Page:Practical Treatise on Milling and Milling Machines.djvu/28

 multiplied. If the work is of the coarser grade, or mere roughing cuts are being taken, a few thousandths of an inch over or under size do not matter; but when finishing a piece that must come within close limits of a pre-determined size, a very small error is often sufficient to seriously impair its quality.

All of the important alignments in milling machines are obtained by scraping, a process consisting of going over the different bearing surfaces by hand with a chisel-like tool, and removing the highest spots until a maximum number of bearing points is secured. Flat bearings are scraped to conform to master surface plates and straight edges, and the boxes of important cylindrical bearings are scraped to fit the revolving piece, which is ground. This work necessarily calls for much skill on the part of the workman, and the care with which scraping is performed largely influences the accuracy of the resultant bearings.

Principal Alignments of Milling Machines. Broadly speaking, the principal alignments of all milling machines are those of the spindle and table. They are, of course, affected by various minor alignments throughout the machine, but it is not essential to take up each of these in detail. The alignments of the table on horizontal spindle column and knee machines should be such that its upward and downward movements will be perpendicular to the spindle axis. Its longitudinal and transverse movements should be in horizontal planes, the longitudinal being parallel to the face of the column on plain machines, and on universal machines when the table is set at zero; and the transverse at right angles to the column.

On universal machines, the table should also swivel in a horizontal plane.

These alignments of the table and spindle of column and knee machines are typical, and it is easy to understand from then what the alignments of other types of milling machines should be.

While we have emphasized the importance of good workmanship in scraping bearing surfaces, in order to obtain accurate alignmnets, it must be understood that certain elements in design are largely responsible as to whether the alignments remain accurate or not. A bearing surface may be scraped ever so carefully, yet the lack of sufficient weight in the casting, or of ample proportions of the bearing surface itself, will quickly result in the alignments becoming inaccurate. Thus it is apparent that if alignments are to be permanent, the proportion of the different parts, including the bearing surfaces themselves,