Locating Holes to be Bored
from Center-punch Marks
The problem of accurately locating holes to be bored on the milling machine has received much attention, and the method generally used when accuracy has been required is the button method, which was previously described. So much time is required for doing the work by this method, however, that numerous efforts have been made to obtain equally good results in other ways.
Fig. 33. Diagram Illustrating Rapid but Accurate Method of
Locating Holes to be bored on Milling Machine
The increasing demand for rapidity combined with accuracy and a minimum liability of error, led to the development of the system described in the following: A center-punch mark takes the place of the button, from which to indicate the work into the proper position for boring. The fundamental principle involved is to lay out, accurately, two lines at right angles to each other, and correctly center-punch the point where they intersect. With proper care, lines may be drawn with a vernier height gage at right angles, with extreme accuracy, the chief difficulty being to accurately center the lines where they cross. For semi-accurate work this may be done with a common center-punch but where extreme accuracy is required this method is not applicable, as the average man is incapable of marking the point of intersection accurately.
The diagram, [Fig. 33], illustrates, in a simple way, the procedure adopted in laying out work by this system. The base E is in contact with a surface plate while the line BB is drawn with a height gage; then with side F on the plate the line AA is drawn. It will be seen that these lines will be at right angles to each other, if the bases E and F are square. Work done by this method must have two working surfaces or base lines, and these must be at right angles to each other. There is no difficulty in drawing the locating lines AA and BB correctly, either with a vernier height gage or with a special micrometer gage reading to 0.0001 inch, the only difficult element being to accurately center-punch the lines where they intersect as at D. It is assumed that two holes are to be bored, so that the intersection at C would also be center-punched.
The scriber point of the height gage should be ground so that it will make a V-shaped line, as shown by the enlarged sketch G, rather than one which would resemble a saw-tooth, as at H, if a cross-section of it were examined with a microscope. This is important because when the lines are V-shaped, an accurate point of intersection is obtained.
Fig. 34. Center Locating Punch
