Radius Dish Help

[Bryan Bear]

I'VE GOT THE 1/8"MASONITE and am going to make this radius dish. Two sheets, bolt pulling the bottom one down. 24" frame. Any hints for success? Looks simple.

The one I made is more cumbersome than I'd like. Mostly because the frame is made for 2x4s I had laying around. Some day, I may get around to making a better one. I think a base of a layer of two of Pharos with a ring between the base and the Masonite would be better. One thing I'd like is to put some HPL (Formica) on the surface to make removal of the worn sand paper easier.

[Barry Daniels]

I always coat my MDF dishes with shellac to make them resistant to glue and solvents.

[Mark Fogleman]

I'VE GOT THE 1/8"MASONITE and am going to make this radius dish. Two sheets, bolt pulling the bottom one down. 24" frame. Any hints for success? Looks simple.

The standard off the shelf radius dish uses a spherical curve where every point in the curve is equal from an imaginary center. You'll be creating a conical curve which is not the same thing. A spherical curved dish allows you to place the top, bottom or the rims anywhere in the dish and the result is the same.

If you go ahead with your design, you can get close to a spherical curve by using an idea I got from Dave Higham on one of the boards, who didn't remember where he got it from, by gluing thin 1X1 square spacers in a spoke pattern originating from the edge of a piece of thick MDF circle going towards the center. Use 3-4 squares for each spoke spacing them about every 3-4 inches. Each square on the spoke would need to be thinner as you move towards the center.

To calculate the thickness of each step there is a calculator here: http://www.liutaiomottola.com/formulae/sag.htm Be sure to convert the "r" to inches instead of feet and use 3, 6, 9 and 12 inches for the as the "L" (distance from the center) figure for a 24" dish. For ex: a 24" diameter dish with a 12ft (144") radius, the outside course (12 in. = L) of spacers would be ~.5" thick, 9" from center course is ~.25 thick, 6" is .125" thick and 3" is .03" thick.

Drill a pilot hole for the center and for each spacer and screw down the 1/8" Masonite to the disc working out from the center. You could put some Bondo or drywall spackling in between the spacers (or more spacers) if you think you need it. The Masonite shouldn't deform that much and average out the force.

Dave is a member and I will pass this on to him. Hopefully he can resurrect photos here.

[Bryan Bear]

I worried about my dish being a spline curve instead of a section of a sphere at first. Then I realized just how small any difference would be and that I would be using it to glue springy pieces of wood together. I think in practice the difference is purely academic IMHO.

[Dave Higham]

I think you're probably right Bryan but as Mark has asked me if I could post the photos here, I will.
They were hosted on *!£§*bucket when I posted the thread on another forum but I never put photos there without keeping the originals myself, thank goodness!
Mark has explained the process so I'll just post the photos and if anything isn't clear just give me a shout.

The formulae.

Difference between a radius and a spline curve (exaggerated).

Theoretical problem with a spline dish (or parabolic or whatever the hell it is)

Raw matériels

[Dave Higham]

Drilling holes in the strips.

Cutting the strips in half (don't try to cut through the locating pin).

Cut the strips into pièces.

Drill pilot holes in the base board.

Glue the spacers to the board.

[Dave Higham]

Spacers all glued.

Screw down the Masonite or MDF or whatever.

[Mark Fogleman]

Thanks Dave!

[Bob Shanklin]

This was the setup I used to make dishes. Blank mounted on a Lazy-Susan bearing on sheet of plywood, rails that router rides on mounted inside frame that also holds dust collection at each end. The router bit spins the dish while cutting the radius, so you only need to go to the center. Took on average about 2 minutes to make a dish. I had different rail sets with several radii, from 12' to 28' as I was making them for friends as well as myself. Before adding the dust collection it was 2 minutes to cut, 2 hours to cleanup. I now use a spline arc side to side, with no radius heal to tail so I rarely use the dishes.



Bob

[Bryan Bear]

Good pictures Dave! I admit, I was shocked when I saw the second diagram. I missed the "exaggerated" tag and couldn't believe how prominent the difference was. I think that is a good approach to making dishes!

One other variable I think we forget about when considering the accuracy of different radius dishes is the fact that the dish radius and the radius of the inside of the plate is also different. We fuss over getting our braces sanded to exactly the shape of the dish then glue them in offset from the dish surface by the thickness of the plate thus tightening the radius. I'm not saying this is an issue, just pointing it out. Technically we are springing the brace a tiny bit tighter than we shaped it.

I'm not saying we should not fuss over accuracy or fit by any means just that we should accept the fact that we are not NASA. We can get the plate braced just the way we want it and close up the box but as soon as it leaves the shop and someone opens a window, that shape will change. . .

[Bob Howell]

I have decided to try turnibg the dish on my lathe. I have the radius dish pdf print out from the library with measurements. Don't have it to scale but plan to draw it out and cut on bs. Its on a legal sheet and I don't see a way to determine the length embedded in the drawing. 28 units across which I think means it is 28 inches but I would like confirmation if someone knows off hand. I will turn it at the edge of a garage with a box fan blowing dust into the blue.

[Bob Gramann]

Bob Howell: That is a lot of spinning mass. Just hope it doesn't get away.

[Bob Howell]

It is turning at 150 rpms now. Use it for sanding. I'm thinking I try 20" first. Experiment.

[Aaron Helt]

They're pretty cheap to buy.

[Bob Shanklin]

I have decided to try turnibg the dish on my lathe. I have the radius dish pdf print out from the library with measurements. Don't have it to scale but plan to draw it out and cut on bs. Its on a legal sheet and I don't see a way to determine the length embedded in the drawing. 28 units across which I think means it is 28 inches but I would like confirmation if someone knows off hand. I will turn it at the edge of a garage with a box fan blowing dust into the blue.

Greg Nelson turned one several years ago, just to see if he could do it, and posted pictures on Mario Dacosta's forum. He said it was pretty scary.

Bob

[Clay Schaeffer]

"Radius dishes and go bars make the work so much easier. They pay for themselves pretty quickly."

Really? You're able to make money building guitars?

[Bob Howell]

I have a large lathe and have turned a lot of bowls up to 18". It is not a problem. I am making smaller guitars for now and will try to live with 18-19" dish.

[Bob Shanklin]

Lee Valley posted a new catalogue that I received today. Lee Valley Luthier Tools and Supplies. They are selling radius dishes.

Bob

[Bob Howell]

The lathe worked well and I have a 15' radius that is 18" round. I did not smooth the surface, as that would produce DUST, not curls.
I will make more sizes over time.

The link to the calculator site is great for checking results.

Thanks