It will be seen, therefore, that one of the principal advantages of the milling machine is its wide range of working capacity, and the accuracy with which the table can be placed with relation to the cutter. This accuracy is obtained by means of graduated dials on the feed-screws, which are read directly to .001 inch, and, by estimation, to .00025 inch. For many years the milling machine was the only tool which supplied these micrometer graduations, but they are now applied to nearly every class of machine tool in which accurate adjustment is necessary. A common method of graduation is by the use of a screw with a pitch of 1/5 inch and with 200 graduations on its dial. In some cases, a screw with a pitch of 1/4 inch is used with 250 graduations, but it is always safe to assume that the single graduation on a milling machine means a movement of .001 inch

Fig. 225. Slabbing Miller-Planer Type Courtesy of Ingersoll Milling Machine Company, Rockford, Illinois.

Avoiding Backlash Error

Lost motion or backlash between the screw and its nut, in any of these adjustments, is a cause of frequent error, and should always be considered. Even for a machine in excellent condition, when the motion of the screw is reversed, the screw will turn through an angle giving the equivalent of about .005 inch movement of the part being fed along, but with no actual movement of the part. As an example, if, in moving the table from the column, the operator carries it .003 inch too far, it will not suffice simply to turn the dial back three graduations. The table should be brought back several hundredths of an inch, and again advanced to within .003 inch of its former position. In order to facilitate the quick and accurate reading of these dials, they are arranged so that they can be readily set to zero whenever desired.

Fig. 226. Planer Type Four-Spindle Milling Machine Courtesy of Ingersoll Milling Machine Company, Rockford, Illinois.

Distinction between Plain and Universal Millers. The movements above described for the adjustment of the work are those necessary for what is termed a plain milling machine. In order to have a universal milling machine, Fig. 224, it is necessary that the table be so arranged that it can be swung upon the saddle in the horizontal plane, so that its feeding movement is not at right angles to the axis of the spindle, Universal milling machines usually have a total working angular movement of 90 degrees, 45 degrees on either side of the normal position.

Fig. 227. Planer Equipped with Horizontal Milling Attachment.

While the milling machine developed from the lathe, through the Lincoln miller, to the standard horizontal universal machine, its development for work on which heavy cuts are necessary took an opposite course.