After finding again the Amateur Telescope Making hobby it was very obvious that I didn't want to grind mirrors and lenses manually anymore. Maybe the main reason is that I have always been a little bit restless person, wanting things happen faster. Just like there would not be enough time or I could not spend time too much. Wanting to see results quicker! Ok, at least my work requires sometimes a lot of my time. :)
Anyway, grinding optics with a grinding machine requires in principle nothing more than manual grinding. The same tools and materials are needed to reach a fully polished optical surface. Still if you start thinking of the differences between those two methods there are a few points which have to be learned in order to suggesfully produce high quality optics. Amateurs' optics can be and many times are high quality! And I keep learning things continuously...
My Grinding Machines
Well, I guess it was somewhere in 2003 when, once again, surfing in the net I found Dennis Rech's building instructions for making Mirror-o-Matic. It led me building two machines. The first one was powered by an inexpensive, small 24 VDC-gear motor to prove for myself that I can do machine grinding. With that machine I made my first Cassegrain telescope (huge 4") for my younger son. The primary blank (Duran50) was purchased over 20 years ago even if it never saw the light on those times.
Also the small machine was almost "the must" for making as small as dia. 30 mm cassegrain secondary. Next there was my 5" telescope's primary which needed re-grinding with the finest grits to remove TDE and many, really many pits left on the surface at the beginning of 80's. Nowadays the telescope produces quite nice images and it will always be there, even if not needed many times :)
I used also that small machine for my friend's 6" F/6.4 primary. Here is a photo of the mirror while figuring, not final yet. He was many times made me small lathe works and making his primary was helping back. After that I was quite convinced that I can with my machine and I digged up the great 8 incher! Once again a Duran50 mirror blank from 80's which actually was coming perfectly by manual method but was left unfinished because of a huge 2" long scratch during polishing! It was too much for me, period! I still remember it very well, it was very close that I would have changed the hobby to throwing one... All in all, the most positive thing was that I cooled down and saved those blanks for the future. Nowadays I have found the blanks as the treasures and without them I might not have begun the hobby again!
It was obvious that my small grinding machine didn't have enough power and room for grinding a 8" blank. Here is a photo of the small machine loaded with a 8" blank. The whole thing is "full of glass"! With my second machine I started by figuring out the needs:
- the machine had to be buildable by my home tools
- the costs had to be as low as possible, ie. junk or second-hand stuff as much as possible
- at least 12" blank had to be doable
- still the over-all dimensions of the machine had to be reasonable...
- ...as well as the total weight
- the grinding style as mirror-o-matic does, turntable spins faster than overarm, sub-diameter tools
- quickly removable turntable for cleaning
- simple machine with quick speed adjust possibilities
Because of my work I can use AutoCad at the work office after a work day. So, with Acad it was simple to make an accurate drawing of the machine to see the main dimensions and needed pulley diameters. The "heart" of the machine is the second-hand, small ABB frequent converter. The biggest 3 phase motor is 0.37 kW (0,5 HP) what it can handle. The converter also has a possibility to connect to the normal wall socket. Over here in Europe we have 1-phase / 230VAC / 50Hz available from wall sockets. The converter takes 1-phase 230VAC in and drives the machine's SEW gear motor 3-phase 230VAC allowing control the frequency range from about 5...80 Hz (I use). The gear motor's output shaft rotates 51 rpm with 50 Hz, so I can select output speed from 5.1 ... 81.6 rpm. The gear motors output shaft directly rotates the turntable so the speed range is the same for the turntable. The overarm speed is reduced 1:5.8 from the turntable in order to match roughly what is recommended by Dennis Rech. I used a 5 mm thick, round and flexible polyurethane O-ring material as a transmission belt between pulleys as seen right-hand on the photo (turntable removed). Normally I use about 70 rpm for grinding and polishing a mirror. 5 rpm is used during manual mirror figuring while the over-arm assemble is lifted up and I am sitting next to the machine pushing a 50% figuring lap on the slowly rotating mirror. The converter also has an option to connect a 12VDC hand-pad to control different actions such as controlling rotating speed. I have used a simple hand-pad to get three different turntable speeds, so I do not need to do any V-belt changing operations if I want to increase or decrease the turntable rotating speed. The machine figuring would require to change the turntable and overarm speeds other way round, ie. for example 5 rpm for turntable, 20 rpm for overarm. That is not possible with my machine yet.
The converter also has a few sophisticated features. For example you can change the parameters so that every time when you switch on or off the machine it accelerates up or down smoothly within 20 seconds. If you have a heavy mirror on the turntable am. feature is more than welcome! The over-load current feature is also enabled, ie. if the motor get a gear-box failure or bearing damage, the converter switches off the motor eliminating bigger damages.The mechanical structure of the machine can not be simpler. The whole machine's frame is made of 15 mm thick finnish plywood. All plywood stiffeners (3 pcs.) are screwed between the cover and bottom plate. Both open ends of the box have a small stiffener to hold the box in shape. The flange bearing (seen on the above photo) and the flange of the gear motor have been bolted together, only the cover plywood plate between them. The overarm's shaft is connected by using two small and inexpensive flange bearings. Finally the turntable's support plate is connected to overarm's pulley by a transmission belt. The support of the turntable has also a groove for the belt, 3 pcs. M6 threaded holes and the center threaded hole (M16) for bolting the turntable. I screwed three bolts upwards from backside of the support plate. The plywood turntable has three equal holes for the bolts locking the turntable in it's place by tightening the center bolt.
Usage of a grinding machine is not always straight forward thing and at least I have had to practice new skills before suggesfully made the first mirror. I think this it is not the correct place to start explaining deeply the machine grinding with Mirror-O-Matic style machine because Dennis Rech's web-pages give a lot of information. Also by joining to the Mirror-O-Matic discussing group at Yahoo gives tons of information of machine grinding and more. The most important thing still is: "Practice makes masters"! There's just no any other way. That's what makes a hobby interesting.
However, I try to list a couple of the things which are good to notify before starting to make a mirror with a machine. Please notify that my comments are only for Mirror-O-Matic style machine and there might be things which are not necessarily exactly workable with different machine styles. And many times we all have own styles to do the same work.
- the size of grinding and polishing tool is always 70...80% of mirror diameter
- mirror is normally on the table - grinding TOT (tool on top)
- good material for making a tool is concrete, plaster or aluminum
- epoxy glued tiles are good for grinding, aluminum works too
- sometimes it's good to glue tiles with pitch (lens making)
- polishing tools pitch squares are made centerly on the surface of the tool...
- ...either one square is exactly in the middle or two channels
Also it's good to remember that some of the old "laws" are valid with machines too: The polishing tool has to be made well, pressed tool with good channels and same-sized squares is everything!
1. Gugolz64 is good pitch. Other grades or types not necessarily needed .
2. Micro-facets on each pitch square are very useful - better contact.
3. From about 6..8 mm thick pitch layer is good value.
4. Thin polishing slurry polishes fast producing better surface.
5. After the polishing the surface of the mirror should be a good sphere figuring easier to make.
6. Correctly done, machines produce very good spherical surfaces.