SkyBadger - Where Astronomy gets personal

Introduction
First pour

Introduction

This is a diary of the first mirror off the MoM.
The MoM as currently sized is capable of mirrors up to about 14" due to the 16" platter.

A picture of the MoM as currently configured for the 6" blank, configured for spin polishing , where the lap overhangs the edge of the mirror by about 15% of the diameter, in order to spin due to differential rotation and polish very evenly and easily with little chance of turned down edge. However....
The large wheel is the turntable for the primary mirror with edging to keep drips in.
Above is the mirror clamp plate with movable clamps to centre the mirror on the axis of rotation
Above that is the lap - a layer of pitch for polishing, a layer of cement for shaping the fresh lap backing plate to the mirror after grinding and the plywood backing plate.
Above that - the swivel foot that fits in the hollow of the plywood backing plate to drive it across the mirror. This foot extends through the drive arm to provide a pillar to mount weights on to make grinding and polishing speed up.

19 March 2023

Mounted 6" blank on MoM machine

Discovered that I couldn't spin polish with the lap as created since it kept on sticking and trying to topple rather than spin. Probably due to the lap being quite tall.

Marked up the lap into 6ths for hand rotation counter-clockwise. Also used scalpel to draw some lines across the lap.

Since I couldn't spin polish, left the mirror spinning and did slow 8 strokes across the mirror before rotating the top a 6th.

After each full rotation of the lap, I stopped the machine and rotated the mirror by 45 degreees or so.

Did 1h 30 of this. Didnt count how many rotations of the mirror that meant.

Centre is polished but less so towards the edges.

Pattern on the the mirror is a trefoil due to the mirror rotation while going backwards and fowards.

Attention was paid to trying to keep the lap on the mirror with no toppling, wobbling or skipping that could lead to turned down edge. Occassionally the lap skipped but mostly it had good continuous drag.

Inspected using hand loupe. fine grain seen at edge , Some small scratches on disk and full polish at centre.

Plan - create another lap that isnt as tall. No Need for concrete base - will need to modify MoM arm or raise mirror stand to support it.

Cut new 4" disk and counter-bored a 20mm hole for the MoM drive swivel foot.

Plan has been executed - cut a range of new 3/4" thick plywood disks from 120mm and up to 13" for use a grinding and polishing backing pads. Poured a new lap onto one of the disks. Warm pressed. Put the old one in the freezer in a bag and then tapped the pitch off it into the bag.Came off nice and cleanly

Contents

Introduction

Mirror Grinding and machine grinding references.