Figure 1-51.—Punch marking mating parts

1. When you hit the punch, you do not want it to slip sideways over your work. 2. You do not want the hammer to slip off the punch and strike your fingers. You can eliminate both of these troubles by holding the punch at right angles to the work and striking the punch squarely with your hammer. The center punch, as the name implies, is used for marking the center of a hole to be drilled. If you try to drill a hole without first punching the center, the drill will "wander" or "walk away" from the desired center. Another use of the center punch is to make corresponding marks on two pieces of an assembly to permit reassembling in the original positions. Before taking a mechanism apart, make a pair of center punchmarks in one or more places to help in reassembly. To do this, select places, staggered as shown in figure 1-51, where matching pieces are joined. First, clean the places selected. Then, scribe a line across the joint, and center punch the line on both sides of the joint, with single and double marks as shown to eliminate possible errors. In reassembly, refer first to the sets of punchmarks to determine the approximate position of the parts. Then line up the scribed lines to determine the exact position. To make the intersection of two layout lines, bring the point of the prick punch to the exact point of intersection and tap the punch lightly with a hammer. If inspection shows that the exact intersection and the punchmark do not coincide, as in view A of figure 1-52, slant the punch as shown in view B and strike again with the hammer, thus enlarging the punchmark and centering it exactly. When the intersection has been correctly punched, finish off with a light blow on the punch held in an upright position. View C shows the corrected punchmark. DRIFT punches, sometimes called "starting punches," have a long taper from the tip to the body. They are made that way to withstand the shock of heavy blows. They may be used for knocking out rivets after the heads have been chiseled off or for freeing pins that are "frozen" in their holes. After a pin has been loosened or partially driven out, the drift punch may be too large to finish the job. The follow-up tool to use is the PIN PUNCH. It is designed to follow through the hole without jamming. Always use the largest drift or pin punch that will fit the hole. These punches usually come in sets of three to five assorted sizes. Both of these punches will have flat ends, never edged or rounded. To remove a bolt or pin that is extremely tight, start with a drift punch that has an end diameter that is slightly smaller than the diameter of the object you are removing. As soon as the bolt or pin loosens, finish driving it out with a pin punch. Never use a pin punch for starting a pin, because it has a slim shank and a hard blow may cause it to bend or break. For assembling units of a machine, an ALIGN- MENT (aligning) punch is invaluable. It is usually about 1-foot long and has a long gradual taper. Its purpose is to line up holes in mating parts. Hollow metal-cutting punches are made from hardened tool steel. They are made in various sizes and are used to cut holes in light gauge sheet metal. Other punches have been designed for special uses. One of these is the soft-faced drift. It is made of brass or fiber and is used for such jobs as removing shafts, bearings, and wrist pins from engines. It is generally heavy enough to resist damage to itself, but soft enough not to injure the finished surface on the part that is being driven. 1-33 Figure 1-51.—Punch marking mating parts. Figure 1-52.—Marking the intersection of lines with a prick punch.