Cool Idea: Making the Perfect Radiused Clamping Caul

I've been scratching my head lately on making a perfect clamping caul to fit an unknown radius from a ukulele pattern. In other words, I can do the convex part OK, but making the concave caul taxes my abilities. I don't want it to be approximate, I want it to be perfect.

I got this tip today from a Stew-Mac technician that I thought was really good: Use the convex part to make a concave mate by using automotive Bondo. Using wax paper, just press the desired convex part into the Bondo, let dry, remove paper and you have the perfectly mated caul. I've been thinking and this might work for making all sorts of strange concave radiouses including molds and sanding bowls. Interesting. I'm going to try it.

You can also use "Friendly Plastic". It's essentially the same sort of stuff that hot melt glue is made from.

I bought a kilo of it some years back for doing these sort of jobs. Came as small beads that you can pop into a bit of water. Place in microwave and heat up. They will become pliable when hot. Mould into the shape you want, taking the precaution of a release (like wax paper). Let cool and you have your clamping caul etc.

Just heat up and re-use the material when you don't require it anymore.

I was thinking the same thing as Allen. They sell Friendly Plastic at LMI.
http://www.lmii.com/products/tools-services/clamps-vises/friendly-plastic

Don't forget to add the thickness of the material you'll be clamping when you create the bondo part. If you just use the inner caul to make the outer caul....it won't be an exact fit when you use it with a top or back plate in between

I'm with you ksquine and I've been thinking about this. The radius is the headblock PLUS the thickness of the sides. The side thickness there at the neck joint will only be 70 but it does add a bit to the radius. It is a small addition to the radius, but is it a significant addition to radius. Ignore it at your peril? My gut says to ignore it. My mind says, oh yeah? > You like neck join gaps Ukulele boy???

Good idea! You could also design it all in computer and use your CNC router to make it!

That is an old trick, though I have epoxy around, not bondo. The other trick is to use insulating foam, it can quickly fill far larger areas.