IN THE following article Patrick A. Driscoll, Lima, N. Y., who went from "A.T.M." to a job as experimental lens maker in the Eastman Hawk-Eye Works in Rochester, tells how to furnish your wife with grounds for divorce for cruelty by casting metal lens or mirror grinding tools in the home kitchen, using the lost wax process. In this you model in wax what you want to mold in metal, then cast in plaster a negative of that model, finally melting out the wax and pouring the space full of molten metal.

As was told in Scientific American, December 1943 and December 1945, in two three-page articles (which do not, however throw practical light on making metal tools for amateur glass pushers, though they are of incidental or background information interest) this ancient method which was once employed chiefly for accurate manufacture of small metal jewelry and dental parts is also being used today on a large scale for making precision castings industry, where it rates high. Driscoll writes:

Into the life of every amateur optician there looms, at some time or other the specter of the iron grinding tool. Requiring a lathe and attachments to cut its convex or concave radii, the iron grinder becomes a thing of complex machining to the average basement glass addict.

As a fellow "TN," suffering from lack of machine shop facilities, I wish to present an alternative solution to the production of the iron grinder. This requires a job of casting. The method is not claimed as new by the writer but its application toward solving the amateur telescope maker's difficulties is original. We shall cast our grinders in zinc, and the process used is termed the "Lost Wax Method."

The low melting point of zinc (715 degrees F.) does not call for a high-temperature furnace. The gas stove of ye long suffering wife will suit the purpose admirably. The following materials and tools will be necessary.

Plaster of Paris

Paraffin

Large tin can

Scrap zinc .

Rotating spindle (temporary)

Glass gage of the desired radius.

We shall assume that a grinder is needed for a 6" (152 mm) mirror. The grinder should be 5/4 of the mirror diameter or 7-1/2" (190 mm), but if the worker prefers the equal-diameter mirror and grinder, let him proceed on that basis. [The philosophy of the 5/4 grinder was explained by Driscoll in the October, 1945, number.-Ed.]

The glass gage should be made from thin glass and as close to radius as possible.

To begin, the paraffin should be melted in a container about 1/4" larger than the grinder diameter desired. After it cools, apply heat around the outside of the can until the block drops out. The aim now is to shape this paraffin block into an exact model of the grinder we are to cast.

At this stage, we must have a rotating vertical spindle. This can be a temporary setup and does not have to be elaborate or complicated. The spindle speed should not be much over 100 r.p.m.-slower if possible. Here the amateur who possesses a polishing machine can leer with satisfaction. The spindle should have a flat, horizontal metal surface attached and on this we place our wax disk. We warm the spindle table, center the disk and allow it to cool. Now with a sharp knife we scrape, not cut, the disk roughly to curve and check the radius with the gage. When the disk is close to the gage curvature, we abandon the knife scraper and to finish the disk to curve we use the gage itself as a scraper. With the spindle rotating, we hold the gage to the surface of the disk and shave off all irregularities.

We have now produced a grinder, modeled in wax to exact curvature in other words, a pattern.

Now our large tin can comes into action. It must be at least 2" larger in diameter than our pattern. The pattern is mounted on a wax pedestal, two 3/8" wax rods are attached, and the whole is placed on the bottom of the can. See the illustration. The can bottom is then heated locally to adhere the wax to the metal and prevent shifting of the pattern. To forestall tipping, be sure that the pedestal and rods are of equal length. The next step will be making the plaster mold. Mix ordinary plaster of Paris thoroughly to a consistency that will just allow it to run slowly out of the mixing pan. It must not be putty-like. It must not be watery. Mixing and pouring must be completed in one minute. Time and plaster wait for no man.

Pour this mixture into and around the wax pattern until every nook and cranny is full and the pattern is covered with plaster to a depth of at least 1-1/2". Allow the plaster to set for 48 hours in a warm room. Do not try to hurry setting by using excessive heat at this stage.

With the mold firmly set we can now invert it and remove the bottom of the can with one of the new-fangled smooth-cutting can openers. The mold is now placed, pedestal end down, in a large pan. The mold must be placed on three blocks to keep it at least 2" up from the bottom of the pan. Be sure that none of the blocks covers the pedestal or the ends of the wax rod. Place the whole setup in the kitchen oven and bake it at 350 degrees F. until all the wax has melted and run out. Continue to bake it until certain that all moisture has been driven out of the plaster. As a test hold a cold metal surface to the hole in the plaster. If moisture collects on the metal, continue to bake until dry. Remove from the oven and allow to cool to room temperature.

We now have evolved a plaster of Paris mold having a pouring spout and two vent holes to relieve trapped air in casting.

Zinc is recommended as a casting metal because of its low melting point and its advantage over lead in point of hardness, but the home optician may substitute lead if he desires. The difference in price will be negligible. Lead will have the drawback of retaining particles of emery or Carbo in rough grinding that may cause trouble in the fine grinding. To the amateur who has access to large quantities of lead, we say use it, making a new mold and casting a grinder for each size of emery. Here again, as was stated here last October in an article on the hand-lever type of machine, may I mention that, by machine, 180, 500, and 1200 emery are all the sizes required to produce a perfect surface for polishing. Finer sizes will tend to scratch when used with a metal polisher.

Place the plaster mold in the oven and heat slowly until it is fairly warm (200 degrees F).

To ascertain the amount of zinc needed, weigh the paraffin model and multiply by 12 or 13.

Melt the zinc on a gas flame and skim off impurities.

With the warm mold resting firmly pour the molten zinc deliberately but do not stop until the mold is full and zinc is expelled through the vent holes Allow to cool naturally, until cold.

Cut the rest of the tin can away from the plaster and then attack carefully with a hammer. Out of the cocoon will emerge a perfect metal grinder having a pedestal and two rods attached. Saw these off and the flat back can then be drilled and tapped for any type of plate or driving stud the worker may decide on.

I have purposely outlined the production of a flat-backed grinder to simplify the job of casting. The amateur with a flair for complications can cast this grinder with its spindle-fitting attachment in one piece, if so inclined.

Having turned my own kitchen into a foundry at one time or other, and knowing the reaction of the better half, may I wish you luck.

THE MANUSCRIPT of the above was sent to Dr. D. Everett Taylor, Willimantic, Conn., with an invitation to comment upon it. He replied: "This casting method-wax-is, as you probably know, the fundamental of the precise process in making dental inlays. Either Ransom and Ransome (Toledo, Ohio) or Kerr Dental Manufacturing Co. (6081 Twelfth St., Detroit, Mich.) dental inlay investing compound would be even more accurate than plaster of Paris, causing less distortion from expansion or contraction and remaining unchanged in high temperatures (2000 degrees F), and the cost would not be prohibitive. If plaster of Paris is used, the addition of 30 to 50 percent sand would improve the mix."

He further offers as a substitute for zinc, lead 1: tin 1: bismuth 1, having: a melting point 250 degrees F, where that of zinc is above 700 degrees F; contractibility, 0.00066 where zinc's is 0.01366; hard-~ ness, 0.042 where that of zinc is 0.018; brittleness, 7, zinc, 5. "This combination," he adds, "is attractive, with it the melting point is greatly reduced, the contraction is reduced to about one twentieth, and the hardness is more than doubled. The brittleness is in- creased somewhat, but, as I see it, the added brittleness is desirable rather than the reverse."

ONE AMATEUR to whom Driscoll's article was shown asked, "Isn't this a lot of work to make a tool?" Looked at in the light of work alone, it is. Telescope makers, being philosophers, don't look at things in quite that light. You might say, "I've always wanted to play with that old lost wax process and making a tool provides a good excuse, with the tool itself thrown in free." As Haviland says in "A.T.M.A.", what is a hobby if not to murder time every moment of which is enjoyable.

URGENTLY, readers of this magazine often prod its book sales department to obtain for them second-hand copies of two famous old books that are out of print-Webb's classic "Celestial Objects for Common Telescopes," which went through seven editions after the first in 1859 (the seventh dates 1898-99), also Barns' "1001 Celestial Wonders," published 1927 and 1929 and patterned on Webb. It is probable that these requests come from readers who have only heard of their fame, since they are listed in numerous other books, but who have had no opportunity to inspect them.

Today the nearest substitute for Webb or Barns is "Norton's Star Atlas." Told this, readers ask "How near?", and think, "Wish I could see all three side by side and decide for myself."

Your scribe, one of whose many vices is a habit of haunting New York's numerous second-hand bookstores, has come across Webb but once, Barns never. Recently a fifth edition of Webb, brought up to 1900, was picket up. With it, Barns, and Norton spread out side by side it is thus possible to give the reader a comparison of these three. This particular three are singled out from other books about the stars one specific feature, their main feature-they contain extensive lists of show objects for users of 2" to 6" and larger telescopes equipped with setting circles; lists prepared to facilitate systematic survey of these sights such as double stars, variables, nebulae, and clusters.

It is true, the Ephemeris contains star lists with coordinates but not arranged as described and not nearly so useful to the amateur.

Webb's book "has done more to introduce the possessors of telescopes to the remarkable objects in the heavens and to create observers than any book ever written." This was said in the Monthly Notices of the Royal Astronomical Society in 1886 when Webb died. It is still true in 1946.

All three books being similar, what are the differences? Not much in presentation. Typical examples-same object, sigma Cassiopeiae:

Norton: R.A. 23, 56.4; 55.29, 5.4, 75; 327 degrees; 3" .1. Grand low power field.

Barns: 235455; D.5.4.-7.5; sep. 3" Teeming with jewels.

Webb: XXII h 53.9 m; N.55 degrees 12';

5.4, 7.5; 323.5; grn.v.bl. wh., tawny.

Glorious low-powered field.

Thus, Webb's descriptions are a little more detailed.

Lesser differences: Norton contains star charts, so does Barns, Webb contains none. Norton is large-9" by 11-3/8"-Barns and Webb are pocket sized. Norton's data are for Epoch 1950, the others earlier.

The chief difference is quantitative. Where Norton lists about 500 show objects and Barns about 800, Webb lists 3177. Of this excess a large part consists, however, of doubles, an emphasis perhaps reflecting Webb's preoccupation with doubles due to his remarkable eyesight. Average observers are seldom equally preoccupied with doubles.

Webb therefore is a book to seek after the hunting in the available Norton has been exhausted, a book for the advanced amateur. For such, it is a treasure.

Lowell's three Mars books, "Mars" (1895), "Mars and its Canals" (1896), and "Mars as the Abode of Life" (1908), all out of print, are only medium rare -in the five-dollar class at secondhand.

"The Splendour of the Heavens" is rare-in the thirty-dollar class. Goodacre's "The Moon," not an old book (1931), is rare and in the same cost class, at second hand and, even then, virtually unobtainable. We know of an amateur who has searched far and wide for it for several years, advertised for it, been ready to give his fortune for it, yet without result. Recently, a small cache of brand new copies has been located where it was hidden away and forgotten during the war and they can be had without sacrifice of fortune -provided they are still there when we transmit your order.

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