Scribes & Punches

A punch is a tool used to exert a significant amount of striking force on a very small area. All punches are made to be struck with ball peen hammers (which have rounded faces specifically designed for use on striking tools) and are, therefore, made of hardened steel. They vary in design and purpose, with some of the more common types include center punches, prick punches, drift punches or drifts, pin punches, alignment punches, and hole (gasket) punches.

Scribes are hardened steel tools used to make shallow marks on metal. They are usually long and thin with a gripping surface on the body. Scribes are used with rules to make straight lines on metal for punch locations or to mark lines for cuts. Scribes should never be struck.

Prick Punches and Center Punches

Prick punches and center punches are primarily designed for making marks or indentations in metal. The prick punch, shown in Figure 16, is a long, tapered punch beveled at the tip to form a point. It is a light to medium duty tool used to make location marks on equipment. Making a set of location marks on a component such as a split casing allows the technician to take the casing halves apart and return them to their original position later.

Figure 16: Prick Punch

Center punches are similar to prick punches, but they are generally thicker and more suitable for heavy duty work. Their primary purpose is to make indentations in metal deep enough to give drill bits a true start. Without the indentation made by a center punch, a drill bit is liable to "walk" across the surface of the work instead of drilling a hole. Figure 17 is an example of a center punch.

Figure 17: Center Punch

Drift Punches, Pin Punches, and Alignment Punches

Drift punches (or, simply, drifts), pin punches, and alignment punches are related because their primary use is removing and inserting alignment pins.

Drift punches are heavy, tapered punches with blunt tips. They are used primarily to drive alignment pins out of equipment, and they are available in a variety of sizes to match the diameters of pins.

The drawback to drift punches is that in order to construct them with sufficient strength to withstand heavy hammer blows, they have to be thick and tapered. This tapering prevents them from following an alignment pin through its hole. To complete the job that a drift starts, a pin punch must be used.

Pin punches, like other punches, are manufactured in various sizes and diameters. Unlike drifts, which taper from a narrow tip to a thick body, pin punches have uniformly narrow shanks that allow them to reach all the way through alignment holes and other circular recesses. Because of their narrow shanks, however, they are not strong enough to be used to break alignment pins free. alignment punches are used to reinstall alignment pins. An alignment punch is long and gradually tapered. It is used to align matching holes in components so that alignment pins can be inserted easily. Figure 18 is an example of an alignment punch, pin punch, and a drift punch.

Figure 18: Alignment Punch, Pin Punch, and Drift Punch

Hole Punches

Hole punches are used to cut circular holes in gasket material, rubber, soft plastics, and even soft metal. Instead of pointed or blunt solid tips, hole punches have sharp-edged circular ends. Hole punches usually come in sets that include a variety of different size punches for cutting various size holes. Figure 19 is an example of a hole punch.

Figure 19: Hole Punch

Safety

The following basic safety rules should be followed when using punches:

	Be certain that the work is held steady. The work should be held in a vise or clamp to ensure that it does not slip when the punch is struck by the hammer. Accuracy is especially important when making starting indentations for drill bits.
	When making an indentation with a center punch, always mark the exact spot where the indentation is to be made before attempting to make it. This ensures accuracy.
	Match the hammer to the size of the punch. The face of the ball peen hammer should be no more than twice the diameter of the striking surface of the punch. Otherwise, the hammer is too small (and unable to supply sufficient force) or too large (thereby possibly damaging the punch). Prick punches should be struck with light or medium weight ball peen hammers; center punches should be struck with heavier hammers, since they must make deeper indentations.
	Never use a carpenter's hammer to strike metal working punches or chisels. The hammer face is hardened, which can fracture and send hazardous metal chips flying.
	Use the right punch for the job that is to be performed.
	Always wear gloves and safety glasses. Any time metal strikes metal, there is the chance that bits of metal will fly. This requirement is so important that some manufacturers stamp it on the sides of their punches.