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Do those of you who use these own two sets? As in one set for sanding, and
one set for attaching the braces to the top/back? Or do you somehow rig a
set with sand paper to also be usable for attaching braces to top/back?

Thanks again for all of your help.

I make and sell these things. So I have a bunch for myself. But now I have limited myself to using one dish for each radius. When pressing in the braces I just place a piece of craft or news paper over the sandpaper. So my suggestion, and what I suggest to my customers, is to use just one of each radius.

Shane

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Canada

I own one set per radius I have them covered i 80 grit. When I use them as backup during glue up I place as 3mil Not 3mm Mylar sheet between the wood and the sand paper. but to tel you the truth unless you really let things get out of hand, on hard woods this is not necessary. as the plate is not going to slipping all over the place and you will be sanding out the out side prior to pore filling anyway. Tops I always use the Mylar to protect the surface.

Great. Thanks for the responses. I was thinking you could just place
something between the top/back and the sandpaper (like you guys are
talking about) when using it for gluing braces, but since I have no
experience doing it yet, I wasn't sure if there was something I might be
missing.

I use a double sided dish with sandpaper on one side and bare on the other. I just flip it over when gluing. I believe the dish on both side also provides some equilibration to keep the MDF from warping.

Actually if you think about it the thinned area in the middle, I would think leaves you open to easer warping. Now some have done what you say by laminating two pieces of 3/4" MDF or A grade Baltic birch ply togather without worry.

I just line my (sandpapered) dishes with a bit of newspaper when I don't want to, y'know, use the sandpaper.

I forgot to say that my dishes use two layers of 3/4" MDF laminated together. I did it this way after dishing a one layer piece of MDF on only one side and then watched it warp.

[QUOTE=BarryDaniels] I forgot to say that my dishes use two layers of 3/4" MDF laminated together. I did it this way after dishing a one layer piece of MDF on only one side and then watched it warp.[/QUOTE]

When I make up dishes I ALWAYS start with a double lamination of 3/4 mdf. I cut individual pieces into 24 in circle with a router jig. Glue them together and then let them sit until a dish is ordered. Then I drill the dog holes and place the disc on the rim sander. Once it is spinning I belt sand the edge to get it perfectly round in relation to the centre shaft of the rim sander. Then I rout the radius into teh top of teh dish. That makes these dishes heavier than most available, but it also makes them more stable in my opinion. When I do double sided dishes (typically one radius on one side and another on the other) I have to do them seperately so I have a another piece of 3/4 mdf that I screw to the dish piece to spin the dish, I still true them round and all of that and when the two dishes are spun I then glue them together. Once everything is done they get two coats of waterbased varathane. I have had mine for a couple of years now and they appear very stable.

I think that it is important to laminate the pieces BEFORE you make the dish. Make the stresses equal rather than trying to pull the dish back into shape after the fact. I find all plywood inherently unflat, it is usually twisted or cupped, even in the higher grades. This is not an issue when making cabinets becasue the edges typically get aligned to something that pulls the piece into form, but to use as falt stable structure, it does not appear to be the best solution for me.

Shane

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Canada

Like Barry I have two double sided dishes, with sand paper on one side and bare on the other, they are 18mm MDF laminated together and have been very stable for several years. One benefit is the extra weight of the dishes when sanding the radius on the box. I also like the ability to better ensure that there is nothing stuck to the dish when I press a soft spruce top into it with 30 go-bars. The bare dish is safer in that respect.

There is actually a guy in England selling double sided phenolic dishes, but I routed my own.

Colin

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

I made my dishes from acrylic, and thus they'll never warp, but acrylic is a lot more expensive than MDF.

For what it's worth, it's been mentioned before that a lot of other factors in the building process are going to have a much bigger and more noticeable effect than your dish warping to a 25.5' radius instead of a 25.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services