(F. H.) Beginning with the demand of the builders of steam engines for machines that would bore cylinders more nearly round than the early, crude machines, machine tools have been developed in the United States to a remarkable degree. The demands of build ers of automobiles has made possible the spending of great sums for specially developed machines. Where tolerances of a few thousandths of an inch were formerly permitted, the practice in 1928 holds the accuracy well within a thousandth or even two or three ten-thousandths of an inch.
The lathe was in 1928, for example, largely used in roughing work to be finished by grinding; milling had supplanted planing in many cases, and was in turn, being supplanted by grinding in some fields. The vertical milling machine was at the same time doing much work formerly done on the slotter, and, for finishing such internal surfaces as automobile cylinder bores, the honing process has almost entirely supplanted the internal grinder. Many of the newer designs of machines had their beginning before 1918, but in most cases the final development that has given them such a per manent place in modern metal working industries, came within the decade 1918-28. Hydraulic feed was built into machines at least 3o years ago, but its successful introduction into machine tool use is quite recent, as an accepted method of securing a posi tive and easily regulated feed. The drilling machine, one of the oldest of machine tools, has undergone many changes. This main development has been in high speed sensitive machines, in heavy duty vertical machines, and in the radial type, in which much of the mechanism has been transferred from the base of the column to the head, and all of the controls are now under the operator's hands, without moving from his working position. The "way drilling" machine, in which spindles are arranged to drill holes in three, four or five sides at once, and at any angle, has been de veloped to handle much intricate work at a rapid rate.
In all machine tools the use of the cone pulleys and shifting belts as a means of securing different speeds has practically dis appeared, except in the sensitive drilling machine. Change gear boxes have become common in most classes of work. The two main developments in sensitive drilling machines have been the re placement of plain bearings by ball-bearings, and the careful bal ancing of the spindles, including the chuck, or drill holding device. This balancing makes for smoother and steadier running at the higher speeds and also greatly lengthens the life of the machine by reducing the load imposed on the bearings. Heavy-duty machines
were developed largely to meet the demand for rapid drilling or boring of large holes such as formed a large part of big gun ammunition and for automobile cylinders. Heavy thrust bearings are usually ball—and both ball- and roller-bearings have come to be a part of the design in these machines. Nor are they confined to single spindle machines but are found with four, six or eight in the case of automobile shops. This is also one of the fields to be invaded by hydraulic feed. In no field however has the advance been more marked than in radial drilling machines. Not only have sizes and weights increased but there are great refine ments in design that make it of particular interest. Among the new features is the centralizing of all control on the head. This includes the locking of the arm on the column, the raising and lowering of the arm and the changing of spindle speeds and feeds, all without manual labour. Roller-bearing spindles and ball- and roller-bearings in the gear shafts, hardened gears of alloy steels. and special oiling systems cover the main features of the newer machines. Multiple spindle and standard drilling machines have both been improved, the bearings of the former being limited by the closeness of spindles. There is also a growing tendency to use multiple spindle drilling heads with fixed spindles on single spindle drilling machines. By this method the same machine can be used on different jobs by merely changing the drilling heads instead of by re-setting the spindles on a multi-spindle machine.