Official Luthiers Forum!

I've got a couple of these Radius dishes.
They are 3/4" mdf, and I've read that it's best to add a 'backing'
for support and to prevent warpage.
My questions are:
What backing material? MDF or plywood?
How to attach? Glue or screws or both?
Any caveats to avoid warping the dishes?

TIA,

Your assumptions are correct.

Back it with MDF, glue and screw from the bottom is best. Usually it's better to glue the pieces up before routing the radius (more stable this way) but if the radius is routed already backing it up will really help.

I'd use epoxy to glue, that way there is no moisture introduced although honestly, it won't make a huge difference in the end. You'll be using the dish to final shape your braces (give them a swipe with a plane before gluing) and to glue the braces to the plate so whatever the final radius of the dish is, it will transfer to the guitar.

_________________
My Facebook Guitar Page

"There's really no wrong way, as long as the results are what's desired." Charles Fox

"We have to constantly remind ourselves what we're doing....No Luthier is putting a man on the moon!" Harry Fleishman

"Generosity is always different in the eye of the person who didn't receive anything, but who wanted some." Waddy Thomson

Like Rod said. All of the dishes I make (I think I am over 100 now and I do them on CNC) are double laminated 3/4 mdf. I use titebond and just glue and clamp. I leave then sit for a good couple of days before I rout the dish. In your case please make sure that you clamp the two pieces together before you screw them together (but if it were me I would just glue and clamp and forget about the screws) as the screws will push some of the mdf through their travel and result in the two plates not coming tight togther. Clamping before screwing them together looks after this issue.

Shane

_________________
Canada

You got two dishes, just glue 'em together. My top "dish" is backed by Baltic birch.

That will work, but you eliminate the nice flat (circular) sanding surface that you get if you stick a sanding disc on the flat side.

Cheers
John

I don't know if you have, or are planning to have, a vacuum capability in your shop, but I thought I would share my solution for anyone that does.
If you have a large radius ,(50'+), a small warp can be a large percentage distortion of the shape. I routed channels in the back, drilled an intersecting hole in the side for vac line, sealed it with shellac, and sucked it down to my granite surface plate. Now it is rock solid, I know it is the radius that was cut and there is un-obstructed access all around it.

Any problem with plywood that is now available not staying flat ? They have found pieces of fencing wire and even soccer balls in Chinese plywood. Lots of complaints about plywood from the large retail outlets not staying flat. Maybe it would be better just to use more MDF to back your discs ?

Yeah, I've had some cheap plywood from the home stores that warped. Could use some good baltic birch, but that is expensive. On mine, I glued another layer of MDF, and coated it with poly. It does make it heavy. If I had it to do again, I think that I would just glue my 15' and 28' dishes together, back to back. Wouldn't be much lighter to move around, but would take up less room.