A method for mechanical machining, in particular for drilling and turning wheels (1) of light alloy, comprising a step (30) of centring and locking the wheel to be machined prior to the steps (40, 50) of drilling and turning the wheel. In the centring step (30) provision is made for the further steps of arranging a seating device for the wheel (1) comprising a plurality of elements having the form of a circular sector (4) in plan view, coplanar with each other and spaced apart from each other angularly, each of the sectors (4) being movable along a predefined radial direction (R), the directions (R) intersecting at a common central axis (Z) running perpendicularly to the radial directions, each of the sectors (4) having a respective seating surface (4a) for a corresponding part of the outer front portion (2) of the wheel (1) arranged in a position facing the sectors (4), and of moving the plurality of seating sectors (4) along the respective radial directions (R) to suit the outside diameter of the wheel undergoing the step of centring, so that the seating of wheels (1) having different diameters varying within a preselected range of values is obtained on the device. An installation operating according to the above-mentioned method is also described.