Back plate question... pieces

[Nathan Ashmore]

Has anyone ever pieced together a back similar to this? I have a nice piece of cocobolo that is two inches short of being long enough to make a back plate. It's dimensions are 6 3/8" x 18" x 7/8". I would use H. rosewood for the center strip and cocobolo for the side strips.

[Barry Daniels]

Yeah, that has been done about a zillion times. Martin was likely the first.

[Doug Shaker]

I don't think the issue of concern is that there will be a center strip of Honduran Rosewood. I think the issue is that each of the two cocobolo panels will be made of two pieces of cocobolo, one small piece near the neck and a larger one for about 85% of the length.

[Alan Carruth]

Piecing along the length is problematic. The FAA allows plywood for aircraft structures to be pieced that way by use of a scarf joint. Iirc, it has to have about an 8:1 slope: if the back's 2.5mm thick the glue surface would need to be 20mm wide. Not easy... OTOH, they're not envisioning any sort of backup on that: it might work with a suitable reinforcement inside. If you can get the glue line to go along the grain at some angle, that would probably be better, and could be made into a design feature. I could see a 'Hippie' theme, with a 'peace' symbol.

[Nathan Ashmore]

I like the scarf joint idea Alan. Maybe using a 22.5º angle would work for the glueing surface. That way I could still brace along the seem on the inside.

[Barry Daniels]

Oh, I thought this was a standard 3-piece back.

[Nathan Ashmore]

Nope, a little more tricky. I would have no problem with just a 3 piece. It's the joining together of cocobolo pieces along the back that worries me.

[Jason Rodgers]

What's that method called, wherein you cut a diagonal through the square/rectangle, slide the two pieces along the diagonal, and glue them to make a longer piece? Like this...

If you did that with your larger rectangles, the resulting pieces would be narrower, requiring wider center strips, but this longer diagonal end-grain joint might be better than the short perpendicular joint in your drawing. Some degree of grain-matching may be possible, as well.

Or, save these pieces for a tenor or uke.

[Nathan Ashmore]

Thanks Jason. Another great idea but I'm still leaning toward the scarf. I'm just wondering about structural integrity. Is a longer glue line better than a shorter one?

[Steve Senseney]

The more dissimilar the materials, the harder the butt joint will be to hold.

If you have the different movement with moisture changes, it will probably be harder to keep the joint together.

The scarf will strengthen your joint, but that will still be difficult to cut perfectly straight, and to hold as you are gluing the pieces, and it will not be a 1:8 angle.

I tried a multipiece back on a guitar, and some of the joints did not hold perfectly.

I would suggest that you lay a wide marriage stripe across the entire area to strengthen these joints. Thin it down further than you might otherwise to avoid excess weight.

[Doug Shaker]

I think I would laminate a sheet of spruce veneer under the whole thing. It would still look cocobolo on the outside and have cocobolo's resistance to dings on the outside, but have some real structural integrity on the inside.

Or, if you wanted to go experimental, a thin sheet of graphite fabric epoxied to the cocobolo on the inside.

[Nathan Ashmore]

Ok.. so I'm thinking if I bookmatch the cocobolo top to bottom with a scarf joint per left and right side and use the rest of the cocobolo as a reinforcement strip over the joint. All of the cocobolo grain would be running the same direction. If not I'll just design a parlon body that is 17.5" long...

[Nathan Ashmore]

And here is the idea so far...

[Alan Carruth]

The whole point of the scarf joint is that end grain doesn't glue well. The closer you can get to side grain joints the better, and 8:1 or so is a minimum. At that, I'm going by memory (never all that reliable), and the requirement might be for an even longer scarf than that. This is aircraft practice, and nobody will die if your scarf fails, but much of what we do has a lot in common with aircraft usage. We're both trying to make light structures that are highly stressed, and similar guidelines seem to apply.

[Nathan Ashmore]

Butterfly key joint might look neat.

[Michael Lewis]

Get another back set. The time you will spend making this one work and still be somewhat iffy in my mind justifies finding another back.

Make something else out of that back. (Jewelery box, head plates, arm rests, bridge plates, or?)

[Clay Schaeffer]

Why not do a back like a Viola da gamba ? Build it with a well defined angle and make it a feature. If the wood is 7/8ths thick you could get 4 slices and quarter match at the angle break. I would reinforce the joint on the inside with a cross brace planed to match the angle.
That piece, as is, would also allow you to do a Gibsonesque LG 3/4 size which can be cool little guitars.

[Nathan Ashmore]

I think your right Michael. I've drawn up plans for a parlor last night that will accommodate the dimensions. I would like to thank everybody for the ideas and theories.
Let me know if you guys are interested in seeing my parlor plans.

[Mario Proulx]

Agree completely with Michael; there's always a smaller instrument that can use the wood correctly so it's best to just seek the correct length material you need for this one.