Hogging Out

Since we got grit, we decided to start "hogging out" the mirror. In the Old Days, we would have used the same plate glass tool throughout the grinding and polishing of the mirror blank, but there have been cost improvements. The very roughest grinding is now its own separate step and is called hogging out.

The purpose of hogging out is to remove a lot of glass quickly with a sub-diameter tool. The tool for hogging out can be almost anything. On advise from another ATM, we thought about using a can filled with mortar.

As an experiment, we used a can without the mortar. I thought that the bottom of the can would be cut away too quickly. You can see it to the left of mirror. However, using the can allowed me to get the feel for what was needed, so I went out to my toolbox and got an 1 1/4 inch socket. This cost $0.25 at a pawn shop, and it was perfect.

Here we see Anna busily grinding. If you have never ground Silicon Carbide (SiC) against glass, prepare yourself for noise. It is loud, but gets quieter as the SiC is ground down.

One of the things that you will quickly learn is when to recharge the grit. There are 2 ways. First, the sound of the grinding changes from very loud to merely loud. Second, you can feel the grit rolling under the tool when you first start. As the charge of grit wears down, you can feel the tool sliding across the mirror.

After you have ground out several chargers of grit, a thick sludge will develop in the hollow of the mirror. This sludge must be removed. We are using a small red tub that we used in the summer to rinse our feet before getting in the pool. Since we don't have a water faucet downstairs, we get our water from emptying the de-humidifier.

We just douse the mirror in the water and gently rub its surface to help the grit fall off.

Here is Anna still grinding away. I decided to cover the top of our grinding stand with plastic. Each time we change grit size, we must thoroughly clean up everything to make sure that none of the larger grit is around to make big scratches in our mirror. With the plastic, we can just carefully fold it up and throw it away.

Here is the mirror blank just as it came from Edmund's Scientific all of those years ago. It has some surface irregularities from the molding and annealing process.

Here is the mirror following about 15 minutes of hogging out. All of the surface irregularities have been ground out, and the face is uniformily pitted. The pitting has made the glass very opaque.

Later, when we are polishing the mirror, we will use a cheap laser pointer to look for pits. The pits will cause the laser light to bounce around and prevent it from going through the glass. As we polish the glass, we will see less and less scatter from the pits.

A very well polished mirror is almost invisible, if both sides are well polished.

Tommorow, more detail on the methods involved on "Hogging Out."

Have Fun.