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John- I've made dozens of bending molds, uke and guitar. they work but I've never made one where the sides actually fit really tight and close to the holding mold and did not require extra work on the hot pipe. Any secrets to share on that one? I currently have about 15 mold shapes. None of them are commercially available shapes. I'd like to start remaking some of my most important ones during this lockdown. Thanks, Bob

you need to call me on this on
I learned a thing or 2 here

I would lay out your pattern on a piece of ply/osb large enough to have both the bender and form. Cut out the guitar shape with a narrow blade on bandsaw as close a possible to the line. Then clean up one of the pieces on a oscillating sander, if you have one, to a final finished outline, Then sand the other piece until it is an exact fit, might have to use some bondo if some areas need filled it to maintain the profile. After an exact fit use the pieces as patterns to route out the needed pieces to complete your bender and form set.

Even if what you end up with isn't an exact match to your profile as long as they aren't too far off the profile and as long as they are an exact match to each other it should work.

these are for Uke
the best patterns have a bit of spring back modeled into them.
so off the center line keep the curves apex in line with the template but tighten the radisi on the bends. About 1/8 in should be enough.
as long as the curve center line is in line with the curves apex you will be able to have a pattern that works with your mold and template.
Remember not all wood bends the same this way you get the most reliable bent and repeatable process

Thanks for the addition/correction, there does need to be some room for the thickness of the side.

the big thing is that you keep the apex of the curves at the same point and by tightening the radius that allows a bit of spring back.

I've made many dozens of bending and holding molds over the the years. Never been satisfied. I want a bending form that is so good that the side sits tight against the holding mold with no clamps. Fat chance of that! Maybe with the press type molds that factories use. Just curious John, do the presses at Martin get the same results with different woods, or do the woods have to be treated differently to get repeatable shapes? I've been to Martin a couple of times, but I don't remember their bending department.

I've been struggling with this as well, but from the perspective of building an outside form. In order to eliminate waste, I've been using the cutouts as the profiles for the bending forms. My understanding is that the shape of the bending form isn't supposed to match the profile of the traced outline off a blueprint or from the tracing of a finished guitar but smaller somewhat to take into account the thickness of the side and with exaggerated curves to take spring back into account.

Correct me if I'm wrong then, but that means that if I trace an outline I should cut about 1/16" or so inside the line. That way I have a little extra material to sand away on the resulting outside form and the cutout. The trick for me has been to maintain a steady rate while moving the ply while cutting on the band saw, as well as keeping my turns smooth so that the blade doesn't wander. I'll admit that my bandsaw isn't the best; it's just a small Ryobi benchtop model.

Martin uses a tensioner on the slates.
they also use 450 degrees for 6 minute

I was thoroughly impressed taking the Martin factory tour. I don't want to describe what I think I remember he was doing here as I'm sure to get it wrong.

I do remember leaving this station thinking "I'm going to try a hot pipe instead of the bending machine" and so far so good sorta. My curves aren't perfect but that's not my goal.

The shot of Bender is what I felt like after getting my 1st sides in the mold.

a few more shots from the tour.
https://photos.app.goo.gl/s1BfR87YP46qvtUh7