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As a few of you know I am still in the process of tooling up for the construction of an OLF MJ and have been struggling with creating outside molds and bending forms out of MDF. An inspection of the mold against the original plans showed that the outline was not satisfactory for the sides to take the correct shape. So I created a more precise template out of clear acrylic and mm in the process of creating a semi-solid mold (for budgetary reasons) out of birch plywood instead, but that's not the issue here.

In the meantime I'm looking ahead at fashioning new bending forms and came across a thread in the archives that implies that the actual size of the bending forms can be slightly smaller than the true outline of the mold in order to accommodate the thickness of the bent sides. Well, I already have a bending form fashioned out of MDF and its profile is actually quite similar to that of my clear acrylic template. while the curve of the molds are not perfect I estimate that there is only a 3mm variation on either side of the line. This raises several questions:

1 - are these bending forms salvagable? If so it would save me time and money, which is what has me dreading on cutting new forms out of birch plywood.

2 - It looks like 3mm (my target thickness for my s. American mahogany sides) inside the line is what I should sand to, correct?

3 - even if I don't get them 100% perfect, will the newly-bent sides be close enough in shape to fit snugly in the mold? How much play in newly-bent south American mahogany can I anticipate for?

...or should I just start over with birch plywood?

Can you post a picture of the template sitting on top the bending form?

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Evan McCartney

! Sure the forms are salvageable. You can always true them up and make a new inner or outer form if they are really off. You could bend and glue some strips of wood in your mold and reshape as needed, You could use Bondo, ( I would not do it on a form that was heated) Or you could just not worry about it. Instruments have been made for centuries by eye, without forms.

You should be ok as far as fitting into forms even if your bending form is not perfect. as long as the general shape and size is right. This because the wood can only bend so tightly. Minor dips and dos will be bridged.

I am only working on my fourth but have found that even if the sides don't fit perfectly in the form this can not be noticed at all in the final product.

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http://goatrockukulele.com

Awwww, FFFFFFFFF

I was just looking over my templates and everything that I'd derived from them and put them up against the original blueprints this time - not the copy. You experienced luthiers will know what comes next:

Basically, the entire plan was stretched along the length of the paper so that my template is a few mm longer than the true outline. I'll have to do my template over. It doesn't seem to have affected the width though. Go figure. I guess it had to do with how the paper was run through the copier; it was one of those oversized feed-in types and the paper was quite warm coming out.

You'd think that somebody who used to work in graphic design would remember these kind of things!

Even though I used an inaccurate template to cut out a rough outline into four 10" x 24" x 3/4" sheets of birch plywood all is not lost though. It just means that I have to do a little more sanding of the MDF bending forms and not quite so much in the birch plywood molds. Even less so because I only cut 4 sheets this time, not 12.

BTW, I still haven't found my camera so unfortunately I can't snap still images to illustrate. All I can do is take a quick movie with my iPod Nano 5G.

Neil,
I have only made 2 guitars off the same mold, 1/4"mdf is a great material to play with. It is strong enough to use as a template, though not as durable as acrylic, but it is cheap and soft. It is great to get started with and make sure that it is right, and if not, you have a whole sheet to play with until it is, then you can transfer to acrylic. The Taylor youtube factory fridays have an interesting video on bending, their system uses overbending to get it right, so basically you can be smaller then just the thickness of the side, the side will not come out perfectly from the bending mold, the more critical part is holding the side to shape as the linings are installed.
Rob

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As far as the bending form usability, can you describe where the 3mm differences are? That's not a large variation when
bending wood. If it's in the waist area, you would want it to be a deeper curve, to account for springback. If it's in the upper
and lower bout curves I doubt it would make any significant difference. And if you have a pipe bender (which you will need
eventually, and it's easy to do with a propane torch and piece of iron pipe to begin), you can touch up the curves to match
your mold.

_________________
Gene

Politicians and diapers must be changed often, and for the same reason- Mark Twain

OK, I found my camera. It was hiding in with the kid's toy travel bag...

Anyhoo, here are some snap shots of the variation on some of the forms:



















FWIW here's how my second attempt at a mold is going. I'm currently working on the master and trying to get it as close to perfect to the plans as possible. I'm really close but have to figure out a way in my budget to replicate it 3 more times for a semi-solid mold.

OK, I think I get it. I'm just going to smooth out what forms I have then and proceed once the molds are done. Thanks for all your help and insights, folks. I can't wait to truly get this build underway.