Anxious Beginner: "How hard ought a pitch lap to be?"

Seasoned Old Hand: "Well, not too soft, or else it'll turn the edge."

A.B.: "Yes, but just exactly what do you mean by 'too soft'?"

S.O.H.: "Why, it ought to be quite hard.'

A.B. (still cherishing hope of getting definite idea that will help): "How would you define 'quite hard,' then?"

S.O.H.: "Well-aaaa, hard enough so it won't turn the edge."

A.B. (beginning to perspire): "But isn't there any exact way to pin it down to definite terms-as you know, I'm only a little fella working all alone."

S.O.H.: "Yes, you may chew the pitch and judge it that way. You bite down on it, slowly but not too slowly, and not to fast, and if it 'gives' too fast it is too soft but if it shatters, unless you have bitten to fast, it is too hard. Then another way is to test it with your fingernail. Press quite hard for quite a time, and if the mark is quite long the pitch is probably quite soft, but if it is quite short it may be quite hard That's all there is to it-it's simple enough. "

This situation has led Joseph A. McCarroll. 521 Palisade Avenue, Teaneck, N. J., to design a pitch tester. Through the common use of such a machine, workers may now, wherever they are, compare pitch hardness in definite, quantitative terms which will mean the same to all from Maine to California, and from Edmonton to Brownsville. A photograph of McCarroll's tester is shown on this page. It is a simple rig, easy to build, and one ought perhaps to be made and kept available at least in every club group. Making one is not much of a task. And here, for once, we favor "standardization," for without standardization of the essential specifications of the little tester the end sought would be defeated. It is not, however, necessary to standardize the materials from which it is made, or the color of the paint on it. McCarroll writes:

"The principle of the test is the penetration of the pitch by a needle of specified dimensions, under a certain weight for a certain length of time and at any give temperature. The test is in all respects similar in principle to the 'Standard Method of Test for Penetration of Bituminous Materials,' as adopted by the American Society for Testing Materials, 260 South Broad Street, Philadelphia, Pa. The standards of this society are 'standard' throughout the entire country, in the engineering professions and in industry and commerce generally.

"The pamphlet issued by the Society known as 'A.S.T.M. Designation D5-25.' copy may be obtained from the Society for 25 cents. This pamphlet includes a description of a needle (smaller than the one be described here) and an outline of the conditions under which the test shall be conducted. It does not describe any machine. That is left to the ingenuity of the inventor. The responsibility for the design of the machine shown on the present page must rest with me. The credit for making a really workmanlike gadget of it is due Mr. Frank Wanderer, a member of our local A.T.M. association.

"The machine shown was made of thin, cold-rolled steel. Aluminum would be better. The arm A is 13 inches long and is provided with a guide piece, so that its end straddles the scale post at the right. It works on the principle of the lever. The arm is balanced on the supporting pin B. It is counterbalanced by weight C. The 1 millimeter needle, supported at E, is separated by a 1-1/4" space from B. The on pound weight D is shown resting in a notch which is the same distance from E as E is from B. This gives a pressure on the needle of two pounds, and each successive notch separated by the same distance, adds a pound to the applied pressure, as shown by the numbers on the lever. The scale is graduated in actual degrees of arc, with the center at B.

"F is a small pan. It contains one half inch depth of pitch. The pitch is allowed t cool to room temperature before a test is made.

"In making the test the sample of pitch is set in place, the needle adjusted, the weight D held in the hand, the temperature noted, the time observed, and at the word 'go' the weight is gently hung on at the two pound notch (usually) and the downward swing of the lever in one minute is recorded. If the sinkage is about three degrees on the scale (using coal tar pitch with two pounds pressure for one minute, I regard the pitch as medium soft, and suitable for making a lap for general polishing purposes. The specific hardness under these circumstances might be said to be '2P3D, meaning two pounds pressure and 3 degrees deflection of the lever. The temperature has no significance in connection with specific hardness, for the pitch must be, say, 2P3D at whatever the workroom temperature is, and if the temperature should rise five degrees it would be absolutely necessary to use a pitch of higher melting point in order to have the test show the same hardness. Any appreciable change in temperature will show in making a test.

"However, if one wishes to describe the general hardness properties of the pitch, it is necessary to specify the temperature at which the test is made. This will show the relation to the melting point of the pitch, which should be known in order to facilitate tempering operations, and, of course, for a complete specification of the character of the pitch.

"The use of a scale graduated in actual degrees of arc makes it easy for the individual to design machines of different sizes, but with the same system of scale notation and needle size. The needle has a flat, round end of 1 millimeter diameter, tapering upward to 5 millimeters in a length of 25 millimeters. The fixed hook G, attached to the pillar and loosely embracing the needle, is simply for controlling the point of contact of the needle on the pitch.

"The intention has been to make something as simple as possible that will do the work expeditiously and as accurately as is needed. There are some mechanical features connected with the design of this little apparatus which might be improved, but they do not introduce any appreciable error in testing. For example, the pivots might have less friction. Also the needle does not keep in an absolutely vertical position while penetrating the pitch, but since a penetration of about 1/16" is about all that is ever required, it is obvious that this feature may be neglected. Better ideas for a pitch tester may be forthcoming."

So much for McCarroll's description of a valuable device The McCarroll tester gives us a way to equate hardness with numbers, as in the one example mentioned- "2P3D." There still remains the gradual establishment of a general consensus regarding how many Ps and Ds of hardness a lap should have-general only, because this always will vary some with individual preferences. Note that there would be nothing here to compel, or desire to compel, any worker to conform to a standard set by others. The machine measures the absolute hardness of the pitch, and its terms are always the same for the same pitch at the same temperature. A may then say or write to B, "I like mine 2P2D," and B may reply, "Give me 2P4D." Each retains his pet hardness but each knows exactly what the other fellow means when he uses the common values of the McCarroll scale.

As a start-off, take a lap your scribe has just been using-made of coal tar pitch supplied by McCarroll. It was a rather hard lap at 70 degrees and was composed of two parts coal tar pitch with melting point 170 degrees and one part ditto with 140 degrees melting point. It was hard enough to permit easy maintenance of an unturned edge (by Everest's very critical diffraction ring test, ATM, p. 371) throughout polishing, and to keep a strong oblate tendency on the mirror. Everest tested this same lap by thumbnail and called it "hard" in terms of his own personal estimate. A sample of this lap, returned to McCarroll, was found to test 2P2-l/2D.

THE picture of a diagonal support, which 1 you saw on the preceding page, is one taken by Cyril Wates, 7718 Jasper Ave., Edmonton, Alberta, Canada, and is inserted with the hope that numerous others will send in similarly clear close-ups of diagonal supports. The idea is to make a collection and, some time hence, publish the lot of them together, since workers often ask us for ideas for designing such supports.

The same Cyril Wates is the one whose telescope is shown on the front cover. Those on the staff of this magazine who are responsible for selecting a front cover picture came to your scribe and begged for a good telescope photograph. About 100 unpublished telescope descriptions were gone over and this was the only one of the lot which would pass every artistic test. Not the telescope, though that, too, is arty enough, but the picture-clouds, river, and all the trimmings of good composition. In addition to the little note on page 169, directed to general readers, the following may interest telescope enthusiasts:

Wates' mirror has a 9-point suspension. He says "it is not quite true, as some old" timers say, that in this region we have 'nine months winter and three months bad sleighing,' but even the Orion nebula loses some of its charm at 30 degrees below. In the summer, on the other hand, the sky is never really dark. On June 21, at mid" night, the sun was only 14 degrees below the horizon, and Vega was the only star visible to the naked eye in Lyra. But we also have skies of exquisite clearness, and a long, lovely autumn season." Wates will not learn, until he unwraps his "Sciam" in mid-September, that his picture is on the front cover.

WE have also been asked by L. J. S. to W publish a collection of close-ups of Cassegrain secondary supports. Can't do it, unless we receive the pictures. Make them small-stop, long-exposure close-ups, glossy.

UP there in the northwest corner of this page is a sort of focogram of the Everest diffraction test described in "Amateur Telescope Making," page 371, made by Daniel E. McGuire, of Shadyside, Ohio, on a mirror which, as the test shows, does not have a turned down edge. Our reproduction does it some injustice, as the left-hand cusp of the ring should come out a bit stronger than it does. That test is probably the best of all the tests for turned down edge. Your scribe confesses not discovering it in A.T.M. until quite recently (must read that book, some time) and doubts whether many others have tried it out yet. Recently, on a mirror job, it was tried many times, in each instance immediately following the Ronchi test and the knife-edge test. It seemed to be the most critical of the lot. Do others find it so? Everest, who discovered the test itself, thinks a mirror which will pass it is mighty good-at least for edge. Any old mirror will easily show the right hand cusp, it is the one on the left that is wanted, but there will be a gap of an inch or so at top and bottom, even on a good mirror. McGuire's focogram, or diffractogram, shows the Everest straightedge, as explained in A.T.M. We have seen some mirrors whose left-hand diffraction cusp looked more like a photograph of a black cat, taken at midnight, in a deep dungeon, when the cat wasn't there. But a really swell mirror we gazed at for a long time, at Wally Everest's test rack in Pittsfield, Mass., showed the ring equally strong on either side. (That fellow has no respect at all for a fine mirror. Someone sent in a proposed new stunt for use of HCF, Everest was asked to give it a try, and chose this same fine mirror for the experiment, about ruining its figure. But he brings them back again, between a couple of smokes, and takes mirrors so lightly that perfect ones are used all over the Everest home as door weights. When bored, he picks up the nearest door weight and turns it into something to envy. We sigh.)

WE hear many rumblings and grumblings about eyepieces of the so-so kind-not very bad, not very good. Well, when a majority of persons tease a dealer to sell them "good but low-priced eyepieces," what can the dealer do? We have now and then been asked where a high-grade eyepiece may be got hold of-one that will not pull a high-grade mirror down. Why not have Kirkham make you one to order, to suit your needs? His eyepieces are beginning to become famous in select circles. But they aren't low-priced-we hope they will always stay high-priced.

LAST month we carelessly reported Hindle as figuring his machine-made mirrors by hand, with small polishers, and now he radios: "I never use hand tools, but figure with part-sized tools machine driven.' As Hindle is the outstanding exponent of the machine for the whole job, we guess we pulled a boner that time! Apology, then, to Hindle. We have here the manuscript of a full treatise, by Hindle, on all-machine figuring. Publication later.

Alexander Elan, 4806 Illinois Ave., N.W., Washington, D. C., asks us to broadcast to Washingtonians the call to organize a club.

IT turns out that the use of coal tar pitch for laps is far from new, though no one seems to have mentioned it to us before we inserted a note announcing its "discovery," two months ago. Even so, at the heart of our little "announcement" was not so much the fact that coal tar pitch laps give smooth surfaces but its hardness controllability, through using mixtures of hard and soft pitch, each at known melting point. By the way, coal tar pitch requires a little more care than pine pitch, in melting. Some which Stoy of Atlanta melted took fire, fell on his hand and cost him two months' time trying to save the member. When melting it in a can, be sure to melt an escape channel for gases down one side first.

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