1938: Mechanical Engineering

High Spots of the Year.

Engineering progress is seldom spectacular. It is usually a growth based on practice or discoveries that have gone before. Perhaps the most noteworthy of 1938 developments is what is known as super-finish, a method of obtaining metal surfaces with fewer defects than heretofore. Yet this is a development of the process known as honing, which has been in common use for some years. The method and the results obtained will be mentioned later.

In a similar manner, local hardening of steel parts, both by the use of flames and by electrical current, have found new fields of application. Hydraulic control of machine movements has resulted in wider applications in lathes, planers, and other machine tools. This now includes the movement both of the work and of the tools. Ingenious mechanisms provide constant or varying speeds and feeds, by either automatic or manual control. Hydraulics have also made their way into a standard automobile in this country for the first time in regular production — the custom built Chrysler Imperial so-called fluid flywheel, built under British patents.

New materials, both metallic and otherwise, have made their way into the manufacturing industries. Newer steels, which with proper heat treatment replace some of those higher in cost, as well as rust-resisting steels that can be machined much more readily than formerly, make possible more durable products at lower costs. New cast alloys are replacing forgings in crank shafts and cane shafts. New Plastics, that are more stable under atmospheric changes and have other desirable properties, and a new synthetic rubber by the DuPonts are all advances that have helped industry. While the new rubber lacks some of the qualities of the natural, it has others that are superior for some industrial uses. New mechanical processes also are coming to the front.

Superfinish.

Superfinish is an outstanding development by D. A. Wallace, President of the Chrysler division of the Chrysler Corporation. It is a refinement of the honing process, made possible by the discovery that speed, pressure, and direction of movement play an important part in obtaining the best surface for reducing wear between metals in contact. This combination differs materially from previous practices in honing. But while the machinery employed is expensive, the desired results are secured at very low cost.

The production of superfinish requires at least three motions between the work and the abrasive stones, and five or more motions frequently give better results. Some machines have as many as ten motions; and some have a multi-motion action, so that the abrasives literally scrub the metal surfaces to a crystalline smoothness.

Other finished surfaces present a series of high and low spots about equally divided. Superfinish removes the high spots and leaves a flat or smooth surface between any minute scratches that may be left below the surface. These scratches are but from one to three millionths of an inch deep.

While the surface superfinished resembles a mirror, the real object is to make it possible to retain an unbroken film of oil between bearing surfaces and so increase the load-carrying capacity, or to increase the wear life with the same load. Used as a finish, this method does not appreciably change the dimensions of the work unless the tolerance is less than 0.0005 inch. It is possible however to obtain the same finish and remove as much as 0.001 inch at the same time. Abrasive stones of from 180 to 500 grit are used, and no extra-fine preliminary finish is required. The superfinished surface is had in from 3 to 30 seconds with a pressure of from a few ounces to a few pounds per square inch of stone area.

Flame Hardening.

Flame hardening has come to the fore as a commercial practice in the hardening of gear teeth and, to some extent, in the hardening of flat surfaces. An early application was the hardening of gear teeth in steel automobile flywheels before separate ring gears were used for this purpose. This was done by heating each tooth separately and rolling it into a hardening bath. The modern method is to have a stream of cooling water just behind the torch to cool the surface immediately. This is largely used in hardening the teeth of large gears which cannot be heated all over, as in the case of small gears.

Another modern application is the hardening of the wearing surfaces of the bed of a lathe, or ways, as they are usually called. This has been developed by the Monarch Machine Tool Company for their lathe beds, to greatly reduce wear and prolong life. The lathe bed is partly submerged in a tank of water, while the heating torches are moved over the surface by a suitable mechanism and at the proper rate. The water from the cooling nozzle follows, as in the case of gear teeth, and cools the bed behind the nozzles. Keeping the bed submerged reduces the distortion to a minimum, the few thousandths of an inch that interfere with perfect alignment being removed by an accurate surface grinding machine.

Somewhat similar results in local hardening are obtained by what is known as the Tocco process. This is being used for hardening the wearing surfaces of crankshafts and camshafts for automobile motors. The method provides for the local heating of the surfaces by electrical contact pieces and the cooling of the piece by flooding as soon as the proper heat has been reached. The effect is quite comparable to flame hardening.

Increased Accuracy in Machine Work.

The increasing demand for greater accuracy in machined parts leads to constantly changing methods in shop practice. This increased accuracy gives us machines that perform more satisfactorily and have longer life. Where a few years ago a thousandth of an inch was considered a fine measurement, we now demand and receive parts made to within a ten-thousandth of an inch of the ideal dimension, and occasionally even split 'tenths' on very fine work. This has been made possible by more accurate machine tools and by new and improved methods of making very fine measurements.

New instruments make it possible to measure defects in the quality of a surface to a millionth of an inch, which opens up a new field in the smoothness of machined parts. We have heretofore had no measure of the quality of a finish that did not depend on judgment of the inspector. The new instrument measures the depth of any defect in the surface and shows the frequency with which these defects occur. The Bureau of Standards has shown that the wear of gages in use depends more on the smoothness of the finish than on the hardness of the steel. It is this quality that makes the new superfinish particularly valuable, as it greatly increases the wear life of parts finished by this process.

The demand for accuracy has led to the abandonment of the use of jigs and fixtures in the machining of some types of machines. For while jigs and fixtures are necessary to secure duplicate work without the use of highly skilled mechanics, it has been found that greater accuracy can be secured by the use of the jig-boring type of machine, with a single-point cutter and a highly skilled mechanic. With the relation of bored holes located by very accurate screws, micrometer dials, and measuring rods, the holes can be bored more accurately than with the average fixture which holds the work and guides the boring bar. Then, too, the single-point tool can do more accurate work than the usual boring cutter. Strangely enough, the jig-boring machine, which was designed to bore holes in jigs, has made jigs unnecessary in many kinds of work.

However, this new use of jig borers in manufacturing would not have been practicable but for the development of the cemented carbide tools which have replaced steel for work of this kind. These tools are almost as hard as the diamond and can be used over long periods without re-sharpening or resetting. They maintain their cutting quality in machining large numbers of holes, which is not only economical but insures the different holes being of the same size. If the tool point wore appreciably while in use, the holes would not be alike and might even be smaller at the end than at the beginning of the cut. This illustrates the manner in which one development makes possible other improvements that could not be utilized previously.

Changing Methods.

Shop methods are constantly changing. Planing gave way to milling in production shops. Milling is being rivaled by grinding and broaching, yet is staging an attempted comeback in the field of turning, which formerly belonged to the lathe. Machines for machining round work by rotating it in front of a milling cutter, and in one instance a revolving broach, are replacing the turning machine in some cases. Honing machines replaced cylinder grinders in automobile shops and are now themselves threatened by the superlinishing machines.

One well known engineer suggests that we may see the grinding machine eliminated from cylindrical work, where it now holds almost complete sway. He feels that we may turn round work sufficiently accurate with carbide tools so that it can go direct to the superfinishing machine and be ready for assembly. While all predictions are dangerous, the many changes that have taken place in machine development and in methods should prepare us for further drastic changes in the near future.