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Silly question. Seems like the preferred material for making a radius dish is MDF, and not plywood. However, all of the molds I have seen pictures of are made from plywood. Why? If MDF is preferred for dishes because they are stable, why would they not be preferred for molds?

Hey Tony,
I have only used MDF for my molds. Many others have I am sure. Sometimes it may come down to what ever is handy - as long as it is stable and flat. I like MDF even though the dust is a pain. Use protection! Routing MDF outside is a great idea, too.

No silly questions, just silly replies from this brotherhood here at the OLF. Brotherhood is not meant as a gender-exclusive term in this case. D Stewart39030.4846990741

The direction of stability for molds is different than for dishes. MDF is easier to deform at edges due to minor impacts than good quality ply This is the reason.

I have used MDF for molds but prefer Plywood for the above reason. At the same time I use what ever scraps are lying around. I've also used MDF for some of my Forms for my Fox style bender and they work there too.

I too only use MDF for moulds as well as radius dishes. I do however use plywood for my bending forms.



Colin

_________________
I don't believe in anything, I simply make use of a set of reasonable working hypotheses.

in some respects mdf may be better for molds, e.g., no splinters, less prep needed for use since it is inherently smooth. and it is much cheaper than good ply.

on the other hand, inter alia, it is very heavy compared to ply, not as strong holding screws, etc.

and some radius dish makers use high quality ply, inclusing our sponsor luthier suppliers if i recall correctly.crazymanmichael39030.5832523148

The molds I've made from MDF are too heavy.

MichaelP - "The direction of stability for molds is different than for dishes. MDF is easier to deform at edges due to minor impacts than good quality ply This is the reason."

Thanks Michael. What exactly do you mean concerning the "direction of stability"? I can see how plywood might be less prone to damage from impact at the edges, but is this a real issue? ...or just preceived because of the potential?

Glen and MichaelM - Yes. I do recognize that there is a weight difference. Could there be any advantage to having the extra weight during the build?

I'm not really trying to argue for or against either. I hope to be making some soon - and was curious. I guess I would like to think that I am giving my best effort.

Colin - I like that back.

MDF is only more stable than ply in x and y directions (length and width) and I need to add only if uncut. Once it is cut it is way more susceptible to moisture absorption. In the Z (depth) it is actually more prone to swelling and racking than ply, and like other have said it is not as good at mechanically holding screws. You can over-come some of these with sealing with shellac but the edges and cut surfaces will always be easy to deform. yes this is an issue when you are using this machined edge as an indexing edge. Molds get tossed around and clamped against with mechanical clamps from may different axis. Bowls only take pressure from one direction. Molds take a bit more abuse on the cut or altered surfaces than bowls do. For me the only reason to prefer MDF over ply on bowls is splintering and delamination when shaping the contour.

Also it is easer to get glue off of machined ply surfaces than off of machined MDF surfaces with out altering the surface. To me this is a big reason for plywood molds.MichaelP39030.5784027778

Thanks for your reply Michael. Appreciate it. I agree that MDF is more prone to deformation in its depth. Dimensionally - it would have the least amount of resistance to growth in this direction.

Raises another question. But I'll do it in another topic -"Plywood Dishes" so this doesn't get de-railed.

Thanks to all.

i see no advantage in having to lift a lot of extra poundage, and the extra weight is not sufficient to obviate the need for clamps or stops whilst working on the rim mounted in it.

but others obviously feel differently since the use mdf in their molds. it is just a personal preference thing. either will obviously do the job.

For the sake of discussion... I'd like to come back to the issue of deformation in the "Z" direction. Would growth in the "Z" direction really matter in an outside mold that much? At least until or unless it affects the form dimenions inside the mold itself. That is the surface we are depending on to remain stable.

???

It could because the MDF can swell a bunch and only in isolated areas that came in contact with moister like from glue. The problem is when it swell it will swell in all directions. If it swelld in "Z" only it would not be such an issue, unless you use that surface as an index in your process such as setting hieght of the mold on a rim sander.MichaelP39030.6338888889

I make molds as you guys well know. Mdf is greta for a guitar or 2 but if you use spreaders they will tend to deform from the stress applied by the spreaders and your guitar tends to shift.
   The weight is also a pain as is the wear from your clamping as screws will fail in short order. To make MDF work better you need to coat it in fiberglass but agian plywood is available and cheaper in the long run.
john hall

I've used MDF for my moulds ever since I started building guitars, the same ones in fact. So far my OM mould (the one in the picture above) has been home to 10 guitars. My moulds have all surfaces sealed with boat varnish, and are still as good as the day they were made. with screws being put in and out at least a couple of times during each instrument, If the screw hole wear (they haven't) just move the screws.

As far as weight is concerned, they are a bit heavier, so what? It's good excercise, we don't do enough of that.

colin

_________________
I don't believe in anything, I simply make use of a set of reasonable working hypotheses.

Thanks John. Makes sense to me.

I don't see how the molds I make (heavy; 2.25" thick, solid MDF) will deform under the tension I put on them with spreaders. I use MDF because it's cheap, easy to work, and is good enough for my non-production-line building. I keep master templates made of thicker ply of pretty much everything used for template routing, and I tend to seal/finish all my MDF molds. I also like the extra mass.