I first built a machine for grinding and figuring mirrors according to the directions in the Sky and Telescope article by Tom Waineo.(1) There are directions for a Waineo machine available on the web.(2) The machine worked, but it was gutless and took a very long time to polish out a 6" blank. I was in a hurry and decided to build a more powerful version of a Waineo machine. The turntable tended to skip in the initial version, so I decided to drive the table directly. Pictures of this second grinding machine are shown below. It used a 325 RPM motor from Herbach and Rademan.(3) The motor is geared down from 325 RPM, to a turntable speed of 24 RPM. The overarm stroke is about 9 RPM. These rates can be adjusted by changing pulleys in the drive chain. I have made a number of mirrors with it this new machine.

I gradually became frustrated with two limitations of this machine: the table rocked, and the stroke length was not continuously adjustable. The table rocked because I was resting it on three wheels for support which is one of the trademarks of a Waineo machine, but I was driving it with a central shaft, and the two support systems were not perfectly aligned. When I made the connection to the central shaft so that it would flex, the support by the three wheels worked very well. The turntable would hold lots of weight, but when sideways force was applied to the table (i.e. when polishing) the table tended to rock and table base lifted up off one of the wheels. This made me very nervous about putting much force on the machine when polishing, and I suspect it also introduced zones on the mirror surfaces. The machine polished out blanks nicely, but the surfaces needed lots of hand work to clean them up.

I rebuilt the machine when I became aware of the Mirror-O-Matic design by Dennis Rech.(4) This machine had two features that attracted my attention: a continuously adjustable clutch, and a turntable supported by a central shaft. I rebuilt my machine to include these two features. The rebuilt clutch follows the design of the Mirror-o-matic, except I made mine out of aluminum rather than plywood. I used cork gasket material as the clutch contact surface. The clutch works great, and was not hard to make.

The turntable rests on a 12" pulley, and I set mine up with a thrust bearing to support the weight. The 3/4" steel shaft extends about 1" above the pulley, and the turntable has a 3/4 bronze sleeve epoxide into a hole in the center. Between support from the pulley and the shaft-sleeve contact, the turntable does not rock. I have polished out one 13" mirror on the newest version of the machine, and it worked very nicely. Pictures of the rebuilt grinding machine are shown below.

I used to attach the mirror to the turntable with an appropriate-sized melamine circle using duct tape. Unfortunately, one needs a new melamine circle for every mirror size, and the if one is regularly checking the figure to see if a good sphere has been produced, one uses a lot of duct tape. I got tired of this method and decided to make an adaptable clamping system. The result is shown in the four lower images listed above. I made a three-fingered chuck. The chuck fingers are made of plywood or redwood and have a hole for a 1/4-20 SS screw. The screw goes through the finger, through a slot in the table, and into a 1" square nut made by taping a piece of aluminum. The nut fits into a ca. 1.05" slot cut into the bottom of the table. Tightening the screw clamps the finger in place. Three of these fingers snugged up to blank hold it in place. The blank is centered by taping a clear ruler to a fixed part of the machine and watching for wobble in the blank when the table is rotated. This is not as precise a proper centering on a lathe, but it gets the blank centered to < 1/32" and should be good enough for mirror making. To clean it up I just wash the whole table. To check the figure I loosen one chuck finger and remove the mirror. The other two fingers stay in place and the mirror can be repositioned by snugging the blank up to the fixed fingers and just retightening the loosened finger. This chuck system is more versatile than the duct tape method, and I don't need to have a different melamine form for every mirror blank size. The final image shows a 12.5" mirror blank being fine ground with a 10" tile tool.

References

(1) R. W. Sinnott, "A Versatile Desktop Polisher," Sky and Telescope, November, 1995, 81-84.

(2) Directions for building grinding machines can be found on the web. Here is a list of relevant web sites.

(3) The motor is item number TM91MTR1775/C and sells for $29.50. Herbach and Rademan can be reached at 1-800-848-8001 or one the web at http://www.Herbach.com/.

(4) D. Rech, "Mirror-o-Matic," ATM Journal, Volume 13, 5-9. The Mirror-O-Matic is also described on Dennis Rech's web site.