Our pesky human brains are fantastic at seeing and forming patterns, and even when we try to do something randomly it’s likely that we’re doing it in an organised way. When it comes to shaping a perfect sphere into a piece of glass, randomness is actually an advantage. So we need a way to trick that human brain to stop it making a pattern and accidentally grinding an unwanted shape into the mirror.
This comes back to why I needed to build the turntable in the last post. The best way to keep the grinding process unpredictable is to periodically rotate the tool one way a random amount (eg. 31 degrees clockwise) and then rotate the Pyrex in the opposite direction a random amount (eg. 95 degrees counterclockwise). So after one turn you might be able to guess the total change in orientation (eg. 126 degrees), but if you can still keep track while turning both discs in different directions every 30 seconds then you’ d probably be better off counting cards and make your millions.
Rough grinding will dig down to remove the majority of the material to turn my flat Pyrex into a shallow spherical bowl. The target depth I want to reach will determine a lot about how I build the rest of my telescope, so getting it right at this stage is important. Too deep or too shallow and I won’t be able change the depth when I move onto finer sized grits. To keep track I’ll use my hybrid-level-dial-doohickey to take depth readings at the centre of the mirror throughout the process. Before even starting, the surface of the Pyrex was a bit lumpy. The centre was 1.69 mm deep, and there was a small divot just off centre 2.14 mm deep. I’ll be grinding deeper than this anyway, so in a way all these lumpy holes have given me a head start!
Before grinding I used a knife sharpening block to round off a bevel around the edge of the Pyrex about 3 mm wide. This is just a precaution to avoid chipping the edge of the Pyrex once I start grinding.
Besides a bit of muscle from me, the main work of grinding will be done by the grit. The first size I’m using feels like very fine sand, but on a microscopic scale each grain is very sharp. As I grind it between the surface of the tile tool and the mirror, those sharp edges will catch and rip off tiny bits of both the Pyrex and the ceramic tiles. In the process the sharp edges of the grit will break down until they are no longer useful for grinding. The result is a muddy mixture of broken grit, Pyrex and ceramic. Each sequence of adding grit and grinding it down takes about 15 minutes and is called a ‘wet’.
To dig as much as possible in the centre of the mirror I held the Pyrex over the edge of the tool while grinding. This ensures that the centre of the Pyrex gets more contact and therefore more grinding than the edges. With each stroke I moved the centre of the Pyrex over an imaginary line from edge to edge across the surface of the tool. As I started to approach my target depth I gradually reduced the overhang so that grinding would start to take more material from the edges as well.
After 3 very messy hours of grinding, and with a kitchen counter covered in mud, I am almost at my target depth. It’s easy to tell where the surface has been ground because of its frosted appearance, and the low points, small holes and divots remained unscratched until the higher areas around them had been ground down.
To finish rough grinding I’ll need to grind out a few more tenths of a millimetre from the centre, and make sure that I’ve ground the surface into a nice even sphere. Simple right?