This page was converted from some files that Tom Waineo made available to amateurs on the old AstroMart ftp site. I was saddened to learn that Tom Waineo passed away in 1996. I never had the pleasure of meeting him, but clearly the care and detail he used in drawing up these notes indicates a great love of telescope making and willingness to share his experience with others. He has also written up a rather nice treatment of null testing which is worth reading.
The Elgin type of grinding and polishing machine is often used in professional optical shops with 4 or 6 machines mounted together in one unit with seperate variable speed stroke and rotation drives with weight or air cylinder for pressure control. Elgins have a triangle hinge fastened to a pivot post that is connected to a eccentric drive. The stroke swings an arc about this post. The length of the stroke and the start position is adjuustable. Those machines require motors, gearboxes, shafts, bearings, pulleys, and belts mounted together in a table. It is possible to simplify all this by a direct drive to a disk and eccentric from a gearmotor. Herbach and Rademan, 18 Canal St, PO Box 122, Bristol, Pa 19007-0122, Orders number 1-800-848-8001 They list AC and DC gearmotors. DC has the advantage of variable speed control. It is usually not necessary for speed control if a slow enough fixed speed is obtained. The drawing motor is item #TM92MTR2222 is 14 RPM with 5/16 shaft 1 3/4 inches long. With a die, the shaft can be threaded to a 5/16-18. The motor has 60 inch lbs of torque, but is not fan cooled. Suggest fan #TM91FAN1847 for $8.95. The long shaft makes a easy installation with a caster wheel and eccentric directly on shaft. This motor was replaced with another Item #TM93MTR2545 18 RPM Shaded pole 75 inch pounds of torque with a 1/2 inch output shaft for $19.95. The motor comes with a cooling fan, but the shaft is only 9/16 long. A 3 1/2 inch diameter V belt pulley with a 1/2 inch bore fits this shaft. A section of 21/32 wide V belt is glued to the V groove with contact cement. This makes a 3 7/8 diameter wheel which drives the disk at 7 RPM. The eccentric plate is bolted on the pulley. The first motor provides 3 RPM disk speed which is fine for a hand figuring table. Either motor does well. There are mounting holes for a 4 inch corner brace. Drill the brace with holes for motor attachment and fasten down to the plywood with wood screws. The other two casters are fixed 2 1/4 inch diameter set on blocks to match the height of the drive wheel. Cement a sheet of rubber or sandpaper to prevent disk drive slippage. If the disk stalls, use another caster to provide pressure on the drive wheel. Details of the motor and mount is not given since you may use another gearmotor to scale up the machine to handle bigger work. The machine as shown does up to 8 inches easy and 10 inch may be a maximum. Make the disk 1/2 blank diameter larger. All the other parts can be obtained from a hardware and lumber yard. For larger work scale the drawing up to a larger size machine. The gearmotors should be in the 14 to 40 RPM range. To make this machine a hand jig saw to make disks, hack saw, electric drill (a drill press would be nice), a 5/15-18 tap with 17/64 tap drill, hammer, and screwdriver are all the tools used.
The eccentric plate has a set of thread holes to give a stroke length. The turnbuckle adjusts the location of the start and end of the stroke. The eye hold a brass nut with thread drilled out for the connecting bolt. Use a couple of washers and a nut to secure the bolt to the eye. The other end is exsended with a metal stip with three holes. Those holes provide changes in the stroke that may exceed the trunbockle capaciuty. They fit in the threraded rod on the stroke beam. The beam is pivoted on a tread rod with a pair of speed nuts with threads drilled out for brushings. A 8 inch hinge is the Elgin triangle. Select one with the least amount of play. You can hammer the hinge tube tighter against the pin is you are carefull. The stroke pin is attached to the end of the hinge. It should be centered on the disk when the hinge is horizonal. If you intend to use full sized laps, there is no need for a offset in the front to back direction. The turnbuckle does the sideways offset.
Note that the disk is not fastened down. In using the machine, you can stop, lift the pin and flip the hinge to the back, and lift off the disk to clean. Use a 5/16 speed nut in center between the flange and the bottom of the disk. This hold a mounting disk to hold the work. A pizza pan serves as a dip pan and is installed between the two disks.
I prefer to present a general idea of this machine with hopes that suggestions would come from other builders. The only part that may be difficult to is the eccentric plate. The rest were all purchased from a hardware store except for the pulley and V belt from Grainger.
To mount optics and laps on machines a disk holder and button is the easiest way. Make a 3/4 inch thick plywood disk the same size as the blank or lap. Seal it by painting with sellac or other sealer. The disk is installed by a countersunk bolt to the speed nut in the power disk with a pizza pan between the disks for a dip pan. Place a pad of felt, rubber, or carpet on the disk for the blank support. Hold in place with duct tape or masking tape using several layers. The upper part has a button for the ball pin to fit in. Buttons can be made from jam jar lids. Scrape the paint from the inside of the jar lid. Epoxy a stack of washers for the pin hole. It the lap dries up, the machine cannot stroke and the pin would jump out of the hole into the jar inside lid. This is a safety feature to give you time to stop the machine and correct the problem. Another safety is the pin stop to prevent the pin from hitting the lower piece if the upper falls off. You could install a upper stop so that the ball could not jump out of the button. A bar can be fastened to the pivot rod and a screw at the other end stop any up jumps. After you become familiar with machine operation, it would not be needede. Buttoms are installed with wax, pitch, or double stick tape. Center within 1/32 or so.
Rough grinding in the 80 and 120 grit sizes is best done by hand with the rotating tool. Those grits do not last long and the combination of stroke and rotation gives a fast grind. Hold the blank by hand in the barrel walker's way. Allow it to slip a bit for rotation.
Machine operation requires a weight to hold the pin down and apply pressure to do the work. Drill a center hole in a tin can for a wood dowel. Place it outdoors in dirt to seal the hole and hold the dowel upright. Melt some lead or fishing weights outdoors and pour in. When cool remove the wood and cut the tin to the level of the lead. Telescope counter weeights can be used. You need a set of 3, 5, and 10 lb weights. Mount on the pin and hold in place with a nut and washer. When stopping the machine, you lift the pin and swing back. No need to remove the weight. Consider a nice weight solution with pennies. Solder a tubing to the center of a large can and fill with pennies. Copper is heavier than iron and you can add or remove to adjust the pin pressure. Top off with a disk held in place with a washer and nut.
A good grit sequence is 80 - 120 - 320 - 20 micron - 9 micron. Set the stroke to start centered and goes over the edge 1/4 diameter stroke length and returns centered. This exposes enough area to apply fresh grit and water with a brush. As the machine is running, hold the top memberr now and then to be assured that the stroking is smooth. Your figures is sensitive to anything that is not right. If you are using a lap that is not channeled, stop the machine and add fresh grit and water to the center every 5 minutes or so. Do at least 1/2 hr per grit and test for complete grind. Time depends on speed and pressure per unit area. In the 320 grit use layout dye or marks-a-lot felt tipped marker to assure uniform area grind.
Polishing reqiures more power than grinding. The spindle drive may stall due to lack of pressure of the disk to the drive wheel. A caster wheel can be fitted to press down on the disk just over the drive wheel. Note that this wheel is mounted on a arm with rubber bands to give the pressing force. The grinding tool can be used for a lap forum or a plaster one can be easily made. Spread a thin film of vasoline over the mirror. Then cover with plastic from a dry cleaners. Try to find something that can have a hole deep enough for the ball to pivot. Epoxy little 3/18 to 1/4 inch spacers to allow the plaster to flow under. Add a pipe section or tube on top to provide a hole big enough for the pin and it's tilts when strokeing a radius. Use a stiff paper, plastic, or metal band around the mirror. Pour in plaster to 1/6 diameter thickness and let harden. Then remove bands and forum should slide off. Bake in a 130 degree oven with the door held slightly open to let out moisture. In 12 hours or so when masking tape can stick to it, it is ready for sealing with sellac and for the pitch lap.
In polishing try to use as little compound as possible. You need drag and friction to get fast polishing and good figure. Optician like to let the lap dry up and then stop and check the mirror. The cleanest sleck free surfaces are obtained by doing so. When the stroke is set center over center, an oblate curve (high center and up edsge) is usually the result. Set the stroke to start centered and stroke 1/5 to 1/4 diameter and returned centered. This generally produces a good sphere. A shorter stroke gives an up edge and/or oblate. A longer stroke may go prolate (low center and down edge). As the machine is doing a stroke hold the upper part to feel the polishing action. You should feel the resistance to the stroke. Weights should be at least 3 oz per square inch or about a 5 lb weight for a 6 inch, Bigger work have heavier laps and may not meed as much weight. Best work is done lap on top. With a fixed speed machine giving more strokes per rotation as this one does, the mirror would polish concave (shorter radius) with either mirror on top or tool on top. With a variable speed machine, less strokes per rotation would convex (or longer radius). When doing flats, a setting of between 2 to 3 rotations per stroke hold a surface flatness or radius. Machines have a individual personality and you learn the best parameters to do a work by trial and error. Once the best setting is found, you need not change it. Some Elgin machines have a second eccentric at the stroke pivot to produce a random or W stroke. I have used them, but returned to single eccentric as been easier to control. When you have a second eccentric, the second stroke length and speed produces so many variables in the polishing that it is harder to find the right combination of parameters to do the work. It you do the first mirror by hand, you can operate a machine. You know what hand work can do and the machine would teach you what each change of stroke does. In starting work, it is best to warm the lap with water and let the machine work it into contact. You should stroke by hand to see if it feels right. If a lap pressing is needed. warm the lap and press by weight or place on floor between newspapers and stand on it. Parabolizing is possible by machine, but it is best to do by hand figuring for better control. I wish I could think of more hints to pass on. If you have one, let me know and I will pass it on with a upgrade to this file. Good luck and have fun. - Tom
All materials on this website are Copyright 2001, Mark T. VandeWettering. Permission is granted to reproduce and distribute these files for non-profit, personal use.
Mark T. VandeWettering <markv@telescopemaking.org>