SILICON CARBIDE
Very Sharp, Long Lasting to Help You Form Your Curve Fast and Accurately. Silicon carbide is an extremely sharp, synthetic abrasive which approaches the diamond in hardness. It is made by
mixing finely ground petroleum coke with pure glass sand. The mixture is loaded into a crucible and heated by electric arc to temperatures in excess of 2000�C. The intense heat of the arc causes thecarbon in the coke and the silicon in the sand to fuse into silicon carbide. By varying the speed of the heating and cooling cycle the final crystal size is determined. A long, slow cooling cycle allows a few crystals to grow to a very large size while quick cooling forms numerous small crystals. The cooled
mass is then broken up and rough graded by screening. The individual crystals are treated by impact,crushing or air blasting to remove brittle edges. Finally, it is graded by screening, water sedimentation,hydraulic flotation or air classification, with the latter used for the very fine grades.
Because of its extreme hardness, silicon carbide is ideal for grinding Pyrex, which is harder than glass,or very deep curve generation used for refractor, Maksutov, or richest field objectives.The very coarse sizes (40 and 60) are used for diameters larger than six inches or deep curves where a large amount of glass must be removed.
Willman-Bell�s silicon carbide is the very best optical grade available. Every step of manufacture and packaging is controlled to ensure the high purity and scratch free qualities sought by the precision optical worker. Accurately graded sizes from 40 to 500 grit are stocked. We recommend this abrasive for use on Pyrex up to the final critical lapping stages.
Telescope Making Supplies
Today, modern technology is capable of producing consistently high quality aluminum oxide which has practically replaced the natural forms for precision optical work. Aluminum oxide, like silicon carbide,is made in the electric furnace. Bauxite (aluminum ore), coke and powered iron are combined, heated,cooled, crushed, treated to produce a uniformly shaped particle and finally graded. This process results in a nearly pure aluminum oxide crystal with a hardness of 9 on a Mohs scale. It is somewhat slower cutting than silicon carbide and is therefore ideally suited for smoothing operations involving the softer optical glasses used for refractors, corrector plates and optical windows.
WHITE ALUMINUM OXIDE LAPPING POWDERS
OPTICAL GRADE POLISHING COMPOUNDS
Until recently there were three principle types of optical
polishing compounds available; Barnesite,cerium oxide, and the iron oxides (or more commonly, rouge). Barnesite production was curtailed when the manufacturer determined that the process could not meet federal anti-pollution controls.
Fortunately, cerium oxide and the various rouges are more than sufficient to meet the needs of the precision lens maker.
All of W-B�s polishing compounds will give a first-class polish, but each differs from the other in
speed, action, cleanliness and cost. By changing from one to another during the polishing and
figuring stages, it is possible to finish a mirror or lens faster and more accurately because the
primary difference is speed. Therefore you may want to polish out your surface rapidly with cerium
oxide and then switch to one of the slower acting rouges that allow you literally to creep up to the
exact figure you are working for without fear of overshooting.
4 oz. 8 oz. 1 lb. 5 lbs.fob
Silicon Carbide
40 Grit* 9.00 20.00
60 9.00 20.00
80 9.00 20.00
120 7.00 8.00 10.00 20.00
220 7.50 8.50 10.50 20.00
320 8.00 9.00 11.00 25.00
500 8.50 9.50 12.00 27.00
Aluminum Oxide
120 7.00 8.00 10.00 20.00
220 7.50 8.50 10.50 20.00
320 8.00 9.00 11.00 25.00
500 8.50 9.50 12.00 27.00
White Aluminum Oxide
25 Micron* 7.00 9.00 12.00 27.00
12 7.00 9.00 12.00 27.00
5 7.00 9.00 12.00 27.00
3 7.00 9.00 12.00 27.00
Polishing Agents
Red Rouge 7.00 9.00 12.00 27.00
Cerium Oxide 10.00 15.00 21.00 65.00
Zirconium Oxide 9.00 14.00 20.00 60.00
Rosin, Pitch, Wax, etc.
Rosin 7.00 9.00 12.00 27.00
Burgundy Polishing Pitch
Hard 9.00 11.00 25.00
Medium 9.00 11.00 25.00
Soft 9.00 11.00 25.00
Tempered 10.00 13.00 35.00
Beeswax 8.00 11.00 16.00
Blocking Pitch
Hard 9.00 11.00 15.00
Soft 9.00 11.00 15.00
*Grit = number of grains laid end-to-end = 1 inch Micron = 1
millionth of a meter
ALUMINUM OXIDE
Slower Cutting, Ideal for Softer Flint, Crown and Low Expansion Optical Glasses. In its naturally occurring crystalline state, aluminum oxide, or corundum has been used as an abrasive for thousands of years. The purest, natural form is the gem-white sapphire. Blue sapphires and rubies are also composed of aluminum oxide but have been colored by mineral oxides. Emery, another common natural abrasive, is impure corundum containing iron oxides. The natural occurring abrasives of this family were subject to large variations in quality. Occasionally, a very fine pocket of aluminum oxide would be mined and provide the optician with exceptional finishes, but, more commonly, quality was very uneven and the material had to be cleaned and separated by the difficult, time consuming water flotation process. As you read some of the older books on telescope making you will understand the serious the quality control problems and the great pains that were taken to purify abrasives.
Super Fine, Smooth Cutting, Scratch Free Final Stage Abrasives. Final lapping is the key to smooth,regular surfaces that polish out quickly with no turned edges. The professional optician knows that a very fine, closely controlled abrasive makes it possible to grind out in a few minutes slight irregularities which would take hours of polishing to correct. But, when this stage is reached, it is absolutely key to have an abrasive that does not scratch. To meet this need a special abrasive has been developed. Because the ore for this product is nearly pure, no additives in the form of coke or iron are needed as flux which significantly lowers the risk of contamination. The intense heat of the electric arc fuses the white aluminum ore and then by controlling the rate of cooling the very small individual crystals are grown to a uniform size, edge and hardness. The result is a pure white discus shaped particle that cuts by a planing action rather than the fracture method of the more common grey aluminum oxides. We recommend a 12 and 5 micron sequence after 500 grit and before polishing.Usually 3 or 4 wets are needed for each mirror, therefore one quarter pound will last all but the most active worker a long time.
Throughout the grinding and final lapping stages, the objective has been to reduce pits and scratches to the smallest possible size. However, no amount of grinding can produce a surface smooth enough or sufficiently transparent to meet the needs of a first-class telescope objective. Different techniques are needed. While there is still considerable discussion on the
subject, it appears that polishing is a result of one or more of the following: removal/wear, athermic surface flow, or the formation of a silica-gel surface by hydrolysis. The result, whatever the cause, is an incredibly smooth surface, if optical techniques and supplies are used.
*** Note...these pages are solely for reference, any prices indicated may no longer be accurate,
all prices are subject to change, please contact William Bell for more information and pricing ***
