Fig. 2030.
Considering the end of the drill, therefore, as a cutting wedge, and considering the cutting edge at the two points c and e, in [Fig. 2030], the end face being at the same angle, we see that the point c has the angle a and point e the angle b; at the drill point there will be still less cutting angle, and it has, therefore, the least cutting capacity. To remedy this the attachment shown in the [figure] is employed, consisting of a frame or head carrying a thin emery wheel, and capable of adjustment to any angle to suit the degree of spiral of the drill flute.
Fig. 2031.
By means of this emery wheel a groove is cut in the flute at the point of the drill, as shown in [Fig. 2031], at a and b, thus reducing the length of c, and therefore increasing the cutting capacity and correspondingly facilitating the feed of the drill. It is found, indeed, that by this means the drill will perform 15 per cent. more duty.
It is obvious, however, that as the thickness of drills at the point increases in proportion to the diameter of the drill, this improvement is of greater advantage with large than with small drills. The reason for augmenting the thickness at the centre with the drill diameter is that the pressure of the cut acts to unwind the spiral of the drill, and if the drill were sufficiently weak at its axis this unwinding would occur, sensibly enlarging the diameter of hole drilled, more especially when the drill became partly dulled and the resistance of the cut increased. By means of the small grooves a and b, however, the point is thinned while the strength of the drill is left unimpaired.
[Fig. 2032] represents Brown & Sharpe’s surfacing grinder, designed to produce true and smooth surfaces by grinding instead of by filing. In truing surfaces with a file a great part of the operator’s time is occupied in testing the work for parallelism, and applying it to the surface plate to test its flatness or truth, whereas in a machine of this kind both the parallelism and the truth of the work are effected by the accurate guiding of the machine table in its guideways. Furthermore, a high order of skill is essential to the production of work by filing that shall equal for parallelism and truth work that is much more easily operated upon in the machine. The machine is provided with two feed motions, the first of which is in a line parallel with the axis of the emery wheel driving spindle, and is communicated (by means of the small hand wheel on the right) to the lower table, which moves in V-guides provided upon the base plate of the machine. Upon this lower, and what may be termed cross-feed table slides, in suitable guideways, the work-holding or upper table, which is operated (by the large hand wheel) to traverse the work back and forth beneath the grinding wheel. Both these feed motions are operated by hand, automatic feed motions being unnecessary for work of the size intended to be operated upon in this machine. The grinding wheel spindle is carried in a bearing carried in a vertical slide, and is fed to its depth of cut by means of the vertical feed screw and hand wheel shown. The spindle passes through the bearing and carries a pulley at the back of the machine, which pulley is driven by a belt passing over idler pulleys at the back of the machine, by means of which the tension of the driving belt may be regulated.