A guide-drill or pin-drill is one which is furnished with a cylindrical pivot-end, such an end being solid with the cutting portion of the tool. A drill of this sort is denoted by Fig. 842, and its pivot-end constitutes a guide that guides the cutting part while in use. For this purpose the pivot is made to rotate in a hole the diameter of which is equal to that of the pivot. The tool is only useful for enlarging holes that were previously made with other drills, and the pivot-end must in all cases properly fit the guiding hole, both to ensure an easy advancement of the pivot along the hole, and to ensure a steady guidance of the cutting edges, in order that the hole may be smoothly formed to its desired shape and dimensions. The pivot has no capability for cutting; consequently, if any portion of the guiding hole is too small, it will not be enlarged by the pivot arriving at that part, but it will tightly grip the pivot and break it off. When it happens that the hole is too large for the pivot, the operation of the drill makes the hole rough and also larger in diameter than the extreme diameter of the drill.

It is sometimes necessary to employ these tools in holes which are larger in diameter than that of the pivots; for such work the pivots are garnished. Garnishing is often necessary, because it is frequently requisite to use the same pin-drill for enlarging the mouths of holes which differ from each other in diameter, and which cannot be all bored to suit the pivot by reason of the different uses to which the holes will be applied.

Garnishing the pivot is done by fitting a small tube of gun-metal or steel to it, so that the outer surface of the tube just easily fits the guide-hole in which it is to rotate, and the inner surface fits the pivot tight enough to prevent shifting during use. When such a tube is to be used for a large quantity of work, it should be made of steel, and very smoothly finished, also provided with a small key to securely fix it For comparative thin garnishers that may be only about a sixteenth thick, a hole should be drilled through both tube and pivot, and a small steel pin driven in. Those that are an eighth or a quarter thick, may have flat surfaces or shallow key-grooves, the lengths of which are parallel with the lengths of the pivots, the keys being driven in at the pivots' points.

Fig. 843 denotes a pin-drill with cutting-edges inclined at a desired angle with each other. A tool of this shape will enlarge the entrances of holes, in a manner resembling that of the drill shown by Fig. 842, but will form conical holes instead of cylindrical ones. By making the taper cutting end of the tool of a great length, it can be used for coning a number of holes which differ considerably from each other in diameter.