An adjustable kinematic coupling for use in machine tools with a tight structural loop

This paper presents an adjustable three-V-groove canoe–ball kinematic coupling design that allows for repeatable opening and closing of the coupling interface as well as nano-, micro- and macro- height and tilt adjustability. This coupling was created to enable the design of machine tools with an extremely tight structural loop. Repeatable opening and closing of the coupling allows access to an otherwise very compact and possibly difficult to access machining area. Macro (mm) height adjust of the coupling allows for different tool geometries. Micro and nano (μm, nm) adjustments enable tool wear compensation as well as micro/nano positioning of the tool relative to the work-piece. Preload is achieved using ballscrews that pass through the centre of the coupling V-grooves, and load washers enable servo control of the preload force. At a preload of 26 kN the coupling achieves a stiffness of 7 N/nm at the coupling interface. By varying the preload on the coupling by ±10%, in-process nm to μm height and tilt adjustment at >95% of the nominal stiffness is possible. When under light or no preload each canoe–ball interface can be adjusted over a range of 8 mm using a wedge mechanism, which, controlled by a separate screw actuator, changes the effective width of the V-groove. While static friction between the elements prevents back-driving the system and thus helps to maintain high stiffness without having to power the screw, mm height adjustment is only possible under significantly decreased preload, and thus stiffness, and is considered a set-up change operation. Furthermore the preload ballscrews can be long and hence when reversed, they can open the machine up by separating the coupling elements and thus raising one part of the machine with respect to the other to gain free access to the workspace.

► A height adjustable kinematic coupling enables a modular machine design with very high stiffness at the coupling interface.
► Feedback control of the kinematic coupling preload can be used for nano and micro meter coupling height adjustments.
► For mm height adjustment care must be taken in the design of the preload mechanism. The ratio of preload mechanism stiffness and coupling stiffness must be sufficiently low.