The draw-cut shaper astonished many by its wonderful performances of great stock-removing feats, and although this may not be what is primarily wanted in a shaper, it may be cited as an example of the performance of a cutting tool under a pulling cut and non-chattering conditions.
In the lathe, planer, standard shaper and boring mill there seems to be good reason for using the tool mounting that is equivalent to the chattering cane. The expression,"cart before the horse," fits the case, but the plow before the horse would be a better analogy.
In order to offset the chattering tendency in machine tools of thirty years ago, a spring tool was used for finishing cuts which required a tool having plenty of rake. Under the varying strains this tool would yield in opposite direction to its frail mounting ; that is, it would spring away from the work under an increased strain, while the slide rests had a tendency to tip over towards the work, which would otherwise cause the tool to "duck in." The yielding of the tool would offset the chattering tendency.
This tendency to chatter has been partly met in present-day machines by making all these slides of stiffest form possible, so that it is no longer necessary to use the old-fashioned spring tool. But the fact remains, that although we have greatly reduced the chattering, we have yet the plow before the horse and the cane ahead of the boy; that is, the tendency remains, and the conditions exist to a sufficient extent to necessitate the use of blunt tools for heavy chips, and to greatly restrict the use of broad tools for forming, taper and irregular cuts.
The detailed description and illustrations of the machine described clearly bear evidence of a more perfect control, which is the result of a very low swing, and an absence of long-distance control and slide on slide tool mounting.
Inasmuch as the dimensions of work affect the design of machine that is, a planer must be used for long work, while the shaper is best for short work, from here on we will limit our discussion to the work under 12 inches and 14 inches in diameter, for which the two machines described in this book are built.
We will take it for granted that the description of the flat turret which is mounted on our tool slide or carriage, with all its schemes of accurate gibbing and accurate presentation of six different tools at the will of the operator, is all clearly set forth and accepted as the best known means for this purpose, and we will pass at once to the consideration of the mounting of the head stock, which carries the work-holding spindle. Of course, any effort to control the tool slide would be futile if we were to mount the work in a light spindle or in a long overhanging chuck. For this reason it has been necessary to depart from the usual proportions of lathes.
For instance, in these machines the swing is only three and three and one-half times the diameter of the spindle.
The mere statement of these proportions convinces the average man that here is something unusual, and that it is quite likely that a machine having corresponding proportions will be found capable of taking its heaviest cut at its maximum swing, and that the work will be most rigidly held by such a spindle. The details of the construction are clearly set forth in the latter part of this book; and it is only the intention here to consider the principles involved in the adoption of this scheme of work and tool control.
In order to get away from the cob-house scheme of design of slide on slide for the tool carriage, in this machine we mount the head stock on guideways running across the machine. In this scheme, of course, the head is gibbed directly to the bed, and since there are no additional slides to consider, it is possible, the same as in the case of the carriage, to adopt an ideal system of gibbing and the stiffest possible design of frame, so that here we have a slide of any desirable shape gibbed directly to the bed.