Making Your Own Radius Dishes
by Dominator

This is the process I use to make domed dishes for building ukuleles with radius tops and backs. Much of this procedure was posted by Mike Chock in the forum over at Hana Lima `Ia.

Things you will need:
1. Router with a 1/2" diameter flush bit and ½" straight bit.
2. A drill and bit equal to the diameter of a shelf pin or a cut off bolt 3/4" in length.
3. Scrap plywood for a platform and 1x materials for the arched rails for the jig.
4. A string or length of rod or wood equal to the radius of whatever size dish you want to make. Example: for a 15' radius dish a 15' rod or string. (My soundboards have a radius of 20' and my backs have a radius of 10').

Step 1:
Start with the work base or platform for the jig. Since I was only planning to make ukuleles I made my platform 24" x 24". If you plan to build guitars then increase the size accordingly. I made my platform from ¾" plywood. Now is a good time to locate and mark the center of the platform before attaching the rails as discussed in Step 3.

Step 2:
To make the arched rails that attach to the platform take your string or long rod and drill a tiny hole through the rod or stick on one end so you can anchor this point. Now, 15' away or whatever radius you decide...fix a pencil. Draw an arc on a piece of 1"x4" (mine were 28" long and extended past the edge of the platform 2" on each end - this is not necessary though. You can make them the same length as your platform) from one corner of the length to the other end. Cut this arc out on a band saw (a jig saw will also work if a band saw is not available) and get it perfect via sanding to the line that you drew. Use the completed arched rail to mark a second 1x 4 and this time cut proud leaving the line. Double stick the two together with the bottoms flush and pattern route with the flush trim bit to produce two perfectly matched arched rails. Hint keep the arc so that it is located high on your 1x4.

Step 3:
Screw each arched rail to the left and right edge of the platform (don't use glue because you want to be able to change the rails for a different radius as necessary). You now have a platform/base with left and right arched rails and open top and bottom. At the center mark located at the end of Step 1 drill a hole 1/2" deep the same diameter as your 3/4" length of cutoff bolt. Epoxy the bolt into the hole to establish an index pin. The platform is now complete.

Step 4:
Now it's time to make an auxiliary base for your router that will ride on the arched rails. I made mine from ¾ birch plywood. I wanted the base to extend beyond the rails on both sides so I made the base 5 ¾" wide by 28" long. Now, find the center and drill a 1' diameter hole (actually any size bigger than your 1/2" straight bit) in the router base. I removed the original router base from the router and used it as a template to drill the small holes used to mount the router to the new base. Since I used the original mounting screws it was necessary for me to countersink the underside of the base so the screws would reach when attaching the ¾" base. Install the ½" straight bit into router and then mount the router to the auxiliary base with the bit protruding from the hole.

Step 5:
Now that the router is attached to the new base place it on top of the arched rails. Line it up so the overhang on each side of the rails is equal and the base is perpendicular to the rails. With a short pencil reach under and put a short tick mark on the base where it meets the rail. Flip the base over onto the router top and using a square extend the mark on the base. Attach a small cleat, made from scrap, on the underside right on the mark you just made. I made my cleats from the same ¾" plywood - 5 ¾" x 1 3/8". Now flip the router and base back over again onto the arched rails. Position the base so the cleat is resting up against the rail. (cleats are located on the inside of the rails). To complete the auxiliary base just add a second cleat to the opposite end. Now the router can only travel north and south along the arched rails. The trick is to get the second cleat attached so there is no significant play (eliminating any side to side movement of the router base) but not so snug that the router can't move easily north to south.

USING THE JIG
I made my dishes from ¾" MDF. You could also use some good quality birch plywood. For ukes I started out by cutting some 15" x 15" blanks. Find the center and drill a 3/8" deep hole same size as the diameter of your pin in your platform. I used a ¼" pin which also matches by circle cutting jig I use with my band saw. So after drilling the center hole I used the band saw to cut the blanks into 15" diameter round discs.

Place one of the MDF discs onto the indexing pin of the platform. The disc should freely. Place the Router with auxiliary base onto the north end of the rails. Move the router base south till the bit is even with the top surface of the MDF. Now, simply adjust the router bit height on your router base to just touch the top edge of the disc. If your discs are larger you can start the dishing in from the ends ½" in or so which will leave a flat edge around the perimeter of the disc.

BEFORE WE BEGIN: Read the hints below before turning on the router the first time.
This process may seem a little unsafe to some people. Personally, I'm one that doesn't like to take risks in the shop. I have had no issues when using this process and if you take it slow you shouldn't either. If you are uncomfortable with this procedure then that just might be justification enough to purchase some discs over the Internet.

Move the router base north so the bit will not make contact with the new dish surface and turn on the router. Nothing should happen except the sound of the router spinning. Now slowly move the router base southward and you are now creating one pass of dust and the dish will want to spin like crazy.

STOP! If you have not read the hints below please do so now before you go any further!

HINT: Use your elbows to control the spin rate of the work piece. Move the router base southward no more than a 1/4" at a time. Chances are the router won't be able to keep up if you feed it more than a ¼" at a time. You will find that you can lock the router base in place easier and control your southward movement by giving one side of the base more pressure than the other. In other words, you don't have to move the router base southward equally. If you apply a little more pressure on one side it will automatically lock a little bit at a time. Keep moving till you reach the center. Keep clearing dust out of your way and stop and take a break whenever you feel like it. Just always keep the router base in contact with both rail surfaces and take mini passes until you reach the center.

With an orbital sander you now can make a quick interior sanding of your radiused dish to smooth out the concentric circles and you will have a professional and accurately radiused dish. Yes, the MDF creates much dust. Be sure to where a good respirator. I found it easier to take breaks and clean up as I went along instead of waiting until I was finished, where I would have been left with one huge mess to deal with. While you are setup be sure to make at least two of each radius. I also back the finished 3/4" radius boards with another ¾" piece of MDF to reduce warping.

Many thanks to Dominator for this domed dish tutorial! Please visit Dominator Ukulele Tabs.