Twist Drills.
Twist drills are superseding flat drills as stronger and better in every way. A twist drill is made with a slight taper toward the shank end. Its cross-section is not quite round, the diameter being reduced from a short distance behind the cutting edge, so as to diminish friction and give the sides of the drill as much clearance as possible. The advanced edges of the flutes are all full circle, so as to maintain the diameter of the drill and keep the tool steady. The advantage of the twist drill is that its cuttings find free egress, while it always runs true, without reforging or retempering. The cutting edges are usually ground to an angle of sixty degrees to the center line of the drill; for brass work the angle should be fifty degrees.
Lathe and Planer Tools.
The manner in which a lathe tool cuts metal is shown in an [outline] which represents a tool feeding a cut along a piece of wrought iron. The removed metal, in its diameter and openness, tells the expert operator both the quality of his cutter and how it is being affected by wear. The principal consideration, says Mr. Joshua Rose, in determining the proper shape of a cutting tool, for use in a lathe or a planer, is where it shall have the rake, or inclination, to make it keen enough to cut well, and yet be as strong as possible; this is governed, in a large degree, by the nature of the work.
How a tool cuts metal.
Beginning a second cut.
Dacotah fire-drill.
Machine Tools: Lathes.
In giving form to wood and metal cheaply and rapidly, machine-tools have within recent years risen to great importance. Of these the lathe is one of the chief. It seems to be descended from the bow drill, the tool which was whirled by a cord wrapped round it, or it may be, that under another sky, the lathe was derived from the potter’s wheel whose axle was changed from a vertical to a horizontal plane. For centuries all lathes had their cutting tools simply laid on a bar, or rest, just as in the hand cutting lathe of to-day. While this afforded opportunity to skill it did not lend itself to large or uniform production. Henry Maudslay, about a century ago, immensely broadened the machine in scope by devising the slide rest which firmly grasps the cutting tool, and automatically moves it toward or away from the axis of the work, as well as along the work in any desired line. This device is equally applicable whether in turning a pencil case, the granite columns for a cathedral, or the propeller-shaft of an ocean steamer.