The Process of Grinding a Mirror

To actually grind the mirror, first some silicon carbide grit (later on we'll use finer sizes and softer compounds) is sprinkled on the tile tool. Then the tool is sprayed with water (mostly for health reasons- so that the ground glass dust is stuck in a slurry and can't be inhaled). Using moderate pressure the glass is pushed across the tile tool surface back and forth. Every few strokes the tool and mirror are rotated opposite ways. Every few minutes some new grit and new water are added. Every twenty minutes or so the glass and tool are both rinsed off in a bucket of water so the grit/glass slurry can't build up too much.

The first stroke, or "chordal stroke" has the glass overhanging the tool by about half, digging out the center of the glass. The "W stroke" is just like it sounds, moving the mirror across the tool in a W pattern. It will take the glass to a sphere, then a parabola, and eventually a hyperbola if done for too long. The "normal stroke" is just forward and backward with the mirror squarely on top of the tool (instead of offset by half like in the chordal stroke). All of these pictures show the chordal stroke.

In the end of this stage the glass should have a spherical curve ground to the proper radius of curvature. This spherical curve will later be polished with finer grit, and only at the end is the sphere figured into a true parabola.

Progress on Rough-Grinding

To measure my progress on rough-grinding the mirror, I mounted a dial indicator in a piece of high-density particle board. I figured that the smooth back of my mirror is the flattest surface I have, so I calibrated my zero point on that. The two markings indicate my target depths, exactly 0.1 inches for an F-5, and about 0.01 inch less than that for an F-5.5.

The rest of these pictures are taken at about 1-2 hour intervals. i.e. This whole page represents about seven or eight hours of grinding the mirror. I started including the tool in the pictures to show how the tiles have worn down from grinding the mirror. As the glass becomes concave, the tile tool becomes convex and by the end of rough grinding they should both hopefully be spherical.

Before I had reached 0.05 inches of depth, I was using the "chordal stroke" to grind. This stroke will aggressively dig out the center of the glass, removing the bulk of the glass rather quickly. The tile tool shows how this stroke wears on the sides of the tool much more quickly than the center. For the mirror it is the opposite, and the 0.05 mark was reached within a few hours of grinding.

Next I moved to the "W stroke," which should quickly bring the shape into a sphere after the center has been ground out. If done for too long, though, the W stroke will go beyond spherical and create a parabola of the wrong focal length. Notice how the tile tool is being worn more and more toward the center.

I had hoped to have every part of the tile tool ground before I reached my target depth of 0.1 inches, but as the final picture shows that didn't happen. I'll have to switch from the W stroke to the "normal stroke," which is meant to maintain a spherical shape but continue digging deeper into the glass and tool. When the entire tile tool is worn through, I can be sure that both the tool and the glass have spherical curvature. By alternating between grinding with the tool on the bottom and the glass on the bottom, I can maintain that curvature at a proper radius.

Telescope Project

As a last hurrah to our undergraduate studies we've decided to each build a telescope in the next few months! Mine (Matt's) will have an 8-inch mirror, with an F-5 focal ratio (less magnification but more light-gathering) for looking at nebulas, galaxies, and star clusters. Cam's will have a 6-inch mirror with an F-9 focal ratio (more magnification but less light-gathering), and will be ideal for planetary and lunar observation. Cam's mirror will actually be made out of old ship porthole glass (picture at left)! This the only really cheap source of thick glass, but at only 3/4-inches it is still really not thick enough for large mirrors. The glass came with 10-inch diameter, but Joe has offered to trepan two 6-inch blanks out of the original portholes. One will become the mirror, and the other will serve as a grinding tool. Because an F-9 is much easier to grind than an F-5, I've gone ahead and started on mine even though Cam doesn't have her glass trepanned yet.

My glass was a gift from Joe. He bought (at a good deal) a mirror kit which included an 8-inch pyrex blank, and an 8-inch plate glass blank. The plate glass was intended to be the grinding tool, but he built a tile tool instead and gave me the glass to use as my own mirror blank (he also made for me a tile tool to use for grinding).

Rather than the traditional method of grinding atop a large barrel or a plywood pedestal, I've employed a workspace which can be pushed under the couch when not in use. This also allows me to watch shows on the Internet while I work. Here is a picture of me just before I marred the flat smooth surface of my mirror blank by grinding. Hopefully the next time it is that smooth it will also be in the shape of a parabola.