Page:Practical Treatise on Milling and Milling Machines.djvu/31

 strength and good wearing qualities, and to transmit the requisite power to the spindle. Cylindrical bearings should be firmly supported, and the boxes should be as long as is consistent with a high degree of efficiency. Those of the spindle are most stable when mounted directly in the thick walls of the frame.

A heavy, well-braced construction is necessary in the knee in order to overcome all tendency to vibrate or sag under the load of the saddle and table during operation. It is also well, to have the back of the knee that fits the vertical column extended above teh top as this gives a larger bearing surface to resist sagging tendences and vibrations under heavy loads.



It has been found from experimenting that vibrations arising during operation are usually manifested first in the table, and are transmitted from there to other parts. One reason for this is the several joints between the table and column. It is impossible to eliminate all lost motion between the bearing surfaces, and still have the parts free to perform their different functions. But weight has much to do with the stability of the table, and in many cases vibrations have been practically overcome by simply adding more weight to this part. It is important, therefore, that both the table and saddle be of sufficiently heavy construction. Transverse braces, however, placed at frequent intervales on the under side of the table often produce the required rigidity without adding unduly to the weight. Efficient clamps on the flat bearings of the knee, saddle, and table also provide means of rigidly fastening any one or two of the table movements that may not be in use, thus eliminating vibrations.