IN THIS DEPARTMENT in the October and November numbers Patrick A. Driscoll described the design and use of a telescope mirror grinding and polishing machine of the vertical spindle, hand-lever type. Now in this and next month's numbers Dr. Henry Paul, 119 North Broad St., Norwich N. Y., will describe his own machine (Figure 1) of the same general type, combined with a polishing machine. The grinding part is seen on the right in the photograph, the polishing part on the left, but the grinding part alone will be described this month. Paul's description is as follows:

It is often the desire of the amateur to have one machine which alone is capable of handling the greatest possible variety of optical work. This combination grinding and polishing machine has very satisfactorily served for the past five years in making highly varied optics ranging from ordinary f/8 paraboloidal mirrors and optical flats to roof prisms and correcting plates for Schmidt cameras. The same motor is used to drive both grinder and polisher. A separating wall in the center of the machine serves to keep abrasives from straying over to the polishing side. The grinding spindle will handle disks up to 12" diameter.

The sturdy table, 2' wide, 3' long, and 3' high, was constructed of 7/8", hard wood. Drawbolts were used to fasten the 3" square legs in place. Lag screws were also used at key points. Thus, joints may be tightened and the table kept solid.

A 1/2 h.p. capacitor type reversible motor is recommended if such a machine has plain bearings, although a 1/3 h.p. motor might be used. For less power than this, ball bearings should be placed at the greatest friction points. The motor was swung on a hinge at one side of its base, which facilitates the use of four interchangeable motor pulleys, 1", 1-1/2", 2", and 2-1/2" in size. While the weight of the motor may hang on the belt alone, a rubber-cushioned wedge supporting a part of the weight of the free side adds to smooth operation.

Starting from the motor, a V-belt drives a 10" pulley (Figure 2) on the 3/4" jack shaft. The latter is supported in plain-bearing, self-alining hangers.

On the right-hand end of the same jack shaft, by means of a flange, is fastened a 10" by 3/8" steel disk, its face machined perfectly true and perpendicular to the shaft. Here freedom from wobble affords beautifully smooth operation and is worth seeking. The disk might well be permanently welded to the shaft and then machined true.

The vertical grinding spindle proper was made from a saw mandrel having standard bronze bearings. Its top end has a standard 1" 8-thread (specified right-hand). The shaft of these sturdy mandrels is usually about 1- 3/16" in diameter.

For the end-thrust bearing of the spindle a thick brass plate, or disk, was fastened over the bottom end of the lower bearing, and a 1/2" hole was drilled or turned just deep enough into the lower end of the shaft so that it would drop over about three fourths of a 1-3/16" ball bearing. After some operation such a ball will wear a small, efficient bearing point in the brass plate. Lubricate well with a large amount of thin cup grease or heavy oil. As a simple alternative the end of the shaft could itself, if turned to a dull-pointed cone, serve as a bearing.

The 10" faced disk drives a 2" by 10" pneumatic tire fastened with rim cement to a heavy wooden disk turned to receive it. Two cast-iron flanges- one on either side of the disk is best- were bored to a close sliding fit on the vertical shaft. A small flat area was filed from top to bottom of this shaft and a 3/8" setscrew from the upper flange against this keeps the rubber-tired, variable-drive wheel at the selected position on the vertical shaft. Without the flat area, setscrew marks will soon result in a wheel that is hard to slide up or down the shaft A knee lever (flat board hinged on one side) permits the spindle to be stopped almost instantly by hard knee pressure; a really quick stop may "save all." Until the switch is off, the tire skids on its driving disk. At the point where the shaft passes through the bottom of the 24" by 6" heavy gage pan is soldered a sleeve 1" high and about 1/4", larger in diameter than the shaft. Under the 1" 8-thread hex nut, which serves as a shoulder on the same shaft, or spindle, is placed a circular washer of soft aluminum turned down at the edges. This effectively keeps water and abrasive out of the bearing. Warning: Put a high-water mark on the pan. A drain plug also would be handy.

The hand lever, or pin bar, should be at least 9/16" or 5/8" in diameter. A 1/2" shaft hanger permits universal motion at the attached end. Three cables, the third permitting a reversing switch (very handy to run laps off their thread) run along this bar (a tubular pin bar with the cables inside would be neater and easier to clean) to the on, off, and reverse switch fastened within reach of the thumb at the hand grip. A bicycle handlebar grip is good here. The best clamp I have found for holding the central pin on the bar was made from a Castaloy laboratory clamp (the former No. 20195, now No. 5-766, Eimer and Amend, 633 Greenwich St., New York, N. Y.). Ordinary cast-iron clamps break easily (goodbye mirror), while a solid support cannot be easily removed for cleaning.

The head plates for attachment of the mirror lenses were made by brazing 1" 8-thread hex nuts to 1/4," or 3/8" thick disks.

Speeds of this grinding head may be varied from 0 to about 400 r.p.m. The operation is smooth and quiet. Ordinary dry soap applied to the tire stops any tendency to squeak.

End of Henry Paul's description of his grinding spindle. Next month he will describe the successful: and nearly unique polishing part of his combination machine, with the mirror rotating constantly in an edgewise position in a bath of rouge water, and working while you sleep.

ATTENTIVE readers will recall that in the November number Driscoll urged not exceeding a spindle speed of 200 r.p.m., warning, "Faster, disaster." They may therefore ask how Henry Paul gets away with double that speed (quadrupled centrifugal force). For one thing, his is an especially well balanced machine. For another, its user is especially well protected by a thick splash pan turned over a heavy ring. And the 400 r.p.m. is, anyway, only its outside speed, seldom attempted. Just exactly what happens by way of disaster? Unpredictable. Asked whether he had ever lost control of a mirror Paul replied, "Yes one flew off and put a dent in the pan but luckily did not chip the mirror, or me." An 8" mirror at 400 r.p.m. represents a peripheral speed of about 10 miles an hour and might only knock the wind out of the recipient. Yet who except the indestructible Popeye, would choose to be hit by such a slug? "Reasonably safe speeds," Paul comments in a letter, "go up to about 400 edge feet per minute, corresponding to 150 r.p.m. for a 10" mirror, 200 r.p.m. for an 8", 250 r.p.m. for a 6".

Leo J. Scanlon of Pittsburgh, who also has used the hand-lever type of machine, reports that "while grinding does proceed at a rate proportional to the pressure applied, speed of rotation, and amount of overhang, and there is a strong temptation to speed up the machine, high speeds throw off the abrasive wastefully. Also, seizure, if it occurs, takes place so quickly as to end in disaster if the grinding surface becomes dry or warm. It is also," he continues, "a delicate venture to entrust the mirror, in the last stage of fine grinding, to a rotating tool, though I have done it frequently. Usually in this last stage and one or two preceding it I do not use the pivot pin but hold the mirror in my hands, with my foot on the motor switch. (Paul too states that he grinds the last ten minutes by hand with mirror at 5 r.p.m.-Ed.) I also try always to avoid passing the mirror over the center of the tool, since that favors seizing, especially if the mirror is not yet spherical. Incidentally we always try to finish a mirror in fine grinding with a slightly raised center. It then is easy to get a sphere by polishing."

Scanlon mentioned, above, the effect of centrifugal force in throwing off the abrasive. Here Paul comments, "Increasing the viscosity of the water used will retard centrifuging off the abrasive before it is broken down, and therefore will decrease grinding time. To accomplish this, household gelatin may be used, or 'Metamucil-2,' a drugstore powder which may be stirred into the water until the desired viscosity is reached. But this thickening of the grinding water is recommended only for the hogging out part of producing deep curves such as f/0.6 to f/2, where working times of 50 to 100 hours have been reported. Here the worker will soon speed up his machine to hurry the job along. Thickening of grinding water offers no advantage at slow speeds and should not be done with fine abrasives at any speed. On shallow curves it is not worth the bother, as these work out in a very short time on the vertical spindle. In general, the hand-lever machine requires more skill to obtain much faster results than other types of machines."

Apropos, not of machines but of Paul, is a distinct contribution of his to the art of rough figuring by fine grinding of aspherical surfaces such as Schmidt corrector plates. "My luckiest find," he says, "is the use of the round, thin, microscope cover glass held down and moved about by the finger and used on the rotating disk just as the thumb is sometimes used in zonal figuring polishing. Such cover glasses are from 1/2" to 3/4" in diameter and only 0.002" to 0.005" thick and are therefore flexible under the finger." This method might prove applicable when rough figuring mirrors of such depth of curve that the greater part of the parabolizing is preferably done in grinding.

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