LAST MONTH in this department Dr. L Henry Paul, Norwich, N. Y., described the grinding half of his combination grinding and polishing machine for making telescope mirrors, optical flats, and lenses. The polishing part was shown in Figure 2 (in the previous installment) and is shown from another angle in Figure 3. Because it is believed that numerous amateurs will find in this machine one particularly outstanding advantage which they may wish to embody in similar machines, enough space will be devoted to it to afford adequate description.

The action of the polishing part of the Paul machine may best be traced by starting at the motor, under the table, which actuates the grinding part described last month. From a 2" pulley (the remainder of the cone pulley seen in Figure 2 never being used) a V-belt passes up through a slot in the table and drives a 5" pulley which is eclipsed in the illustration. This pulley is attached to a short horizontal shaft, in this instance an old grinding head. On the left-hand end of this shaft (in Figure 3) is a head plate to which the mirror is attached with pitch. The mirror rotates constantly in a bath of rouge water and therefore requires almost no attention. This in turn requires that it be turned in an uncommon position-in a vertical instead of the familiar horizontal plane. Speeds of 70, 105, 140, and 175 r.p.m. may be chosen by selecting suitable motor pulleys.

Suspended on an upright pin bar is the lap, which is held against the mirror by means of a spring or large strip of inner tube stretched to the desired tension as measured by a spring scale. The pin bar is actuated, pendulumwise by the horizontal overhead shaft, and this in turn is kept rocking by a belt-driven jack shaft and crank arm through an articulated pitman and pitman bar which is hidden in Figure 3 but visible in Figure 2.

Stroke length may be varied by moving the crank pin along the crank arm. Stroke position is altered at will by readjusting a clamp at the top of the pitman. Unorthodox as it at first appears to be, this machine includes all the necessary elements and motions of an orthodox machine and, in fact, it is an orthodox machine turned up on its side in an unorthodox manner in order to make possible the added advantage of the rouge bath. If, for illustration, you take the commercial machine shown on page 83 of the 1944 printing of "A.T.M.A.," (Ferson's new chapter on prism and flat making) and tip it over on its side like a kicked-over table, you then have about the same principle. On the Paul machine, however, polishing is accomplished somewhat more by rotation than by cross stroke.

"THE HORIZONTAL polishing system was first seen in the shop of Dr. Henry Ketcham, Johnson City, N. Y.," Paul states. "It offers certain distinct advantages, the major one being that, after initial adjustments on the job, it requires little attention, being semiautomatic. Since the lower edge of the mirror and of the lap dip as much as half way to their centers in the trough (old inner tube) of rouge water, the bother of applying rouge to the lap by hand is eliminated. It is true, the rouge water splashes all over the mirror and tool but a large washer photograph by Dr. Henry Paul Figure 3: Paul's polishing machine on the drive shaft, running just inside the trough, keeps most of it where it belongs. Another eliminated factor is temperature gradients due to evaporation effects.

"Turned edges of any consequence have not been encountered (when the system was properly used). An added advantage is the fact that in the earlier stages of polishing, the trough that holds the solution may be moved up close to the mirror, thus keeping all the rouge in operation for rapid polishing. Then, as the end of the polishing is approached, the trough may be lowered, permitting the coarser particles to settle to the bottom so that the later stages of polishing may be accomplished by finely suspended rouge. It is believed that one reason for the outstanding lack of scratches by this method is the fact that heavy particles tend to stay in the bottom. This may also be why ordinary optician's dry rouge (B. and L. 21-90-61) works well. Since much of it is used, five-pound containers are obtained, but at relatively low cost.

"The polishing progresses somewhat slower than by conventional methods but this is compensated in advance by thorough fine grinding with very fine emery, such as American Optical No. 305, the last application being worked down extremely well. Finest emeries should be levigated and mixed, one-tone, with levigated drugstore talc. I have never had a scratch from this method of polishing.

"I have often started a mirror polishing in the late evening and leisurely after breakfast stepped down to the shop to find a completely polished surface within a few wavelengths of the ultimate. Thus I call this the nightshift machine; it works while I sleep. (Once I let it run 24 hours. No harm resulted.) To avoid the heartache, only too well known to all mirror makers, of fortuitously reaching a perfect curve before polishing is complete, and then helplessly having to watch its demise, I prefer to do the figuring on a finished polish. A small condenser bulb pen light and a 7X to 10X magnifier is used to best check for complete polish.

"The free-running lap or mirror under the swinging arm pin is usually made about 5/6 to 9/10 the diameter of the driving lap or mirror on the polishing spindle head. Most often the convex form, whether lap or mirror, is on the driving spindle head and the concave form turns free on its supporting pin. Flats have been made both ways-most often, however, with the flat on the polishing head and the smaller polisher free-turning. The center of the free-turning disk is usually kept somewhat above or below the center of the polishing head. Often the edge of the free-turning lap comes within 1/8" of the edge of the disk it is polishing. This tends to avoid turned edge. Laps need only simple channeling-one groove across and three equally spaced channels at right angles to this.

"Between adjustment of the length of the stroke the position of the sweep across the disk of the polishing head, the size of the outer versus the inner disk, the distance the center of the rotating disk is held above or below the center of the polishing head disk, and the reversal of position of the lap and glass, almost any desired effect in spherical or flat surfaces can be produced.

"No attempt has been made to parabolize or produce aspherical surfaces on this polishing head. All non-spherical surfaces have been made by using small polishers including the thumb and ball of the palm, on the vertical hand-lever spindle on which the grinding was done. Despite the quantity of abrasive lying around, this spindle has been used as a polisher for all types of figuring without a scratch. Abrasive doesn't get up and fly. However, the precaution of cleaning the cross bar and, particularly, the central pin and clamp, must always be observed before using this vertical head in figuring.

"Often, in figuring on the vertical spindle, the small polishers are held in the hand and applied to the spinning glass, using a ruler held in the left hand to gage the place of application. When using the finger alone as a polisher on a narrow zone this must be done more cautiously, as operation may be rapid. When the pin is used in the back of the lap, effects will vary according to whether the lap is free-spinning, partially braked by the left hand, or totally stopped from rotating. See Dévé, 'Optical Workshop Principles,' pages 70-72.

"The 'trapeze,' or pipe frame, set up to support the moving mechanism of the polishing assembly has worked extremely well. It is constructed of ordinary 1/2" pipe fittings, the list being 12 6" lengths, four 3-way connectors, ten T's, and four flanges. The respective sides can be constructed as units Here arises the interesting and slightly humorous problem, sometimes handed to unsuspecting apprentice plumbers, of 'closing a circle' with pipe fittings. The apprentice discovers that screwing one end of the fourth side into one elbow automatically screws the opposite end out of the other elbow. One solution is to screw the left end in extra far, cut off all but two threads from the right end and then turn that end in, completing the square. This will leave the left end a bit loose but a setscrew will tighten it. My less elegant solution was to turn a blowtorch on it and run solder into the threads.

"The cross-connectors are now screwed in and the same problem arises in three other places and is similarly solved. Or the two units may be welded or brazed together.

"The 1/2" T's drilled out with a 3/4" drill serve as bearings. Holes should be drilled in the T's for lubrication. These bearings are very sturdy and entirely adequate for these slow-moving parts. Regular 3/4" shafting and collars were used throughout.

"The driving crank contains a slot in which slides a bolt which may be clamped at any position and the end of this bolt drives the pitman bar. A crank range of 0" to 3" (6" stroke) is adequate. At the right end of the horizontal overhead shaft (Figure 3) is a sturdy clamp tightened on the shaft with a setscrew which allows change of the position of the stroke in polishing. The stroke is usually less than one half the diameter of the glass disk on the head, the bar pin should never cross directly over the rotating driving center.

"The yoke which permits the bar pin to be swung out in an arc, up and back overhead out of the way, was carefully machined from solid material, but a 3/4" self-alining shaft hanger, if sturdy, should work well in its place."

ONE MORE method of making a pitch lap, this one more especially for a deep-curve mirror but equally applicable to other mirrors, is offered the telescope making fraternity with pitch in their hair by John P. Tyskewicz, 142 Seymour St., Hartford 6, Conn., who foresaw that for a deep mirror enough pitch would be squeezed out to start a taffy pulling party. So he devised his "flapjack" lap, precast and uniform in thickness allowed to cool, and applied warmed-over afterward.

On a sheet of waxed paper he laid a ring of rubber-insulated, twin-conductor type lampcord as a dam for the pitch.

He melted his pitch and let it cool a little so that it would not unwax the paper by melting the wax, poured the dam level full and left it to cool into a flat flapjack, or disk, of uniform thickness.

While the tool was warming he worked the flapjack loose with a long knife, leaving the dam on.

He next warmed the flapjack with radiant heat (water would prevent its adhesion to the tool), swabbed the tool and flapjack with turps, and lowered the flapjack, still in its dam on the tool, letting it touch first at the center. Gravity plus thermodynamics did the rest.

Now he removed the dam, wet the mirror with soapy water and work the lap down to perfect fit in the usual manner.

For making channels Tyskewicz uses a 100-watt soldering iron with V-shaped scoop attached, makes plenty of facets not larger than a tenth diameter of lap, and leaves their ends uncut to retain the rougy water longer.

CASUALTIES-(1) "I lent one of our engineers an 8" cast iron flat," a reader reports, "to use as a base for crocus cloth to finish a mechanism. I discovered the man grinding gate valve disks on the flat with valve grinding compound." (2) Another reader writes "Some men at a military field borrowed my flat and, when returning it, remarked with a hint of pride and virtue 'It got pretty badly scratched up, so we refinished it for you."' Suspicious and alarmed, the owner asked what technique was used. The casual reply was, "Buffing wheel." (3) A green firm buffed an amateur's mirror then aluminized the ruined figure. At a lawyer's "suggestion" they settled, $100, but still felt injured.

WOODEN tubes for reflecting telescopes suffer from one almost unsurmountable objection, according to F. N. Hibbard, Richmond, Va.; their tendency to warp. "A friend of mine,' he writes, "made a 10" reflector with a wooden tube and says he has to readjust the mirror every night he uses the telescope. Even a slight warpage produces flares on one side or the other of the star images.

"The problem is not to eliminate atmospheric unsteadiness. There will be more or less of that practically every night. There will not be a dozen nights a year when 75 or 100 diameters per inch of aperture can be used to advantage. The main problem is to eliminate flares and unsteadiness due to the tube itself.

"I would build any telescope up to 2' in diameter of metal, and line the metal with cork or quarter-inch strips of light wood such as balsa or Douglas fir. The metal would give the necessary strength and solidity to hold the mirror in exact position and the wood or cork would keep the metal temperatures from getting inside."

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