Grinding & Polishing the Mirror

July 16, 2007

This is my grinding and polishing workshop. Yes, an honest to goodness 50 gallon steel drum in the basement.

My 14" fused quartz blank has been in polishing with Cerium Oxide (CEO) on 55 Gugolz pitch (basement was a bit nippy when I started polishing!). The mirror now has 25 hours of polishing and it's nearly finished. The very center continues to show just a little too much reflection from a laser for my tastes.

To get the blank to this point, which is f/ 4.9, it took

1 lb #60 SC and 4.5 hr of coarse grinding
5 lb #80 SC and 25.0 hr of coarse grinding
.6 lb #120 and 4.5 hr of coarse grinding
.25 lb #220 and 3 hr of medium grinding
.25 lb #320 and 2.5 hr of medium grinding
1 oz #25 WAO and 3 hr of fine grinding
1 oz #15 WAO and 3 hr of fine grinding
1 oz #9 WAO and 2 hr of fine grinding
.5 oz #5 WAO and 1.5 hr of fine grinding

I would definitely use more #60 or #40 SC next time, but I didn't have much #60 to start and didn't want to wait on it. Shoulda! All the coarse grinding was performed with a 25 lb flat barbell weight on top to provide extra pressure.

The FL was a bit under my intended f/5 but I couldn't lengthen it with #120 and #220 doing exclusively tool-on-top (TOT) grinding. Probably, that's due to not having weight on the stack.

I did 2 hrs of CEO with polishing pads on the tile tool just to experiment with. Although the inner 9" looked polished, a laser revealed a whole lot of scatter from the center areas.

It can take all of my 210 lbs of body weight to push the tool over the mirror when the pitch tool and mirror are well-mated and getting dry. Hard work! At most I can polish for 120 minutes continuously, then I just have to rest.

The mirror spends its free time in a little coffin I made for it. Keeps it protected from dust and dogs. I am paranoid about mirror scratches!


The pitch tool also has its protection. It's some sort of artist's portfolio that my wife scared up. Good for keeping the tool in a moist atmosphere and not having anything touch its surface. The down side to both storage containers is that they have to lay flat...and take up lots of surface area.


This is the pitch tool's second incarnation. The first lasted 25 hours; then the pitch squares started to pop off. The "potholes" result from pressing the channels with a warm steel bar. That puts ridges on the squares and then pressing with the mirror creates the potholes as it flattens them. It's only temporary. After an hour of polishing, the potholes start to close up.

Here's how I spell "good contact"! Fifty pounds of weight for 30 to 45 minutes, rotating the tool a little every 15 minutes. The temperature in the work area is about 70F now in July. When I remade the tool last week, I added some harder pitch because the 55 Gugolz is pretty soft at higher temperatures.

mid-August update on mirror

It's been HOT. Even in the basement. Given the soft pitch, I chose to work on the Telekit mount rather than the mirror. Once the weather consistently cools, I'll figure the mirror.