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I agree that MDF is more prone to deformation in its depth. Dimensionally - it would have the least resistance to growth in this direction.

I asked in another place about why MDF is preferred over plywood for making dishes. I was told that “dimensionally” it is more stable than plywood. But concerning the stability in the "Z" direction - it would seem that MDF should not be the preferred material - stability wise. The "Z" direction would be the most critical for our puposes. Issues of splittering and delamination during construction aside (easy to address and fix) - plywood would seem to be prefered - over time. Unless we are saying that these problems with “Z” are more common in plywood – generally speaking over time.

Why is this an issue? …either will work just fine. In this case – I don’t think there is a right or wrong, perhaps only better and best. If I have a choice, in my mind - I want to make the best choice.

Also it is getting real hard to find top grade birch ply in many locations that does not have voids. I believe these voids are an other reason MDF has become the preferred bowl material.

[QUOTE=tony] I agree that MDF is more prone to deformation in its depth. Dimensionally - it would have the least resistance to growth in this direction.

I asked in another place about why MDF is preferred over plywood for making dishes. I was told that “dimensionally” it is more stable than plywood. But concerning the stability in the "Z" direction - it would seem that MDF should not be the preferred material - stability wise. The "Z" direction would be the most critical for our purposes. Issues of splintering and delamination during construction aside (easy to address and fix) - plywood would seem to be prefered - over time. Unless we are saying that these problems with “Z” are more common in plywood – generally speaking over time.

Why is this an issue? …either will work just fine. In this case – I don’t think there is a right or wrong, perhaps only better and best. If I have a choice, in my mind - I want to make the best choice.
[/QUOTE]

MDF is only unstable in Z if its finished edge is machined. This will lead to swelling and racking over time if not sealed. The issue with ply in this direction is two fold. many layers of the ply structure are exposed in the machining process there by exposing many possible paths of failure or many places for possible delamination. both materials short-comings can be over come with sealing.MichaelP39030.6109143519

Thanks again Michael. Very good of you to share your wisdom.

Voids are easily overcome with the judicious use of Bondo. I have both MDF dishes from OLF vendors and plywood dishes made by yours truly. I bought the MDF to see if the quality were any better than what I could do with plywood. My plywood with voids were fixed as described above and then sealed with shellac.

Boy Tony, all I have posted seems as if I am putting down MDF. I am not In fact my bowls are MDF because it is less work to create them. I am only pointing out the possible and sometime probable short comings of MDF. Every material has its drawbacks I think phenolic is the closest to perfect material for both applications but it s cost and machining properties it its major draw backMichaelP39030.640775463

   The MDF I use is cermaic tile backer board . This is a full 1 inch thick and more stable than the 3/4 and more stable than ply.
   Seal the MDF with urathane and it will last a long time.
john hall

Ha! Strange that you should say that Michael. I was challanging you based on my impression that you were defending MDF's use.

I really didn't think you were doing either. In fact, I know that you were being frank in discussing the issues of using both, from your perspective. And that is what I have appreciated coming from you.

Gasawdust - I was thinking that I would do the same about filling voids, etc. - when I said "easy to address and fix". I wasn't think bondo, but wood filler. I would think that either would work fine.

It's winter time in Alaska - been sub-zero to the teens for most of the last few weeks with snow/ice on the ground. Cutting, shaping, or sanding MDF outdoors is probably not going to happen for a while, and I can tell you for sure - it won't happen indoors. I can see benefits of using plywood for weight purposes, better suited for hardware attachment, with no real compromise in strength or stability. So, I intend to make my forms and dishes from 3/4" plywood - 2 pieces back to back - filled, sanded, sealed, and probably finished.

[QUOTE=MichaelP] Every material has its drawbacks I think phenolic is the closest to perfect material for both applications but it s cost and machining properties it its major draw back[/QUOTE]


Michael, a British supplier sells moulded phenolic dishes ready made. ?44.95 ($85) each for a 25' and a 15' dish or they also do a double sided 25'/15' for ?64.95 ($120). I still make my own from MDF, though everytime I see their catalogue I'm tempted.

Colin

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

[QUOTE=tony]
Why is this an issue? …either will work just fine. [/QUOTE]

I just re-read what I wrote and realized that what I said could very easily be mis-understood. The question was intended to be rhetorical - poised as though I had ask myself "Why is this an issue?" (...with me) and follow up the question by commenting that either will work just fine.

Just my lame way of acknowledging what some might have been thinking when they read through my questions. I was not trying to be accusatory towards anyone.

Anyway... I enjoy this place and want to keep the friends I make. Thanks.

[QUOTE=Colin S] [QUOTE=MichaelP] Every material has its drawbacks I
think phenolic is the closest to perfect material for both applications but
it s cost and machining properties it its major draw back[/QUOTE]


Michael, a British supplier sells moulded phenolic dishes ready made.
?44.95 ($85) each for a 25' and a 15' dish or they also do a double sided
25'/15' for ?64.95 ($120). I still make my own from MDF, though
everytime I see their catalogue I'm tempted.

Colin
[/QUOTE]

Could you provide the source colin? Sounds interesting. Personally I love
my Baltic birch plywood dishes I got from Luthier Suppliers. They have
not warped at all and work awesome.

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John Mayes
http://www.mayesluthier.com

John they are available from a company called Craft Supplies, they are not listed on their website but are in their catalogue. I have just spoken to them and they are going to put something on their website and let me have the address. I'll let you know when that happens.

Colin

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

[QUOTE=Colin S] [QUOTE=MichaelP] Every material has its drawbacks I think phenolic is the closest to perfect material for both applications but it s cost and machining properties it its major draw back[/QUOTE]


Michael, a British supplier sells moulded phenolic dishes ready made. ?44.95 ($85) each for a 25' and a 15' dish or they also do a double sided 25'/15' for ?64.95 ($120). I still make my own from MDF, though everytime I see their catalogue I'm tempted.

Colin
[/QUOTE]

Ditto...Colin, would that be Touchstone? Craft Supplies?

I've had my dishes for over 4 years...bought them from Dave Mitchell who used to be on the forum way back when this place only had a handful of people. Whatever happened to Dave?

Anyway, they are sealed and I backed them up with 3/4" ply which makes them much heavier...like adding power steering to driving the bus. And they haven't warped in that time.

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JJ
Napa, CA
http://www.DonohueGuitars.com

THe dishes I make are doubkle laminated 3/4 inch MDF. As soon as I make them I seal them with two coats of waterbased varathane. They are very stable! I think that with mdf the benefit over plywood is that they are denser (heavier) making them do most of the work when in use rather than you. Also, they are more homogenous in structure, aside from voids in plywood you could a 'wave' effect between different densities in laminations and between the wood and the glue. In my most humble opinion, filling voids and reshaping a dish after it has been made could result in a bit less (but probably not really important) accuracy. Although mdf sprays dust EVERYWHERE when shaped, the dust is heavy, settles quickly and is easy to clean up.

MDF for me....

Shane

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Canada

[QUOTE=Mattia Valente] [QUOTE=Colin S] [QUOTE=MichaelP] Every material has its drawbacks I think phenolic is the closest to perfect material for both applications but it s cost and machining properties it its major draw back[/QUOTE]


Michael, a British supplier sells moulded phenolic dishes ready made. ?44.95 ($85) each for a 25' and a 15' dish or they also do a double sided 25'/15' for ?64.95 ($120). I still make my own from MDF, though everytime I see their catalogue I'm tempted.

Colin
[/QUOTE]

Ditto...Colin, would that be Touchstone? Craft Supplies?[/QUOTE]

Mattia Craft Supplies are the people, as I said they are not on their website but are in the catalogue.

Haven't heard from you for a while, glad to see they are keeping you busy!

ColinColin S39032.2202893519

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

I have Shane's dishes and they are heavy indeed, wouldn't want to drive a bus all day long with them but love the feeling of strength i got when i sanded my last rim!