The employment of these sockets preserves the truth of the bore of the drilling machine spindle by greatly diminishing the necessity to insert and remove the shank from the drill spindle, because each socket carrying several sizes of drills (as given with reference to lathe work) the sockets require less frequent changing.
Fig. 1729.
Drill shanks are sometimes made parallel, with a flat place as at a in [Fig. 1729], to receive the pressure of the set-screw by which it is driven. To enable the shank to run true it must be a close fit to the socket and should be about five diameters long. The objection to this form is that the pressure of the set-screw tends to force the drill out of true, as does also the wear of the socket bore.
These objections will obviously be diminished in proportion as the drill shank is made a tight fit to the socket, and to effect this and still enable the drill to be easily inserted and removed from the socket, the drill shank may be first made a tight fit to the socket bore, and then eased away on the half circumference on the side of the flat place, leaving it to fit on the other half circumference which is shown below the dotted line b in the end view in the figure. The set-screw is also objectionable, since it requires the use of a wrench, and is in the way and liable to catch the operator’s clothing.
There is, however, one advantage in employing a set-screw for twist drills, inasmuch as that, on account of the front rake on a twist drill, there is a strong tendency for the drill, as soon as the point emerges through the work, to run forward into the work and by ripping in become locked. This is very apt to be the case if there is any end play in the driving spindle, because the pressure of the cut forces the spindle back from the cut; but so soon as the drill point emerges and the pressure is reduced, the weight of the spindle acting in concert with the front rake on the drill causes the spindle to drop, taking up the lost motion in the opposite direction. In addition to this the work will from the same cause lift and run up the drill, often causing an increase in the duty sufficient to break the drill.
If the spindle has no lost motion and the work is bolted or fastened to the table or in a chuck, the drill if it has a taper shank only will sometimes run forward and slip loose in the driving socket. This, however, may be obviated by feeding the drill very slowly after its point emerges through the work.
