Fig. 1678.
In the Sellers drilling machine, [Fig. 1678], the drill spindle when in single gear is driven by belt direct, producing a uniform and smooth motion that is found of great advantage in drilling the smaller sizes of holes. The back gear is arranged to drive the spindle direct without the power requiring to be transmitted through a shaft, which induces vibration. The drill spindle is provided with variable rates of self-acting feed, but may also be moved rapidly by hand, and is counterbalanced. The work table is capable of revolving upon its axis, and the arm on which it is carried is pivoted in a slide upon a vertical slideway on the front of the main frame, so that the table and the arm may be swung out of the way for work that can be more advantageously rested on the base plate of the machine. A central hole is bored in the table, being true to the drill spindle when the arm is in its mid position, and clamps are provided to secure the circular table against rotation when it is set to place, and also to secure the swinging bracket to any required position. This form of table, like the compound table, has the advantage of permitting all parts of the table being brought in turn under the drill, but the motion is not in right lines. Holes are provided in the circular table to admit holding-down bolts.
The rates of feed are proportioned to the kind of drilling to be done. When the back gear is not in use and small drills are to be driven, the range of feeds is through a finer series than when the back gear is being used, and large drills or boring bars are to be driven.
[Fig. 1679] represents a drilling machine of English design. The cone pulley a is provided with back gear b placed beneath it, the live spindle driving the drill spindle through the bevel gears c, one of which is fast upon a sleeve d through which the drill spindle e passes. The feed motions are obtained as follows:— i is the feed cone driving cone j, which drives a worm and worm-wheel at k. In one piece with the worm-wheel is a ratchet wheel l, and at m is a handle with a pawl that may be engaged with or disengaged from ratchet-wheel l. When it is engaged the handle, which is fast upon the vertical feed spindle n, is revolved by the worm-wheel and the automatic feed is put in operation; but when the pawl is disengaged the worm and worm-wheel revolve in the bearing while the spindle n remains at rest, unless it be operated by the handle m, which obviously revolves the spindle n more quickly than the worm and gives to a corresponding extent a quick motion to the drill spindle. Spindle n is provided with the gear-wheel o, which drives gear p, which is threaded upon the feed screw f and has journal bearing at q. The sleeve d has journal bearing at g and at h. At r is a hand wheel upon a horizontal shaft at whose other end is a bevel gear engaging with a bevel gear on the vertical screw for the knee t which fits to the vertical slides v. The work table w is fitted to a horizontal slide upon the arm x, which is pivoted to the knee t at y, the handle for operating the screw of the table being at z.
Fig. 1680.