Grain-matched control cavity covers

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Grain-matched control cavity covers

Postby Steve Sawyer » Mon May 14, 2018 11:57 am

I've admired the guitars with control-cavity covers that are cut from the back of the body blank so the grain matches the back of the body (usually secured with rare-earth magnets).

Since I'm working on a chambered body, with both the front and back being "caps" on the center portion to be band-sawed for the chambering, I'm wondering if I could give this a shot.

It would seem that I could simply use a very fine blade in my scroll saw to cut these out of the back cap. The question would be then, what is a good technique for providing a "ledge" on which these covers will rest? I can think of a number of methods (some of them pretty janky! :) ) but so many have done this, I'm sure there are some tried-and-true methods. I did a search for this, but didn't seem to get any useful hits.

Would appreciate any pointers to threads, videos or other resources to help me figure this out.

Thanks!
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Re: Grain-matched control cavity covers

Postby Freeman Keller » Mon May 14, 2018 4:24 pm

Since my solid body and chambered electrics are all built out of wood that doesn't have strong grain (ie mahogany) I just use another piece of mahogany to make my covers.

Image

Image

And with a totally hollow body you certainly could glue a piece of wood inside to support the covers (and their screws or magnets) I just route the cavity to leave a lip.

Image

If I ever have a totally beautiful piece of wood that I want to perfectly match the grain I'll probably take it to a friend with a CNC laser to cut the pieces out
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Re: Grain-matched control cavity covers

Postby Christ Kacoyannakis » Mon May 14, 2018 6:05 pm

When I bought my mahogany body blanks, I asked the seller to sell me slightly thicker blanks, and then he cut a slice off the back. Since this is pretty straight grained mahogany, the grain on the thin slice should match pretty well. I think you technique should work well too. By the time you put finish on the back and the covers, there should not be much of a gap at all.
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Re: Grain-matched control cavity covers

Postby David King » Tue May 15, 2018 1:06 pm

I take a slice off the back for my covers but I've also done the cutouts with a 1/16" end mill in the router and then bound the edge with a contrasting veneer to make up the kerf.
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Re: Grain-matched control cavity covers

Postby Steve Sawyer » Wed May 16, 2018 3:09 pm

Thanks, folks.

Freeman - I may just back off and use your technique. I should have some nice scrap from cutting the chambering that should match the back well enough, and I've already COMPLETELY violated my KISS intentions when I started this build, so maybe I should think about doing the grain-matched covers on a future build, particularly if I use some heavily-figured stock. This is ribbon-stripe sapele that I'm using, so it should look fine, even if the grain doesn't match exactly.
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Re: Grain-matched control cavity covers

Postby Randolph Rhett » Thu May 17, 2018 9:40 pm

Slicing a piece off the blank before you build is the only sure fire way to get a tight fitting grain matched cover. Even with a scroll saw, the kerf will probably be bigger than you are looking for. I don’t have a picture, but I’ve used a cutoff of the top rather than try to match the back wood. Of course, you are sandwiching two drop tops, so that wouldn’t work for you.
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Re: Grain-matched control cavity covers

Postby Steve Sawyer » Sun May 27, 2018 11:50 am

Freeman - any advice on a thickness for the covers? I'm thinking about 1/8".

Also, how wide a supporting ledge should I shoot for? Since I'm thinking flat-head screws to secure the covers, it would seem I'd need 1/4" at least.
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Re: Grain-matched control cavity covers

Postby Freeman Keller » Sun May 27, 2018 5:17 pm

Steve Sawyer wrote:Freeman - any advice on a thickness for the covers? I'm thinking about 1/8".

Also, how wide a supporting ledge should I shoot for? Since I'm thinking flat-head screws to secure the covers, it would seem I'd need 1/4" at least.


I'm pretty sure I used some cutoffs from bending acoustic guitar sides - that would make them 0.075 or 0.080 - I just used what I had. On the big cover I was worried that it might split so I glued another piece cut out to fit the cavity with the grain running at right angles

Image

I don't remember what the sizes are but I left a bigger area for screws in the middle of the sides

Image

I remember that Les Paul covers are not symetrical and it was a real PITA to make the covers to fit (notice the router bobble in the 2nd picture). I would jig something up differently next time to use the same template for the body route and the cover. Maybe you could do something in your modeling software.
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Re: Grain-matched control cavity covers

Postby Steve Sawyer » Sun May 27, 2018 6:53 pm

Freeman Keller wrote:I remember that Les Paul covers are not symetrical and it was a real PITA to make the covers to fit (notice the router bobble in the 2nd picture). I would jig something up differently next time to use the same template for the body route and the cover. Maybe you could do something in your modeling software.


Yah. I spent the day making templates for just this purpose. As I was doing it, I kept wondering if this is insane, and that the more experienced folks around here would say "WHAT in the Sam Hill are y'all DOIN' there boy?"

However, it seems that while a bit time-consuming it should work really well. I'll post the process with pics when it's completed if you're interested.

Good suggestion on the cross-ply reinforcement. Especially if I go as thin as you do. That's less than 2/3 what I was thinking.
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Re: Grain-matched control cavity covers

Postby Beate Ritzert » Sun May 27, 2018 6:57 pm

Maybe this is also a viable option for someone?

Image

Image

I did that today. With a mechanical fretsaw i would have probably more precise. The sawing gap will be covered with contrasting wood, in this case ash (just because i have a sufficient stripe of veneer; maple would be an even better match because the top will be flamed maple.)

The idea: resaw the cover from the remaining piece, also the posts to fix it to the body. The hole will be turned into a recessed grip. That's why i made it that large.

Another possibility might be using a bottom plate for an acoustic guitar in addition to a top plate. Or resaw the bottom plate from the body slab.
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Re: Grain-matched control cavity covers

Postby Freeman Keller » Sun May 27, 2018 8:34 pm

Steve Sawyer wrote:
Yah. I spent the day making templates for just this purpose. As I was doing it, I kept wondering if this is insane, and that the more experienced folks around here would say "WHAT in the Sam Hill are y'all DOIN' there boy?"

However, it seems that while a bit time-consuming it should work really well. I'll post the process with pics when it's completed if you're interested.

Good suggestion on the cross-ply reinforcement. Especially if I go as thin as you do. That's less than 2/3 what I was thinking.


As I recall my LP plans show the normal Gibson size and shape - I think they give some measurements. I used the plans to make a quick and dirty router template for the basic shape. The holes themselves were drilled out with Forstner bits. After routing the recess I simply traced inside the template on a piece of mahogany and cut it out. My thinking was that if it didn't work I could always drop in the black plastic Gibson items (not!).

Beate - I'm impressed. I can't imagine doing that - its going to look stunning
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Re: Grain-matched control cavity covers

Postby Steve Sawyer » Sun May 27, 2018 9:58 pm

I'll second Freeman's comet, Beate. Not sure I'd have the patience or persistence to cut a piece like that with a fret saw!

That's going to look awesome.
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Re: Grain-matched control cavity covers

Postby Steve Sawyer » Thu May 31, 2018 8:53 pm

As I promised to Freeman, here is the method I used to rout the control cavities. As I mentioned, this seems to be very involved, but I have to say that the results were better than I'd expected.

First, I should mention that when I did my first build (a Telecaster copy) I made extensive use of 1/8" stainless steel dowel pins and 1/8" doweling run through a quick-n-dirty doweling plate for alignment. This was in part because I made a template that not only included the pattern for the outline of the body, but also for the neck pocket, pickups and control cavity, all of which turned out to be a mistake. However, I really like the ability to guarantee that the templates always go on properly aligned, so I continued this practice with this build, especially since I am doing a chambered body, so every piece in the "stack" for the body would be properly aligned along with the caul I made for vacuum pressing the whole thing together. Gotta love aircraft drill bits! :)

I placed bores for alignment pins at the center of each of the four control pots, the dead center of each pickup rout, and the location for the pickup switch. I only carried two of the four holes for the control pot shafts through to the back out of laziness if nothing else.

The next step was to make a pattern for each of the control cavity covers out of MDF, carefully cut and smoothed so they are exactly the shape desired. Each of these pieces also had alignment pin holes so that they could be placed accurately onto the body.

Control Cavity Rout 1.JPG


Clamping the original pattern on another piece of MDF long enough to encompass both cavities, I used my favorite hand-drilling jig to place the appropriate alignment holes.

Control Cavity Rout 2.JPG


Placing the patterns for the covers onto this piece of MDF, I traced their outline with an extremely fine (03 gauge) drafting pen (I have a whole set of these and LOVE using them in the shop for marking).

Control Cavity Rout 3.JPG

Control Cavity Rout 4.JPG


I used a scroll saw to saw as close to the inside of the lines as I dared, then brought the pattern right up to the marked edge using an oscillating spindle sander. I kept test-fitting the cover patterns into these holes until the would go in without effort, though I did have to have them aligned perfectly to be able to slip them in.

Control Cavity Rout 5.JPG


Continued due to picture limits...
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Re: Grain-matched control cavity covers

Postby Steve Sawyer » Thu May 31, 2018 8:54 pm

Part 2:

Next step was to use the alignment pins to attach the cover patterns to the back of the guitar body, and to use these to align the cavity cover opening pattern to the back using double-sided tape.

Control Cavity Rout 6.JPG


As you can see in the next picture, the fit is pretty good with the exception of a bit of an excess gap on the "head" end of the control cavity cover pattern. I thought of filling this gap in the pattern with some slips of paper and CA glue, but decided that it would need to be cleaned up afterward with a sharp chisel, and that will be easier to do on the cover than it is in the opening, so I'll put some masking tape or some thin veneer along that edge when I make the cover, and adjust that to fit the slightly off opening.

Control Cavity Rout 7.JPG


And here we are, after removing the cover patterns, ready to cut the control cavities.

Control Cavity Rout 8.JPG


I hogged out much of the control cavities with some Forstner bits and a trim router with a 1/4" spiral down-cut bit, then proceeded to cut the opening for the covers on both with a 3/8" guided bit.

Control Cavity Rout 9.JPG


As you can see, the cover patterns fit perfectly into the routed openings. Believe me, I had to hold my tongue right to get them to slip in - if they're not held perfectly parallel to the back of the body, they ain't goin' in! :) However, I figure this is perfect as I can adjust the fit with a sharp chisel or even a sanding block after I get the covers cut and planed to thickness.

Control Cavity Rout 11.JPG
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Re: Grain-matched control cavity covers

Postby Beate Ritzert » Thu Jun 14, 2018 6:53 pm

Steve Sawyer wrote:That's going to look awesome.


Unfortunately not, at least not as good as i hoped - the blade of that fret saw was fairly wide and produced a wide slot. In addition i made a few mistakes and cut a bit off the line in a few places. Which means that the cover will need a binding wider than planned. Several years ago did something similar on a bottom plate of 5 mm thickness and fitted the cover using a binding of 0.3 mm thickness on each edge. Here i will need at least twice the thickness.

Meanwhile i cut the cover from the remaining block of wood and then two posts to hold the cover which i fitted into the cavity.

Image

Image



A huge amount of work, much more than justified for the effect. next time i'll need to find a quicker way. Moreover, the cavity must be extended into the top as well:

Image
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Re: Grain-matched control cavity covers

Postby Steve Sawyer » Thu Jun 14, 2018 10:55 pm

I was wondering how you planned to support the cover! :)

Hadn't thought of binding the cover, but that could be an interesting design element...

I like your router plane. Does it work as well as it looks?
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Re: Grain-matched control cavity covers

Postby Beate Ritzert » Fri Jun 15, 2018 3:32 am

It would work even better if i was able to get the iron really sharp ;-)

But i am actually satisfied with the surface i got on that strongly flamed maple...
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Re: Grain-matched control cavity covers

Postby Beate Ritzert » Fri Jun 15, 2018 10:11 am

Another option - a lot easier if the control cavity comes into the upper horn:

Image

In a shop with appropriate tools (e.g. large and precise bandsaw with thin blade) it might be an option to cut a slice of about 4-5 mm from the body blank and then use an electric fretsaw or a router with a narrow bit to cut the cover. Then glue the slice back to to body.

If the tools are absent, the wood dealer might be able to do the cut for a fair price.
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Re: Grain-matched control cavity covers

Postby Steve Sawyer » Fri Jun 15, 2018 10:23 am

That was originally my plan, but faced the saw-kerf width issue. The smallest scroll saw blade I could find was .022"/.5mm, and that doesn't take into account the hole needed to start the cut.
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Re: Grain-matched control cavity covers

Postby Beate Ritzert » Fri Jun 15, 2018 10:34 am

But You can provide a cutout for the fingers and place the hole there. A clean cut of 0.5 mm should be the width you need to provide for such a gap anyway.
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