Section.
Fig. 2026.
In [Fig. 2025] a cylinder r is shown lying in a conical recess, and end views of the cylinder are shown at v and w. Now suppose the line of contact of the roll or cylinder upon the recess represents the cutting edge of the drill, and that we consider the clearance at the outer end, and at that part that in revolving would describe the circle q, and on referring to circle v and the outer circle of the recess, and also to circles w and q, it is seen that there is more clearance for v than there is for w, and that the clearance of the latter would be still less if q were of smaller diameter, and it follows that the clearance is less in proportion as the point of the drill is approached. In determining the amount of clearance, therefore, we are compelled to make it sufficient for the point of the drill, and this under this system of grinding is excessive for the outer diameter of the drill, causing it to dull quickly, it being borne in mind that as the outer corner of the cutting edge of a drill describes the largest circle of any point of the cutting edge it obviously performs the most cutting duty in removing metal, and furthermore revolves at the highest rate of cutting speed, both of which cause it to dull the most rapidly. In [Fig. 2026] we have a cone r lying in the coned recess, an end view of the cone being shown at v and w, and if we again consider the line of contact of the cone on the recess to represent the cutting edge and the circumferential surface of the cone as the end surface of the drill, we observe in the end views v and w that the clearance is equal for the two positions, or by varying the degree of taper of the cone we may regulate the amount of clearance at will. It is found preferable, however, to give more clearance as the point of the drill is approached so as to increase the cutting capacity; hence, in this case, the outer corner of the drill has the least clearance, which greatly increases its endurance for the reasons already mentioned, and which were further pointed out in the remarks upon drilling in the lathe. There remains, however, an additional advantage in this method of grinding which may be pointed out, inasmuch as that the clearance produced by the method shown in [Fig. 2019], while capable of being governed from end to end of the cutting edge, yet increases as the heel of the land is approached, making the central cutting edge (c, [Fig. 2028]) more curved in its length so that it approaches the form of cutting edge of the fiddle drill and this enhances its cutting capability.
Fig. 2027.
Fig. 2028.
Referring again to the general view of the machine in [Fig. 2019], the chuck is supported or carried by the shaft having the ball lever f, which is clearly seen in the rear view, [Fig. 2027], and the rod carrying the sleeve b (which holds the centre for supporting the shank end of the drill) is secured to the back of the chuck, as seen in the same figure. When, therefore, lever f is moved over, the drill is moved through an arc of a circle of which the axis of the shaft of f is the centre, and this it is that gives clearance to the cutting edge of the drill.