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I have just made a 15ft radius dish using two sheets of 20mm MDF bonded together. Is it sensible to rout the 15ft dish on the other side or should I
1. Bond another 20mm of MDF before doing it or
2. Make a completely separate 15ft dish?

I really prefer to have separate dishes for each radius. I do know of people who make them with a radius on each side, but your dish blank needs to be thick and ridged otherwise it will likely warp a fair bit when you cut the radius into the back. I make my dishes out of a 3-piece laminate that is 1 ¼" thick before I cut it. After testing a variety of materials and laminating combinations this is what I have found to be most stable. If you were to cut a two sided dish I would want it considerable thicker than 1 ¼".

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Josh House

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I put the same radius on both sides of my dish. My idea was one side would be for sand paper and the other for glue up. As it turned out I could have just done with the sand paper side. I never use both.

My blank was two laminated pieces of 3/4" MDF. I haven't seen any warping in either my 15' or my 25'.

I find that the problem of warping when you cut the radius surface can be effectively eliminated by laminating a thin piece of Masonite to the under side of the dish before you rout it. This doesn't add as much weight or thickness as the full lamination, and it's just as effective in my experience.

Of course, a warped dish will flatten out when you put the go-bars on it. I have to think that a piece that dished on both sides would also deflect under pressure, and change the radius. Besides, if I'm going to do the work of making two dishes I'd like to have access to both of them at the same time. What happens when you want to glue braces on the top and profile the edge of the rim while the glue dries? MDF is not expensive, and dishes don't take up much storage space, but the labor to make them is costly, so it makes sense to maximize the utility of the ones you make.

I wish Al had given me that advice a couple years ago. In any event, I totally agree with his conclusion.

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Under Compensated Nut!

I have 15' on one side and 10' on t'other. Works well, but never use the 15'

I vote separate as well. Good to have the other while one is stuck in the glue deck.

You can make two radius dishes with half a sheet of 3/4" MDF, or about $8 each. Cheap enough to make separates. Put 2-3 coats of spirit based varnish on them. Water based will make the MDF swell. I used some old Minwax poly that was too old for anything else to seal mine.

Alex

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"Indecision is the key to flexibility" .... Bumper sticker

Thanks all.

With a dish both sides I would still have 24mm of material left at the middle so deflection would seem unlikely. However, as I already have some spare MDF, I am going to make a separate dish, as suggested by Alan C and others, so that I have the option of using both at the same time.

My 'bought' MDF dishes can arch up about 1/8" around the edges at some humidity levels, but they go down flat when I put on the go-bars. Why would you expect less deflection than that?

Vs 200 lb of go bars, I would say likely. As for single-sided dish warping, probably not a big deal for go barring. But, I wouldn't want to rim sand with it.

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Pat

MDF is notoriously flexible. And the forces add up quickly under go-bars. I only use two radius dishes so I don't mind having two around. If u don't use them for glue up (under go-bars) then yes, shape both sides

This was a great question!

I'd leave the other side of the dish flat. I actually put sandpaper on my first one, and used it for various things. I made it out of plywood, and it worked good.

Until I discovered Baltic Birch plywood. I like the weight of the radius dish with Baltic Birch; very solid. Expensive, but I've since changed all of my jigs to Baltic Birch, and even created new ones. Very different from working with plywood or mdf jigs. Just sayin.

OK, so. I am going to get deflection. If the dish deflects then the front/back has to deflect the same amount as has the brace. Assuming the brace is making full contact with the front/back, the length of the arc on the underside of the brace remains the same as when undeflected. So, when the glue has cured and the piece is removed from the dish surely it would return to the radius original created on the underside of the brace.

Just being argumentative. I made a second separate dish.

"So, when the glue has cured and the piece is removed from the dish surely it would return to the radius original created on the underside of the brace."

No.

"Just being argumentative. I made a second separate dish."

Good...