Figure 1 shows a machine; made by Loyd A. Magruder, 800 Ashman St., Sault Ste. Marie, Mich. It has direct gear drives and is of all-iron construction, made, as Magruder points out, largely of junk and with very few tools. The stand consists of two old auto flywheels bolted to legs sawed from old auto frames. The oscillating cross member is driven from two crank arms, one of which shows in profile. The rest is a matter of gearing. Possibly, as someone has pointed out, the bevel gears and pinions could be backed up more closely with bearings, to get away from flexure and chatter, yet the actual forces involved in working a small mirror are rather low and the proof of the pudding is that this machine has been a success.

Magruder's design embodies some brass castings, in making which he found that the brass could be melted at the hot point of a home furnace stoker, using a crucible improvised from waste electrodes obtained for a few cents from a carbide company. He plans next to tackle a ruling engine. [Commenting on this, R. E. Clark, author of the chapter on small lenses, "ATMA," says: "Several years ago I tried to make one. Making the master screw is a heart breaker, and trying to make the thing perform is still worse." Practical instructions for making the screw are contained in Vol. IV of Glazebrooks' big "Dictionary of Applied Physics."]

Concerning the large machine shown in Figure 2, we have no data, beyond the fact that one of its builders was Geo. I. Moe, 4118 N. 16 St., Tacoma, Wash., and that it was built for use in grinding a 24" mirror of Pyrex which for several years has been theoretically under active construction by the Tacoma Amateur Astronomers.

The machine shown in Figure 3 is the one used at the Warner and Swasey Co. shops, in Cleveland, Ohio, for grinding and polishing the 80" mirror for the McDonald Observatory in Texas.

A long time ago we asked whether anyone had made a first mirror on a machine, with no previous hand experience to learn the "feel" of the work, and John MacDonald 828 N. Hibbard Ave., Jackson, Mich., sent us the photograph shown in Figure 4, with the comment that "the answer is emphatically, 'yes.' I have a 10" reflecting telescope in the course of construction which is primarily designed for photographic work. The focal ratio of the Pyrex mirror is 6.5. It was 'machined' on a similar disk of Pyrex through the various grades of grinding. When grinding was complete, the tool was covered with the conventional pitch lap, and on this the polishing and figuring were completed. The 'shift' of the knife-edge in the Foucault test, determining the difference of focal length between the central and outer zones of the paraboloid, can be lengthened or shortened at will, without any alterations in the shape of the facets.

"The machine was designed and built by myself, and is of the transverse bar type. It is adjustable to any stroke desired. The difference in the speed of rotation between mirror and tool can also be regulated. The mirror and tool both rotate in the same direction, but at different speeds. The rotating action of mirror and tool is produced by over-running clutches, which prevent any ratchet action and consequently do not permit any synchronism of the intermittent turning action of mirror or tool. The transverse bar slides on hardened steel rollers, and can be raised or lowered to accommodate any thickness of glass, and the grinding pressure is adjustable from nothing to 60 pounds."

The little machine in Figure 5 was also made by MacDonald and is for grinding small lenses. It may be run vertically for grinding and polishing, and horizontally for centering, edging, and testing such work. Hindle's alligator machine (so-called because the part on top which moves the mirror back and forth resembles an alligator in shape, when seen in plan) is becoming justly popular. We learn of two on Long Island, and Figures 6 and 7 show another, probably as finished in design and appearance as a machine could be made. The maker is the same Emir Kelley we mentioned in the October number-the Kelley who built a grandfather clock which automatically raises a flag and blows a whistle on his wedding anniversary, as a warning not to forget. Of this machine, Kelley (of Kelley and Stewart, South Brownsville, Pa.) writes in reply to our request: "It has a three-point base for leveling. The turn-table may be thrown out of gear, permitting easy rotation by hand for centering the tool. The tool is clamped in place by three eccentric, knurled disks, with a scrap of aluminum between disks and tool, making a very rigid chuck. The turn-table and cranks are mounted on ball bearings, reducing friction and lost motion to a minimum. Figure 7 shows the central part of the alligator, with mirror floating between adjustable rubber-tipped push arms. The frame is adjustable for height, to provide a straight-line drive to the mirror."

Another Hindle alligator machine is shown in Figure 8, a photograph sent us by Franklin B. Wright ("ATM," chapter on accuracy in parabolizing, and "ATMA" chapter on theory and design of Schmidts), who says the machine was built by Carl E. Wells, 419 Oak St., Roseville, Calif., also that "the most beautifully figured mirror I have ever gotten a knife-edge in front of was made by him." Wells, not Wright, is included in the photograph.

In the May, 1937, number two amateurs designated as "Castor and Polltux, the Gemini Twins"-namely, Edward P. Woolcock and W. E. Lester, 319 Hermosa Ave., Long Beach, Calif.-told how they were working on a 20" disk by means of a simple vertical spindle. Later they built and substituted for this the machine shown in Figure 9. Woolcock describes this as follows:

"The grinding machine is of the Hindle type and uses the familiar alligator principle, as explained in 'ATM.' We were able to make the machine for a total cost of $20, exclusive of the motor. Flat belts and pulleys are used to drive the three spindles at varying speeds. The diagram shows our belt and pulley arrangement, and Hindle pp. 234-240, 'ATM,' gives the explanation.

"As Hindle states, the machine gives an ovoid stroke, which can be varied at will by changing the bolt on the eccentric drive shaft. The stroke overhang can be varied by placing the drive bolt in different holes in the alligator. The side swing can be varied by changing the bolt on the other eccentric shaft. The rubber cleats on the alligator are not used to hold the mirror, but they are spaced about 1/4" from the mirror's edge and act only as pushers. Their operation is unique for, because of the spacing between cleats and mirror, the mirror actually turns in a direction opposite to that of the tool which is rotating beneath it. This amount of mirror turning can be varied by changing the separation between the rubber pads and the mirror. At each swing of the stroke the mirror is pulled at a place about l/2" from where it was last touched by the rubber cleats.

"The ovoid stroke helps to keep abrasive and moisture on the tool-we were able to grind for ten minutes at a time without replacing grit or using extra water. In the finer grades we ground for 20 or 30 minutes at a time, only occasionally squirting water from a toy squirt gun along the edge of the tool. The ovoid stroke quickly carries any moisture to the center of the tool, where it is dispersed equally over the grinding surfaces.

"We were able, by means of this machine, to grind from No. 80 through emery in two days. By hand this would have taken two weeks, We ground for 1-1/2 hours on each of six grades of Carbo., and fine emery. This time is actual abrading time and does not include time out to change grit, etc.

"Ronchi tests indicate that we have a perfect sphere with the strokes we have been using.

"Our favorite polishing speeds have been near 4 r.p.m. for the tool, 40 r.p.m. stroke, and 10 r.p.m. side play. All the speeds have been varied from time to time, using strokes from 4" to 8", with a 2" to 4" overhang and a mirror rotating speed of about 3/4 r.p.m.

"As can be seen in the picture, we used a small drill press to supply our power (1/3 h.p. motor) and to give us our first speed reduction and variable speed arrangement (by shifting the V-belt on the two multi-step pulleys).

"We had time to read 'ATM' and'ATMA' during work and we don't feel a bit tired after 5 or 6 hours of polishing! The various motions of the mirror during polishing are equivalent to pushing the mirror over the lap at the rate of 1 mile per hour."

Figure 10 shows a small machine of the Lee type ("ATM," p. 160) made by W. B. Hiner, 123 Cleaves Ave., San Jose, Calif., polishing a 6" mirror. It makes 40 strokes per minute, of length from 2" to 10" (adjustable) and with tool and mirror turning in opposite directions, mirror on top. It will take up to a 12" mirror and has been used by Hiner on three mirrors. The Lee type of machine is generally similar to one described in an ancient book, "The Telescope," published in 1861 by the younger Herschel (Sir John), and designed and used by his father, Sir William Herschel.

We greatly regret that another group of machine descriptions, sent us earlier than the above, was lost in the mails when temporarily lent to an amateur who was building a machine, and was never recovered.

LAST August, F. M. Garland of Pittsburgh described here his methods of marking mirrors by different kinds of etching. We now discover that Lyman Nichols, 118 Liberty St., New York, N. Y., an amateur telescope maker, has developed special inks, black and white, for writing in white or black on glass and other smooth-surfaced kinds of materials. It does not etch but it is otherwise very solidly affixed.

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