THAT contributions can an amateur make to archaeology? Many people collect arrowheads. Not a few have dug into the works of the ancient mound-builders. Some own collections of artifacts that run into the thousands. But for archaeology much of this work is worse than wasted, because collections not adequately documented are worthless, and often the digging destroys invaluable information.

Frank H. H. Roberts, Associate Director of the Smithsonian Institution's Bureau of Ethnology, explains in the following how the hobby of collecting artifacts can be made a scientific avocation deeply rewarding both to the amateur and to his professional colleague:

"Archaeology fascinates most people. It is the means by which the past is made to live again, and it opens up new and exciting perspectives of time. The study of man's early history, growing out of the pleasant pastime of gathering antiquities for curio cabinets, long ago passed the stage where stone axes were thought to be thunderbolts and arrowheads were looked upon as 'fairy stones,' when the man who found a pot was called an archaeologist and he who found two became a great archaeologist. It has become a very complicated subject with numerous ramifications.

"The material remains which an archaeologist is lucky enough to find are only the starting point. They must be studied in relation to the environment in which they are found. The place where the people who made the objects lived will tell far more of the story than the objects themselves, because its climate and natural resources at the time were determining factors in the growth and development of the cultures which produced the artifacts. Detailed studies of the soil in which the objects lie, and a complete record of what is found there, are absolutely essential, for the process of excavating destroys the source of information. For that reason professional archaeologists have been loath to encourage laymen to make a hobby of archaeology and have insisted that excavations should never be undertaken except by an experienced person. This attitude on the whole is not snobbishness, as many amateurs believe, but is the outgrowth of a real concern for irreplaceable information.

"What constitutes the difference between a professional and an amateur is not easy to define. Is the mark of a professional full-time employment at the job, or is it organized training and skill? Some men who make their living as archaeologists are purely amateur in that they have had virtually no training in the techniques of the science, while on the other hand some of those who are concerned with archaeology only as a pastime would qualify as professionals in skill and experience.

"To most people the word archaeology is synonymous with digging. Its glamour as an avocation certainly comes in no small part from the fact that in many of us there still lingers the small boy's delight in hunting for buried treasure. But an amateur can also profitably engage in other phases of archaeology which will not only give him satisfaction but contribute to knowledge. One can carry out many interesting archaeological projects without touching a shovel.

"A person interested in the traces of aboriginal occupation of the area in which he lives can obtain considerable pleasure from preparing a map showing their location and character. Getting the necessary information for such a map entails walks about the countryside and enjoyable chats with the local residents. Many farmers keep the 'curios' turned up by their plows and generally are delighted to show and talk about them. The research may even require a bit of reading in a library to determine what Indian tribes lived there in former days or to find out if there are references to the archaeology of the area. In the course of tramping about the fields the investigator may find an occasional arrowhead, other stone implement or potsherds. A record giving information as to how and where they were found will make a useful supplement to the map and may prove helpful to some professional when a comprehensive study is made of the region. If you find a number of artifacts, it is useful to sort them according to types and show their distribution.

"A map of this kind becomes still more valuable if it indicates the types of vegetation and the general character of the topography. By comparing it with old maps, often available in local historical society libraries, you may be able to find changes that have occurred in the terrain since the earlier maps were made. The earlier character of the ground may have had a definite bearing on the location of an Indian village, and often it explains conditions which might otherwise be puzzling. Old maps also occasionally lead to the discovery of archaeological features which were destroyed by cultivation and are no longer apparent on the ground.

"Collecting arrowheads of course is one of the most popular hobbies. Some people collect them mainly by purchase or trade. Those who buy their artifacts run considerable risk of having fraudulent objects palmed off on them, as there are a number of men in the U. S. who are experts at making 'Indian' things. In any case, a collection by purchase has little actual value, because there is no record of where the artifacts were found. Occasionally a large collection containing very fine specimens is offered for sale after the death of its owner and, much to the disappointment of the heirs brings far less than expected, even less than the original cost of the items, solely because no information accompanies them. Every person who makes a hobby of collecting arrowheads or other aboriginal objects should keep a careful record of where and how each was obtained, asking the seller for this information when a piece is purchased. Each specimen should be numbered and listed in a notebook, so clearly that anyone can readily identify it.

"In recent years there has been an encouraging trend toward the organization of societies by laymen interested in archaeological subjects. Many of these groups seek the help and advice of professionals and are making a serious effort to follow accepted archaeological procedures. Some of the local groups have formed state-wide societies, and in the eastern U. S. several state societies have combined in a regional association.

"State organizations and local societies have developed programs of investigation and contributed considerably to the archaeological knowledge of their own regions. In Connecticut and Massachusetts, for example, some excellent excavation projects have been carried out by amateurs under the general supervision of a few professionals. In Missouri amateur societies have cooperated with the University of Missouri in making surveys to locate and record all archaeological manifestations throughout the state. Their work has been particularly helpful because it has been done in areas which will be flooded before long by the construction of large dams. In Texas local societies have salvaged materials which were being destroyed by construction operations and have kept a valuable record of their activities.

"Contrary to a rather widespread impression, the professional archaeologist is not opposed to the amateur and is not continually seeking to keep him from what can be a worthy avocation. What the professional is anxious about is that the amateur should learn the best procedures and do his work properly. The sincere amateur will find that most professionals are more than willing to advise and assist him with his problems and to suggest ways in which his efforts can be of service."

THERE IS virtually no literature on the making of precision screws for ruling engines, which were described in an article in this magazine last month. Some of the basic principles of the manufacture of these screws are illustrated in the drawings herewith. Roger Hayward, who drew them, is himself an amateur mechanic; he wrote the sections on the lathe and on molding and casting in John Strong's Procedures in Experimental Physics, widely known among amateur telescope makers because of its excellent chapter on precision optical work.

In the group of drawings on the left, shows a spiral spring which illustrates what a mechanic means when he speaks of a "drunken" screw-thread. In a perfect spiral the helix angle, or angle of the threads with regard to a plane perpendicular to the axis, is everywhere the same. Here part of the spring is bent to show the nature of the distortion of a drunken thread: the helix angle increases and decreases with each half-turn. A screw-thread may be thought of as a spiral inclined plane around a cylinder-when the spiral is even and regular, the thread if unwound would form a straight line; when it is drunken, the line would be crooked.

The next drawing (top right) shows how a short nut would wobble in advancing on a drunken thread. In a ruling engine this wobble would produce a periodic error in the spacing of the grating grooves and put "ghosts" in the grating spectra.

The third drawing (second row) describes with necessary exaggeration a faulty screw of varying diameter and varying pitch. Pitch is defined as the distance between the corresponding points on two adjacent threads. The drawing shows how diameter and pitch are closely related and, in fact, additive and not separately identifiable. In this particular screw the varying pitch compensates for the varying diameter. A single short nut could be moved along its length with equal friction-an early test for uniform diameter that was not quite watertight. All would seem well until the nut was connected with the diamond of a ruling engine. The screw would put progressive errors ("errors of run") on the grating.

Now suppose that two separated short nuts are placed on the screw, as shown in the same drawing, and the screw is rotated within them. The nut at the left would travel faster than the one at the right, because the pitch is "greater than ideal" in the left-hand part.

Next, suppose that the nuts are rigidly interconnected, in fact are parts of a single long nut, and emery grains in oil are introduced. When the screw is rotated, grinding action will take place on some of the threads to correct the pitch. This, of course, is the method used to refine a screw.

The third row of drawings illustrates one method of measuring the diameter of a screw. The circles are cross sections of wires of known diameter that are placed in the valleys of the thread. The flat sections are the anvils of a micrometer delicately touching the wires. The screw on the right is an example of varying pitch in the threads, which is not revealed by the wire method. The root diameter is uniform along the screw, but the screw's major diameter increases, as shown by the outer pairs of dashed lines. Yet the micrometer reads the same. The reason is that the greater pitch at the right-hand end of the screw allows the wires to enter farther into the valleys, compensating the discrepancy. On the other hand, in the drawing at the right the pitch is uniform, though the root diameter and the major diameter are not Now the tapering of the screw is revealed by a difference in the space between the anvils. The wire method of measuring slightly compresses the screw at the two small areas of contact with the wires. To avoid this, Dave Broadhead, whose method of making screws for the Strong ruling engine was described in this department last month, measured with micrometer anvils in contact with tips of 20 threads at a time. This method must be used with caution, else it will damage the delicate thread-tips. Micrometric methods of measuring are used only down to one-10,000th of an inch. Thereafter the interferometer is used. The screws on the Strong engine are 1-1/4 inch in diameter, threaded over a length of 10 inches with 40 threads per inch (.025 pitch) at 45 degrees.

After making an apprenticeship screw precise in diameter within one-500,000th of an inch and others for the Strong engines Broadhead felt that "it is a good gamble if the screws are uniform in diameter within one-10,000th of an inch after systematic lapping with a long lapping nut and no cheating with a short one." He calls the control of on end of a long lapping nut on the thread at the other end a "telephoning effect."

"The effect at one end," he adds, "is modified by this control from the other provided the ends are in contact with the screw-and here lies the rub, or rather the lack of it, for the ends are actually connected only to the extent that the lap and threads are stiff. The smaller the errors dealt with, the stiffer it must be Getting contact between the mirror an lap is a must in mirror making, but i not as easy with a metal lap since it can't be cold-pressed. Thus the lap wears to a bell-mouthed shape (shown in the lower left-hand drawing) and then will no longer span enough of the screw to reduce long errors of run. It is really a short lap, yet you may foolishly think of it as a long lap.

"Bell-mouth lap is more difficult to avoid than turned-down edge on a mirror. In the first place, the fresh grit reaches the ends of the lap first. I countered with an extremely thin abrasive mixture, two grains (by weight) of 303-1/2 emery to a gill of thin oil, and used just enough of it, or half a teaspoonful each three hours, with clear oil dripping on the lap between times, to keep a little black color, indicating removed metal, in the drippings from the lap. In the second place, the thrust of the screw tends to give the lap halves a rocking, oscillating motion, despite the push-pull screws that hold them together. These screws are 'rubber' when we work in such minute dimensions. I countered with lapping in vertical position and trying to reduce the thrust by careful counterbalancing and reduction of friction on the guides. Thirdly, no matter how carefully the screws are adjusted, one end of the lap and later the other will be sure to be tighter than the first. Bell-mouth lap is the inevitable result. I countered by using three pairs of push-pull screws on the nut halves and adjusting each by watching a milliammeter in series with the armature of the highly efficient, sensitive ball-bearing electric motor that rotates the lap. The current is proportional to load (torque).

"When initially threaded, the lap fits the screw tightly, but it starts wearing out of fit at once, because the screw gets smaller and the lap larger in radius. The push-pull screws can take up this wear in one dimension (the vertical in the lower right-hand drawing) but cannot take it up in the other. Soon the lap contacts the screw along only two small and opposite areas. To counter this loss of 'wrap-around' I sometimes channel the center of the lap lengthwise, thus giving it four 'halves,' and sometimes remove the central 'facets' along its length.

"The meanest thing on a screw, much worse than scratches on a mirror, comes when emery and debris roll up in tiny balls and bend the tips of the threads before the lap can be stopped. The bent tip then becomes an error which spreads by telephoning effect to the entire end portion of the screw, affecting the lead while it is being reduced by lapping. A damaged thread cannot be lapped individually. When the tape recorder announces that a thread-tip has been bent, you are set back a week in your progress, even if you stop the motor at once.

"The lapped screw is polished after 'figuring,' instead of before as in a mirror, by running it in soft grit, and finally with oil alone, for many hours. No true polish is possible, since the threads cannot be moved in various directions in the nut.

"In screw-making we stand today where the old-time mirror makers were before the invention of the knife-edge test."

The ruling-engine article last month called attention to the great difficulties connected with bringing the geometric axis of the pivots near the end of a screw into coincidence within one-millionth of an inch with the axis as determined by its helix of threads, to prevent the nut from wobbling in the ruling engine and putting periodic errors on the grating. J. A. Anderson rates this five times as difficult as making the screw itself. The axis-coincidence job was formerly accomplished by correcting one side of one pivot, which left it out of round so that it had to be ground round again in its own bearing, and then correcting the other end similarly, and by alternating ends until no error could be detected with an interferometer. In place of this inelegant method Strong invented the automatic mechanism shown on right.

Surrounding the screw is an internally threaded "nut drum." Surrounding this nut drum is a "nut drum housing," with an open space as thin as tissue paper between the two. This narrow space is filled with liquid grease. Lapping abrasive is applied only to the "lap for cylindrical bearing" (see drawing) and "lap for thrust bearing."

Here is how the invention works. When the screw is rotated the nut drum moves easily in a lengthwise direction but with great difficulty sidewise due to the wobbling screw, because the grease film in the space must be mashed and squeezed laterally in the space between the drum and housing to overcome high viscous friction. The reacting force is exerted on the sides of the pivots that need abrasion. Automatically the axes are driven toward coincidence, as shown in the illustration above.

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