Hogging is where we take a flat piece of glass and grind it roughly to a desired spherical shape. I am working the three 14" blanks in parallel with the idea that one of them will be a very good mirror, and I can afford to experiment with the other two (in my wildest dreams there exists a bino-scope :)

If you lay a straight edge across a mirror you can measure its depth by noting what size drill bits will slide under it. I used Excel to make a table of drill bit sizes and sagitta depths for a 14" mirror. I have a set of bits going from 1/16" to 1/4" by 64'ths so I can track progress. The target is 13/64.

That will tell me how deep the center is, but not how spherical the rest of the mirror is. For that we need a sphereometer. I cut a half-inch off the bottom of a coffee can, routed a wooden disk that fits snuggly inside and put an old micrometer head through the center. This works great. I don't worry about measuring the sagitta, but just take readings to see whether I need to work the center or outer zones.

I have a turntable powered through a series of pullies by a treadmill motor that can be controlled to run at speeds from 'creep' to 'discus throw'. If you ever salvage a treadmill be sure to save the electronics for the speed controls.

First attempt on blank 1 was to spin-grind with a 70% tile tool and 60/80 lapidary grit. I cast several different tools and worked very hard to center them on their pivot but they just would not spin. They chattered a lot and sometimes rotated backwards. I added weight and varied the turntable speed without success. The result was a lot of flat grinding with little deepening of the center.

Then I purchased a cheap 7" diamond saw blade and tried that. Boy it grinds fast! But unfortunately it also grinds FLAT. After many hours of frustration blank 1 had lost 1/8 inch thickness and was only halfway to the desired center depth. But one of the tile tools was performing better so to save what was left of blank 1 I switched to blank 2.

More frustration. Blank 2 lost very little edge but progress was very slow. Reading from other atm sites I see people have HAND-ground mirrors faster than this :-/ Time for a new tack.

A cheap 4-inch diamond blade, segmented style, for an angle grinder came up for sale at Harbor Freight for about $7 so that became the new tool. I attached it to a wooden disk and laid some metal weight on that. Then running the turntable at 15-20 rpm I proceeded to hand-grind blank 1 using elliptical strokes in the opposite direction to the turntable. This worked very fast and blank 1 finished in a few hours so I moved on to blank 2. Progress was fast at first but diminished rapidly. Was the blade worn out?

A better quality 4.5-inch blade, brand Husky, was purchased for $13 at Home Depot. It is the turbo style, has bigger diamonds and more area than the 4-inch. I also poured an 18 pound slug of lead in a 5-inch saucepan and screwed the new blade to that. I call it "the Pig" and it grinds fast! Blank 2 finished out and I moved on to blank 3.

Again blank 3 started fast but progress fell off rapidly. Examination with a high-power lens showed many of the diamonds had disappeared. Comparing the unused side with the used one revealed that over half the diamonds were gone. The blade was flipped over and grinding resumed. Again fast but this time after an hour there was hardly any grinding. Inspection showed that nearly all the diamonds were gone on the second side.

The problem is that these blades are directional. The diamonds are supported mainly in one direction and going backward and sideways pops them off. Dragging a blade across a glass face works them in the wrong direction a lot of the time and they can't stand it. One side of the blade is more wrong than the other and it fails more quickly. I flipped the blade back over where it worked a little better.

The blade slowly worked the center down but the outer zones were just too much, so I alternated with it and with the 7-inch blade which cut the outer zones pretty well but did nothing for the center. Progress kept slowing and looking over my log at one point I realized I had pretty much wasted the last 2-3 hours, so I went back to the store and got a new 4.5-inch. Within 25 minutes I reached the target depth on blank 3.

These diamond blades are made for cutting, not grinding. A new one will grind fast and probably would completely hog a 14" blank, but do not count on it doing more than one. I recommend the turbo style which has a solid outer edge with segmented areas that allow water and sludge to pass through. The continuous and segmented rims have thinner areas of diamonds. Blades available in your area may differ, so look for one that has the coarsest diamonds and the most area with them.

Set the turntable to 'creep' and use elliptical strokes going in the blade's preferred direction. Short thin ellipses to work the center, long fat ones to work the outer zones. My wets lasted 3-5 minutes with the smaller blades, 1-2 minutes with the bigger one and once I got the whole surface grinding (1-2 hours) I stopped every 20 minutes to take measurements. Even when things were slowing down the sphereometer always showed progress, which is very encouraging. Keep a log of three measurements - edge, middle, and center; and note each time you sink another drill bit, you will find it very gratifying!