In all planer work it is an essential in the production of true and smooth surfaces that the tool be held as close in to the tool clamp or tool box as possible, and this forms one of the main advantages of tool holders.
Chapter XVIII.—DRILLING MACHINES.
Power Drilling Machines.—The drilling machine consists essentially of a rotating spindle to drive the drill, a work-holding table, and means of feeding the drill to its cut. The spindle speed and the force with which it is driven are varied to suit the work. The feeding is sometimes given to the spindle, and at others to the work table. In either case, however, the feeding mechanism should be capable of varying the rate of feed and of permitting a quick withdrawal of the drill. The spindle should be supported as near to its drill-holding end as possible. When the table feeds to the work the spindles may be held rigidly, because of their not requiring to pass so far out or down from the bearing supporting them; but when the spindle feeds, it must either pass through its bearings, or the bearing, or one of them, must either be capable of travel with the spindle or adjustable with relation to the machine framing.
In using small drills in a machine it is of the first importance that the amount of pressure necessary to feed the drill be plainly perceptible at the hand lever or other device for feeding the drill or the work, as the case may be, as any undue pressure causes the drills to break. To attain sensitiveness in this respect the parts must be light and easy both to move and to operate.
[Fig. 1671] represents the American Tool Company’s delicate drilling machine for holes of 1⁄4 inch and less in diameter. It consists of a head fixed upon a cylindrical column and affording journal bearing to the drill-driving spindle, which is driven by belt. The table on which the work is placed is carried by a knee that may be fixed at any required height upon the same round column. The knee and table may be swung out of the way, the column serving as a pivot. The table has journal bearing in the knee, and is fed upwards by the small lever shown.
[Fig. 1672] represents Elliott’s drilling machine for drills from 1⁄32 inch to 3⁄4 inch in diameter. The work table may be revolved in the arm that carries it, and this arm may be swung round the column or post. It is operated upwards for the feed by the hand lever shown. The conical chuck shown lying on the work table fits into the hole that is central in the table, and is used to receive the end of cylindrical work and hold it true while the upper end is operated upon.
The construction of the live spindle and its cone are shown in [Fig. 1673]. The drill chuck q is attached to and driven by a one-inch steel spindle 19 inches long, which is accurately fitted through the sleeve bearings, within which it is free to move up and down, but is made to revolve with the cone by means of the connection o, one end of which slides upon the rods l. The drill is held up by means of the spiral spring m acting from the bottom of cone to the collar o. The weight of cone and spindle is carried upon a raw-hide washer, beneath which is the cupped brass p which retains the oil. The thrust of the feed lever g is also taken by a raw-hide washer r.