John M. Pierce's HobbyGraph Articles

HobbyGraph #1

Amateur Lens Making.

      Fig. 1 shows a lens grinder, suitable for the amateur for making lenses up to 1" diameter. It is also useful to those interested in grinding and polishing minerals and stones into cabochon gems for rings and other jewelry.
      Formed laps may be cut directly on the machine with a hand tool. Any motor from 1/10 HP up may be used. The motor pulley should be 1 1/2" and a 3/8" vee belt should be used. Get them from Sears Roebuck or a hardware store. The grinder should be oiled frequently.

The Lens Blank.
      Small lenses may be made from fragments of a spectacle lens or from plate glass from a broken mirror or auto window. Don't try to use non-shatter able glass, as it is not solid glass. If polished glass is used the original surface may form one surface of the lens, thus halving your work of making a lens. Pitch the glass to a stick or "dop" as in Fig. 4. Grind it round, and rough out the curved surface with #2 abrasive (120 grit) and water on either side of the Vee pulley.
      If you have a drill press you may cut out your blank by grinding a circular groove in the glass with #2 grit and water and a tubular cutter held in the drill chuck. A piece of small brass tube works well, or an empty cartridge case may be used. The inside diameter of the tube should be slightly greater than that of the lens desired. Pitch the glass to a block and put onto the table under the tube. Apply grit and bring the cutter down onto the glass. Raise and lower the cutter frequently to let the grit into the groove that soon deepens and cuts through the glass. The core is the lens blank.
Grinding The Lens.
      Before starting to grind the lens must be mounted on a dop stick slightly smaller than the lens and about 3" long. Melt some pitch and at the same time warm the lens blank over the fire. Dip the end of the dop stick into the pitch and press the lens blank against it. Mold the pitch around the glass with the fingers to make it secure.
      Most eyepiece lenses are plano-convex in form; that is, one side is flat and the other curved. The focal length of a plano-convex lens is roughly twice its radius of curvature. You must provide a template of the curve desired of cardboard or sheetmetal. A copper cent is a perfect for a lens of 3/4" focal length.
      A tool for turning out the spherical hollow in the lap may be made from an old file. A 6" square file is excellent. Grind two opposite sides down to about 1/8" thick for a length of a half inch, as shown in Fig. 2. Then grind the edges to the form shown. Be sure that the edge slants in all around the bottom for clearance. The top edge is the cutting edge. Keep the file wet while grinding. If it discolors from heat it will be no longer have a temper left and will be useless.
      Put a brass lap blank (see Fig. 1) into the grinder and adjust the tool rest to support the cutting edge of the tool at the center of the lap. Start the motor and apply the tool to the top of the rotating lap. You will find that it cuts the brass easily and the curve desired can be readily turned out. Stop the motor at intervals and apply the template. Make the curve fit the template exactly and be about 1/16" larger than the lens. Make a small hole at the center of all laps. As the center does not cut appreciably, it is best to remove it. The hole may be almost 1/4 the size of the lens.
      We will now grind the convex surface of the lens. Mix a past of #3 (220 grit) abrasive and water and anoint the lens blank with it. Press the lens blank onto the rotating formed lap. Rock the dop back and forth and around while the glass grind in the hollow of the lap. Add more abrasive as needed. Grind until the whole surface has reached the curvature of the lap. Add more abrasive as needed. Grind until the whole surface has reached the curve of the lap. Then clean lap and lens and repeat with #4 grit, then with #5, and last with #6. Grind for about 10 minutes with each grad or until all pits left by the coarser grade are gone and the surface is perfect when viewed by a magnifying glass.
      It is important that the lens should be the same thickness all around the edge. If grinding is continued until the convex surface cuts the flat one in a sharp edge as in Fig. 5A or if car is taken to keep the edge even in as in Fig. 5B, no centering will be necessary.
      During the fine grinding you should test the lap occasionally with the template. If it wars out of shape swing up the tool rest and reform it.
Polishing The Lens.
      Polishing is done with rouge and water on a pitch lap. Mix water with rouge and in proportion of 3 water to 1 rouge in a covered jar. Get a short handled brush that will go into the jar. Keep the cover on to avoid scratches due to dirt in the rouge.
      Make a pitch lap as follows:- Put the pulley on the pitch lap holder and put into the grinder. Melt pitch and fill the holder heaping full. When the pitch cools enough to hold its shape start the motor, dip the lens blank into the rouge, and mode the pitch in the rotating lap with the lens blank. Keep wet with rouge or it will stick and rough up the pitch. Be sure the hollow you form is central in the lap. When pitch is hard, use the point of a knife on the rotating lap to trim the edge to the size of the lens and to remove the center.
      You now proceed to polish, using the same technique as in grinding. Brush rouge onto the glass frequently. Continue until you have a perfect polish with no pits or scratches visible with a magnifying glass.
      Cabochons and poor quality lenses are sometimes polished on felt glued to the brass lap instead of pitch. Tin oxide is better than rouge for polishing porous stones. Rouge may stain them permanently.
      Concave lenses are made on convex laps and flat surfaces on flat laps. Procedures are similar to those already described.
Grinding Convex Lenses On Tubular Laps.

      This method of making spherical surfaces has long been in use by lapidaries when making spheres of rock crystal and other minerals. It is suitable and simple way of shaping the short focus lenses used in eyepieces.
      The dopped lens is rocked back and forth in the open end of a tube whose inside diameter is about 1/2 that of the lens. Since a spherical surface is the only form that will rock evenly in a circular support, an accurate spherical surface will be generated. Start with #3 grid and work through the grades to the finest. Polishing is done on a pitch lap as already described.

Re-shaping The Lap For The Final Grind.
      Whether the lens is made on a tubular lap or a formed one, it is advisable, for best results, to give a final grind on a specially prepared formed lap. This is especially necessary if the lens must be made to close limits as to focal length.
      To prepare this finishing lap you need a mating lap of equal but opposite curvature. This can be turned out on the grinder with hand tool and template, or if a ball bearing of the right size is available it can be pitched onto a dop stick and sued with confidence that the curve is perfect. The two laps, convex and concave, are held in the two hands and rocked together after receiving a coat of fine abrasive and water. Continue to wo9rk them together until the surfaces are in contact all over, as shown by the ground areas. Since spherical surfaces are the only ones that will rock evenly together you will get a perfectly spherical lap.
      Flat laps may be trued by grinding them on a piece of plate glass.
      If a finished lens is off center, as shown by an edge of varying thickness, it may be centered as follows:- Pitch the lens to the end of a tubular lap slightly smaller than the lens itself, as shown in Fig. 3. Put into the grinder and rotate by hand while you watch the directions of a lighted lamp bulb from the two surfaces of the lens. While the pitch is still quite soft press the lens with the forked stick as in Fig. 3 to adjust it, until both reflections remain stationary as the lap revolves. Remove from the grinder and re-heat if the pitch hardens too quickly. Then start the motor and with a piece of wet emery stone, grind the edge until it runs true. This should make the edge equal all around.
      Lenses may be removed from a dop with a knife and cleaned with turpentine.
      A cardboard or sheet metal shield should be erected around the lens grinder to keep the splash from the walls and clothes.
      If you have no motor, drive the spindle with a bow - Boy Scout style.