jig (manufacturing fixture). A strong, heavy framework used to hold a component while it is being assembled. Parts are assembled in jigs to assure their interchangeability. Airplane wings, for example, are assembled in jigs, and any wing having a given part number can be interchanged with another wing having the same part number.
If the parts were not assembled in a jig, they might look the same, but they would probably not interchange; the bolt holes would likely not line up.
jigsaw. A type of saw that uses a very narrow blade moved back and forth across the work being cut. A jigsaw is used to cut small-radius curves in wood, metal, or plastic.
Jo-bolt (special aircraft fastener). A patented, high-strength structural fastener used where strength requirements are high. The Jo-bolt is passed through a close-tolerance hole, and a sleeve is pulled up over its tapered shank to form a blind head inside the structure.
joggle (sheet metal). A small offset near the edge of a piece of sheet metal. It allows one sheet of metal to overlap another sheet while maintaining a flush surface.

Johansson block. An accurately ground gage block used as a reference in precision machining operations. A Johansson block (Jo block) is usually ground to an accuracy of at least 0.000 01 inch (0.25 micrometer).
Johnson bar (aircraft controls). A type of brake control used on some early airplanes. A Johnson bar is a long handle located between the pilot’s and copilot’s seats, easily accessible to both.
Pulling straight back on the Johnson bar applies both brakes equally. Pulling the bar back diagonally applies the brake on the side to which the bar is pulled.
jointer (woodworking tool). A woodworking power tool used to smooth the edges of a board or piece of wood.
joule. A measure of work in the kilogram-meter-second system of units. One joule is the amount of work done when a force of one newton moves an object one meter in the direction in which the force is applied. A joule is also known as a newton-meter. One joule is equal to 0.1020 meter-kilogram, or 0.7375 foot-pound of work.
journal (bearing surface). A hardened and polished surface on a rotating shaft that rides in a plain bearing. The bearing journal provides a smooth bearing surface and wears very little.
joystick (airplane control). An old name for the control stick used to control an airplane. Movement of the joystick to the front or back moves the elevators to control the pitch of the airplane. Movement from side to side moves the ailerons to rotate the airplane about its longitudinal axis (to roll the airplane).