240 THE MACHINE-SHOP jig consists of two cast-iron trunnions, on which swing two indexing plates. Channel irons which connect these plates carry all the bush plates, and quick-acting clamps, the compensating beams, and locations. The con- struction is such that the revolving parts are light, yet rigid, so that no de- flection can occur during drilling. The whole jig can be revolved easily, and indexed to allow for drilling from four sides. Foot treadles are arranged in convenient positions for starting and stopping the machines. The result of these economies is that the 112 holes in the frame are drilled in twenty minutes.