"IN the March issue of Scientific American you mentioned your hope of hearing from those who are grinding 20" Pyrex mirrors. Here is our bid for membership in the Twenty-Inch Club.

"We received our solid disk on January 9. The disk weighs about 125 pounds and is slightly over 4" in thickness. The raw slug had 34 flat sides where the ceramics used in the mould had joined to form as nearly a circle as possible. One side of the disk was marked with grooves, indicating that it was apparently the bottom of the disk when cast. The other side had several large surface bubbles and was far from being flat. Unlike the smaller Pyrex mirrors, which are smooth and transparent, the 20" disks are more frosty or opalescent and do not have as much amber tint.

"We anticipated lots of labor before we tackled actual work, and our beliefs were well founded. The large disk was a good deal harder than the smaller sizes. Carborundum is a good abrasive for plate glass or even small Pyrex mirrors, but it breaks down too fast when grinding the harder type of Pyrex. We spent many hours trying to face our disk with silicon carbide and aluminum compound abrasives. We had fair success using cast-iron filings. Crushed Steel, however, proved to be the best bet in grinding. It does not break down, cuts four times as fast as Carborundum, makes one tenth the mess, can be used over and over again (one wash and it's as good as new), grooves more than it pits, and above all it is practically as cheap per pound; because it can be used over and over, it is actually much cheaper than any other abrasive for this type of grinding.

"We rigged up a vertical spindle to rotate the disk as it was being ground. An iron plate fastened to the spindle, with three roller bearings to take the weight of the mirror, constituted our grinding machine. With a rotating speed of 30 r.p.m., we used a flat, 12" cast-iron tool weighing 75 pounds. This tool was used for facing both sides of the disk, and was later turned to curve and used to hog out the center of the mirror. Because the edges were so uneven, we decided to grind the disk to a circular shape and remove the 34 little flat edges. Two parallel, 4" wooden rollers mounted on a 2 by 4 frame and swung over a rotating, horizontal iron plate, made up our edging machine. [A sketch was submitted but could not be reproduced. The principle is the same as shown in "A.T.M.," fourth edition, page 135-if the separation of the rollers D,D were adjustable the drop of the larger disk, and therefore its bearing on the rotating plate, would be variable at will.-Ed.] By spinning the mirror over the rollers by hand, and by feeding No. 60 Crushed Steel between the iron plate and the edge of the disk, we were able to true the mirror edge in about five hours.

"We have spent about 15 hours a week working on the mirror. In 6 weeks, or 90 hours, including time for adjustments, tests, experiments, and so on, we have accomplished the following results: The mirror has been faced on both sides, and the edge has been ground into a perfect circle. We have hogged out the center of the mirror, and we are smoothing the curve by hand, using a 1" thick, full sized plate glass tool and No. 60 Crushed Steel. We tried a marble tool, but it was too soft. Polishing will probably be done by machine and figuring by hand.

"Let's hear from the rest who are making 20's."

THE two workers having omitted to describe the machine shown in their photograph, we asked for more data and received the following, dated two weeks later:

"We have been working steadily at the job, and we are now just about ready to enter fine grinding. Like all TNs, we changed our minds several times about the focal length. At first we thought we would make it f/7, hoping for ease of grinding and figuring. The mounting difficulties of a long focus mirror did not bother us as much as the fact that we wanted an all-purpose mirror if possible-one that could be used for nebular, planetary, and photographic work. We finally decided to make the focal length f/5. With this focal length we can get a rich field for studying faint nebulae, a good light for photography and, by using it as a nonperforated Cass, we can have sufficient power for planetary studies.

"You asked for a little more detail on our grinding machine. It is hardly worthy the name of machine. Basically it is only a rotating vertical spindle with an iron plate on top for holding the mirror. The bearings under the plate are old automobile ball bearings. These bearings are strong and steady and, although they support the weight of the rotating mirror, there is no side play during grinding. A 1/4 h.p. motor with a 3" V pulley drives u 5" pulley on a shaft-on this same little shaft (Sears Roe buck polishing support) there is another 3" V pulley which drives a 14" pulley mounted on a pipe shaft and supported by 2 by 4 wooden bearings. A 2-12" flat-belt pulley is also mounted on this shaft, and a 2-1/2'' flat belt connects this pulley with a 12" flat-belt pulley mounted on a 1" vertical spindle which supports the mirror.

"We have no mechanical device for moving the tool across the mirror. The 2 by 3 R braces and many wooden braces which you see slung across the table are merely supports to hold the tool. Our 12" cast-iron tool has a 1-1/2." hole in the center. We plugged this with a handle from an old shovel. The handle then stuck up about 10". A 3/8" hole was bored through this handle and a steel rod was passed through. This rod served to keep the tool from twisting, yet provided a means for sliding the tool back and forth across the mirror.

"After having ground the edges with Crushed Steel, as explained in our last letter, we fine-ground the edge with 220 and 280 Carborundum. A piece of thin sheet of metal was mounted as a band around part of the mirror's circumference. One end of this tin strip was fastened to a board and the other end was kept taut and snug against the edge by means of a spring. As the mirror rotated, we painted fine Carborundum on the edge. With this simple device we put a fine finish on the mirror's edge in less than one hour. This little time spent on finishing the edge improves the mirror's appearance many times.

"After having roughed out the mirror to an approximate f/8 on the machine, we have since been grinding by hand on our plate- glass tool. Using Crushed Steel, we were able to take the focal length down to f/5 with comparatively little trouble. At first two of us pushed the glass across the tool, but later we found that it was rather easy for one man to handle the job. So now we each take a ten-minute try at grinding, alternately with a ten-minute rest period. With this method we can grind for a much longer period and with less fatigue. We are having such good success with hand power, in fact, that we may not resort to the machine again, except possibly for some preliminary polishing. The machine, however, was indispensable or almost indispensable when truing the rough blank, edging, and hogging out.

"Incidentally we have taken several feet of motion pictures showing our crude but effective method of attack on the 20" disk. Someone might be interested in viewing the action."

THE rest of the Twenty-Inch Club appears at present to consist of C. R. Tinsley 3017 Wheeler St., Berkeley, Calif., whose job we described in the February number; A W. Everest, Pittsfield, Mass., Walter L. Moore, Coral Ridge, Ky.; Lew Lojas, 1510 White Plains Road, New York and Amos H. Huff, Escuela, Ariz., but w don't know at present writing (March 16) how the three last-named of these member are making out. There are probably other workers on 20" Pyrex disks, either the solid variety (less expensive) or the waffle variety (more expensive, being replicas of the 200" disk), but we hope these comments will smoke them out. With these disks purchasable at reduced rates, in clubs of six or more, there is likely to be quite a future of development in 20" telescopes and an interesting one at that-especially when it comes to the mountings.

WE sent the two preceding letters to Everest, who returned them with comments on his 20" disk-which, by the bye, was given him for Christmas by Mrs. Everest (Wives! Buy your husbands 20" disks, and you will then know they are safe and at home.-Adv.). Everest wrote:

"Been puttering around with the 20". Weighs 115 pounds, less a couple of pounds of bubbles. Hogged her out to an f/5 curve, 200" R/C. This meant 1/4" deep in the center, and the removal of over three pounds of the hardest glass I ever worked on. Took nine pounds of No. 50 Carbo, ten hours hard labor, and gallons of sweat.

"She was roughed out with a washer tool, which is about four times faster than a glass tool. This is made by cementing large iron washers over the face of a glass tool having the proper radius of curvature. In (this case it was a 12-1/2'' Pyrex blank which had been used for an f/8, 12-1/2" mirror 200" R/C. The washers wear very little, give the fast action of the channeled cast-iron tool, and can be knocked off for the fine grinding, where glass-to-glass gives slower but much smoother action. There is a little trick to getting them on with a uniform film of pitch. This is accomplished by dropping them into a pan of pitch and brining them up to almost the boiling point. The tool is cleaned with turpentine, placed face up with the handle down in the milk bottle [See "A.T.M.A.," page 43.-Ed.] and the washers hooked out of the hot pitch one at a time and placed in position. If hot f enough, most of the pitch runs off and the balance of the surplus is squeezed out by pushing around in 1/4" circles until the pitch is nearly set after placing on the tool. This pushing around also gives them a (better toe-hold. I always start by putting one in the middle, six around this spaced 1/4", and so on.

"To start the central depression, a circle of washers 8" in diameter was cemented on the tool, the extra diameter of the glass merely representing useful weight for this spell. 4" strokes were used until the depression spread to 10" diameter. When it had reached about 8" diameter it was found that the depression lacked nearly 1/2" of being concentric with the rim of the mirror. So the side that needed it was favored a bit for the remainder of this spell. Didn't worry too much about exact concentricity at this point, however, as the surface was so far out of flat, to start with, that this concentricity was sure to shift a little one way or the other before the depression spread to the rim of the mirror. At 10" diameter the spit test ["A.T.M.A.," page 26.-Ed.] showed the R/C about 10" too long, showing that the center washers had worn a little faster. This was a natural result of their being the only ones in contact at the start.

"At this point, I made a little experiment I have always wanted to try, in order to determine the difference between Steel grit and Carbo. I never could get any speed out of the Steel on Pyrex without such tremendous pressure, over two pounds per square inch, that astigmatism was sure to be ground in. It always sounds as if it were cutting fast, but unless there is pressure enough to get a crushing action on the surface of the glass, not much of anything happens. Steel is not hard enough to get the shearing action of Carbo.

"I tried 2 oz. of Steel grit first and ground for 15 minutes with about 25 pounds' additional pressure. There was no noticeable reduction in size of the grit. Took tool off mirror and left them to dry, after which the grit and gunk were carefully brushed into the scoop of a balance scale. The total weight was 2-1/2 oz., meaning that l/2 oz of glass had been removed.

"Same experiment now tried with Carbo: 2 oz. ground down to the point where it was useless for further rough grinding in less than four minutes. On weighing up, however, I found that 5/8 oz. of glass had been removed-more than with the Steel and in one fourth of the time. Several more trials with different pressures gave about the same results with regard to the amount of glass removed, the time taken being in about direct proportion to the pressure. This seemed to indicate that it was going to take three times as much Carbo by weight as glass to be removed, and this was found to be actually the case after the rough grinding was completed. Carbo, of course, was selected for the job, and I used all the pressure possible with my hands at the start, gradually tapering down to 10 or 15 pounds at the finish.

"After the Steel grit experiment, a larger tool was in order but, being suspicious of those center washers, I knocked them all off and replaced with a new set covering the whole face of the 12-1/2" tool.

"With the washer tool, the mixture of Carbo and water must be exactly right. If too much water, the Carbo will be quickly pushed off over the edge. If not enough water, it will pile up in the center. A few trials when throwing water between tool and mirror to wash out the gunk, will show bow much water to leave when sprinkling on fresh abrasive.

''Continuing with the grinding from the 10" depression, 4" diametrical strokes were used till it spread to 12". Spit test showed the R/C back where it belonged and examination of the tool showed a uniform wearing away of the black oxide from the surface of the washers. There was no danger of the tool getting out of shape from now on.

"The remainder of the rough grinding the fining, and the preliminary polishing to date have been done with the zigzag stroke shown on page 35, "A.T.M.A.," Fig. 30 left. A job this size must be done on the barrel-as much as I hate it-taking a short step to the left at the completion of each cycle of the strokes shown.

"Zigzag strokes inside about an 8" circle were used to spread the depression to 15". R/C shortened 2".

"Strokes bringing center of tool to within 2" of the edge of the depression were used to 17" diameter. Spit test showed R/C back at 200" with about 1" turned down edge. Otherwise, zone-free sphere.

"Strokes shortened to within 3" of the edge of the depression and ground to 19" diameter. Edge a little cleaner and R/C shortened 2".

"At no time, till now, had the depression been truly circular, since the surface was not flat to start with. So, at this point, a handle was cemented in the middle of a 28" square of plate glass, and the flat rim of the mirror was ground with No. 240 until it was in contact all over. The local applications of abrasive tried first pushed out from between the two surfaces about as fast as applied. Filling the whole depression with Carbo and water corrected the trouble, allowing the plate glass to pick it up and drag it over where wanted about as fast as required. I didn't time this but it took about 20 minutes, leaving the depression with an exactly circular boundary.

"The depression was spread to the rim of the 21" disk with strokes to within 2" of the rim, to prevent shortening the R/C, which it did. But the rough grinding wound up with turned edge plainly visible under the spit test, due to the fact that there is no suction with the washer tool or with such coarse abrasive, and this allows the tool to rock over slightly as it comes nearly to balance over the rim of the mirror with such long strokes.

"The washer tool is not recommended for fining-too harsh action and liability of scratches. So the washers were knocked off and the glass tool was used with the regular sequence of Carbo and fine emery, 1-1/2 hours of each. Although the long strokes were continued, in order to get sufficient abrasion way out, the edge cleaned up during the 280 stage where no turn could be detected either under the spit test or by the bubble test [see "A.T.M.A.," page 35.- Ed.].

"Well, there is the story of the 20" to date. Have given her two hours of HCF on the 12-1/2'' tool: surface semi-polished, zone free, ellipsoid with about 25 percent of full correction. Probably will be most anything else when the preliminary polishing is completed, but I am keeping an eye on her and won't let her get too far out. Requires frequent addition of rouge or water, but nothing like the rapid drying of a full size HCF lap on top of, say, a 10" mirror."

Everest had not yet edged his disk when he wrote. His photograph, which appears opposite, was taken by one of our special secret service sleuths, E. Dayton Thorne, Patchogue, Long Island, who snuk up him one hot summer's day at Stellafane with a candid camera disguised as a seidel- note the smile. Since Everest would not furnish his own photograph for the end his chapter in "A.T.M.A." (which, by the way, is going very well) this is how we circumvent his innate modesty: we suggest that you cut out the picture and paste it on page 48, "A.T.M.A." That in fact, is why we left that space. With Ellison's loss it seems to us that Everest will now slide into place as the leading mirror expert.

Obviously, Everest and the Lester-Woolcock partnership are in disagreement about the relative virtues of Crushed Steel and Carbo. On seeing Everest's letter the two came back with: "Steel is at least ten tines better than Carbo. Of course, you must have plenty of weight on the tool. We used a 75-pound tool and rigged up a lever which we sat, so that we had about 150 pounds' pressure. Don't worry-the 20" can take it."

Everest had mentioned above that, with too great a pressure, astigmatism would be ground in; he speaks of 2 pounds per square inch as "tremendous." The pressure used by the other two figures about 1-1/2 pounds. But your scribe is not foolhardy enough to inject any snap opinions into this Battle of the Abrasives; let 10,000 other amateurs make 20" mirrors, try each kind, and turn in their votes. Woolcock and Lester continue: "When we used Carbo we had to stop very often and clean off the milky residue and replenish the grit. With Steel, however, we merely rubbed a paint brush over the mirror to stir up the particles. We use Diamond Crushed Steel obtained from the Pittsburgh Crushed Steel Co. When you see the grooves that steel makes when used with plenty of pressure, you will give up any other abrasive for rough grinding of Pyrex."

As we go to press the California pair have he final word but maybe, if Everest had it, would grin and add, "Yes, and when you see the astigmatism...." Or perhaps he wouldn't. It would seem to depend on whether we grant the premise on which the California workers appear to base their argument-that there is no risk of astigmatism.

THIS scribe's face is red, for he omitted a name from the Twenty-Inch Club's list, given earlier. Architect Clarence L. Jones and son Art Jones, of Chattanooga, have been making one, and Marion P. Wall designed the mounting. The job is nearly done. Judging from some newspaper pictures we have seen, this is a swank job. It is to be a community telescope, a labor of love by the makers.

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