My Mirror Grinding Machine

For a long time I've wanted to build a machine for grinding telescope mirrors. Dennis Rech's M-o-M designs finally inspired me to just get up and do it. Over the years, I'd been collecting motors, gear boxes, pulleys, etc. Most of them came from industrial junk yards. To use these parts, I had to modify some of Dennis' plans. How I did it is described below. This machine is the result. It is capable of grinding up to 20" telescope mirrors. Pictured is a 10".

 

I quickly discovered that changing the original plans was not easy, so I hacked out a 3-D drawing program to help me visualize it. I'd been doing similar work professionally for 16 years, so it wasn't a big deal. These are some of the M-o-M drawings I made in the very beginning. I still work on this program as a hobby.

Leaping forward a few years, I've decided to make the program and the source code public, no strings attached. I call it EZ3D Designer. It is an enormous improvement over the original. It's easy to use, quite handy, educational, and can be downright entertaining. Some examples are included in the download. "Quick start" instructions are available on line.

One of the M-o-M features I really like is the use of a power head that sits inside of a base-box and is free to move around. This makes it easy to change pulley sizes and still keep the belts tight. The clamps you see are temporary and will be replaced by springs in tension.

The motor is 1/3 hp at 1750 rpm and the gear box is a 25:1 reducer. I found both in a salvage yard. If you don't have a gear box, you should stick with the M-o-M power head design. In the configuration shown, the reducer shaft output is 50 rpm, the eccentric rpm is 11, and the turntable rpm is 67. This is typical for grinding and polishing. For figuring, I have another set of pulleys that spin the eccentric at 25 rpm and the turntable at 11 rpm. I can make a pulley change and not have to change belts (hence the clamps). The bearing boxes house 1"x 3" cylindrical bronze bushings. The shafts are 1"x12" chrome plated steel (McMaster Carr). They are supported by a shaft collar riding on a bronze thrust bearing. Note the beer. It's important.

The biggest difference between my machine and the M-o-M is in the upper arm assemblies. Dennis chose the Elgin design which employs a bell-crank mechanism to drive the overarm. I chose to go with the Zeiss design which uses an eccentric arm connected directly to the overarm. I felt the Zeiss style arms would experience less stress and be cheaper to build.

The two-axis U-joints you see are made of a soft 2"x2" pine which costs about $1.80 for 8 feet. A 3/8" hole and a 3/8" smooth shank hex head bolt makes for a surprisingly good hinge bearing. The catch basin is a 20" dish used for holding potted plants. About $5.00 at Home Depot.

The overarm drive pin is attached to the tool with a 3/8" swivel foot and pitch. The eccentric arm is attached to the overarm with the same kind of swivel foot (McMaster Carr). It rests in a 1-1/4" hole on the overarm that is bored about halfway through. The block with the V-shaped notch is a sliding hatch that serves to keep the swivel foot from lifting up out of the hole. Sliding friction alone keeps the hatch in place so that the eccentric can be quickly disconnected.

Shown here are the bottom of the turntable and the eccentric. They both rest on 8" pulleys fastened to the shafts with set screws. Dogs transfer the torque. The groove in the turntable near the edge is a water barrier. Also shown are some T-nuts. I attach my mirrors to a plywood backing disk using pitch and then bolt the disk to the turntable through the T-nuts. The eccentric crank is a carriage bolt outfitted with nuts, washers, and steel spacers. It screws into a 3"x3" square nut that rides in a channel. The square nut is made of soft wood with a T-nut hammered into it.

The mirror in these photos is a 10" Corning molded Pyrex blank. I hosed it off just before I took this photo so the slurry appears kind of thin. The dark spots are the pitch that fastens the mirror to the backing plate.

Not long after I took the above photos, I added still another overarm eccentric. It was an interesting experiment. Have a look at the details.

 
 

The 10-inch mirror wound up in the optical tube assembly shown here. The scope is really too big for the Celestron equatorial mount, but my son takes deep sky photos using lots of short exposures and stacking them. Some of his photos are on-line in flickr. He has a number of scopes so look for 10" F/6 Newtonian.

This is a large image. Try clicking on it (twice).

The only special tools that I used were a drill press and a router. All in all, I spent about $100 on the plywood, about the same for pulleys and shafts, about $30 for bolts, glue, paint, etc. Almost everything came from either Home Depot or McMaster Carr.

I have a few things that aren't relevent to the topic at hand but an ATM might find interesting.

  * Print a Couder mask with CouderPrn  
  * Print a Ronchi screen with PrintaRonchi  

Following line added 12/25/12.

Try Parab, a new mirror analyzer for Windows inspired by Sixtests and FigureXP.

I am now working on a 15" x 1.5" Pyrex blank cast by Horace Davis. Here is some more about Horace.

I enjoy communicating with fellow enthusiasts. Building telescopes and programming for Windows are just some of the thing I do. If you have any comments, please send me an e-mail. I have plenty of time to respond.

Tom Stokes