Abstract: An electro-pneumatic machining jig is adapted to be attached to the bed of a machine such as a milling machine, grinding machine or the like. The jig holds a workpiece, such as a cylinder head, for machining in a desired position relative to a tool head. The machining jig has a pair of jig stands (10) for clamping the workpiece therebetween, with the jig stands (10) mounted in selected spaced relationship on the bed of the machine. Each jig stand (10) has a workpiece connection means (22, 25) mounted on an adjustable stand (16, 17, 18). The stand is supported on a base (11, 12) which includes air-float means (54, 55, 50) and electromagnetic means (13, 53). The air-float means (54, 55, 56) permits alignment of the clamped workpiece relative to the tool head whilst the bases of the pair of jig stands (10) are floating. When the air-float is terminated, the electromagnetic means (13, 53) are energized to clamp each jig stand (10) to a mounting plate on the machine.