This invention relates to devices for enabling an operator to center a work piece with respect to the drive member of a machine tool, such as a vertical mill, a horizontal mill or a horizontal boring mill. It commonly takes a great deal of time and effort to perform the centering operation, and devices which facilitate that operation are therefore of great utility.
The usual manner of centering a work piece on the work platform of a machine such as a vertical mill is to employ a holding bar assembly which is affixed to the machine spindle and to which is attached an indicator. The structure and function of such holding bars and indicators are well-known, as evidenced by the radially-adjustable indicator disclosed by Bradley, U.S. Pat. No. 3,129,918. Such combinations can, depending on the characteristics of the work area of the machine, be utilized to center a work piece in either of two ways.
One manner of centering the work piece while using such a holding bar and indicator arrangement is to first secure the work piece to the work platform and then adjust the indicator so that it engages one of the outer edges (first edge) of the work piece. The radial position of the indicator on the holding bar having been noted, the indicator is then moved radially away from the machine spindle so as to clear the first edge of the work piece, and the spindle is rotated approximately one-half turn so that the indicator is now in proximity with the opposite edge (second edge) of the work piece. The indicator is then returned to its previous position on the holding bar, and by reference to the distance between the indicator and the second edge of the work piece, the work piece is moved relative to the spindle so that its position more closely approximates the desired centered position with respect to the axis of the work piece which lies between the first and second edges.
The indicator is then moved along the holding bar so that it engages the second edge of the work piece and its radial position is noted. Next, the indicator is moved radially outward to clear the second edge of the work piece, and the machine spindle is again rotated approximately one-half turn. The above procedure is then alternately repeated on the first and second edges of the work piece until no adjustment of the position of the work piece is required to permit the indicator to engage each of the first and second edges of the work piece while the indicator is at one particular radial position. When these steps are completed, the axis of the work piece which lies between the first and second edges will be centered with respect to the machine spindle.
The entire procedure is then duplicated in order to center the axis of the work piece which lies between the third and fourth edges with respect to the machine spindle. When both axes of the work piece are simultaneously centered with respect to the machine spindle, the work piece itself will be centered with respect to the machine spindle.
The second way of centering the work piece is in principle the same as the first, the difference being that the work platform of the machine with the work piece clamped thereto, rather than the machine spindle, is rotated.
Both centering procedures consume a great deal of the operator's time, inasmuch as they require numerous approximations of the desired position of the work piece due to the trial-and-error nature of the procedures. Moreover, because the operator centers the work piece by visually approximating a one-half rotation of either the work center or the machine spindle, these procedures have inherent inaccuracies and cannot be relied upon to locate the exact center of the work piece.