BRIAN MANNING, a British laboratory technician, has the distinction of being the first amateur to make a ruling engine that generates diffraction gratings of unsurpassed optical quality. A spectrograph fitted with one of the homemade gratings easily splits the green line of the mercury spectrum into its constituent pattern of finer lines. The spacing between one pair of the finer lines amounts to only .03 angstrom, about 120 trillionths of an inch.

The gratings consist of grids (two by three inches in size) of straight, uniformly spaced, parallel grooves ruled in the aluminum coating of flat glass mirrors. The precision required of the machine that makes the grooves is little short of awesome. The ruling engine must position a specially shaped diamond tool on the aluminum with enough pressure to displace the metal, push the diamond in a straight line to make the groove, lift it from the aluminum for the return stroke and simultaneously shift the aluminum sideways 66 millionths of an inch, plus or minus not more than half a millionth of an inch. Moreover, in ruling a grating the machine must repeat this operation precisely at least 45,000 times without stopping.

Manning, the persistent amateur who designed, constructed and debugged the engine, can be addressed in England at "Moonrakers," Stakenbridge, Churchill, near Kidderminster, Worcestershire. The project has absorbed most of his spare time for the past 21 years. A thick book could scarcely contain a full account of the construction details, but he summarizes the work as follows:

"My interest in diffraction gratings was aroused in 1954 by the desire for a grating to use in a spectrohelioscope. I knew virtually nothing about diffraction gratings except that they were expensive and that a successful one had never been ruled by an amateur. The idea of making one with a ruling engine of my own construction was attractive. I had completed several reflecting telescopes in my home shop, which has a three-inch engine lathe and a drill press in addition to the conventional tools and materials for telescope making.

"I knew that ruling engines resemble the shaper, a common power tool in machine shops. The shaper has a reciprocating ram that carries a chisellike tool in a straight line. The chisel makes straight cuts in the workpiece during outbound strokes. The workpiece is displaced sideways automatically by its screw-driven carriage during each return stroke of the ram in preparation for the next cut.

"A well-maintained precision shaper can work to a tolerance of about .0001 inch. A ruling engine must work within .000001 inch! As the late Albert G. Ingalls of Scientific American, who popularized telescope making as a hobby, once wrote: 'On the scale of ultraprecision with which we must deal in a ruling engine we may regard the machine as made of rubber!' Ingalls pointed out that the screw in shifting the carriage can be elastically compressed as much as .00001 inch-10 times the error that can be tolerated by the grating it pushes. This is why pioneering developers of ruling engines spent much time eliminating friction of the stick-slip type. The framework and the other parts of the ruling engine are similarly flexible because no perfectly rigid material exists.

"More than 80 percent of all gratings employed in research have been ruled with engines of purely mechanical design. The uniform spacing of the rulings, and hence the optical quality of the grating, can be no better than the quality of the screw and its mountings. Like many amateurs I tried without success to make and mount a screw of the required accuracy. Finally I hit on the idea of achieving the desired precision by using optical measurements of carriage displacement to control the rotation of a crude screw [see illustration at right]. A similar scheme had been undertaken seven years earlier by George R. Harrison and his colleagues at the Massachusetts Institute of Technology, but I did not learn of that work until my engine had been finished.

"As the first of several experiments to measure carriage displacement I set up a Michelson interferometer on a cast-iron plate. The movable mirror of the interferometer was temporarily mounted on the end of a screw-actuated carriage of the kind that serves as a milling attachment on engine lathes. The light source was a neon lamp.

"When the instrument was in proper adjustment, it displayed a pattern of alternately dark and light interference fringes in the form of concentric circles that expanded or contracted as the carriage moved forward or backward. One difficulty was that the interval between bright fringes varied with the position of the moving mirror. Moreover, the conversion of the movement of the circular fringes into electric pulses with a photomultiplier tube turned out to be difficult.

"In time I learned of the Twyman-Green modification of the interferometer. In this scheme Michelson's extended light source is replaced by monochromatic rays that diverge from an illuminated pinhole placed at the focal point of a simple lens [see illustration at left]. The lens bends the diverging rays into a parallel bundle.

"By adjusting the mirrors to be virtually parallel the experimenter can produce a constant difference in the path of the interfering rays over the entire aperture of the interferometer. The pattern of concentric fringes is replaced by an illuminated field that varies sinusoidally from light to dark. The pulsating light falls on a photomultiplier tube. The electrical output of the tube is correspondingly sinusoidal, and the pulses vary in proportion to the displacement of the mirror.

"Generating the interference effect throughout a three-inch excursion of the movable mirror requires that the interferometer be illuminated with light of a single color. The efficiency of the lamp must also be as high as possible to minimize heat that would expand the engine and ruin the grating. The only adequate sources of monochromatic light prior to the advent of the stabilized gas laser were gas-discharge lamps, particularly those filled with argon and mercury vapor. They were expensive. For some years a relatively inexpensive krypton lamp was available, but its brightness was barely adequate.

"In the end I built a cadmium lamp. To avoid the difficulty of sealing electrodes in glass I excited the lamp with radio-frequency energy. My first lamp operated for about 10 minutes before turning black. After much experimenting I hit on the right combination of temperature, filling pressure and glass.

"Most recently I have made the lamps with an electromagnetically enriched isotope of cadmium (Cd-114) that I get from the atomic energy establishment at Harwell in England. The light emission of this material is largely free of the fine spectral structure that broadens the blue emission line at 4,799.92 angstroms when ordinary cadmium is used. This line matches the spectral response of my Type 931A photomultiplier. The improved lamps have a minimum operating life of 1,000 hours.

"Having developed the optical measuring system, I proceeded with the construction of the first version of the engine. The principle of its operation was simple and remained so as the engine evolved. The movable mirror of the interferometer is fixed to one end of the carriage that supports the grating. All other parts of the interferometer (indeed, all other parts of the engine except the driving motor, the stabilizing flywheel and the electronic components, including a regulated power supply) are mounted on a thick cast-iron base and are enclosed in an iron box with wall quarter of an inch thick.

"The screw that drives the carriage is coupled to the motor through a magnetic clutch. During the return stroke of the ruling engine's ram the clutch is temporarily engaged to rotate the screw until the electronic circuit counts seven interference pulsations. On the count of seven the clutch disengages, thus positioning the aluminum film to receive the next groove. The operation is repeated automatically until the grating is finished or something goes wrong. Seven pulsations correspond to a carriage displacement of 66.14 X 10 inch, or 15,120 rulings per inch. The interferometer of the first version of the engine usually went out of adjustment after about half an inch of the aluminum had been ruled. The main reason was that the slideway were not rigid or straight enough.

"The carriage slideway of the present machine consists of a straight steel bar six inches long and one inch square. This form was selected because it can easily be tested for straightness on an optical flat. I made three six-inch optical flats of Pyrex glass expressly for this purpose, although in the end the bar had to be finished by observing the tilt of fringes with the interferometer as the carriage traversed.

"Rotation of the carriage around the slide bar is restrained by outriggers carrying ball races that roll on straight cylindrical rails. A spring assembly on one of the outriggers can be adjusted to remove most of the weight from the slide bar. The lead screw has 26 threads per inch. It was cut on my three-inch lathe and lapped for smoothness, but no attempt was made to correct errors of pitch.

"At the end of a ruling stroke a cam on the crankshaft that operates the ram closes the electronic circuit. A second cam on the crankshaft communicates rotation to the input of an electrical clutch. The clutch rotates the lead screw through a train of reduction gears. After the required fringe count has been made electronically the clutch disengages. The angular rotation of the lead screw averages about .6 degree of arc per ruling.

"The ram that carries the reciprocating diamond tool slides on a hardened bar of steel eight inches long and 3/4 inch in diameter. A friend ground the bar for me between centers on a precision lathe. The bar came off the lathe straight and cylindrical to within .0001 inch and required only slight lapping.

"The ram, consisting of an aluminum L section, is supported on the horizontal slide bar by four bearing pads of Graflon. This remarkable substance appears to be an intimate mixture of polytetrafluoroethylene (Teflon) and graphite. Not only does it have the lowest coefficient of friction of any solid but also its static coefficient of friction is lower than its dynamic coefficient. Therefore when Graflon slides, it does not exhibit stick-slip friction. The chattering movement that has been the bane of builders of ruling engines for more than a century is thus eliminated.

"The bearing pads were cut from a sample of Graflon that was donated to me by the Morgan Crucible Company. The crank that drives the ram operates through a rocking-arm linkage that I devised to conserve space on the baseplate.

"The diamond tool or stylus [see illustration at left] does not cut the aluminum. It displaces metal sideways. The shape of the tool resembles the keel of a boat. The diamond is supported by a dop made of copper wire. A small hole to receive the stone was drilled axially in the square-cut end of the wire. Surrounding copper was then crimped over the diamond and fastened with silver solder. Excess metal was cut away from the mounted stone to expose areas that became the working facets.

"The tool bears on the aluminum with a force of only about a gram, but in effect the minute working facets exert a displacing force of several tons on the metal. I first attempted to use the natural faces on a crystal of Carborundum, as the American physicist R. W. Wood had done when he pioneered the production of gratings for the infrared portion of the spectrum. When this effort failed, I polished flat facets on the Carborundum with a rotating copper lap charged with diamond paste.

"My first good grating was ruled with one of these tools. Its dimensions were 5/8 by 3/16 inch. Although the grating was small, it easily resolved the D lines of sodium.

"Ultimately, however, I accepted the fact that a diamond tool is essential. I obtained a fragment of diamond from a shattered grinding wheel and fastened it to a mounting with cellulose cement. Through beginner's luck the resulting tool worked splendidly until the cement gave way.

"I then attempted to cut facets on the diamond. The project almost drove me to distraction. The diamond consistently cut deep grooves in the laps, but no facets appeared on the stone. A visit to our local reference library produced the explanation. Diamonds can be polished in a reasonable length of time only if the direction of cutting is appropriately oriented for the crystal plane that is being abraded.

"The keel shape of the tool requires a minimum of three facets. The procedure for determining the cutting directions of each facet is complex and tedious. It is fully described in the technical literature.

"During the final stage of polishing the facets are examined under a microscope with intense vertical illumination at a magnification of roughly 400 diameters. The intersecting edges of the facets should appear as perfect diffraction lines devoid of spots or thickening. The faceted diamond and its dop are assembled in the pivoted tool holder at an angle close to the desired 'blaze,' which is the slope where the reflecting surface of the ruled grooves concentrates maximum light in the spectral order of interest.

"The grooves of blazed gratings are saw-toothed in form. A grating of 15,000 grooves per inch, in which the reflecting surfaces are inclined at about 10 degrees with respect to the surface of the aluminum and with which the second surfaces make a right angle, is blazed for the first spectral order. Initially I place the diamond tool as close as possible to the calculated angle. Then I rule and test a small grating, readjust the tool accordingly and repeat the procedure until the required slope is closely approached by successive approximations.

"Rulings blazed for the first order that are 66 millionths of an inch wide are about 12 millionths of an inch deep. As I have mentioned, the tool makes grooves by deforming the metal. For this reason the aluminum coating should b at least 30 millionths of an inch thick t allow unrestricted plastic flow. I had difficulty obtaining films of this thickness from companies that specialize in coating mirrors, and so I set up my own aluminizing apparatus. It includes a six inch bell jar, a pair of piston backing pumps connected in series, a diffusion pump and the kind of evaporating coil previously described in these column [see "The Amateur Scientist," SCIENTIFIC AMERICAN, March, 1960].

"Incidentally, I devised an inexpensive valve for operation between the backing pumps. Experiments demonstrated that an oil film across a small aperture prevents the flow of gas at pressures of up to about two torr. To make the valve I inserted a piece of gauze between the pumps. The gauze is wetted with oil during each stroke. During the evaporation of aluminum the optically flat glass blanks are rotated continuously by a synchronous motor that revolves at the rate of 30 revolutions per minute.

"In order to prevent outgassing, the motor was thoroughly cleaned, rewound with 20 turns of thick enameled wire and lubricated with silicone vacuum oil. It runs on one volt and three amperes. I have no difficulty exhausting the system to a hard vacuum.

"Glass blanks were made originally by figuring rectangular pieces of plate glass. Unfortunately the errors of flatness and parallelism of the new 'Float' glass are so large that the material must be ground, polished and figured with the aid of an optical flat.

"The electronic system consists of the photomultiplier, an amplifier, a neon-lamp circuit that transforms the sinusoidal output of the photomultiplier into a series of flat-topped pulses, a flip-flop pulse counter and a thyratron switch that controls the magnetic clutch lower. All these circuits use old-fashioned valves (vacuum tubes). Friends chide me for not switching to solid-state devices, but I am content to stick with my red-hot wires in glass bottles. They don't blow up if I make a wrong connection!

"About two days are needed to rule 45,000 grooves in an aluminized blank three inches long. The ram makes 16 strokes per minute. Variations in the temperature of the engine must be minimized throughout the 48-hour interval to prevent dimensional changes that would ruin the grating. I keep the engine in a room on the north side of my house. A bimetal thermostat and an electronic relay control an electric room heater. The thermostat is close to but not inside the housing of the engine. I do not attempt to rule a grating during extreme variations of outside temperature. The controlled heater confines the temperature of the engine to excursions of less than .1 degree Celsius.

"Variations of barometric pressure equivalent to one inch of mercury can cause an error in groove spacing of about 11 millionths of an inch per inch of grating length. The problem can be avoided by sealing the tank with a lid and a gasket and maintaining the pressure at 27.6 inches of mercury, plus or minus .003 inch. A small barometer in the tank is fitted with a photoelectric detector, which switches on a small pump to make up for leakage into the tank. The pressure is a little lower than the lowest barometer reading and ensures that the lid is always held on. The mass of the tank also averages out short-term temperature variations and damps vibration.

"To rule a grating I position a suitably prepared blank on the carriage. About 100 grooves are ruled to check the mechanism, including the action of the rocking-arm linkage that lowers the diamond into contact with the aluminum at the beginning of the ram's traverse and lifts it for the return traverse. The electro-optical system is similarly observed to ensure that the carriage is indexing in appropriate increments. Ruling is then stopped.

"The electric heater continues to operate for roughly 10 hours until all parts of the machine reach a predetermined temperature. The engine is then started, but ruling is delayed for 30 minutes. During this interval final checks are made of the adjustment of the mirrors of the interferometer and the output of the photomultiplier.

"To start ruling, the diamond is lowered. During the ruling procedure the output of the photomulitiplier slowly decreases owing to fatigue of the cathode surface. Full output is restored at intervals by increasing the brightness of the cadmium lamp. The adjustment causes small but acceptable errors in the spacing of the rulings.

"Although the interference-control system is capable of detecting the position of the carriage within a hundredth of a fringe (a tenth of a millionth of an inch), it is not easy to construct a mechanism that will advance the carriage with this accuracy. Precision is degraded by the effects of friction, the uncertain driving action of the screw and nut and the elasticity of the metal. The cam that drives the magnetic clutch is shaped so that for each advance of the carriage the rate of fringe counting is initially high and decreases almost to zero as the last count is approached. Even so there is some random overshoot. These small errors can be tolerated because they are random.

"On the other hand, diffraction gratings that contain periodic errors of groove spacing display false spectral lines known as 'Rowland ghosts' that flank the parent line. An example is depicted by the accompanying spectrograms [this page]. The upper illustration displays the green spectral line of mercury at 5,460 angstroms together with flanking ghosts that are almost as intense as the parent line. This spectrogram was made with a diffraction grating that was ruled on an engine made by Henry A. Rowland at Johns Hopkins University. The lower photograph displays the same spectral line of mercury as diffracted by a grating that was ruled on my machine.

"Without interference control the gratings ruled on my machine would scarcely diffract recognizable spectral lines. Even with the control the engine is not without limitations. The carriage and the ram can rule aluminized blanks up to two by three inches in size, whereas the best professional machines accept blanks at least three times larger.

"One of the most difficult problems with my machine was confining the motion of the diamond to a single straight line as it reciprocates several tens of thousands of times. Putting a lubricant between the ram bearings and the slide bar introduces problems of variable film thickness. On the other hand, dry bearings operating at light loads acquire a high polish and start to wring, or adhere to the slide bar by molecular attraction. The Graflon bearing material shows a minimum of this effect along with low wear and freedom from stick-slip effects. Even so the ram is still subject to minute deviations."

Bibliography

PROCEDURES IN EXPERIMENTAL PHYSICS. John Strong et al, Prentice-Hall, Inc., 1938.

THE PRODUCTION OF DIFFRACTION GRATINGS-I: DEVELOPMENT OF THE RULING ART. George R. Harrison in Journal of the Optical Society of America, Vol. 39, No. 6, pages 413426; June, 1949.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.