BYO Top Braces

Bones

I've been working on my first Selmac with a pliage and I have the top bent but not joined.

I have been thinking a lot about the arch radius of the horizontal braces and I decided to calculate the theoretical radius assuming that the top is a straight line from the neck block to the pliage and again from the pliage to the tail block. The angle of my pliage is about 7 degrees (as bent) with 2.5 deg at the neck end and 4.5 degrees at the tail. That is assuming those angles are relative to the top edge of the sides of the guitar and also assuming that the edge of the sides are straight.

So assuming that we are going to pull the edges of the top down to meet the sides and that the sides are much stiffer than the top in the direction we are pulling the top then almost all the bending will be done to the top (i.e. the sides are very rigid in that direction). Also assuming that we are trying to keep the pliage angle roughly the same as it is right now in the 'free' condition (i.e. not constrained to the sides of the guitar) then the edges of the top will have to bend down to meet the sides everywhere except at the neck and tail blocks (i.e. down the center). Hence, the need to 'radius' the horizontal braces.

I calculated the theoretical radius for each brace at each of the 5 (or 4, I have not yet decided if this will be a 4 or 5 brace) horizontal brace positions for my geometry (short scale, 14 frets clear).

If 'x' is the height of the top down the center (relative to the tops of the sides) and 'y' is the distance to the sides from the centerline at each brace location (half of the width of the brace) then the arch radius of the brace will be:

r = x/2 + y^^2/2x (from our geometry class, Pythagorean theorem for triangles)

If I assume a 4 fret 'D' hole style I get these radii:

r1 = 6'4"

r2 = 3'11"

r3 = 4'8"

r4 = 5'7"

These numbers are WAY different than what they have in the book by Collins (r1 = r2 = 12', and all the others are 7')

I admit I need to double check my measurements and calcs but just curious what the other luthiers out there are using for their horizontal brace radii and pliage angles. Also, if I don't make the top of the sides perfectly straight (i.e. hump it up near the waist area) that will change things a lot but looking at the Charle plans the sides of the Selmac are straight (but I have never examined an original Selmer).

Thanks

Bones

PS- I scaled/calculated the radii for the first and third braces off the Charle plans for the oval hole and r1 = 5.6' and r3 = 8.6'. Looks like he has about a 6 degree pliage. Way different than I am getting for my geometry but I have a bit more pliage angle and also mine is a short scale 14 fret so.....

Hmmm....???

Bones

Ok I've thought a lot about this over the last few days and it is starting to make a lot of sense to me now. I need to set some fixed parameters and the rest will define themselves. If I assume that the top edge of the sides are in a plane like on the Charle plans it makes everything a bit simpler (of course the neck and tail blocks and linings will need a slight angle to match the pliage angle and lateral curve of the top/braces).

Does anyone know if the original Selmer sides are straight (i.e. NOT humped up at the waist area)? I've never actually seen an original. My Gallatos look straight though.

Thanks

Bones

Still thinking this issue over so decided to work on some other stuff in the mean time. Here's some pics of the sides with neck and tailblock assembled and the mortise for the neck routed.

Bones

Since I'm doing a 14 fret, short scale it kind of makes the distance from the pliage to the tailblock a bit long so I am planning to go with a 5 brace structure like a petite bouche even though I'm going to make this one a grande bouche. Soooo, taking all that into account these are the arch radii that I end up with for each brace (brace #1 being closest to the neck). This assumes that the edges of the top curve down to meet the straight/flat top edge of the sides (i.e. no curvature built into the top edge of the sides as shown on the Charle plans the edge looks straight).

Brace 1 radius = 8' 7.1"
Brace 2 radius = 4' 7.9"
Brace 3 radius = 5' 10.7"
Brace 4 radius = 6' 5.7"
Brace 5 radius = 11' 9.4"

Since the Charle plans don't seem to address this level of detail I'm curious if any of the other BYO'ers out there have gone thru this math as well and if these numbers seem to make sense??? Feel free to chime in or PM.

Thanks

Bones

Ok, sorry for the delay on the update. I hemmed and hawed about it and decided to go with my calculations even though it seemed to conflict with the Charle plans and it seemed to work out fine. Made some templates out of MDF and routed out the radii as accurately as I could but still needed to do some custom fitting to get a good mate with the top in the go bar deck. Here is a picture of a brace fixtured in the go-bar deck and also a pic of the top just sitting on the sides prior to gluing and clamping. With the brace radii the top edges didn't really need to be forced down much at all to meet the sides except there was still a slight hump vestige at the pliage but that pulled down with very little pressure.

Bones

Overall I'm very satisfied with the result. The angle of the top at the neck block is spot on at the 2+ degrees I was shooting for. The total pliage angle ended up at around 5 degrees which is a little less than the 6 shown on the Charle plans. My bending fixture was made to 7 degrees intentionally to account for some flattening of the pliage when the braces pull the edges of the top down. Here is a side view of the pliage after gluing the top to the sides and also a picture of the neck blank roughed out which I am currently working on along with the back bracing.

DjangoJay

Fascinating read Notice this is an old post, have you made anymore progress on your build?

Bones

Yeah interesting stuff trying to replicate how it was done without really knowing. It's strung up "in the white". Turned out great. Need to lacquer it and do the final setup. Next build 18" archtop cutaway, then 16" archtop non-cut, then a petite bouche. No time right now though. I'm sporadic out in the shop.