Dumb Guitar mold question

[michael o'malley]

So I'm building my first bent-side acoustic, after many solid bodies, and I'm making the mold. I have a good form and a couple matching layers ready

Dumb question: Should the mold be bigger or smaller than the sides? For example, if the sides are three inches, should the mold be less than three, or more than three?

I'm thinking less, because you need to true up and level the sides and glue the top and back on and if the mold is deeper than the sides you will need to prop the guitar sides on something. within the mold. But if smaller, how much smaller? This is my first effort, I expect to make many mistakes

Thank you!

[Bryan Bear]

I like to have the mold smaller by enough to level the rim and clamp linings in on each side of the mold. With the taper to the body, I have more room (less support) near the tail than I do near the heel on the back side. The thicker the better for keeping the sides nice and true as long as you have room on either side to level and clamp. I have made quick and dirty molds before that are much thinner; they work but you have to be more careful. For example, my ukulele mold is 2 layers of MDF so it is 1 1/2" thick and the tail of an uke is much deeper than that. I probably should have made it deeper but at the time didn't expect to use it too many times.

My first guitar mold is made from 4 layers of 3/4" (3 inches) and that worked well for me. When I came up with a new shape I used 15/32" plywood and 6 layers. I wanted the 6 layers because I wanted to be able to link them together with alternating layers. 15/32" gave me 2.8125" and that little bit of extra room on the heel end seems to be a bit more convenient for how I work. I have a bit more room to get my high tech clothespin lining clamps in there. . .

[Rodger Knox]

4 layers of 3/4" ply for me...
You can use the offcut to make a bending form.

[michael o'malley]

Thank you I'm not planning to make a full size flattop. more of a thinline archtop, probably 2-3 inch side.

Good idea about clamping the linings--need to keep that in mind. I can see where bigger would be better

[Bryan Bear]

. . .Good idea about clamping the linings--need to keep that in mind. I can see where bigger would be better

I'm not sure if we have our lines crossed or not on this. To clarify, for me, bigger is not necessarily better. bigger offers more support but an interfere with clamping my linings in. Unless you are planning to clamp to the mold itself but that would take a lot of clamps. Most people use many small clamps and clamp to the side above the surface of the mold.

IMHO, the difference between 2 inches and 3 inches is much larger than between 3 1/2 and 4 1/2. If I were doing something 2 inches I would probably go with 1 1/2" for the mold but that would only be half the depth if I went up to 3" so I would want more support. You could make a 1 1/2" mold with 2 layers of 3/4 for when you want to do a 2" body and make another layer that can be screwed on accurately (for a total of 2 1/4") when you want to do a 3" body.

[Stephen Neal Saqui]

I agree with Bryan. Have room to clamp the lining. Good bending equals stable sides and with that you don't need a deep form.

Another thing to consider are form fitting cauls for the inside of the form at the waist. They're held by a turnbuckle which supplies just enough pressure to hold the waist in place. They're very handy! I'd show you photos but haven't figured out how on this forum!

[Peter Wilcox]

I'd show you photos but haven't figured out how on this forum!

Have you seen this? http://www.mimf.com/phpbb/viewtopic.php?f=19&t=1656

Irfanview works well (in Windows) for resizing pics.

[Bob Hammond]

michael -- there are many 'dumb' answers, but there are no 'dumb' questions.

[Stephen Neal Saqui]

I agree! This is a very good question!

[Todd Stock]

Three ply works best for me - more runs the risk of interfering with profiling and top/back glue, and less results in a more flexible mold that is less positive in fixing neck and tail block orientation. Plywood versus MDF - allows a 2-1/2" side width, which is sufficiently stiff while still being light enough and handy enough for one-hand movement around the shop.

[David Boehnker]

I think mine are more like 3.75" to 4." I use narrow cauls and slide the sides up and down in the mold just enough to glue the linings and profile them. Just another way to do it.

[Mike Ryan]

3 plies of 3/4 (not) Baltic birch for my molds. This last mold is for a parlor guitar. Seems to work fine for me.
Without "dumb questions" how do we work out smart answers. Smart quick answers which I often get in a vacuum without discussion or much challenge end up being dumb answers for me.
I also like photography and at a rather upscale camera club yesterday I was told one of my cameras was junk and my impression of that person becomes that of an elitist and elitists are not that much fun and I don't tend to want to enter discussions with them much.
A humble guy with a lot of talent AND experience is not always easy to find.

[Matthew Lau]

FWIW, that's not a dumb question at all.

Thanks for asking it for the rest of us.

[Doug Shaker]

I build one half-mold (with full caul, at the full height of the sides, or a bit higher, and use that to laminate my tone-wood outer sides with my spruce inner sides.
Then I build one full mold, about 2.5 to 3 inches in height, that I use for assembly.