had originally imagined, so it didn't really "interfere" with these recesses at all. But I think that would have worked out fine anyways and I would have still used this same method, probably cutting the recesses prior to carving.

 

[Ripthorn]

By cove, I assume you mean a core box, round nose, or dish cutter. A cove has the bearing in the middle.

 

[dwizum]

Yes, it was a 1/2 diameter core box bit. I guess I was "in the zone" when I typed cove and not really paying attention!

 

[bdplaid]

Bringing this back to life, How would one do this for an instrument that already has holes drilled for the pots?

 

[stevensivak]

I would imagine that it would not matter. But to be sure, mock it up in scrap...

 

[bdplaid]

the alignment is an issue, to get it centered.

 

[stevensivak]

If the body is clamped, and you’re using a drill press, you don’t really need to worry about centering

 

[bdplaid]

Thanks, but let me be clear: I want to dish out existing holes. They have to be centered. Yes, drill press and clamps. My thinking is to align the press using a drill bit, clamp everything down, then insert the bit into the chuck and go to it.

 

[stevensivak]

Look at the shape of the bit that he used. There was no centering mechanism and thus it’s the same situation as what you’re doing I think anyways.

 

[T_Bone_TL]

Given the actual process used here (following a hole in a template) you'd make a disc the size of the hole in the template, with a dowel the size of the hole in guitar centered in that disc, (or just turn the appropriate shape in one piece on your lathe,) poke the dowel into the guitar and set the template around the disc. Clamp the template (shimmed if need be due to contours) and then remove the disc/dowel. Then go to with the collared router and core box bit in the template hole.

 

[stevensivak]

Why did he even make a template in the first place?

 

[T_Bone_TL]

The recess was cut with a 1/2" core box bit. The recess is blatantly larger than 1/2", therefore the core box bit was run around in a circle, following a template.

 

[stevensivak]

But he could’ve used the right size router bit without a template, correct?

 

[rickwrench]

Bringing this back to life, How would one do this for an instrument that already has holes drilled for the pots?

To cut recesses where there are already holes, use a big cove bit with a guide bearing. The most common guide bearing sizes are 3/8" and 1/2". They swap easily. 3/8" guide bearing fits inside CTS style pot holes, 1/2" fits in the bigger toggle switch holes.
The bit in the pic has the 1/2" guide bearing on it.

Works well. Centers the dish very nicely, too.

Rick