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I have a few errands to run this weekend, but if time allows I'm hoping to make a couple of radius dishes. I don't have the necessary materials on hand, so a trip to the lumber store will be in order. Baltic Birch plywood and MDF seem to be popular choices, but it would be great to know more about the pros and cons of each. How thick do dishes need to be? Also, is it better to laminate two pieces together, or to use thicker stock? Any input would be greatly appreciated.
Thanks,

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George :-)

George,

I say double laminate 3/4" MDF, then carve your dish. This is the way I do it and here is why. The outer surfaces of both MDF and Plywood are "harder" than the material inside. Once you remove one face the material destabilizes and you longer have control of the dish radius. Although I think the difference is small and I also think that having a 15' dish rather than having a 15'2" dish probably makes absolutely no difference, I still believe that you should get what you aim for. By laminating the material you create a mechanical joint that resists distortion much the same way that a brace keeps the curve in a guitar top, by shear resistance. I prefer MDF over baltic birch for several reasons. It has a much more homogeneous structure, I believe, from my own experience, that it is more stable, it is heavier which aids in the sanding process and it will last beyond your working life if you look after it. Oh ya, and MDF is cheap! Although I apply a finish on the dishes I make once I have them completed, I don't believe it is really necessary. These dishes should not really be subjected to much environmental change if they live in a guitar making shop. I do my dishes on my cnc machine now but I made and sold many by using a router sled on the rim sanding machine that I built (search the archives if you are thinking of a rim sander, Brad Way's is my favourite!) and I was proud of that product.

Good Luck and be prepared for a LOT of dust!

Shane

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Canada

I have never made one of these (I wish I did though) but I have worked with MDF many times. Make sure you get yourself a good mask as MDF dust is horrible stuff.

+1 to that. If you don't have adequate dust collection you might want to figure out a way to do this outside.

Bob

Shane is right, judging from my experience. A 24" diameter 15' radius dish is .400" deep at center. 3/4" MDF is thickest I could find, and without laminating two pieces, would be pretty thin in the center.
If you want, I could give you dimensions and specifics from my setup. It took me a while to figure out. Chris Paulick's great tutorial helped tremendously. Pretty simple really. Since you didn't ask about router sled, I will assume you already figured that part out.

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It is better to try and fail, than never try at all

Thanks for all the replies. I have the router part figured out -- I think. ;-) And I've worked with MDF before, so know what that's like.

So you guys laminate the MDF layers before you rout the dish. Good to know.

Robert - I'm planning on making a 15' and 25' dish. I found a post in the Tutorial section that lists their depths as .4004 and .2401. Does this match your own measurements?

Thanks again,

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George :-)

I didn't go that far, but yes. .400" and .240"
The only thing that I would change about the way I did it, is make sure my box is wide enough to get my hand around the side of the dish. It will take off on you, if you don't hold it firmly. Motorized would be pretty cool also.

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It is better to try and fail, than never try at all

What Shane said and.
When I did mine I used a single piece and then laminated it to another after. One thing that helped was I made some hold downs on either side of the router sled set up. These kept the mdf disk flat to the surface it was rotating on. This was important as the disk was warping slightly as it was being machined and the hold downs kept it flat to the table and subsequently made the back of the disk true to the dish of the dish if you will. So when I was done the dish was a little warped for the reasons Shane stated but when I glued it to another piece of mdf it flattened out and was true. As long as you can keep it flat when you machine it, it really doesn't matter if it warps. You can screw it down to you go-bar deck to flatten it for use. Of course if you want to use it to sand a stationary guitar rim assembly it does matter and adding another piece is helpful.
I also used MDF light. This reduces the weight considerably. I also coated the whole thing with West Systems epoxy. The machined surface really drinks the epoxy and I put on a couple of coats. These were applied thinly and allowed to soak in so there was no epoxy build up to change the radius or accuracy of the dish.
Link

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Cut to size.....Beat to fit.....Paint to match.

Link,
Thanks for the good tips about keeping everything flat while routing and again when clamping in the go-bar deck. I just returned from my local lumber yard, where I purchased a sheet of 3/4" MDF. Even though they take good care of their stock and the yardman helped me select the flattest looking piece, all of the MDF was a little warped. I was pondering how to deal with that and I really appreciate your advice.

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George :-)

I use 2 pieces of 3/4" MDF glued together. I then put a dish on each side. Have done 15/25, and 15/28. They remain stable with one on each side. When making 12' for a friend, it was rather deep so it was a single.

Bob