A broach-drill is analagous to the conical pin-drill indicated by Fig. 843; but is without a pivot-end, therefore not steady in its action, unless it is what is termed, nearly parallel. The tool is shown by Fig. 849, and is rotated in drilling-machines in the same manner as other drills. It is of no use for commencing holes, through having no point properly shaped for the purpose, but is available for all holes that have been previously bored with other drills. It is of very simple form, and easily made, consisting merely of a thin drill having a very long end for cutting along its sides, which end is easily shaped to the proper angle with grinding. The comparative great length of this portion renders it suitable for making conical holes of various diameters and depths.

The principal uses of broach-drills are, roughly enlarging mouths of holes that are intended to contain rivets, and tapering holes along their entire lengths to avoid broaching with other tools. For making a deep conical mouth to a hole intended to contain a rivet, the tool is very efficient, the rotation of it in the drilling-machine rendering the operation very easy, and the thin cutting end removing the shavings with but little friction. For cutting brass and gun-metal the tool is used dry, but for iron and steel oil or water is applied, with a much slower rotation. For broaching holes along their entire lengths, the tool is more efficient in short holes than in long ones, especially if the metal being cut is iron or steel. It is also requisite to apply only a gentle pressure during this broaching, to prevent the comparative thin end breaking off with the friction resulting from a great length of cutting edge being in contact with the hard metal.