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My favorite moulds so far have been, 1" ultra light MDF sandwiched between BB.

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Pat

Like Arie, I've done moulds using both 3 or 4 layer pre-glued blanks and with individual sheets pinned together after cutting and even a combination of both where the customer wanted to be able to add a layer as required for deeper bodied guitars. Both methods work equally well in my experience and both have plusses and minuses.

The pinned method allows you to do a few things like build a hinge right into the mould. With my work method it takes significantly longer because I use vacuum to hold down the sheets, leave a .1" skin at the bottom and then free the parts on a router table with a flush trim bit. For a 3 layer mould that's 6 pieces that have to be freed on the router table.

Glue up in advance works well but, there's more prep work and it takes a lot more glue to keep the larger sheets together. Additionally, dust collection is more difficult because you have to cut more than 2" deep.

Remember that there's a very broad spectrum of CNC machines out there, from commercial machines designed to hold a tolerance of .0005" or better when cutting steel to fully home made machines that can barely cut the MDF they're made of and can't cut a round circle if their business depended on it. And of course - a lot in between. If individual sheets of a mould are not lining up after being cut, that's a problem with the particular CNC machine being used, not a problem with using CNC in general.

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http://www.birkonium.com CNC Products for Luthiers
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I to like MDF for molds with a protective coat of shellac I've never had an issue.

I hate splinters and plywood is just not what it uses to be....

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Michael Lloyd

“I was born to ignorance, yes, and lesser poverties ...
I was born to privilege that I did not see ... I didn’t know it, but my way was paved” – John Gorka

My favorite is domestic plywood:
- The ply layers about about 1/8" thick, so fewer glue layers, so easier on cutters.
- Much higher quality/uniformity than import/big-box ply, not quite as good as BB.
- To get a stiff mold at a given weight, it's more effective to make the outside a little bigger than it is to use a stiffer (and heavier) material like BB -- remember the cube rule. Domestic ply layers are SPF, I assume -- much easier to machine than BB or MDF, and modest dust.
- Price is about halfway between import ply and BB.

The LMI design is nice because the parts are ready for glue-up right out of the CNC, and no extra hinge or clamp pieces are needed (other than the pin), and every layer is the same design, just flipped.

I CNC one 3/4" layer at a time and have great results using dowel pin alignment. 2 pins isn't enough -- I'd suggest about 6 per half (every 5"-6" along the perimeter), plus the hinge pins at each end. I check squareness of the sandwich on a flat table, but have only rarely needed to tweak it. As Andy said, the CNC machine needs to be reasonably good.

After glue up, I trim all edges with a 1/8" round-over bit, sand, and seal... no splinters.

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David Malicky

Plywood is a lighter material. I like that aspect of it. However, I do have MDF moulds.

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Under Compensated Nut!

The nicest molds I have seen are made by a friend Rick Maguire (maguireguitars.com) who uses high density fiberboard and MDF in a sandwich, MDF in the middle. They are really nice...
I use home-center grade plywood for molds. It works just fine.

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"I want to know what kind of pickups Vince Gill uses in his Tele, because if I had those, as good of a player as I am, I'm sure I could make it sound like that.
Only badly."

Ive used solid pine layered one at a time with a router. No alignment problems, light, cheap, non of the problems the other materials have and because they are layered different expansion tendencies seem to be countered by other pieces. Theyve stayed flat and smooth for years in an un-moderated garage.

An OM version of what I plan to make, I've decided to go with mdf as after playing with the CNC router I've discovered it sticks to the vacuum bed better.

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Formerly JJH

I learn more from my mistakes than my successes