Someone donated an old microwave to our church office. The plastic frame of the turntable guide became distorted after many years. The white rollers continually came off of the ends of their axles. That caused the turntable to stop and to tilt.
We did not want to spend around $20 for a new turntable guide (even if we could find one the size we would need), plus still more dollars for shipping and handling.
Step 1: Measure Your Microwave's Bottom
You will need to know the outer diameter of the circle scribed by the rollers as the turntable revolves. In this Instructable I usually refer to the plastic rollers as white. Here one is shown in cross section as pale yellow for the sake of distinguishing the roller from other items in the graphic. The bottom of the microwave has rises and falls stamped into the metal, as you see here. When you decide on inner and outer diameters for the ring that will hold the plastic rollers, you will need to evaluate how much space you have so the ring does not bind unduly between the microwave bottom and the glass turntable. A cross section of the fiberboard ring is shown here in a copper color. The turntable drive is at the center of the fiberboard ring and the turntable. It is not always easy to get a clear view for measuring when peering into the microwave.
Step 2: Cut a Circle From 1/8 Inch Fiberboard
On our microwave the circle scribed by the rollers is 10 5/8 inch at its outer diameter. I cut a disc from the fiberboard 11 1/2 inches in diameter. Mark the center of the disc as precisely as possible. Lay out three lines radiating from the center spaced 120 degrees from each other. I scribed another circle on the disc 10 5/8 inches in diameter (the outer diameter of the roller track). I scribed yet another circle inside this circle just a little smaller. It was smaller by the thickness of the white plastic rollers plus just a tiny bit of "wiggle room."
I also cut a circle 8 3/4 inches out of the center of the disc to leave ring as shown in the graphic.
Step 3: Mark Holes for the Rollers
Mark the size of rectangular holes needed for the plastic rollers to fit inside those holes when the rollers rest on their edges as they did in the old turntable guide. These holes are on the 120 degree radial lines. Drill two holes to remove most of the material needed to make the rectangular openings. See the rectangle with the two holes at the left of the graphic.
Step 4: Clear the Holes With a File
Use a small file to clear the rest of the material from the rectangular holes. The plastic rollers should slide through the holes comfortably without a lot of extra play.
Step 5: Keeping the Rollers Vertical in Their Holes
The turntable guide needs to be lifted off of the bottom of the microwave a bit. It also needs a larger face to act against the side of the rollers so they stay vertical. Cut some rectangles from scrap fiberboard and glue them as shown. One will be above the ring you made and one will be below it. This may seem a little primitive, but it works.
Step 6: Install and Check
Put your new turntable guide into your microwave and test it. It should not hang up, but should work smoothly just like the old one did before it failed. Although viewed through the speckled glass, you can see the rollers and how they are held vertical by the scrap pieces glued in place.
Step 7: A Failed First Attempt
Parts of a microwave turntable need to be strong enough and yet should not be made of metal. My first attempt was to use a ring of 3/8 inch plywood with six holes for glass marbles. The marbles would act like ball bearings in a lazy Susan. Note the side profile at the right of the graphic.
This attempt failed because glass marbles are not perfectly round, even though they may appear that way. The turntable frequently hung up.
Step 8: A Partially Successful Second Attempt
For the second attempt I chose to cut down the 3/8 inch plywood ring I made for the marbles. I used nylon screws from the hardware store to hold the plastic rollers in place and to act as axles.
This worked fine for heating a cup of tea. But, if the microwave ran five or ten minutes, the nylon screws absorbed enough of the radiation and turned it to heat that the nylon melted. The plastic rollers were still fine, but the nylon melting created a useless turntable. In the photo you can see how the nylon screw melted and caused burn marks on the wood. Also shown is a new nylon screw.
Step 9: What I Would Like to Try
Everyone knows you should not have metal, like a spoon or aluminum foil, in a microwave while it is in use. The metal creates a lightning storm as charges generated in the metal by the radiation try to get to an electrical ground in the metal frame.
True as all of that is, I would like to try a ring made of steel with axles welded to the ring. The ends of the axles would be threaded and a nut would keep the plastic rollers from coming off of the end of the axle. There should be no lightning storm in the microwave, if the metal ring is electrically grounded to the metal frame of the microwave. See the smaller drawing to the lower left of the ring. A piece of light spring steel would be attached to the metal bottom of the microwave with a pop rivet. The spring steel would gently ride on the bottom of the metal ring. I have not tested it, but it should work.