Punches - Aircraft Structure Repair | Aircraft Systems

Punches - Aircraft Structure Repair

Punches are usually made of carbon steel that has been hardened and tempered. Generally classified as solid or hollow, punches are designed according to their intended use. A solid punch is a steel rod with various shapes at the end for different uses. For example, it is used to drive bolts out of holes, loosen frozen or tight pins and keys, knock out rivets, pierce holes in a material, etc. The hollow punch is sharp edged and used most often for cutting out blanks. Solid punches vary in both size and point design, while hollow punches vary in size.

Prick Punch

A prick punch is primarily used during layout to place reference marks on metal because it produces a small indentation. [Figure 1] After layout is finished, the indentation is enlarged with a center punch to allow for drilling. The prick punch can also be used to transfer dimensions from a paper pattern directly onto the metal. Take the following precautions when using a prick punch:
  • Never strike a prick punch a heavy blow with a hammer because it could bend the punch or cause excessive damage to the item being worked.
  • Do not use a prick punch to remove objects from holes because the point of the punch spreads the object and causes it to bind even more.

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Figure 1. Prick punch

Center Punch

A center punch is used to make indentations in metal as an aid in drilling. [Figure 2] These indentations help the drill, which has a tendency to wander on a flat surface, stay on the mark as it goes through the metal. The traditional center punch is used with a hammer, has a heavier body than the prick punch, and has a point ground to an angle of about 60°. Take the following precautions when using a center punch:
  • Never strike the center punch with enough force to dimple the item around the indentation or cause the metal to protrude through the other side of the sheet.
  • Do not use a center punch to remove objects from holes because the point of the punch spreads the object and causes it to bind even more.

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Figure 2. Center punch

Automatic Center Punch

The automatic center punch performs the same function as an ordinary center punch, but uses a spring tension mechanism to create a force hard enough to make an indentation without the need for a hammer. The mechanism automatically strikes a blow of the required force when placed where needed and pressed. This punch has an adjustable cap for regulating the stroke; the point can be removed for replacement or sharpening. Never strike an automatic center punch with a hammer. [Figure 3]

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Figure 3. Automatic center punch

Transfer Punch

A transfer punch uses a template or existing holes in the structure to mark the locations of new holes. The punch is centered in the old hole over the new sheet and lightly tapped with a mallet. The result should be a mark that serves to locate the hole in the new sheet. [Figure 4]

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Figure 4. Transfer punch

Drive Punch

The drive punch is made with a flat face instead of a point because it is used to drive out damaged rivets, pins, and bolts that sometimes bind in holes. The size of the punch is determined by the width of the face, usually 1⁄8-inch to 1⁄4-inch. [Figure 5]

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Figure 5. Drive punch

Pin Punch

The pin punch typically has a straight shank characterized by a hexagonal body. Pin punch points are sized in 1⁄32-inch increments of an inch and range from 1⁄16-inch to 3⁄8-inch in diameter. The usual method for driving out a pin or bolt is to start working it out with a drive punch until the shank of the punch is touching the sides of the hole. Then use a pin punch to drive the pin or bolt the rest of the way out of the hole. [Figure 6]

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Figure 6. Pin punch

Chassis Punch

A chassis punch is used to make holes in sheet metal parts for the installation of instruments and other avionics appliance, as well as lightening holes in ribs and spars. Sized in 1⁄16 of an inch, they are available in sizes from 1⁄2 inch to 3 inches. [Figure 7]

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Figure 7. Chassis punch

Awl

A pointed tool for marking surfaces or for punching small holes, an awl is used in aircraft maintenance to place scribe marks on metal and plastic surfaces and to align holes, such as in the installation of a deicer boot. [Figure 8]

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Figure 8. Awl

Procedures for one use of an awl:
  • Place the metal to be scribed on a flat surface. Place a ruler or straightedge on the guide marks already measured and placed on the metal.
  • Remove the protective cover from the awl.
  • Hold the straightedge firmly. Hold the awl, as shown in Figure 9, and scribe a line along the straightedge.
  • Replace the protective cover on the awl.

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Figure 9. Awl usage

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