Blank thickness for glue up

[Graydon Stephens]

I recently purchased flamed maple back and sides online. To my surprise the back was 0.25" thick and the sides 0.23". I have no access to a thickness sander or planer. So i built a router sled and using a 1.25" mortice bit it is working out well, so I want to go as far as I can on the back blanks prior to glue-up. After glue up further thicknessing would be done with a cabinet scraper. My question - how thin can I go prior to glue-up? Can the glue joint be structurally weaker if blanks are glued too thin? I ask because a wood supplier told me they were making their blanks thicker at the request of customers and it had to do with gluing the blanks. Thanks.

[Chuck Morrison]

It depends on how thick you want the back to be. I try to get back sections pretty close to final thickness before gluing, realizing that I need to get the joint very close to perfectly aligned. If I don't succeed, then I end up with a back that's thinner than I'd wanted.

Do you want a thin vibrant back or a stiff reflective back ? With maple I would use a finished thickness of ~3.2 mm (1/8") for a reflective back. So hoping for a good joint I'd get the pieces down to 3.5mm or so before gluing. That doesn't leave much room for overlap. When I was into vibrant backs I would often go down to 2.2mm thick or less.

[Graydon Stephens]

Thanks for the input. My first build was a walnut back 0.085 thick. I am targetting 0.095 - .105 with this build and was hoping to only have to remove about 0.010 after joining.

[Tim Allen]

How thin you can go before glue up depends on your set-up for gluing and how skilled you are at using it.

I can get backs pretty thin before gluing by using my go-bar deck to hold them flat. I hold one side from moving with a batten screwed to the deck, and press the other side inward using eccentric cams along the outside edge, which are just 2" discs of wood screwed to the bottom of the deck. The screw is about 3/8" off center so turning the cam pushes the wood toward the joint. I put cheap plastic packing tape under the joint to keep the back from being glued to the deck. This aligns the pieces pretty well, but even with using bars all up and down the joint I think the joint is still a little more than .01 off in places.

My recommendation would be that you glue some scraps of 1/8" masonite or something, using whatever set up you have, and see how close you get to perfect. Then you'll get the answer that applies to you.

[Graydon Stephens]

Thanks. The set-up I used on my first (and only) acoustic build was an old school baton-and-nail press, as outlined in Jonathan Kinkead's book. The joint was perfect (beginner's luck??). Got the same result with the spruce top, so I'm using that method again. This may sound dumb, but in regards to whether you can go too thin for gluing I thought someone might respond that your likely to get a more stable joint if you glue-up at say 0.140 and then reduce to your final thickness. And my thought would be, yeah I was afraid of that... I glued-up thick on my first build and did not enjoy removing all that material with a hand plane and scraper.

[Tim Allen]

Well, if the joint is perfect, that's probably good enough. : D

I've never heard that making the joint in thick wood and planing it down makes it a better joint. I think the practice of joining it a little thick it so leave room to plane if there is minor mis-alignment.

Typically backplate joints aren't much of a problem in any case. It's tops that seem to want to split at the center seam. I think you're good to go.

[Alan Carruth]

One of my students once got a maple back that came in sanded to 2.6 mm thick. Since I like to make a back like that at 2.5 it didn't leave up much wiggle room. It took three tries to get it; fortunately it was wide enough.

I do much of my thicknessing with a Wagner Safety Planer. Making them thinner is not too hard, but I have yet to figure out a good way to make them thicker...

[Dick Hutchings]

This is where a nice thickness sander comes into play. I find I use mine quite a bit and bringing a guitar top or back from 1/4 to 1/8 is no issue at all. I tried doing it with hand planes because I love using them but for backs and tops, my thickness sander is my favorite tool.

[Graydon Stephens]

Gents, thicknessing with the router sled went fairly well. I took the back blanks down to 0.125 (+/- 0.005 in spots). After glue-up there was some surface misalignment, not 100% flush in spots. After final leveling with cabinet scraper, I think I'll be at 0.105 (+/- 0.005). Tim, so much for "perfect" joints, they don't exist. : )

[Clay Schaeffer]

" Making them thinner is not too hard, but I have yet to figure out a good way to make them thicker..."
Another layer of veneer, and some West System epoxy....