Bracing the Back (Making Cauls)

[ngerty]

Hello,

I'm preparing to brace the back of my guitar and need to make four cauls with the radius of the braces in them. I know how to do this, but am unsure if the braces all have the same radius. I stack them together, put them on a a flat surface to get them even and then feel the bottoms to see if the curves are the same. They're close, but not exactly the same. Can I use one as a template for all my cauls or should I use each one to make its own caul? I'm leaning towards making one for each, but just thought I'd get some input before going ahead with anything more.

Thanks for your help,

-Neil

[kencierp]

Normally the back braces are all the same radius since the back plate is shaped like a section of a sphere. The rim also shapes the back along the length of the body and is contoured to a matching radius. Here's a simple way to make perfectly matching gluing cauls;
http://www.kennethmichaelguitars.com/contourtool.html

[kencierp]

Here's a little tutor showing the complete back plate assembly with the cauls in place:
http://www.kennethmichaelguitars.com/ki ... embly.html

[klooker]

I'm working on my first build - a Martin dreadnought kit.

Martin supplies 4 CNC made back braces. When making cauls, I discovered that 3 of the braces had a 15 foot radius but the shortest one had an 8 foot radius.

This is total speculation, but I think it may be because it is more difficult to bend a shorter piece - try to bend a piece of wood that's only an inch long vs. one that's a foot.

Besides being curved, the braces also had a profile on the top that were next to impossible to clamp against using plain bar clamps. I took a scrap of wood an cut a slot or dado in the center about 3/16" wide which the top knife edge of the brace can fit into. This gave me a flat surface for my clamp. See photo below.

Kevin Looker

[Freeman]

Hello,

I'm preparing to brace the back of my guitar and need to make four cauls with the radius of the braces in them. I know how to do this, but am unsure if the braces all have the same radius. I stack them together, put them on a a flat surface to get them even and then feel the bottoms to see if the curves are the same. They're close, but not exactly the same. Can I use one as a template for all my cauls or should I use each one to make its own caul? I'm leaning towards making one for each, but just thought I'd get some input before going ahead with anything more.

Thanks for your help,

-Neil

Normally the back braces have the same radius - remember that many people build in radius dishes instead of against a caul. For my first couple I simple plotted 16 and 24 foot arcs using Autocad (but you can do it with a piece of twine and a pencil), then I cut some 36 inch pieces of 2x2 on a band saw to clamp against. You can use these to sand against also - uses some 100 grit sticky back sand paper.

Make your cauls long enough to span the longest part of your guitar (normally end to end) - you can put some sand paper one end later and use it to bevel the kerfed surfaces before gluing the plates on - poor man's substitute for sanding in a radius dish.

I leave the tops of the braces square and un-scalloped while I do all the clamping - that way I avoid Klooker's problem - but I still put a little piece of wood on the top before appling the clamp or go-bar. It is much easier (and lots more fun) to shape the brace when its glued to the plate - just us a nice sharp chisel.

After fighting with cauls on my first couple I started using a go-bar deck with Ken's card trick, finally for my last couple I broke down and bought a couple of radius dishes. Wished I done that for the first....

[ngerty]

Thanks everyone. I think I'll use the card trick for the top. For the back cut out four radiused cauls that are longer than the longest brace. I then fastened them in roughly the same way the braces are glued to the back (spacing wise) and now have a nice radiused support board for glueing. I think a go-bar deck is plenty easy to make and use so that's what I'll do for brace glueing.

Thanks,

-Neil

[Freeman]

Good instruction at StewMac on building a go-bar, you can buy everything you need at Lowes or Home Depot