Abstract: The head of a plano-milling machine has two jointly rotatable, perpendicularly-related collet spindles journalled on bearings in a box-like housing which can be mounted on the slide of the machine so as to be indexable about the axis of the vertical collet spindle to present the horizontal collet spindle to different vertical faces of a workpiece.The head is journalled to the slide by a combined indexing means and hydrostatic bearing which includes means to lock the head in a chosen angular position. Provision is made for numerical control and automatic tool changing.