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Thin a top

I built this mandola a few years ago from a nice Cohiba box. Turned out well; the neck is bolted-on and it's held the tension of the full eight strings for several years.

I've started in improving it, however. I've removed the glued-on copper-wire frets and I'm going to install real frets.

I would like to experiment with thinning the top down. It's solid cedar, and about 7mm thick. I do have it cross-braced.
I'd like to take that down to about 4mm to see if the sound improves.

I don't have access to a surface planer... I considered just putting sheets of sandpaper on a glass surface and starting to sand, but that's going to be tedious.

Anyone suggest a more efficient method?

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Permalink Reply by ChickenboneJohn on August 10, 2017 at 6:45am

If you can take the top off, remove the bracing and simply reduce the thickness with a hand plane. It's quick, clean and accurate - no fancy power tools needed, just a good sharp blade. Using a router just makes the job more complicated and it's the wrong tool for the job. Sorry, but it really bugs me when people suggest power tools instead of more appropriate hand tools for a simple job like this.

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Permalink Reply by bigmcgiv on August 10, 2017 at 9:30am

I would use a trim router harbor freight has one for 30.00 pre drill your depth and go crazy on that lid. clean up wit a sharp chisel or razor knife sand if needed

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Permalink Reply by Paul Craig on August 12, 2017 at 9:22am

Sharp chisel and tap with a light hammer.

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Permalink Reply by Taffy Evans on August 12, 2017 at 11:17am

Hi again, I would like ask two things. What is meant by "go crazy with the the router"? What technique jigs are used? And, even if you use a very sharp chisel it's blade would have to be long enough to span at least half the box width. I would suggest a plane as it it is what is used traditionally for thinning tops by hand.
And I would imagine having to finish off with a sander, sanding block and/or cabinet scraper.
As the top is cedar you could use a sanding block. Hog off cross grain until close then go finer with the grain.

I do my acoustic guitar tops final tuning/thinning using sanding block and cabinet scraper, after it is glued to the rims.
Just wondering how you used those methods, not saying it's wrong.
Cheers Taff

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Permalink Reply by bigmcgiv on August 12, 2017 at 11:45am

I agree with taff. Every one has a cabinet plane in their shop to work on their Stradivarius. Just a little humor.

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Permalink Reply by Mark Werner on August 15, 2017 at 8:23am

So, here's what I ended up doing:

As I said, I realized that I had a "router" of sort, one of those side-cutting drills which takes router bits.

So I bought a surface-cutting bit and built this jig:

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Permalink Reply by Mark Werner on August 15, 2017 at 8:26am

Here it is with the top in place:

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Permalink Reply by Mark Werner on August 15, 2017 at 8:27am

And finally the mostly-finished top:

What a mess... Should have used my shop vac as I was cutting....

Now to re-fret.

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Permalink Reply by Paul Craig on August 15, 2017 at 9:46am

That will work. Great job with the jig.

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Permalink Reply by Taffy Evans on August 17, 2017 at 12:19pm

Hi, did you get a more responsive top from this exercise as you wanted? Just one comment I would make. I did not realise you would be removing wood from the inside of the top leaving bracing in place, my comments were relating to thinning the whole top from the outside or inside surface, but you would have lost the logo.

My comment/advice is, for those who choose this method, is to think about removing the bracing and thinning the whole top and then replacing with same size or modified bracing that would further enhance the response of the top. I admire your dedication to achieving better sound, but that little extra work removing bracing may provide an even better outcome. Well done anyway.

Cheers Taff

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Permalink Reply by Mark Werner on August 17, 2017 at 12:26pm

We'll see how it turns out. I'm waiting for the new fret material now.

I did want to leave the top intact, both for the looks and to leave the action unaffected. If I cut away the outside, I'd have had to construct a new bridge.

Also, those braces were glued on with Titebond III, a permanent glue. I didn't want to risk damaging the top.

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Permalink Reply by Ron "Oily" Sprague on August 19, 2017 at 12:43pm

I like that jig! Please provide materials and dimensions.

Also, about those braces. Take a lesson from commercial git bracing. Thin those down from the middle toward the ends with a plane or sand paper, and also, give them a more triangular cross section: thicker at the bottom where they are glued, and thinner "upward" as in the pic - some sand paper, a sanding block, and elbow grease will remove the material fairly quickly. You'll get the same bracing, but the top will be even more responsive, so the git will be just that much fractionally lighter.