Adjusting the rod or bar for drilling consists in placing the planed boss-end upon a parallel ring or parallel block situate on a drilling-table, as seen in Figs. 886, 887, or 897, such packing being necessary to keep the boss high enough above the table to prevent contact with the drill-point. While the boss thus remains it is adjusted to place it exactly beneath the drill for one of the holes to be drilled; or if only one hole is to be drilled, the dot showing the centre is placed accordingly. Adjusting an article for drilling should always be performed by rotating the drill and observing the dot which shows the centre of the hole to be drilled. If the point of a drill which is fastened in the machine-spindle, and the axis of the spindle's rotation were exactly in the same straight line, the centre of the hole to be made could be put directly under the drill-point while it is at rest, and the adjustment would by the same act be completed; but because the drill and the spindle's axis do not coincide with each other, it is necessary to put the machine into action when the drill is tightly fastened, and then gradually shift the object beneath until near enough to easily observe the relative situations of the drill and dot. During this observation the drill-point will be seen to move in the path of a small circle's circumference, which may be only an eighth or a quarter of an inch in diameter, the size depending on the care with which the drill was made and straightened. The diameter is seldom less than an eighth of an inch; but, whether larger or smaller, the centre of the circle traversed by the point is the exact place in which the centre of the hole to be made must be put. The article is known to be properly situated when the drill-point travels in the path of a circumference whose centre is exactly over

2p2 the centre-dot of the hole to be drilled; consequently, the article must be gradually shifted (o the place either with a tin hammer, with the screws of the poppets that hold the article, or with the traverse-screws, if the table is provided with them. To enable the operator to see plainly when the work is adjusted, the drill-point is allowed to be as near as possible to the piece without touching it.

Circular gauge-lines marked on a drilling-table are also useful for adjusting. These lines resemble gauge-lines on a slotting-table, being concentric with each other, and with the centre of the table, as indicated by Fig. 899. They are only serviceable for adjustment when the centre of the drilling-table is exactly beneath the axis of the drill's motion; consequently, a table which slides to and fro on vee-slides, or dove-tail slides, must be moved until the gauge-lines are concentric with the drill, as required. This position is known by marks or dots, and when properly placed, any one of the gauge-circles may be selected and considered a standard ring in the centre of which the hole to be drilled shall be located when adjusted. Some drilling-machines are provided with tables which are fixed, therefore the gauge-circles of these are at all times available for adjustment. Such a table may also have a circular hole truly bored at the middle, in which packing-rings having holes of different diameters may be placed. Upon a ring of this class, a boss-end to be drilled can be fixed, if small; but if too large for a ring the boss-face can be put into contact with straight parallel blocks, which are put at any desired distance from the centre, to suit the diameter of the piece to be drilled. In some cases the parallel packing beneath the piece can be entirely dispensed with, the hole in the middle of the table being deep enough and wide enough to provide ample room for the drill-point to disengage from the metal at the conclusion of drilling.

Although an end of a rod or bar can be adjusted with a tolerable approach to precision by means of these circular lines, it is always proper to adjust the object by observing the rotation of the drill-point, in case any irregularity or wear of surfaces prevents the gauge circles being concentric with the drill-spindle, as required. As soon as the piece is correctly placed beneath the drill by some means, it is also in a suitable position for causing the hole to be drilled in the desired right-angular position with the length of the rod and with the two planed boss-faces, because these are put square to the drill either by means of the parallel ring, parallel blocks, or the surface of the drilling-table. The correct placing of the piece being now effected, it is next finally tightly fastened, and the drill-point is put into the centre-dot for commencing the drilling. If it happens that the drill has been carefully straightened, the point will properly enter the dot without any guiding; but it usually requires guiding into the dot by being pushed sideways with a lever at the moment the point is caused to touch the metal. The distance which the point is thus moved sideways, is exactly half the diameter of the small circle in which the point rotates while free from the metal, as when it was referred to for adjusting the piece of work; therefore, if the circular path is a quarter of an inch in diameter, the drill-point is moved an eighth of inch, by which movement it is put exactly into line with the hole to be made, and also with the drill-spindle's axis. In this condition the drill is maintained by its continual contact with the metal during drilling, because the rod or bar is tightly bolted to the table, and because the drill is flexible enough to allow the necessary straightening while it rotates. During the drilling, the drill-point continues in very nearly the same relative position into which it was first pushed, unless it gets out at the beginning of the drilling; to remedy which a gouge-chisel is used to again put the point to its proper place. The use of a chisel for this purpose is mentioned in page 144.

The drilling of a boss-portion is executed with care to allow sufficient metal for a subsequent slotting of the hole to the finished dimensions; therefore, the drill is not allowed to obliterate any of the gauge-lines on the face, but is caused to cut as near as possible to them, that but little slotting may be afterwards necessary. If a hole several inches in width is to be made, a comparatively small drill should be used, with which a number of small holes may be made near the gauge-lines in order to remove most of the superfluous piece in one lump.