Pins are those small straight pieces of machines which hold or connect portions of machines together. Pins are distinct from bolts through having no heads, and are of any size to suit the work for which they are made; they are either cylindrical or conical. The greater number are conical or taper; and these are now specially referred to. All taper pins are of steel, unless they are several inches thick, some of these larger ones being of iron.

The turning of taper pins is usually effected by means of temporary square holders or handles. Each pin is provided with one of these at the thickest end, in order that it may be held and rotated in the lathe while turning, and that it may be held while being fitted or tried into the pin hole. A pin having a handle is shown by Fig. 927. Fig. 928 represents a pin in use, being tightly fixed through a boss and spindle.

Small pins of only about half an inch thick, and smaller sizes, have their handles consisting of extra lengths of the wire or rods of which the pins are made, the handle of each being as large in diameter as that of the pin. Large pins require their handles to be smaller in diameter than the pins, both to avoid the trouble of turning any portion of the handles, and to economise steel.

A taper pin which is used for permanently holding a boss on a rod or axle, must be tightly and properly fitted to the pin-hole, and be finally driven into its place with hammering, that it may not become loose and fall out in consequence of the shaking of the boss and spindle while in motion. To ensure a good fit, the hole must be nicely broached, and smoothed ; and during the fitting the pin is gently hammered into its hole a few times with a tin hammer, that the pin may be thus marked to show which part was in contact, and consequently, which part needs reducing to obtain a proper bearing. It is also necessary to make the pin-hole only very slightly taper, the angle of its sides being only about two degrees.

Those pins which are very taper, are sometimes used for quickly connecting and disconnecting a rod, bar, plate, or joint. Most of those belonging to this class are furnished with ornamental knobs or handles; therefore each pin is forged with its handle in the desired form, and no temporary handle is required.