It is apparent that the machine must be made to swing, say, 18 inches, for it is the custom to consider these dimensions in considering the price. They are therefore selling dimensions and must not be reduced. Just as dolls are sold by the length, and when carried home will be found to lack the proportions of the real baby, just so lathes are sold by dimensions that would be deceptive if this were a new subject. As it is, the machinist knows that he must have an 18-inch lathe to do lathe work on a 5-inch or 6-inch shaft.
Many good results have been obtained in special forms of lathes for a given class of work that we are now to take as object lessons, and which may serve as pointers to indicate the direction of the development in machine shop evolution.
Long-distance control, cob-house design, gibs and gib screws without tension, except when taking working strains, any one or all of these are bad conditions. It is not enough to know that the tool mounting is capable of withstanding heaviest cutting strains without breakage or even perceptible quiver. We must know that the controlling carriages are short, compact and unaffected by strains varying from light to heavy cuts. A tree may be capable of withstanding a hurricane and yet be waved by a light breeze. A child can deflect a wagon spring. Tool holders and carriages are nothing more nor less than springs in equilibrium and should be of the shortest, stiffest possible design.
In view of the foregoing, it would be safe to assume that the correct scheme is one that gets around the long-distance control and cob-house construction of slide on slide. The foregoing is offered as one of these propositions no deeper to solve and having no more indefinite conclusion than the two and two example in arithmetic mentioned in the beginning. It is needless to say that a machine approaching the ideal is fully described in the strictly commercial part of this book. Each feature of this machine may be readily understood.
The machine set forth gives a control of work and tool that greatly reduces the tendency to chatter, and thereby makes possible the use of ideal tools for all cuts. Sharper tools with plenty of rake for cutting the metal may be used, also broader tools for light and heavy cuts where chattering has been the limiting element.
We are fully aware that it is useless to make a machine with solely large stock-reducing qualities; that although this is almost a supreme test, yet since it is so general to consider cost of metal as well as labor, the average work requires mostly accurate turning at medium chips, and the stock-reducing feat is to be performed only in extreme cases. But it seems to be clear that accurate work requires a machine in which a light broad cut or a heavy stock-removing cut can be taken with the minimum chatter.
Chattering is caused by conditions that are fundamentally objectionable. Every boy
Stock reducing only one-half of the problem knows that he can make a cane chatter along the sidewalk by pushing it ahead at a given angle, and that it will not chatter when dragged at same or any other angle.