Fixed Post Grinding Machine

Introduction

Grinding telescope mirrors involves rubbing a mirror blank against a "tool" with grinding powders in between (usually suspended in water). It can be done by hand, especially with smaller mirrors, but a machine can be a great help. There have been various designs for machines over the years but I decided upon a Fixed Post Machine.

A fixed post machine has a driven axis (basically a turntable) and a fixed post adjustable to be offset from the center of rotation of the table. One element (mirror blank or tool) is fixed to the turntable and rotates with it while the other is constrained to revolve around the fixed post. The element attached to the fixed post is not driven independently but receives a rotary impulse from the friction between it and the rotating element on the turntable (with the grinding powders in between). This differential motion between the two elements accomplishes the grinding action. The concept is simple and best illustrated by images below.

One of the principles of grinding telescope mirrors is that the mirror and tool will achieve matching spherical curves with one becoming concave and the other becoming convex. The rate of change of the radius of curvature (ROC) depends upon specific techniques in the grinding/polishing strokes and those used for rough grinding (achieving the basic designed ROC) are different from those used to fine grind and polish the mirror to its final state. Reversing the positions of the two elements allows for precise control of the ROC.

One of the useful (but also limiting) properties of the fixed post machine is that the ROC tends to change very slowly with grinding time. This means rough grinding is usually done by other means than the machine. For smaller mirrors rough grinding by hand is recommended. Once the approximate ROC is obtained by rough grinding the remaining stages are to eliminate the pits and roughness of the rougn grinding and to refine the ROC to the desired value. This is where the fixed post machine shines.

Fine grinding uses a series of smaller, finer grinding grits to remove the pits from the preceding stage until a surface smooth enough to be polished is achieved. Fine grinding can change the ROC with diminishing rapidity with finer grits and a precise final ROC can be achieved. The fixed post machine saves a lot of physical effort (but not necessarily time - but the time passes easily).

The Machine

I started with donated turntable that featured a 12 inch diameter disk. It required a motor and pully and an overhead rail for the fixed post. A donated treadmill motor with motor control was adapted to drive the turntable The overhead rail was fashioned from 1 inch square tubing with holes drilled for the 1/2 inch steel post. A moveable fixture for the post was added later.

To affix the element to the turntable a set of 3D printed adapters were designed and printed to attach to the backs of the elements. A 1/2 inch hole in the adapter accepts the post while a hexagonal boss allows fixing the element to a hexagonal hole in an adapter attached to the turntable. A few pictures are worth a thousand words.

The machine is very helpful with fine grinding and polishing. See below for example of fine grinding 4.5 inch mirror for Schiefspiegler telescope. Up to 10-inch mirrors can be accommodated.