Mirror Grinding and Polishing Machine - by Rainer Jakob - 22 Janury 2012
Having made two 150mm f/8 and one 150mm f/24 mirrors and being busy with figuring another 150mm f/4, I decided to move on and tackle something bigger. How about a mirror 400mm in diameter? Maybe I would also have these glorious views of the wonderful objects of the night sky that my ATM colleagues are always talking about?
I have been dreaming of a Nasmyth (constant level eyepiece) design for a long time. This is not only because I like sitting better than standing on a ladder or bending down, but I actually find the mechanical part of the scope more interesting and challenging to construct. Here I must admit that I am one of the few members of the ATM group that prefer to fiddle and build things, rather than to stand into the early hours of the morning and study the night sky. I just enjoy to build something and to see how it works.
I spoke to the experienced members of our ATM class about making a 400mm mirror by hand and very soon realised that this be rather left to the younger generation. Instead I would build a mirror grinding and polishing machine. I realised it would make no sense to build such a machine for only one mirror, so I would build a sturdy piece of equipment that could be used for many years to come and hopefully make many mirrors.
But what machine should I build, what were the critical elements? I had no idea. My biggest obstacle was my time constraint. I did not have the time to go and research the many types of grinding machines that have been built. It would have to be a team effort by the ATM members. I approached Chris Stewart of our ATM and he was immediately enthusiastic to help. I put it bluntly to him: I will build the machine but you guys have to tell me how to.
He gave me the following brief :
It must be an Elgin style machine with a single swing arm
The turntable must be supported and turn on three bearings and not use a single central column as is found in many such machines
The turntable must have a ring fastened to its underside and the driveshaft must be tensioned against this ring and turn the turntable by friction
Off I went and my design stage went something like this:
If the machine must make many mirrors, it will have to be in many different places. It must therefore fit through a normal door with some space on either side. That makes it 550mm wide. I will have to transport the machine and it must fit across in the back of my panel van: maximum length 1200mm. The height of the turntable would be the height that was comfortable for me to grind a mirror: about 850mm. That was it.
The turntable and the eccentric would each have their own motors which would run from a speed controller. To make the machine universal for all mirror sizes from 200 – 600 mm, the turntable would be regulated to speeds from 2-60 rpm using a six pole 0,37kW single phase 220V motor with a forced fan to keep it cool at low rpm’s. The eccentric would make 4-45 strokes per minute also using a 0,37kW 220V six pole motor linked to a reduction gearbox with an output of 20 rpm. This I luckily found in my storeroom.
I decided to do away with direct drives or chain drives and opt for a set of pulley systems for the gearing, in order to give the drive systems plenty of chance to slip in case something should go wrong. As the motors are strongest at full rpm, but lose torque below about 40% of maximum rpm, I put in 8 gears on the drive side and 3 gears on the eccentric using the pulley system to give the necessary output speeds. The driveshaft turns the ring on the underside of the turntable by friction from a rubber bung. This is the weakest link in the system and the first to slip should something go wrong.
The biggest challenge was to fit everything into the confined space, and still be accessible. In order to change the speed on either drive, it takes less than 3-4 minutes to change a gear on the pulley system.
The turntable always remains at the same height, but the eccentric arm can be lifted for height adjustment as determined by the thickness of the mirror in order to be in the horizontal position during operation. The eccentric can also be adjusted to give variable stroke lengths and can be adjusted to an off-centre position in two directions perpendicular to each other.
The one side of the frame stands permanently on wheels, but the heavier eccentric side stands on the ground with a height adjustment on the one leg for levelling purposes. This side can be lifted up with a lever to insert swivel wheels underneath for easy manoeuvrability. All the electrical cables run inside the tubing of the fame work and are out of harm’s way. Most other features can be seen on the video clip:
http://youtu.be/R97QsBI5spU (Note: size is ~ 66Mb; Length: 15.29; Video taken by Dave Hughes )
Under construction at the moment is the clamping device which will centre and hold the glass in the middle of the turntable. A hogging attachment which will grind a rough curve of required depth is also in the process of being built.