ABOUT A YEAR AGO a number of 16-inch Pyrex glass blanks were offered to the readers of this magazine at a fraction of the market price. Within a short time the last of more than 400 blanks had been sold, two thirds of them to amateur telescope makers throughout the nation. One might assume that each purchaser at once started grinding a mirror, and that perhaps 250 16-inch telescopes emerged at the same time. A few of those who bought the blanks indeed began to work soon after receiving them, but it takes a long time to complete a 16-inch telescope. A majority are holding the blanks until they gain experience with smaller mirrors.

Some of those who own the 16-inch blanks have asked for suggestions about working them. Four methods will be discussed: face down against equal-diameter tools, (1) by hand and (2) with the Hindle machine; and face up against subdiameter tools, (3) by machine and (4) by hand.

Since a 16-inch blank weighs about 50 pounds, the first method would prove strenuous. The glass tool should be channeled as described by Fred Ferson in Amateur Telescope Making-Advanced (printings since mid-1944) to eliminate the risk of sticking and for other advantages, or else a similarly "broken" tool of glass or tile blocks pitched to metal may be used.

For the second method a large and rugged Hindle machine capable of throwing around a 50-pound blank would be needed. One such machine that John H. Hindle of England built, all metal and wire-rope driven by a heavy motor, was illustrated in SCIENTIFIC AMERICAN in March, 1943.

For the third method there are several sound types of machines. One that is seldom built by amateurs, perhaps because it requires two cranks geared together, is the triangular type shown in Amateur Telescope Making at A on page 163.

The arc-stroke machine shown at B on the same page enjoys a good record. In highly disguised but mechanically identical form it is widely used commercially. If the lengthwise arm of the machine were discarded and the arc arm forcibly oscillated we would have the "Arsenal sidewinder" used in the Frankford Arsenal and illustrated on page 83 in Amateur Telescope Making-Advanced (printings since mid-1944). As a commercial product the latter costs several hundred dollars. A. H. Johns of Larchmont, N. Y., embodied the same-sidewinder principle, together with caster buffers, in a machine known, because of various resemblances, as the "Johns banjo."

There remains in the subdiameter tool, mirror-face-up category, the Draper type of machine. During the war Dave Broadhead of Wellsville, N. Y., designed the machine shown at the left directly from the original 1864 drawing by Draper as reproduced in Amateur Telescope Making, page 165. For several days I watched this machine at work in Broadhead's cellar shop. It was designed so well and worked so efficiently that it is offered with confidence as suitable for making 16-inch mirrors.

"Nearly all machines used in precision optics," Broadhead states, "are basically related to the Draper, with the following adjustments:

"1 ) A turntable rotating at a rate that balances accomplishment against heat.

"2) A reciprocating arm to drive the tool over the work ('stroke' in the drawing) at a rate perhaps five times that of the turntable, and means to vary its length.

"3 ) Means for varying the 'offset' (see drawing).

"4 ) Means for 'decentering' the driving pin so that the stroke takes place a fraction of an inch more on one side of the center of the work, in order to soften and spread a zone.

"5) Variable driving speed to permit the same linear speed in polishing of different diameters."

As the machine shown was designed for 12-inch maximum work diameter, a few inches should be added to its length for 16-inch mirrors. The height should be changed to suit the worker's own height and that of his high stool. Vertical legs may be substituted for the A-frame legs, but if this is done the worker will no longer be able to stand close to his work. (In the drawing he would behind the machine, facing the reader.)

On 12-inch work the 1/8-horse motor has proved more than sufficient. A 1/3-horsepower motor was ample when the arm was temporarily extended to a separate spindle for making two 36-inch mirrors.

The motor's long V-belt with its sag on top improves the smoothness of the drive. The gear reduction may be obtained from the Chicago Gear Works or from the Boston Gear Works. Broadhead's was an old one, salvaged as per amateur tradition. The Ford transmission can be obtained from Sears, Roebuck and Company, but the one used cost $7 at a junk yard. Greasing suffices to lubricate it, no oil bath being needed. The gears shift readily without stopping the motor.

Pulleys of 3 inches and 15 inches diameter, or sprockets and chain, would improve this machine, Broadhead states, by giving a smoother drive to the turntable.

The reciprocating arm may be lifted entirely off the machine without first loosening anything. In his version of Draper's original, Broadhead embodied the illustrated provision for shifting the arm laterally to cancel zones without stopping the machine.

Since the driving pin is free to rise or fall within its oiled guide, the tool loading remains uniform and perfectly controlled; none of the weight of the arm rests on the tool.

The husky turntable shaft has a 7/8-14 SAE right-hand thread and a shoulder. This shaft should barely rise above its bearing.

On both vertical shafts the top lateral bearings are common floor flanges carefully bored out on the lathe. At the low speeds of these shafts relatively little power is lost in plain bearings. Power is lost, however, when shafts extend above their bearings, a grotesque fault of many machines, including Draper's. Fancy bearings will not recover this loss, which is due to cramping caused by flexure. The avoidance of this waste, together with the efficient transmission, largely explains why the midget motor has been adequate. The solidly bolted-in 4-by-4 puts the pipe-flange bearing high up, close to the "ears" of the crankshaft head. Thrusts up to 600 pounds are obtained in lowest gear. Nine speeds, from 1.2 revolutions per minute on the turntable and 6 strokes per minute on the arm up to 13.5 revolutions and 68 strokes, respectively, are possible by shifting the motor belt on the step-pulleys in combination with the gear shift. Thus Broadhead can always obtain his favored slow eight-feet-per-minute for polishing, and slower speeds for figuring. For roughing out and for fine grinding much higher speeds can do no harm.

The movable dimmer switch on the floor serves not merely to control the motor; it assures quick stops in emergencies when the hands are otherwise occupied. It also frees the hands during halts.

One way to learn to operate such a machine is to try tools of various sizes with varied strokes until, after perhaps 100 years, the art is learned empirically. There is, however, a science to it that others have recorded. In his book entitled Optical Workshop Principles (Jarrell-Ash Co., Boston, 1943) Charles Dévé of France has set forth some of this science, though his book will call for hard study of geometrical principles.

Instead of going through this procedure with his Draper machine, Broadhead excavates with a two-thirds-diameter tool to full sagitta at the outset. He uses strokes that pass near the center of the mirror but do not extend the tool more than barely past the edge.

With the curve thus roughed out, 5/6-diameter tool is used until figuring begins. As Driscoll has pointed out, a 5/6 tool has long been known in the optical industry as the best for holding the curve almost to the radius with which it starts. With this diameter of tool, correct setting of stroke and offset will suffice to increase the curvature, decrease it or hold it precisely. This "method of uniform wear," as it was called by Dévé, enhances the attractiveness of the subdiameter tool method. A drawback is the need to guard against grinding astigmatism into the mirror: the worker must start by grinding the blank flat, support it on Brussels carpet and turn it.

"So now," Broadhead writes, "we must discover experimentally the 'neutral position,' the precise combination of stroke and offset that will hold the curve at one radius. Do not change to the third abrasive until the neutral stroke has been found and can be held. Then grind and polish with this stroke."

Broadhead makes a combination roughing and grinding tool by cementing chamfered or sloping-sided blocks of glass to the steel backing out to 2/3 diameter, and surrounding these with a ring of thinner blocks extending to 5/6 diameter and of such thickness, ascertained perhaps by consulting a psychic medium, that the outer ring will come into contact just as the curve reaches the sagitta.

Now he adjusts the stroke to about .35 length of mirror diameter and .2 offset (trying .5 and .3, respectively, in case the medium was wrong), until the outer blocks reach contact. If the spherometer, preferably made with a dial indicator, shows that the curve is shallowing again, we may try .2 and .1.

In mirror polishing there are two common techniques: the wet-lap technique and the dry-lap technique. In the first, water is used very freely and heavy weights bring work and tool into something like contact. But, as Broadhead points out, gobs of water on one side of the work often lift it unevenly, and capillary prevents the water from squeezing out. It is feared that too many amateurs follow this method, also using gobs of rouge. In the dry technique, observed in many commercial optical shops, the lap runs almost dry for lack of attention and squeals like an agonized pig, and this often causes sleeks.

When the worker can concentrate his full attention on one spindle he can use a third method-the moist technique. "In this I cold press," says Broadhead, "with no more water on the lap than might be deposited by breathing on it. To prevent sticking, I lay a damp cloth, not around the edge to produce a cold zone, but on the weight, which should be the same as the one to be used on the drive pin. On a 14-inch surface I use only three pounds. To maintain humidity, cover the work with a cake cover."

I watched Broadhead closely while he figured two mirrors. He put a spoonful of Barnesite in a glass of water and stirred it with a tiny artists' brush that had very few hairs. On the newly made lap he brushed a little of the pink liquid, washed the mirror and all but dried it with his hands. Accent throughout was strongly on little abrasive, little water.

After a few minutes' work with the machine a hint of a squeal was heard. A tiny droplet-less than a drop of pink water-was dabbed on either side of the moving tool. This quieted the incipient squeal. Perhaps a quarter of a minute later, as the pig threatened to squeal again, two more stingy dabs were granted it, and so on throughout polishing, while the machine groaned from exertion. The aim is to maintain a strong and unvarying attraction, or "suction," due to extremely intimate contact during work. Into this the weights enter but little. At no time after the first stirring was the water in the glass stirred.

This moist technique, with its powerful adhesive forces, calls for a rigidly built machine and for tools having maximum rigidity and minimum weight.

Broadhead figured with agonizingly slow strokes, deliberately stretching the operation out to avoid heat caused by the low-moisture method and to maintain calm control. He used a star tool of .5 diameter. Such tools work fast and must be used very cautiously. He deliberately overcorrected a little, then cold-pressed the 5/6 lap on the mirror and ran it a few minutes at "neutral" stroke to smooth the figure.

"What you call my technique came largely," Broadhead protests, "from reading Driscoll's articles, talking with Ferson, and watching closely at Frankford Arsenal during a wartime visit."

Suppliers and Organizations

The Society for Amateur Scientists
5600 Post Road, #114-341
East Greenwich, RI 02818
Phone: 1-401-823-7800

Internet: http://www.sas.org/

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.

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