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 and the goal 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 shaft through

the center hole. This works great. I don't worry about measuring the sagitta, but just

take readings across the mirror to see how even it is and 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 launch'. 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 only very slowly ground to depth.

Reading from other atm sites I see people have HAND-ground mirrors faster than this :-/

Time for a new tack.

A cheap 4" 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 little better 4.5" blade, brand Husky, was purchased for $13. It is the turbo style,

has bigger diamonds and more area than the 4" blade. I also poured an 18 pound slug of

lead in a 5" saucepan and screwed the new blade to that. I call it "the Pig". It is

heavy enouth that I have to lift it on and off the blank with the base of my hands

instead of my fingers but it slides easily across the glass. It also 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 in one

direction only and going backward or 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.

Persistance will out. I continued grinding with the small remaining diamonds, rotating

the Pig occasionally to find sweet spots. The center of the mirror ground ok but the

large outer zones were taking forever. Remembering how the 7" blade worked before, I

placed the Pig on that and resumed grinding. This worked fast and brought down the

outer zones quickly, but again would not bring the center down. So I alternated the two

blades. After some more hours blank 3 was finished.

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!