Telescope Mirror Grinding and Polishing Machine
After I had made the 41-cm telescope mirror by hand I had decided that I needed a grinding and polishing machine for the bigger mirrors I wanted to make.
Specification
The machine must fullfill the following needs:
- It must be as compact as possible.
- Must be able to grind and polish mirror of up to 76 cm.
- The grinding table must be able to swing from horizontal to vertical in order to test the mirror. During testing the mirror will be supported in a sling.
- Must be able to cut new mirror blanks from glass.
- A crane is needed to move the larger mirrors when finished.
Design
The most compact machine is the Zeiss and Elgin types. The Zeiss type has a very complex motion which make it difficult to decide what effect it is going to have on the mirror. The Elgin has a semi-circular stroke which is made nearer to the traditional hand polishing. Therefore we choose the Elgin for the machine type. It will be based on the Elgin machine[1] described in the Maksutov Circulars. The frame of the machine will be based on the machine (see photo below) that Prof Ritchey[2] used to ground and polish the 60-inch Mount Wilson mirror. It gives us the ability to swing the grinding table from horizontal to vertical for mirror testing. An accessory will be connected to the bottom of the grinding table axle to make provision for the cutting of new mirror blanks.
Eccentric arm drive
The eccentric arm is driven as follows:
| Motor Power (W) | Motor speed (RPM) | Gearbox ratio | Output speed (RPM) |
|---|---|---|---|
| 500 | 1425 | 50:1 | 28.5 |
|
Gearbox axle pulley Diameter (mm) |
Gearbox axle pulley Circumference (mm) |
Arm axle pulley Diameter (mm) |
Arm axle pulley Circumference (mm) |
Ratio |
Speed (RPM) |
|---|---|---|---|---|---|
| 70.0 | 219.9 | 280.0 | 879.6 | 4:1 | 7.1 |
| 87.7 | 275.5 | 265.9 | 835.3 | 3:1 | 9.5 |
| 108.3 | 340.2 | 248.8 | 781.6 | 2.3:1 | 12.4 |
| 131.4 | 412.8 | 228.8 | 718.8 | 1.74:1 | 16.4 |
| 156.7 | 492.3 | 205.6 | 645.6 | 1.31:1 | 21.8 |
| 181.5 | 570.2 | 181.5 | 570.2 | 1:1 | 28.5 |
The 70.0 mm pulley was made from aluminium and the rest from blockwood. The circumference of the pulleys was used to make the pulleys. Some were too big for my vernier caliper. With the circumference measurement one gets a 3.1 times magnification of the diameter and thus one gets a more accurate measurement.
Grinding table drive
The grinding table drive consists of 2 sets of pulleys which gives 12 different speeds.
| Motor Power (W) | Motor speed (RPM) | Gearbox ratio | Output speed (RPM) |
|---|---|---|---|
| 500 | 1425 | 50:1 | 28.5 |
| Pulley set 1 |
| Number |
Gearbox axle pulley Diameter (mm) |
Gearbox axle pulley Circumference (mm) |
Middle axle pulley Diameter (mm) |
Middle axle pulley Circumference (mm) |
Ratio |
Speed (RPM) |
|---|---|---|---|---|---|---|
| A | 70.0 | 219.9 | 265.2 | 833.2 | 3.79:1 | 7.5 |
| B | 91.9 | 288.7 | 249.1 | 782.6 | 2.71:1 | 10.5 |
| C | 117.9 | 370.4 | 228.6 | 718.2 | 1.94:1 | 14.7 |
| D | 146.9 | 461.5 | 203.5 | 639.3 | 1.39:1 | 20.6 |