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Nearly all spherically dished workboards that I see out there, both DIY and commercially available, are made of MDF. I have seen a few made from plywood. Has anyone on the OLF experimented with other materials for these workboards? For instance: Maybe a less expensive variety of phenolic? Just curious. I am thinking about experimenting with something more durable. Thanks for any input.

Hi Don,

I’m curious about your durability comment? Are your MDF dishes breaking down somehow? The dishes from Canadian Luthier Supply are laminated MDF and are completely sealed. I’ve used mine on the last 35 or so instruments and they still look new.

Best, M

PS hope you are healing up quickly!

Michael—

I have the same dishes. They are excellent! However, I had some MDF peel up on one of them when I tried to change the adhesive-backed sandpaper. It may be the result of my using them in a motorized dish spinner. Who knows? Maybe I need to put some heavier duty coating on my dishes. That might be the best solution. Once you start looking at other materials, the prices ramp up fast. But I did find some phenolic that might be feasible.

I’m doing pretty well; thanks for asking! Four weeks post surgery. Still healing, but making good progress.

Ah, that makes more sense. I’m also using mine as a motorized dish which I love! I’ve never tried to replace the paper, but I’m also a long way off from needing to.

Glad to hear you are making progress!

Cheers, M

Michael--

Yeah, the only issue I have run into with the CLS dishes is the sandpaper change. They really are the best made dishes out there. But there is a limitation with the material itself. If I stick with MDF, I think I will "super seal" it with a very thick and durable coating. But I also might order some of this phenolic and experiment with dishing it. I'll post with a follow up if I go that route.

There’s another product called Medex that may be an improvement? https://www.roseburg.com/medium-density ... mdf/medex/

Another way to make a "radius" dish and avoid a lot of weight and MDF dust is to cut a circle of 3/4 inch plywood and another of 3 mm plywood and either a bunch of tapered wedges or ring segments to follow the perimeter of the dish. The 3mm plywood gets screwed in the middle to the 3/4 inch plywood and the wedges or ring segments hold the perimeter of the 3mm ply the required distance off the 3/4 inch ply to form a dish, which probably has more of a spline curve cross section, rather than a true radius, but is close enough for guitar work, and in some ways better.
The dish pictured used wedges, The next one I make will have a continuous ring around the perimeter. I will butt together "segments" rather than cut out a continuous ring to save on materials. That dish is mounted to a bowling ball balancer and is very "aggressive".
Hi Don,
Using a heat gun to soften the adhesive might help with removing the old sandpaper.

Definitely. I was trying to do that, but some MDF still lifted. The material itself has a tendency toward being flaky, like the best kind of biscuits. That's why I want to either go with something more durable or more heavily coat the MDF.

We used to "harden" the edges of MDF router templates (where the bearings would ride) with CA. To do the entire dish surface might be a noxious job best done out of doors, but might work.

Wouldn’t that be a perfect job for marine grade or Baltic birch ply?

It's a good question, Ed. Is plywood (even good plywood) avoided for this job because dishing it out could set off stresses that warp the wood? I have no idea. I just wonder if there are reasons (other than cost) for the near universal use of MDF for these dishes. I know from the Gore/Gilet books that they have used plywood. One of the suppliers used to offer plywood as an option (but no longer). Why don't folks use a decent grade of plywood for these dishes?

not all MDF is the same. I use a product that is used in commercial pattern making . It is a tempered MDF . I have tried plywood and found that this will move considerably. Anything from Home Depot or Lowes is low end. The key to keep them from pulling up is SEAL THEM. I use shellac and polyurathane.
This helps a great deal. I have some that are over 20 years old and still doing the job. I tried to heat corian ( expensive ) I tried machining it I don't think you can sell them to the average builder at the price. Martin uses them machined from Fortel ( aircraft ) Aluminum. You can also do a formica sheet on the MDF this will make them almost indestructible. That would be a reasonable cost for the trouble.

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

I made my dishes from good Baltic birch plywood about 15 years ago. They have been stable ever since.


Steve

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Steve Smith
"Music is what feelings sound like"

That’s encouraging! I trust BB plywood, and I can get it easily enough. I made a flat 24” disc with it, but my dished plates are MDF. Maybe I’ll just try BB plywood, laminated to the right thickness, and very well sealed with a tough finish. That should prevent the problem I recently had with the MDF flaking.

I have a pair of baltic birch dishes are from Luthier Suppliers and really like them.

I’ve also moved my sandpaper from dish to dish so much over the years it actually doesn’t have a lot of tack any more. Pressing the part down into the dish seems to be what really keeps it on. I’m not sure if you use spray adhesive to keep it in place, but maybe a simple X will do rather than fill coverage?

The main issue I’ve had with dishes is keeping them flat. I think hollowing out the radiused side effects the tension in the material and makes it cup (could be wrong). The only solution for that I’ve found is to glue on an additional layer of 3/4” baltic birch or just clamp them down to the table and “drive” the body in the mold rather than the bus.


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I laminated two pieces of BB together with Titebond. They’re about 1 1/4” thick.

Edit: was just in the shop and measured them. Exactly 1 1/4" and the BB we get is metric so two pieces of 15mm ply laminated together.


Steve

_________________
Steve Smith
"Music is what feelings sound like"

I would expect if you started with a 2” layup of Baltic pitch any rendered dish would be very stable…

baltic birch is now over $300 a sheet

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Regional differences, I guess. I can get it for less. But yes, it is expensive, compared to other plywood.

how many plys? IF you are using 7 ply or less expect this to warp. You need the 3/4 13 ply and is a 5 by 5 sheet.
A more economical is the formica on the tempered MDF . This MDF is only offered Commercially in
my area. It is stable stuff.

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

I would second John Hall's suggestion of skinning the dish with Formica (HPL). If you use the thinner vertical grade stuff used for skinning cabinets it should conform to the shallow curve of the typical dish. HPL being relatively solvent resistant you could use lacquer thinner to remove most adhesives.

I’ve been using this stuff I get from Paxton lumber. It’s from India. It’s somewhere between ultra high quality BB and the 5 ply Home Depot stuff. I think it is $60 for 2x4 sheets.



Don, if you want me to drag some over to you next week let me know. Although I suspect you can get what you need in WBGV.


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When I've made radius dishes they always curl upward a bit, producing a tighter radius (by a little bit) than I want. I surmise there's some built-in stress in the MDF. A layer of 1/8" Masonite on the underside before you cut the radius keeps them down. I have not tried Masonite on the inside of the dish to harden it up, but see no reason it wouldn't work. Yes, it would reduce the radius by 1/8", but this isn't a LIGO (Laser Interferometer Gravitational Observatory) setup...

Twenty years ago I doubled up 3/4” marine grade plywood. Dished 15’ on one side 28’ on the other side.
Applied a sealer coat of shellac to all surfaces before before applying ( I think) 60 grit to each side. They’re still very serviceable.

I was concerned about this happening with my MDF dishes, so varnished them with 5 coats of Rustin's gloss yacht varnish.
I've had no problems replacing the abrasive (I used a heat gun on it)
They are all backed up with 18mm MDF or constuction ply, back side gets varnished too.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.