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Grinding Machines, Jobs for Telescope Makers |
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by Albert G. Ingalls |
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"The machine shown in Figure 1 was constructed almost entirely of junk parts and the cost was surprisingly low. The framework is built of 2" X 2 1/2" X 1/4" angle-iron. The motor F is a second-hand washing-machine motor. The main flywheel came from an ancient treadle-drive dental engine.
"The main drive shaft G is 3/4 " steel shafting, but it would be better to use 1" shaft, as the lighter shaft has some tendency to whip at the center. The upright shaft H is driven by a worm gear from an old car. Shaft J is driven by a bevel-gear taken from a discarded differential. "With regard to the bearings, of which there are six, it was found that standard pillow blocks would run into quite a sum of money, so a makeshift was devised consisting of short sections of 2" pipe welded to bases of heavy bar iron. The various shafts were centered in these pipes and babbitted, grease-cups being inserted in the four upright bearings, and oilholes bored in the horizontal ones. All speeds being low, these bearings should run without appreciable wear for a very long time. "The working platform D consists of two layers of 3/4" board, glued and screwed and then given several coats of paint. It is fastened to a flange at the upper end of the 3" pipe E. At the lower end of E is a short bolt running in a plain bearing. This bolt is fastened into a plug of oak secured to E by means of wood-screws. The platform turns very slowly, being driven from the shaft H by means of a bicycle chain and sprockets. "At the tops of the upright shafts are adjustable cranks, K and L, which control the motion of the 'alligator.' The elaborate construction of the right-hand crank is an unnecessary refinement. It can be seen that the crankpin L is controlled by a long screw with a ratchet-wheel M at one end. This ratchet encounters the double pawl N at each revolution, thus providing a continuous variation in the length of the swing. "The alligator, A, is a framework of 1" x 1/2" channel-iron, welded together. The alligator is built around a square frame which embraces the mirror. Four lengths of 1/2" pipe are welded into holes in the corners of this frame. Sliding in these pipes are 1/2" steel rods, one of which is seen plainly at B. The rods are tipped with rubber buffers--carried by most hardware stores for walking sticks. The push-rods are locked in any positions by means of set-screws threaded into the alligator. "The crankpin K drives the alligator from any one of several holes in the center brace of the framework, permitting adjustment for overhang, as in the Hindle design. "Two adjustable guides are seen in the photograph at P, P. These prevent rocking of the alligator. Only one guide is necessary and the right-hand one has now been removed. The contact between the alligator and the guide is the one place where, in spite of lubrication, squeaking occurs. This was overcome by fastening a leather strap on top of the guide. The weight of the alligator should not rest on the guide but on the adjustable collars on the crankpins. "Crankpin L works in a long slot formed by two facing angle-irons at that end of the alligator. Therefore, crankpin K provides the drive, while crankpin L gives the swing. "On top of platform D is a simple drip-pan, made by soldering a strip of galvanized iron to a disk of the same material. The tool (or mirror) is held in place, slightly off center, by blocks secured to the platform by wood-screws. A sheet of paper under the blocks makes cleaning-up a very simple job. "When in operation, the push-rods move the mirror in a series of elliptical strokes over the surface of the tool, these ellipses traveling from side to side under control of crankpin L in its slot. Obviously, the length of stroke and the swing can be changed as desired, from a very long stroke for roughing out the curve, to a short stroke for bringing the disks into spherical contact. "The most important feature of the Hindle machine has not yet been mentioned: the method of rotating the mirror, C, between strokes. This motion is provided by adjusting the push-rods so that there is a clearance of about 1/4" between the rubber buffers and the edge of the disk. This clearance means that, at the commencement of each stroke, one of the buffers will touch the mirror before the other, thus giving it a slight turning movement. This action is similar to what mechanics call a continuous ratchet. "I wish to call attention to an error in the construction of this machine as pictured. A study of the various gears shows that the mirror turns in the same direction as the tool. To correct this condition, the shaft G with its worm should be placed on the other side of the shaft H. I have not considered it necessary to make this change since the mirror and tool move at quite different speeds. If they moved at the same speed, astigmatism would, of course, result. "When roughing out the curve, the machine provides the overhang recommended by Everest in his article in 'Amateur Telescope Making--Advanced.' The stroke and swing should be as long as possible consistent with the avoidance of tipping. The machine should be operated by pulling the belt until the operator is quite sure that the mirror will not tip, before starting the motor. "As Everest explains, the elliptical-overhang stroke results in the center of the mirror being hogged out, while the edge is scarcely touched. If, therefore, the rough grinding is continued until the center of the mirror is deep enough, the outside zone will be practically flat, giving a shape like the inside of a 'tin hat,' and it will be impossible to bring the curve out to the edge without deepening the center. To avoid this, grinding should be stopped when the center is about two thirds the correct depth, and the stroke and swing gradually shortened. With a little care the right depth can be reached just as the curve reaches the edge. "The elliptical stroke pushes the abrasive off the edge. For this reason no paper is used on the platform during rough grinding, so that the mess may be scraped off and settled after each spell of grinding. Carborundum and water may be added from time to time as the machine operates, but the disks should be washed off about every 15 minutes. A weight of about 8 ounces per square inch is advised for rough grinding, tapering to 4 ounces for fine grinding and zero in the last stages of the same process. "The well-known practice of reversing mirror and tool between each stage of fine grinding is highly recommended as a sure method of bringing the disks to spherical contact. I can also recommend a minor discovery of my own for testing contact. Buy a tube of artists' 'Black Stumping Chalk.' Smear a little of this across a diameter of the clean, dry tool. Lower the mirror centrally into place and rub gently without pressure. When the mirror is removed its surface will show black streaks at all points of contact. "Much may be done in reducing high zones by smearing abrasive on the zone to be operated on, and using a very short stroke. It should also be remembered that the greatest abrasive action takes place at the points where the edge of the tool comes at the end of a stroke. Repeated changes in the adjustment of the cranks is therefore the secret of avoiding gross zonal errors. This applies even more strongly in polishing. "Polishing will be done with pitch, pitch with wax facets, or HCF, according to the taste of the operator. Long spells of polishing can be carried out by using HCF combined with rouge and soapy water. As Hindle points out, the use of soap obviates the necessity of scoring the facets. With regard to figuring, in spite of popular belief to the contrary, it is perfectly possible to complete this operation on the machine without any hand work at all. "Crank shaft J rotates at 60 R.P.M. H at about 13 R.P.M. and the platform at about 2 R.P.M. These figures are reduced by one half during rough grinding by the use of a smaller pulley on the motor. "The 12 1/2" Pyrex mirror for the telescope shown in Figure 2 was ground by hand before the machine was built, but polishing and figuring were done on the machine, including the correction of a very bad hyperbola. The mounting of this telescope which is designed for the University of Alberta, is a modification of the familiar double yoke, with the declination axis off-center to provide greater access to the polar regions. This necessitates a counterweight of about 125 pounds to balance the tube. The head of the tube moves on ball bearings, being turned by a handwheel (not seen) engaging with a circular rack.
"The entire telescope weighs about 750 pounds. The foundation and upright are, of course, temporary, as a concrete pier will be provided. The 24" wheel at the top of the polar axis provides means for the installation of a drive designed along the lines of the one invented and described by Mr. H. Boyd Brydon in a recent issue of the Journal of the Royal Astronomical Societ1y of Canada. "Dignity and Impudence" are exemplified by the 4" Richest Field Telescope seen in the lower right hand corner of the picture. "The Hindle machine is a labor-saving rather than a time-saving device. Rough grinding will take as long, possibly longer, than by hand, since we cannot increase the speed much beyond one stroke per second without nullifying the tendency for the upper disk to become concave, and changing abrasive is a slower job than in hand work. Some saving of time may be expected in fine grinding, and there is less temptation to skimp this vitally important item. Polishing is much faster than by hand, principally because long spells of work are possible without interruptions for rest. "Perhaps the greatest advantage of machine-work lies in its perfect regularity. Despite popular opinion, there is no special virtue, per se, in the irregularity of hand polishing. The ideal lies in what may be called 'uniform irregularity.' With the Hindle machine the cranks are adjusted to give any desired stroke and, as long as the machine is running, this stroke is maintained with perfect uniformity, but never twice over the same area of the tool or mirror. The slightly eccentric relation of the disks, the movement of the swing-crank, the rotation of tool and mirror, all combine to give an almost infinitely varied distribution of abrasion within the limits set by the chosen adjustments. Since these adjustments may be changed at will, it will be seen that we have here the mirror-maker's ideal of complete flexibility combined with absolute accuracy." JOBS. Amateur telescope makers who may think of getting jobs in the industries which now are making optical products for defense may obtain, gratis, from this department, "personal history forms" which, when filled out and returned, will become part of a roster available to these industries. As a matter of fact, a number of the amateur telescope makers already have been working in such plants, and for some time past, and they have been successful. One of the early crop of amateur telescope makers was Winston Juengst, whose photograph appears on page 402 of "A.T.M.," a photograph taken years ago when he was a youth. Juengst subsequently completed a full course in optometry at the University of Rochester and now is supervisor of the School of Mechanical Optics, at Montague and Henry Streets, Brooklyn, N. Y. A number of men, starting with little or no optical experience, have passed through this school and on into industry.
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The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds. The Society for Amateur Scientists At Surplus Shed, you'll find optical components such as lenses, prisms, mirrors, beamsplitters, achromats, optical flats, lens and mirror blanks, and unique optical pieces. In addition, there are borescopes, boresights, microscopes, telescopes, aerial cameras, filters, electronic test equipment, and other optical and electronic stuff. All available at a fraction of the original cost. SURPLUS
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NO MACHINE IS NEEDED FOR grinding and polishing telescope mirrors, and
only a minority of amateur telescope makers use machines, since one's
two hands are a good machine. Nevertheless, among advanced mirror makers
who finally tire of pushing glass by hand, or who, following Yankee tradition
need less than half an excuse to build a machine of any kind, machines
are becoming more and more common. No machine has been copied so often
as the one described by Hindle in the present (fourth) edition of "Amateur
Telescope Making." This machine-now firmly established-is a pronounced
success. Such a machine was built by Cyril G. Wates, 7718 Jasper Ave.,
Edmonton, Alberta, Canada, and described in the January, 1941, number
of The Journal of the Royal Astronomical Society of Canada, 198
College St., Toronto, Ontario, a journal which, by the way, is entirely
amateur even if "royal," and the following is a slight adaptation from
that description. 
