By astrokumar; Published 21 Dec 2008
[ i know its after half a million yrs i am continuing, my apologies]
I have already mentioned in the previous section, how to get the stuff. so, i assume you already have it. Lets begin. start by preparing the workplace. This is nothing but a high stool, or barrel or anyplace that you can turn around, and keep an absolute mess and wont get shouted at.
Keep the thicker glass on the table or the stoll you have decided to work on and draw a circle around it with a marker.
now, take the cleats [ three wood or plastic pieces about 0.5" square] and nail them at approx 120 degrees from each other around the circumference of the circle. place the glass in between and check for the fit. see to it that it just about fits right enough and the cleats shouldn't introduce any stress on the glass. now that you are done.. pat yourself on the back. you are ready to begin. By the way, just a little detail before you start. keep and old bucket of water and a couple of waste pieces of cloth nearby. you need this to clean your glass after every charge.
Take the 100grade [ or 80 grade] silicon carbide [SiC] and apply a tablespoon on the glass. The thinner one should be in between the cleats. spread evenly, place the thicker glass on top of it, offset by about 1/3rd diameter and push forward and backward [ rubbing action] the glass should not go more than 30% up or down. Apply pressure all the while.
You will notice that after about 15-30 strokes depending on the pressure applied, the powder has broken down and the glasses begin to stick.Stop now. Tkae the bucket of water nearby, and dunk the glasses in.[ NOT DUMP, DUNK. DIP]. Its to clean the glasses of the waste SiC and the little glass that has been ground. Yes, slowly and slowly the glasses get ground and appear dull. Lots of scratches will appear first, and more glass gets ground later. after cleaning the glasses, place on the table and repeat the above procedure. But before grinding[ rubbing] again, shift your position, along with the above glass about 90 degrees from initial position and then start grinding. Remember the 30% up and down motion and the 1/3rd offset ok? well, after about three hours of work on a 6" diameter glass, you will notice a slight curve appear on the thicket one. a concave curve. Now thats what you want. there is a way to find out for what focal lenght, what should be the depth of the curve you have made. This depth we shall now call "Sagitta". It is given by s = D/16f where D= Diameter and f = Focal Ratio. and f = Desired focal length/ available diameter of glass. so, for example, for a 6" diameter glass of 60 inch focal length,
s = 6/ 16*10 f = 60/6 = 10 so, the s = 0.035 inch, which is roughly 1mm in SI Units. This depth will take you, a beginner about 6-8hrs to reach. This depth can be measured by either a dial gauge or even a small length of nail of the required thickness [1mm] place the nail in the centre of the glass [thick one] and using the edge of a straight steel ruler, check if the edge of the ruler touches both edges of the glass and the nail sitting comfortably between, with vobbling. then you are done. I shall post more, keep watching