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At 3.5" is my external form too thick? I've heard that once you have your sides aligned and clamped into your form you shouldn't move things around until it is time to take the completed box out. Yet I have to move the sides up or down a little to have room to clamp the linings and I have to take the sides right out of the form to clamp the side struts while gluing. After I've got the side struts done, then I put the sides back in the form and make sure they are all square and the edges sit correctly on the sanding bowls etc to be ready to glue on the top and back plates. Seems to work, but am I missing something?

Thanks,
Pat

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Hi Pat,

Most molds are made from 3 pieces of 3/4" plywood or mdf which of course renders a 2-1/4" thick mold.

I like mine a little bit thicker and I use a nominal 3/4" birch plywood (which actually measures out to 11/16") and I use 4 pieces which gives me 2-3/4" thick.

I do have two molds that are 4 pieces of 3/4" MDF which is 3" thick and I found that just a bit to thick for OM size guitars.

When the mold is to thick you will have a hard time sanding the rims with a sanding dish as the sides might not sit high enough for the sanding dish to clear the outer edges of the mold.

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What Rod said, plus, if it's too thick, you won't have room for your lining clamps.

Pat

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formerly known around here as burbank
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http://www.patfosterguitars.com

I do it all the time, if only to glue the linings and side braces and clean things up. I mark the top & back sides of the mold on the rim with white pencil.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

Pat, my forms are two layers of 3/4", and I clamp the work in with deep throat clamps to small cauls inside, so the wall of the of the mold is only 1-1/2" wide. I have to shift the clamps from one side to the other when lining and dish grinding, but the sideset stays in the mold until the plates go on.

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Dan
http://www.acme-archtops.com

I prefer 3 inch molds ..... My guitars are generally thicker than most though, even my OMs are more than the std Martin would be, at 4 1/2-5/8, and 3 3/4 at the heel. Bigger guitars are all 4 5/8 deep at least and 3 7/8-4 inches at the heel. Sanding the rim can sometimes be tight on a narrower guitar, but I just slide the rim up, and use the spreaders (2 sets) to hold it tight while bowl sanding.

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Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

I like 3" moulds, especially when gluing in the head and tail blocks. For gluing on the linings and dish sanding, I just loosen the stretchers and push the mould down onto the bench, so that either the top or back is protruding more then retighten the stretchers. Once my linings are glued on I don't really need the mould any more, as I use solid laminated linings, but old habits die hard and the mould gives some protection from the idiot that occupies my shop.

Colin

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Dan,

I prefer narrower forms too, though I still use spreaders.

Here's my joint. It requires a socket to tighten, but it leaves the block areas clear for clamping the sides for gluing. I've since added half-exposed vertical dowels to provide location in the horizontal plane (there's a term for that type of indexing, but I can't recall it at the moment).



Pat

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formerly known around here as burbank
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http://www.patfosterguitars.com

Pat, I started out not using spreaders because of f-holes, and the relative insecurity of the jackscrews staying tight under the vibration delivered by a powered sanding dish.
Thanks for the picture - your mold is just about right; have you considered lining the inside faces with 150 - 180 sandpaper for added traction? Perhaps not, if slippage hasn't been a problem, but I like it so I can use LESS spreader pressure, where I do use spreaders.

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Dan
http://www.acme-archtops.com

Yep, works great, don't it?

Pat

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formerly known around here as burbank
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http://www.patfosterguitars.com

I have yet to buy/build a sanding disk (radius dish??). I have one OM body waiting to be fit with the neck. Am I missing a step here?

Mike

I make a body form out of 3 to 4 layers of 3/4" ply and all stacked and drilled in 6 locations around the perimeter 1" holes, then drive in 1" dowels and glue them in to the bottom layer, I make the dowels about 5" long and the other layers can be adjusted up or down on the dowels and spaced according to the body depth, they usually require a rubber mallet to persuade them where I want but they hold tight and are very strong. The top end is hinged and the bottom is latched with a bolt and wing nut.
I must have about 13 or 14 of these with matching side jigs.

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Nehemiah Covey
www.coveysacoustics.com