Page:Practical Treatise on Milling and Milling Machines.djvu/32

26 Another point that influences largely the rigidity of the table is the size of the flat bearing surfaces in the saddle and on the knee. It is essential that the table bearing in the saddle be wide and sufficiently long to prevent too great an overhang when the table is at the ends of its traverse, and the top of the knee be of ample width to easily support the weight placed upon the table.

Other features which conduce to rigidity are: a large overhanging arm with a support the the outer end of the cutter arbor, and an intermediate bearing on the larger machines, also arm braces that firmly tie the overhanging arm and knee together.

Speeds and Feeds. It is rare that the conditions surrounding any two jobs on a milling machine are the same. Sometimes the work is of the heavies class to which the machine is adapted, requiring gangs of cutters operating at a comparitively fast speed and coarse feed; again it is of a lighter type, requiring only one cutter operating at a fast speed and fine feed. The shape of the piece sometimes demands that the cutter be fed through faster or slower than would ordinarily be done in milling a plain surface. Different materials cannot be milled at the same speeds and feeds. Cutters of large diameter are employed for some jobs, and to get the proper peripheral speed, they must be rotated at a slower rate than those of smaller diameter. A finishing cut with the same cutter is usually taken at a faster speed, and correspondingly lower rate of feed per revolution of spindle than the roughing cut, in order to obtain a smoother finish. All these, and many other conditions, make it necessary that a machine have a wide range of spindle speeds and table feeds. Furthermore, there must be many intermediate speeds and feeds between the highest and lowest in the ranges. In many cases it is also advantageous to have the speeds and feeds independent of one another, so that the spindle speed may be changed without disturbing the rate of table travel. This is possible in the constant speed driven machine,