Official Luthiers Forum!

Hi all.

Building a 12 fret slope shoulder dread in dalbergia Spruceana back and sides. I was sure I had thinned the back to just under 2mm before bracing and gluing it onto the sides. I did think it felt very heavy and stiff but it's my first rosewood build so I carried on. Just measured the overhang before trimming the plate and it's 2.5mm - much heavier than I wanted. Any tips for thinning evenly? Hands sanding? ROS? I'd braced the back very lightly to try for an active back, grr...

Had a similar problem, and I found a nice sharp card scraper made it easier to try and keep the material removal reasonably even as you can see the material being removed.
ROS tends to remove more material very quickly where the plate is supported by the sides and braces, and sanding by hand, you've got quite a bit to go.
With the top, on you could track the change in coupled back, etc, resonances as you go in Gore style.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I would probably use my ROS but like Colin said if you have one and don't have a lot of experience with it you can remove a lot of wood fast. What you do is draw lines across the back with a pencil making like some sort of grid pattern. That way you can observe the evenness as you remove material. Remove all the pencil lines and repeat as necessary. When I thin the edges of the tops I use a block plane followed by an ROS.

Thanks Colin and jf. Despite my previous lack of success with a scraper I successfully sharpened it up and now have sore thumbs and a nice pile of shavings. Unfortunately the shavings are so thin I can't get a measurement on the micrometer so I think I may risk the ROS. Any tips on grits, technique and avoiding swirls appreciated.

Rather than worry too much about having the back evenly thicknessed I would be inclined to graduate it from edge to center, leaving the center slightly thicker. ROS sanders do tend to leave swirl marks, but as the grit becomes finer the swirls become less noticable. Hand sanding with the grain can remove the swirls left by the finest grit.

I start off with 80 and go through 320 on my ROS. On spruce I would start with 120.

So I go 80-120-150-180-220-320 and when it's done it's like glass. Once you get through to 150 then you are just polishing out the previous swirl marks. But be careful with the 1st three grits because those are the cutting ones, especially the 80.