Radius question to the builders

LarsEmil

Hi there builders.
Im a luthier student who just finished lutherie school where i built two selmacs.
Im on my 3rd gypsy jazz build now and am trying to do everything according to the francois plans.
Just finished laminating some beautiful sides and im gonna start laminating linings soon. My question is this:at the school, we used radius dishes for sanding the sides with the kerfing/lining on both the top and the back. which created a dome. but from what i hear (correct me if im wrong) the selmers had a cyllindrical shape on the back and the dome on the top was produced by the pliage and not vertical arching of the sides. so are the sides just a straight taper from tailblock to neck block? and how do you produce the angels the kerfing/linings need to have in order to accept the top and back. I do own the michael collins book but i wasn't able to make much sense of that.

Thanks you guys.
Lars

Craig Bumgarner

LarsEmil wrote:

....we used radius dishes for sanding the sides with the kerfing/lining on both the top and the back. which created a dome. but from what i hear (correct me if im wrong) the selmers had a cyllindrical shape on the back and the dome on the top was produced by the pliage and not vertical arching of the sides.

Yeah, basically. I would argue the arch is actually formed by the braces and the pliage is just a means of forming the top to the braces, but that is a minor technicality.

so are the sides just a straight taper from tailblock to neck block?

Yes, this can be clearly seen in the Francois Charle plans you have.

and how do you produce the angles the kerfing/linings need to have in order to accept the top and back. I do own the michael collins book but i wasn't able to make much sense of that.
Lars

Michael's book shows a good method, using a sanding board and a piece of wood lengthwise on top of the rim to support the non-sanding end of the board at the right angle. Problem is the thickness of the piece of wood and its distance from the centerline only works if you follow his plan exactly.

Suggest instead that you mock up the arched braces using scrap and masking tape to support them. Or even go to the extent of fitting the real braces to the liners before gluing to the top. I know this isn't the way it is usually done, but it can be really helpful in working out the geometry of the top. Not only the arch of the braces and the angles on the linings, but most importantly, that all critical neck angle and bridge height. If you are careful, you can glue the braces to the top after fitting to the linings and top will drop right in when you are done. Regardless, if you mock up with scrap or actually set up the braces, than the angles are easily found. I still use Michael method but set the thickness and distance as needed to get the angle my mock up tells me I need.

Nothing wrong with radius dish construction unless it conflicts with a replica or your image of what the guitar should look like. In my own guitars, I sometimes use the radius dish system on the back if I am not trying to make a "replica". Works good, strong and light.

All the best,

noodlenot

nothing to add to Craig´s excellent post,really, but maybe luthier´s forums are also a good place to ask. Here are some links (and different ways of doing the same thing) to get you started:

http://www.luthiersforum.com/forum/view ... lit=selmer
http://www.benoit-de-bretagne.com/phpBB ... hp?t=10687
http://www.anzlf.com/viewtopic.php?f=1& ... r&start=50

take care,and good luck with your future builds (and career)!

cheers,
miguel.

Craig Bumgarner

LarsEmil wrote:

.... the selmers had a cyllindrical shape on the back and the dome on the top was produced by the pliage and not vertical arching of the sides.....

Just a followup comment on the "cylindrical" shape as opposed to a spherical shape as induced by a radius dish: You probably already know this, but if the sides are straight and the back (or top) braces are arched, the result is neither cylindrical or spherical. More of a cylindrical shape in the middle with drooped ends.

Joli Gadjo

Craig Bumgarner wrote:

Yeah, basically. I would argue the arch is actually formed by the braces and the pliage is just a means of forming the top to the braces, but that is a minor technicality.

Just to stir up the discussion:

So how does the arch in the back of my Castelluccias stays then... if there are no braces to support it.
I know luthiers use some of those heat / molding techniques, but is that really enough to support the arched back? Shouldn't it collapse?

Craig Bumgarner

Joli Gadjo wrote:

So how does the arch in the back of my Castelluccias stays then... if there are no braces to support it. I know luthiers use some of those heat / molding techniques, but is that really enough to support the arched back? Shouldn't it collapse?

I'm pretty sure the Castelluccia backs are laminated over a domed mold. The shape is molded in and would hold it free standing. The backs are about 25% thicker than typical Selmer laminated back. Because the shape is arched and secured at the edges of the sides, the arch is retained. Castelluccia also used deeper (taller?) linings than Selmer, likely to help secure the edges.

Those unbraced Castelluccia backs are pretty flexible. Some call this an "active" back. When done right, active backs are thought to participate in the sound production by vibrating in sympathy with the top. They have a reputation for being difficult to synchronize with the top, but Castelluccia obviously got it right. Not only do the guitars sound great, but they are feather light.

Full disclosure: As some of you probably know, Joli & I play together in the HC of DC and I have been listening to his (4) Castelluccias a couple nights a week for the last couple years. They really are great sounding guitars. He has generously allowed me to document them and over the last six months, I have built three guitars inspired by them, especially the one for sale now here on DB which is particularly impressive. I have not yet attempted a molded unbraced back....., on my list for next fall. 8)

LarsEmil

Craig Bumgarner wrote:

LarsEmil wrote:

.... the selmers had a cyllindrical shape on the back and the dome on the top was produced by the pliage and not vertical arching of the sides.....

Just a followup comment on the "cylindrical" shape as opposed to a spherical shape as induced by a radius dish: You probably already know this, but if the sides are straight and the back (or top) braces are arched, the result is neither cylindrical or spherical. More of a cylindrical shape in the middle with drooped ends.

thanks for great replies. You are absolutely right! but is that the way selmer did it?

Craig Bumgarner

LarsEmil wrote:

......! but is that the way selmer did it?

Well, yeah..., at least that's my guess, I wasn't there of course. Also keep in mind that Selmer did things differently over the years so there is not one right way.

noodlenot

Craig Bumgarner wrote:

LarsEmil wrote:

.... the selmers had a cyllindrical shape on the back and the dome on the top was produced by the pliage and not vertical arching of the sides.....

Just a followup comment on the "cylindrical" shape as opposed to a spherical shape as induced by a radius dish: You probably already know this, but if the sides are straight and the back (or top) braces are arched, the result is neither cylindrical or spherical. More of a cylindrical shape in the middle with drooped ends.

on my first guitar,that´s exactly what happened... on my 2nd i made the middle arch a bit higher (higher sagita) and touched up the sides a bit with a block plane (a la cumpiano), and it was much more cylindrical, but still not there... i´ll keep trying.


cheers,
miguel

Craig Bumgarner

I have been meaning to mention that it can get tricky when pulling an arched back down to straight sides at the tail block. When you go from arched to straight, you end up with a little extra material which can make the back ripple a little here. If this were clothing, you would put "darts" in it. I solved this by arching the back less and allowing the sides to rise just a little. Molding the back during laminating would no doubt solve the problem as well.

CB

Joli Gadjo

Can't wait for you to build a mold!!