Over the past few years I have been making telescope mirrors by hand. I was fortunate enough to take a mirror making class from Carl Zambuto out in Bellingham, WA at a Telescope Optics Workshop (TOW). Carl was a great teacher (and hard task master) and was kind enough to answer questions via email after that workshop.
Learning by the hand method taught me how the tool & blank interact with each other and what to look for. But hand work is exactly that - WORK!
I attended several TOW's over the years. One year I met this character who had this strange little machine to grind telescope mirrors. That character was Dennis Rech and that strange little machine was one of his early Mirror - o - Matics. I took several pictures of it for future reference.
The next time I was at TOW Dennis was back with a much larger and refined machine - his Big - o - Matic. Dennis was also selling plans for his smaller machine, so I bought a set. Dennis has a website you can look at the machines at and order plans. It is http://www.mirror-o-matic.com/ There is also a Yahoo discussion group at http://groups.yahoo.com/group/Mirror-O-Matic/
As usually happens in life, those plans sat around for a few years. From time to time I would pick them up & reread them. When I would go to surplus places or get my surplus catalogs I would always look for items that I could use for my M-o-M. I found some used AC gearmotors in very good condition at a very good price. A friend of mine had given me a piece of aluminum plate 1/2" x 22" x 98". I ran across a good deal on flange bearings. Slowly but surely I gathered the materials & parts together. About 6 months ago I started construction and finished up a couple of weeks ago (mid May 2004.)
I used Dennis' 20"plan for the most part. General size, shaft placement, shaft sizes were all pretty much the same as the original 20" M-o-M. But I changed a number of items. They are:
The following pictures are of various parts of my M-o-M. You can click on them for a larger image.
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This is a picture of the machine in general. In it you see the turntable with mirror blank on it, the swing arm with weights on it, the eccentric assembly, and an IV drip bag on on overhead arm. |
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This angle shows the switches and the open end to get at the motors. There is a cut-off switch (red) near the bottom and then two button switches, one for each motor. You can also see the back of the swing arm assembly and the shaft that it rotates on. |
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This is a close-up of the swing arm assembly. The spindle is made of 5/8" Stainless Steel rod and I machined the bottom of it round. It fits into a greased 5/8" round bottomed hole in the tool. I mounted the hinges this way as to prevent the arm assembly from dropping onto the mirror. The tool in this picture is a 4" diameter diamond cup I bought on ebay that I use for "hogging" out the glass. Once I got the radius of curvature (ROC) I wanted I made a tile tool. See below. |
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This is a picture of the motor end. I have a separate motor for the turntable and the eccentric. I found some used gearmotors and a surplus store in the area and decided to use them. This eliminates the need for a series of belts & pulleys to reduce the motor speed. |
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This is the turntable motor. It is 1/4 HP and has a shaft output speed of 96 rpm. The belt is twisted 90º. I used a notched automotive belt to do this as it twists easier than a solid belt. The motor is mounted on a platform that can slide and belt tension is applied by a spring on each side of the motor. |
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This is the eccentric motor. It is 1/8 HP and has a shaft output speed of 36 rpm. I needed to mount it on its side, which is not a really desirable thing to do. I extended the oil/vent hole and then ran it up 90º so the top where the vent is is higher than the motor. This motor also slides and uses springs for belt tension. |
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This is the tile tool I made to do my fine grinding. The base is 1½" plywood. In order to conform to the curve of the mirror I placed a piece of waxed plastic sheeting over the mirror. I then mixed up a batch of marine epoxy and added a thickening powder to it. I then poured this on the center of the plastic sheet that was sitting on the mirror. I placed the plywood disk on top the epoxy, and then put a weight on top of that. This squeezed the epoxy out from the center to the edges of the disk. I let it dry for a day, removed it from the mirror, trimmed the edges, and then epoxied the hexagon tiles on it. A final coat of straight epoxy was given to all uncoated surfaces to seal them. |
So far I have used this tile tool to grind a 12½" mirror using 80 grit, 120 grit, 220 grit, 320 grit, 500 grit, and 12 micron and 5 micron abrasives. It is still in very good shape and I could probably use it again for a mirror of the same ROC. Using the epoxy mix to mate the curve to the mirror works very well and is quite easy. Just make sure you use a high quality marine grade epoxy like System Three or WEST.
My next step is to make a pitchlap and start polishing. So far everything has worked quite well and I am very pleased with the results. And it is much, much easier than walking around the barrel grinding & polishing by hand.
UPDATE 05/29/2004 - Since I last updated this site I have made my pitch lap and have started polishing. The following are some photos of my progress.
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This is the pitchlap I made. Like the tile tool, the base is 1½" plywood that was mated to the mirror with an thickened epoxy mixture. (See tile tool above.) I then sealed the entire disk with marine epoxy to seal it. I then heated up my pitch, made a 3/8" masking tape dam around the edge, poured my pitch, and then presses it to the mirror which had a thick layer of cerium oxide on it so the lap would't stick. I used a new pitch that recently came to market. It is a synthetic pitch called Acculap and is sold by Salem Distributing. |
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Here is the pitchlap in action. I polish for ½ hour at a time, let the mirror cool for a bit, and then do a Ronchi test. I use a digital camera to image the test to keep a visual history of my progress. I found that to get a good image I needed to manually set my camera to 2 f-stops under what it would normally take. Most digital cameras have this exposure adjustment. |
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