The drill being chucked, the emery wheel is brought up to it by means of the hand wheel e, which moves the frame c laterally, the grinding being done by the side face of the emery wheel. On the same shaft as e is a lever which may be used in connection with the stop or pin (against which it is shown lying) to enable an adjustment of the depth of cut taken by the wheel separately when grinding each lip, and yet to permit both cutting edges of the drill to be gauged to the same length.
Suppose, for example, that the point of a drill has been broken so that it requires several cuts or traverses of the emery wheel to bring it up to a point again; then when this has been done on one cutting edge the lever may be set to the stop, so that when the grinding of the second cutting edge has proceeded until the lever meets the stop both edges will be known to be ground of the same length, and will, therefore, perform equal cutting duty when at work.
The depth of cut being adjusted, the lever d is operated to pass the side face of the emery wheel back and forth along the cutting edge of the drill, this lever rocking the frame c on which the emery wheel is mounted back and forth in a line parallel to the cutting edge of the drill. Different angles of one cutting edge of the drill to the other are obtained by swivelling the frame carrying the shaft of lever f. The emery wheel is cased in except at a small opening where it operates upon the drill, and may, therefore, be liberally supplied with water without the latter splashing over. Water is continuously supplied to the emery wheel by an endless belt pump, which also delivers water on the end of the drill, enabling heavy grinding cuts to be taken without danger of softening the drill at the cutting edge, which is otherwise apt to occur. The following is the method of operating the machine: Open the jaws of the chuck by means of the hand wheel a, insert the drill from the back of the chuck towards the face of the stone, letting the end of the drill rest on the lower jaw, with the cutting edge just touching the end stop; close the jaws temporarily, while the back centre b is run up and clamped; then release the jaws, hold the drill back against the back centre b with the left hand, at the same time rotating hard against the two side stops on the jaws; then tightly closing the jaws, clamp the drill by means of the hand wheel a, using the right hand for this purpose. Throw ball-handle f part way back, and by means of hand wheel e feed up the stone until it just touches the drill. Bring ball-handle f forward and give additional feed; pass the stone over the face of the drill, back and forth, by lever d, moving ball-handle f back a little between each two cuts. This slices off the stock to be removed; then when entirely over the face of the lip being ground, hold lever d stationary, and rotate the drill against the stone by means of ball-handle f. By this means a heavy slicing cut can be taken and a final smooth finish obtained without any risk of drawing the temper of the drill.
When one lip has been thus formed, slack up the jaws of the chuck, turn the drill half around, pressing its lips as before against the side stops on jaws, and at the same time be sure to hold the drill firmly back against the back centre b (pay no attention to the end stop, which is only used in locating the drill endways in the first setting), tighten chuck, and grind the second lip without any readjustment of the stone. The lips will then be of equal length. During all these manipulations the stop that is arranged in connection with hand wheel e can be slack, and may rest against the pin in the bed made to receive it.
[Fig. 2027] represents a rear view of the machine, at which there is an attachment for thinning the point of the drill, which is advantageous for the following reasons. In [Fig. 2028] we have a side and an end view of a twist drill, and it can be shown that the angular piece of cutting edge c that connects the two edges a and b cannot be given sufficient angle to make it efficient as a cutting edge without giving clearance and angle excessive to the edges a and b.
Fig. 2029.
In [Fig. 2029] we may consider the angle of the cutting edge at the corner h and at the points f and g. First, then, it is obvious that the front face for the point h is represented by the line h h, that for f by line f f, and that for g by g g, and it appears that on account of the spiral of the flute the front face has less angle to the drill axis as the point of the drill is approached.