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a tool known as the " optical flat." This consists of a polished piece of flat, clear glass, one surface of which is very accurately flat. The principle is, that if a piece of clear glass with an optically flat surface is laid on another more or less flat surface, and is illumined by monochromatic light, dark and light bands will be observed on the lower surface. If these bands are parallel and equally spaced the surface under inspection is flat. If the bands are curved, then the surface below the glass is curved, the reason being that a difference in the thickness of the film of air between the adjacent surfaces causes a variation in the direction and spacing of the light bands. By observing certain conditions of light and position, flatness to within one millionth of an inch may be plainly observed with the naked eye.

The diagrams shown on Plate I., fig. 4, indicate how light bands, or " interference fringes," look when an optical flat is laid on sur- faces of different degrees of flatness. The optical flat may be used for comparing the height of two pieces laid side by side. If one is higher than the other the arrangement of the light bands will instantly indicate it. Curved surfaces may also be as easily inspected and the amount of error estimated. Complete apparatus for accu- rately measuring with optical flats is made by H. L. Van Keuren, Boston, Mass.

Among recent developments in the small-tool field, those of importance in lathe and planer cutting tools have been slight, apart from the research work of the late Frederick Taylor, of Philadelphia, and the advances in steel making and treating which make the tools more durable. Taylor spent many years and much money investigating the proper shape, clearance and cutting angles for lathe and planer tools. He also designed and built for the market a machine for grinding these tools according to the shapes outlined in his charts. He also did a share of the work in evolving the Taylor-White process of hardening high- speed steel, which made possible higher speeds and heavier cuts than had previously been imagined. This discovery profoundly affected machine design as well as production methods, since it compelled the building of heavier, speedier and more powerful machine tools to meet the severe demands placed upon them by properly hardened high-speed steel. Except for this strengthen- ing, standard lathes, planers and shapers have really advanced but little. One feature, however, may be noted, and that is the increasing use of air-operated chucks on turret lathes (monitors) and chucking-machines.

An unusual type of single-purpose lathe was designed by Lucien I. Yeomans, of the Amalgamated Machinery Co., Chicago, for which he was awarded a medal by the Franklin Institute of Philadelphia. This type of lathe was intended for making large shell of from 6 to 16 in. in diameter. The head-stock and body of the lathe were cast in one solid piece, with holes cored out for the spindle and ways. The ways were merely accurately ground lengths of round steel shafting so placed that the ends projected through the cored holes in the bed! They were properly located by means of huge master jigs, and then type-metal was poured into the cored holes and around the ends of the shafting. This held them securely in place. The carriage and cross-slide guides were made and located in the same general way. The lathe spindle turned in a machined bushing which was set into the cored hole in the head and secured with type-metal. This method of construction saved an immense amount of machining, as there was no work put into its construction except such as could be turned or bored. The makers were enabled to turn out a large number of machines in an astonishingly short time, and they agreed to furnish any order for such machines at the rate of 10% per day, beginning with the confirmation of the order. The American Car and Foundry Co. had 173 of these machines in one battery making shell, and other large installations were placed. A number of large gun-boring lathes were also made on the same plan, but the close of the World War put an end to this work, as the machines were not suited to other pur- poses. A wartime machine designed and built by Yeomans was a ' concrete planer " with a 93-ft. table and 185-11. bed, used for gun- carriage work. All of the heavy parts, including the bed, table and housings, were made of reinforced concrete with inserted metal ways and facings. Five of these machines were made for Government war work. At the time the Armistice was signed the bed for a planer 500 ft. long had actually been almost completed.

Closely allied to the lathes are the machines of the vertical boring-mill type. Most of these are well known, but modern manufacturing methods have induced the design of certain advanced types, such as the duplex boring-mills made by two English houses, Webster & Bennett, Ltd., of Coventry, and G. Wilkinson & Sons, of Keighley (see Plate I., fig. 5).

Rapid production has also been responsible for what is known as the vertical " station-type " of machine. These machines are an outgrowth of the vertical boring-mill or " vertical turret lathe."

A typical example is the " Mult-Au-Matic " (Plate I., fig. 6) made by the Bullard Machine Tool Co., Bridgeport, Conn. In this machine there are six " stations " or chucks, though other holding devices may be used. The chucks are mounted on a table which " indexes " or makes J of a revolution at set periods, bringing the work under different tools each time. Each chuck revolves or remains stationary on its own centre, making it possible to do drilling, boring, facing, turning and other work either simultaneously or successively. One station is used for the insertion and removal of the work, and afver the chucks have once been filled all round, a piece of work is removed and another inserted at each indexing of the table. This machine is mechanically operated throughout; but a similar machine, in which all the feeding and indexing movements are hydraulically operated, is made by Giddings & Lewis, Fond du Lac, Wis.

Drilling-machines of the station type are also made by several concerns. One made by the Cincinnati Automatic Machine Co., Cincinnati, Ohio, is shown in Plate II., fig. 7. Both of these types were foreshadowed by the five-spindle automatic screw machines made by the National Acme Co., Cleveland, O., whose machines have been on the market for over 25 years. The general principle is the same, the main difference, as in the case of the lathe and the boring-mill, being that one is horizontal and the other vertical, the latter being of much later date in each case.

Apart from the station-type drilling-machines, others now in common use include those of the " gang " and the " multiple spindle " types. The gang types consist mainly of several in- dividual drilling-machines of a like size bolted side by side, or mounted on a single base. The multiple spindle type may have a considerable number of spindles mounted on a single head, the spindles being run by worms, gears or universal joints.

Gear-driven spindles lend themselves well to mounting in heads which may be used in the spindle of ordinary single-spindle drilling- machines. Holes of varied arrangement or " pattern " may be simultaneously and quickly drilled in this way. The worm-driven spindles are more conveniently used if mounted on a cross-rail, making it possible to drill a large number of holes in a straight line, although the spacing between the various holes may be varied. The universal joint-driven spindles (Plate II., fig. 8) furnish the most flexible means of arrangement of all, as they may be set to conform to almost any arrangement or pattern of holes. The National Automatic Machine Co., Richmond, Ind., and the Foote-Burt Co., Cleveland, O., have made many machines drilling up to 100 or more holes at once. Some of their machines feed a large number of drills in from several angles at once. For instance, one model simultaneously drills from 5 to 20 holes in each of five sides of a cast-iron box.

Milling-machines form attractive subjects for tool designers, and new forms are constantly being evolved. The more modern forms include the continuous milling-machines, which are of two principal types, the rotary and the reciprocating.

The reciprocating machine is simply a modification of the regular type of milling-machine. It carries fixtures for holding the work at opposite ends of the table. While the cutters are acting on the work at one end of the table the fixtures at the other end are being emptied and reloaded. The feed of the table is reversed as soon as the work is milled, and the cutters immediately begin to cut on the work in the other fixture. The operator then steps to the other end of the table, removes the finished work and reloads the fixture, and so on.

The rotary continuous milling-machines may be of the horizontal- table-vertical-spindle, or the vertical-table-horizontal-spindle, type. On Plate II. are shown examples of the former, the mqre common (fig. 9), another form, with work-holding fixtures in place (fig. 10), and of the vertical-table, or " drum type " (fig. n). In all of these shown in the last three cuts the table moves continuously, the work being removed or inserted while in motion. A machine built on a slightly different plan is shown on Plate III., fig. 12. In this the table is tilted and the motion is not continuous. The table movement more closely resembles that of a station-type ma- chine in that it indexes and remains stationary as the cutter is fed to the work by the sliding spindle head. Milling-machines of the planer type shown on Plate III., fig. 13, are made in a variety of forms, largely for automobile work. They may carry from one to a dozen cutters placed on spindles at various angles. In some cases the tables are made to reciprocate and one end of the table may be emptied and reloaded while the cutters are at work at the other end, after the manner of the lighter types previously referred to.

A highly developed type of milling-machine is shown on Plate III., fig. 14. This is an automatic profiling-machine designed for the sinking of forging dies and the like. In the illustration a model or master-die is shown below and a finished forging die above. Any number of dies may be made from the same pattern and they will be all alike. A " finger " is automatically fed over the surfaces of the model and as it moves a revolving cutter cuts corresponding