From the time of the birth of the slide-rest lathe it has been customary to have the head stock rigidly affixed to or a part of the bed, and to get all of the relative motion between work and tool by mounting the cutting tool on the necessary slides. The first slide resting on the lathe bed is called the carriage. To the saddle of this carriage there is a tool-carrying slide which runs transversely to the travel of the carriage. In addition to this, it is frequently the practice to add a swiveling slide for traveling at any desired angle, which has been called the compound rest. More than one-third of the lathes used to-day are provided with the three slides.
A glance at the engine lathe carriage shows it to be of frail design. The guiding V's of the bed, on which it rides and by which it is controlled, are a long distance from the point of the tool. The average carriage has four bearings on the shears. The front part of the carriage consists of a bridge which spans the distance between the two bearings on the front V. In the same way there is a bridge at the back that connects the two contacts on the back V. Now, from these bridges on the front and back V's another bridge runs across, making the whole form in the shape of an H. This may seem an elementary description to the average lathe hand, but the object will appear later on.
There are other types of carriage saddles. The H-shaped are most reliable for very light cuts, and others, although uncertain in action in all cuts, are generally better for heavy stock-reducing operations.
There is no opportunity to make either the carriage or the tool mounted thereon of sufficient rigidity to withstand the working strains, nor of suitable section to properly hold their gibbing, which of course serves as a means of holding each slide to the surface on which it travels.
We will assume that the bed of the standard engine lathe is all that it should be. We know that a single slide can be stiffly designed and securely gibbed to a piece having dimensions similar to the engine lathe bed; that the limitations and troubles come in when it is necessary to cut away that slide, giving clearance here and there for the swing of a pulley or a large shaft and the addition of other slides, all of which must come within certain dimensions, regardless of the effect upon the stiffness.
The natural process of reasoning for the reader at this point is that inasmuch as attention has been directed to these weaknesses, it is merely for the purpose of introducing a machine in which these weaknesses do not exist. There is, however, a more important point to be brought to view. It is that the engine lathe, with this great handicap, is the very machine with which we have obtained our experiences and formed our opinions regarding the limitations of a cutting tool and the quality of work obtainable under the various cuts.
No conception of the performance of a tool under heavy cut in a properly proportioned machine can be obtained by the performance of a tool in the engine lathe large enough to take a similar cut. The engine lathe takes its largest cut on work about one-third of its swing, and instead of the tool being controlled by guiding surfaces close to its maximum diameter of most efficient work, these guiding surfaces are three times as far off in a direct line, and many times farther off in the line followed by the metal supporting the tool.
The extent of vibration of the tool point of lateral yielding is governed, of course, by the stiffness of the carriage to resist the working strains at that point, and as this stiffness is inversely as the square of the distance between the sliding base and the cutting tool point, it becomes apparent that
Increasing dimensions do not compensate for indirect design a tool working at this handicap may be forced to crowd off the metal, but while doing so the cutting edge vibrates laterally, making it necessary to use only blunt tools, which add greatly to the "pulling-out-by-the-root" process of removing the metal, and although such lathes can be forced to push off the metal, there is an absence of the cutting action that leaves a true and smooth surface. Furthermore, such lathes are very susceptible to chatter when taking light cuts such as a scraping broad tool, because a certain amount of pressure is required to take up all the slack of their parts and the spring due to long-distance control. This is not a case where the doctors disagree. Lathes are built, by people who understand the work thoroughly, but they are built to supply a certain demand that calls for a standard lathe known to every machinist, and it is necessary in offering these lathes for sale to give swing over both shears and carriage, and price.