Off Axis Grinding Machine
Metrology Frame
A “solid” metrology frame is hard to envisage on a large three axis machine tool. The challenge was to devise a method of implementation which could be realised in practice at acceptable cost, whilst still giving sufficient accuracy over the large workzone.
Laser interferometry in combination with reference straightedges allowed the practical realisation of an accurate method of relating the workpiece surface to the cutting edge of the tool and to the measuring point of the profilometer.
Principle Components of the Laser Metrology Frame
Two glass reference bars are mounted nominally parallel to the worktable of the machine and co-planar. A third reference bar is mounted to the travelling gantry, parallel to the Y axis and with its reference surface vertically above the measuring stylus for all X, Y positions. The system calculates the distances between the reference surfaces and the interferometers and makes corrections in the X, Y and Z directions accordingly.
The reference bars are fabricated in Corning ULE glass 2.75m long and of 100mm x 307.5mm cross section. The bars have been lapped and polished by Eastman Kodak who also carried out their characterisation by interferometry. The reference surfaces have an aluminised stripe to enable them to function as plane mirrors in the machine laser interferometer system.
The metrology frame carrying the two co-planar reference bars takes the form of an essentially hollow square member with the reference bars mounted on opposite sides of the square. Each bar is kinematically mounted at its points of minimum deflection with provision for fine vertical adjustment. The frame itself consists of a highly stress relieved steel weldment filled with epoxy concrete for high stiffness and damping.
The metrology frame and the worktable must both be located on the base structure of the machine in such a way that neither forces due to varying workpiece/fixture mass, nor distortion due to movement of the large machine gantry, distort the metrology frame. Neither should forces due to varying workpiece/fixture mass unduly affect machine geometry nor distortions due to movement of the machine gantry affect the worktable (and thus the workpiece geometry).
Laser Interferometer Positioning and Metrology Frame
Tweet
Cranfield Precision operates to ISO9001 quality standard and is accredited by Lloyds Quality Register.
A Division of Cinetic Landis Ltd.
Registered in England
Registration Number 05577045
Registered Office: Cross Hills, Keighley, West Yorkshire, BD20 7SD