Build Advice When Making a Side Bending Mold ?

[PHFawcett]

I'm in the process of building a side bending Fox style setup. I need advice on the sizing of the body form but I could be over thinking the process.

I understand the need for a reduction to allow for the side thickness but what about the protective wrapping and metal strap thickness ?

[runamuck]

Stainless steel first (1/32nd or less), the wood wrapped in paper (3-6 thousandths or so, wood heat blanket, ss.

So every thing that would change the radius amounts to 1/32nd or less and that can be considered irrelevant.

More importantly, it's a good idea to make the bouts and particularly the waist a little tighter than you want to finish up with. There will be spring back.

[phavriluk]

My opinion:

I see advice about the bent sides conforming to . . . what? If the sides can be persuaded to fit in the mold, next step. Nobody's measuring anything and if they did they'd be wasting their time. So long as the geometry is right between the neck and body, and all the bits are in harmony, it's good. No wood is going to jump into the mold without persuasion, so any minor measurements about the perimeter dimensions and shape will not be precise anyway. Don't get agita over small fractions, go build. Nobody will notice or care.

[Neil K Walk]

I am not a pro but a hobby builder, so take my advice with a grain of salt. I’ve spent the last few years “getting jiggy with it” making my own forms, molds and jigs so I feel like maybe I have something to contribute to this discussion.

I've made my own bending forms by tracing finished guitars on 3/4” birch plywood. Then I cut about 1/8” inside the line to factor in for the thickness of the side but I’m nowhere near scientific about it. I’ll clean up the smoothness of the cuts (learning to use a bandsaw is a process that in itself requires practice IME) using a spindle sander - which in my case is a spindle mounted on a bench top drill press. As others have said, this is an opportunity to factor in for spring back but I’ve found that you want the curves to be “gentle” so that when you slide the cauls/shoes you want the motion to be smooth.

I dealt with cupping on my first build so I’m not a fan of the molds that have metal dowels and large open areas in the profile; I prefer making a solid mold that is at least thick enough for a dreadnought. Some woods will spring back yes, but as long as you let the side “cook” for about 10 minutes then let it cool overnight while still in the “sandwich” on the bending machine the bend should hold. Just put it in the mold and leave it there until you either put the top or back on. Otherwise I don’t ever let it get above 350°F at the extreme (I typically only go up to 305°F) so there’s not much concern about it bursting into flames. I admit that I prefer to do such things outside though.

I also take the opportunity to keep the outer cut and make my own mold. If you have a mold already, then trace it and cut inside the line as I described above. Because of this, I don’t just make 2 outlines; I make 8-10. I will fine sand one of each (inside and outside) then use it as a temple to refine all the others with a flush cut routing bit on a router table. I typically screw each individually to the template so the screw hole then becomes a pilot hole so that I can then use a brad point bit to make holes for a wooden dowel.

Even still, I only have one bending machine so I will typically do one side on one day and do the other the following day.