MOULDING A POLISHING
Making a polishing lap is I consider the worst job in telescope making. Cutting
the facets is the problem. Sawing the channels and then vee-ing them. The
saw bogs in the pitch and the teeth clog. If you wobble sideways with the
saw a facet or part will chip off. Then the edges of the facets have to be
bevelled with a knife. Another operation likely to chip of sections of the
facets. When finished, though it is perfectly satisfactor for its purpose
it looks untidy.
I used Allyn J. Thompson's book, "Making Your Own Telescope" as my guide in making my first telescope mirror. In it he describes a moulded lap with small round facets. I made one. The problem is that with pressing and polishing they flatten out and it has to be channelled after all. Perhaps I had the pitch too soft. As it was my first mirror I was on it a long time and the time factor came into it.
That was long ago. Now I decided I would attempt to mould a channelled lap. A mould was machined out of aluminium to cast a thin matrix. This could be used for a flat lap or curved to conform to a spheroid. Tin was used as I had some available. Problems which I won't go into were encountered before I cast one successfully.
The matrix was laid on an aluminium plate. A vinyl band was wrapped and taped to form a dam about 10 mm. Deep. This was sprayed over with Silicon Mould Release. The lap material, which in this case was resin, wood dlower and castor oil, was melted over a gas burner and thoroughly stirred and then poured over the matrix to a depth of approximately 6 mm. The lap backplate of aluminium which was heated up and wiped with terpentine to ensure bonding was placed on top of the soft lap mix and pressed down. The lot was allowed to cool. A blow with a block of wood seperated the bottom plate. Now the matrix had to be removed and here is where the trouble began. It would not release. Compound was covering it in several places. As the matrix did not conform perfectly to the bottom plate, the lap compound got under it. The matrix and lap were destroyed. I am now of the opinion that the resin was too hot allowing it to get under the matrix.
As a lap was required to start polishing optical flats a lap was poured by the conventional method. Instead of cutting the channels, the lap was heated up and another matrix placed on top and weighted to press the grouves into the lap. However on removing the weight and matrix instead of flat top squares they were rounded. Also the matrix did not sink in for the full depth. The channels were deepened with a knife. The fine ground pyrex blank was placed down, face up and coated with a cerium oxide slurry. On top of this was placed a piece of nylon mesh. The lap was heated, placed on top of the glass and weighted. After half an hour the lap was slid off and the gauze peeled off. It was a very presentable lap with a fine grid pattern on it.
However I was determined to mould a lap. l reckoned that if the matrix was made of a flexible material, it could be peeled out of the lap. I decided try making one out of Silastic. This would resist the heat of the lap resin and have the necessary flexibility to peel out. The mould was sprayed with silicone. The grouves in the mould were filled with Silastic and a flat plate also sprayed with silicone release was pressed down onto it. Time was allowed for the Silastic to set. Difficulty was experienced getting it apart and removing the matrix. It was too flimsy to use. I should have realised this before attempting the job.
I then decided to go back to the to matrix, but instead of using an open grid, to cast it with a 3mm. Backing to give it strength. Another mould was machined up. This is illustrated in the drawings below. The mould was placed on a level surface heated up and the tin poured in. On cooling the matrix was secured in the mould with three screws. The mould was held in the lathe and turning from the centre out the thickness of the matrix was reduced in thickness to make it more flexible. It was necessary to hold a piece of wood against it with the tailstock centre and the screws removed to allow machining to the outer edge.