SMALL GRINDING AND POLISHING MACHINE.
This machine was designed and built to grind and polish Cassegrain secondary mirrors as I considered my other machines too big for the job. I had to purchase a piece of aluminium for the base plate but the rest of the machine used material and parts I had in the workshop. The spindle housing with a modified spindle is used on my faceting machines. The motor, cowl, pulley, water bottle and drip tap and tube is off an old model faceting machine. The legs are used on current models. I found enough pulleys from other projects on the shelf.
The motor is a shaded pole fan motor running at 1350 rpm. To reduce the speed two belt reductions were necessary. To avoid another shaft housing and bearings, pulleys 4 and 6 (see Fig. 1) which are pinned together are run lose on the Crank Shaft. The drive is from 1 on the motor to 6: from 4 to 9 on the spindle. Then from 8 to 5 on the crank. 6mm. diameter polyurethane belting was used. This gives great flexibility with regard to pulley sizes and shaft centre distances. Also a smooth vibrationless drive.
The bowl was made out of an aluminium frying pan. The spindle nose is the same as on my other machines but with a'/2" UNC internal thread. This is to take the small mirrors and tools. The post for the hand stroke lever is removed when the machine is run automatically. Similarly the stroke and stabaliser arms are removed when hand operated. Details of components are shown in Fig.1.
A post with a height adjustable sleeve over it carries a sweep arm that has a spindle for mounting the secondary mirror or diagonal. This is driven by a small 12 volt DC motor with reduction gearbox and a belt drive. The motor plugs into a 12 Volt DC power supply and speed control. The motor and gearbox was out of an obselete photocopier and the power supply and control was one used on a faceting machine I made. The sweep arm is swept across the diamond plate and fed down with the vertical adjustment knob. The block clamped around the sleeve which has the pivot spindle for the arm can be moved up and down and clamped to accommodate larger mirrors. The arm can be swung up and over to rest on stops and hold it in a vertical position.
The diamond plate is a 100 grit commercial lap plate supplied to gem faceters. Another copper plate with 600 grit diamond rolled into it is used for fine finishing. Mirrors are fixed to the adaptors using Superglue. Instead of a knob with screw to hold the lap plates on the back plate, a disc of thin rubber is used to give drive. If a knob is used there is the danger of bumping the mirror against it and knocking it unstuck. A small stream of water is fed onto the plate to keep the plate free of glass residue and to keep it cool.