Borer-Bits.
—The form and the dimensions of the cutting tools, variously described as “drills,” “borers,” and “bits,” used with machine rock-perforators are matters of great practical importance. The dimensions are determined mainly by two conditions, namely, the necessity for sufficient strength in the shank of the tool, and the necessity for sufficient space between the shank and the sides of the hole to allow the débris to escape. Experience has shown that the latter condition is best fulfilled when the distance between the sides of the hole and the shank of the tool is from 3⁄16 inch to 1⁄4 inch, regard being had to the former condition.
The form of the cutting edge is determined by several conditions, some of which have been already discussed in relation to hand drills. The form first adopted was naturally that possessed by the hand drill, namely, the chisel edge. To increase the useful effect of the blow, the cutting edge was subsequently doubled, the bit being formed of two chisel edges crossing each other at right angles. This bit, which from its form was called the “cross” bit, was found to penetrate the rock more rapidly than the straight or chisel bit. The gain in speed was very marked at the commencement of the hole; but it diminished gradually as the hole progressed in depth, owing to the difficulty with which the débris escaped. To remedy this defect, the cutting edges were next made to cross each other obliquely, so as to form the letter X. In this way, the two chisel edges were retained, while the breadth of the bit was considerably reduced. This form, described as the X bit, cleared the hole much more effectively than the cross, but not in a manner that was altogether satisfactory. Another modification of the form was, therefore, made, and this time that of the Z was adopted, the upper and the lower portions of which were arcs of circles struck from the centre of the bit in the direction contrary to that of the rotation.
This form of tool, which is known as the Z bit, readily cleared itself of the débris. But besides this advantage, it was found to possess others of an important character. With the chisel-edge forms, the corners of the bit were rapidly worn off by friction against the sides of the hole. With the Z form, this wearing no longer occurred, by reason of the large surface exposed to friction. Another advantage of the Z form of bit lies in its tendency to bore the hole truly circular. Generally then, it may be stated that this form satisfies most fully the determining conditions. The form of bit, however, that is most suitable in a given case will, in some degree, be determined by particular circumstances. Of these, the nature and the character of the rock will operate most strongly to influence the choice. Thus the cross bit will generally be found the most suitable in fissured rock, while the single chisel edge may be used with advantage in rock of a very solid and hard character. Indeed, on the judicious selection of the most suitable form of cutting edge, the success of machine boring largely depends. The chisel bit, the cross bit, the X bit, and the Z bit, are shown in [Figs. 24] to [27].
Fig. 24.
Fig. 25.
Fig. 26.
