MIMF

I'm making my first acoustic guitar, an OM-style with sapele back and sides, following instruction from Tradition and Technology, and referencing the OM plans from LMII. I successfully hand-planed the back and top to the proper thickness, but got a little over-zealous with the sides - I was aiming for .085, but in some places they're as thin as .06. Is this too thin to be structurally stable? My current plan is to use cutoffs from the neckblock and tailblock to make side braces, four per side, that would extend through the kerfing and be made of the same wood and with the same grain pattern as the sides to prevent possible shrinkage issues, but if the sides are too thin for this to help I'd rather just order new sides now.

Instead of using cutoffs to brace the sides, I'm also considering using the bending practice sides I ordered with the rest of the wood to laminate the curved areas. However, the practice sides are pretty nice Indian rosewood (there's a knot hole near a corner making them imperfect, but the hole is very close to the edge. I could cut it off and still use them to make a guitar more than four inches deep), and if this wouldn't be a better solution than the more standard side bracing I'd rather keep the practice sides and make my next guitar out of nicer wood for slightly cheaper.

Thanks for any input! This forum has been super helpful, just going through old posts and reading stuff from people who've already had the same issues as me. I searched for information on this topic but didn't find anything, my apologies if this has already been covered at length.

I would say that is too thin (1.5 mm) as you will also do final sanding once the box is assembled.
You could possibly laminate with some other wood, but still you will need to be careful in your sanding to not go through.

LMII will sell you another set of Sapele sides. Sapele is eager to crack when bending, so replacing sides alone isn’t uncommon. You still get to practice bending with the sides you have. I like my sides to end up in the .076 to .083” range, lower in the range for Sapele. Sometimes, if I’m doing a really tight bend, I’ll make the side a bit thinner just in that place (it gets thicker when bent).

Thanks for the input! They're not a uniform .06, just in a couple small spots - I stopped planing after I realized how thin I had gotten it. They're mostly .075-.08. I think what I'll do is bend them first, trying to keep the thinnest parts near the tight waist bend, and measure afterwards to see if it's swelled sufficiently. If not, I'll laminate with the rosewood. I only planed on one side, so if I have the factory side on the outside, I don't expect I'd need to do much more sanding than with the higher grits to prep for finishing, and if it goes through the sapele to show the rosewood, it'll just be a cosmetic issue - I never expected my first guitar to look perfect.

If I needed to buy individual sides from LMII, would I just email them? They only sell sapele as a set on the website.

Email them. They try very hard to please their customers.

Some classical guitars have sides that are only 1/16th inch thick which is what you are dealing with in only a few spots. OM guitars aren't any deeper. You might add a couple of extra side braces, but if they bend o.k. I would use them.

I found this thread on Mandolin cafe:

https://www.mandolincafe.com/forum/thre ... -thickness

There is one thing to keep in mind when you use thin sides. You need to keep your bindings flush or a hair proud. Some people suggest leaving the bindings slightly set in, and sanding the sides to meet the bindings. If your sides are already at the lower limit you want to avoid much additional sanding (but you already knew that).

Well, they bent beautifully. I was worried because I've read so much about how sapele is difficult to bend, but these were easier than the rosewood practice sides, possibly because of how thin they were. They indeed swelled up a bit after bending, they've gained .005-.010 inches all around (especially near the waist and upper bout) after letting them sit for five hours, so I'll use them as-is with a bit of bracing and some masking tape wrapped around the router bit when I cut the binding channels. I would redo the sides if I was making this for someone else but since it's my first guitar and just for me, I'll just be prepared to get some experience repairing cracked sides in a couple years. Thanks for the help and suggestions everyone!

They won't crack. They will be fine. One thing you should have learned from this experience is that thin sides bend easily.

Note that side tapes are also good for keeping sides crack free. If the sides cupped at all in the 'straight' part below the waist you might want to use a wood fillet there. I put any such things on after profiling the sides and before putting on the liners, so the reinforcements end up under the liners.

Try to see if it cracks when pressure is applied.

I'm going through something similar with my sides, on my first build. They're about .065". So far they've bent and assembled into the garland well. I may glue muslin to the inside to guard a little against cracking, but I might not need to.

https://www.facebook.com/photo.php?fbid ... 000&type=3

Remember that you will remove more from the sides as you install and scrape bindings and then during final sanding. Your .065 may turn into .055 sides. I would be tempted to laminate a .023 standard veneer on the inside. This will do no harm and may make the guitar sound much better.