I finished the 'M-O-M 20' in July 2011 and within only a couple of days I went through the whole grinding proces of a 10 inch f/6 mirror. I had it already hogged out by hand for about 80% and after about 2 hours of machine hogging it was deep enough and pretty good spherical, so that I could proceed to the next grit. Fine grinding was very easy and went without any problem, thanks to the M-O-M manual and the very helpfull advises of experienced members of the M-O-M Yahoogroup. I could finish the fine grinding within about three hours. The result is a nice spherical and good looking surface without any pits or scratches.

The powerhead

The powerhead - motor and two shafts plus pulley's - is pictured below. My first motor was a 1/3 HP Dayton split fase, 1425 rpm motor, running at 220 volts. But it was very noisy and vibrated heavily. Therefore I bought another one, a 1/2 HP Dutchi 3-phase, 1440 rpm motor, running at 400 volts. I also bought a variable frequency drive (VFD). This motor is much (!) quieter and the VFD gives me the possibility for ramp up and ramp down. With the motor running at its nominal speed, the shaft next to the motor - only the 10" pulley on it can be seen - runs at 213 rpm and the other shaft, visible on the front side of the photo, at 32 rpm. By changing pulleys on these two shafts the speed of the turntable (shaft and pulley in the right corner) and the excenter (left corner) can be adjusted. And of course the VFD gives me the possibility to get any speed I can't get using the pulley's alone.

The drip pan

The drip pan (photo below) under the turntable is a 650 mm diameter flowerpot saucer. Because it is too shallow (the turntable throws slurry over its edge) I made a kydex ring, glued to the inner side of a wooden ring. Three bolts through the wooden ring fit into the three holes visible in the rim of the saucer, making the ring easily removable from the drip pan. The saucer is made of polypropyleen, which cannot easily be glued and I had to glue a piece of pvc pipe in it's center, around a hole fitting over the turntable shaft. Berthold Hamburger pointed me to a loctide product, which I did use. It is a combination of a polyolefin primer, loctite 770 and a glue, loctite 406. It worked very well. The small hole to the left of the shaft, between the two pvc rings, is the water outlet.

Grinding

On the picture below the machine is fine grinding an 8 inch mirror with an 71% tool (143 mm diameter) and 9 micron aluminium oxyde. The bent lines on the mirror are the effect of the tool turning - with respect to the mirror - faster at the central part of the mirror as compared to the edge zones. The speed of the turntable is 45 rpm.