Help needed with pitch lap - what is going wrong??

[Sfarndell]

Hi all

I've finally got the motivation to finish an old mirror started 4 years ago . At the time of stopping, the mirror was FULLY polished, with a lovely smooth surface and no TDE or astigmatism and I'd started parabolising (+-30%), but now I'm ruining it! I was fastidiuos about hygiene, but on polishing little chips of pitch came off and now the surface of the mirror is pitted everywhere as if I was still using Aluminium oxide, and there are also a number (10-15) small scratches.

My process was as follows:

My new lap was made at my local ATM club which was made a little harder given that we are now going into our warmer months. I admit I never made my 1st lap either, so my experience in lap-making is limited and the ATM-ers have been doing this for years.

The lap channels were made by using a silicon template on top of my mirror (protected by a few layers of newspaper to prevent cracking the mirror) so the channels were cut and the lap hot-pressed on making the lap. I then chipped off the rough edges and scrubbed the lap for a minute or so with a wire brush and cleaned it with water and nylon brush then cold-pressed for an hour before using it. (cold pressing on a small amount of Cerium Oxide, etc).

THe Cerium oxide was double bagged inside a sealed container then mixed with water and allowed to settle before I poured into the mirror, so clumping is unlikely.

Could the pitch be too hard, allowing the little pitch chips (which are likely embedded with CeO) to grind, rather than polish? the lap is a 67% subdiameter lap (18" mirror, 12" lap)

Any help would be much appreciated!

Scott

[Peter Drew]

A pitch lap that is too hard certainly can give rise to scratches, other than that you seem to have taken all the usual precautions.

[Sfarndell]

Thanks Peter.

I have always been very pedantic about following correct pressing of lap/hygiene around errant grit/dust, so it's unlikely (but not impossible) to be external contamination. From the advice here and on another forum, the only solution is to re-make my lap and go back to repolishing to a sphere until all the micro-pits are gone. Only another 10 hours or so .

[FraserClarke]

What is the base of the lap made of? Is that clean/sealed enough, or could you be getting contamination from there?

I've certainly had an old plaster base start disintegrating and dropping bits of scratchy crud all over the mirror...

[Astrobits]

I don't think that I can add anything to the advice above. I think that the best help you can get is from the guys at your local ATM class. They have done many mirrors and can look at yours and the tool/cerium oxide and give better advice than we can from 9000 miles.

Personally, I have used hard pitch laps without any significant problem, but then, I have always avoided dry cerium oxide which I know can give small sleeks. I have read that it is best to prepare the dry cerium oxide + water mix at least a day before using it to allow the agglomerates ( produced in the drying process ) to break down. I have always purchased Regipol 900 series. It is a slurry which is the manufacturing stage before drying and I cannot remember ever getting a scratch.

Nigel

[Sfarndell]

Thanks guys

This is a 38mm thick plywood which I varnished twice. I also wash my tools with soap and a nylon brush, followed by a no-soap rinse before using it. Something went badly wrong here, so I'd rather just restart my lap again and take it from there. I'll post an update next week to let you know if this all worked out

Scott