Elevating 'A' frame doubles stiffness

Tom Shelley reports on a novel mechanical geometry in a horizontal machining centre doubles stiffness, acceleration and the first natural frequency of vibration. The spindle is mounted at the apex of an 'A' frame whose outer extremities run in almost vertical tracks, slightly inclined towards each other. The idea is the brainchild of Professor Michael Valasek, based in the Department of Mechanics in the Faculty of Mechanical Engineering in the Czech Technical University in Prague. Development started in June 2000. Professor Valasek says that what he describes as a "good traditional machine tool" has a stiffness of 60N/micron, can achieve 0.6g acceleration, and has a 50Hz first eigen frequency. "In 10 years of experiments with parallel kinematics machines, people could increase acceleration to 1, 2 or 3g, but in the majority of cases, they then decreased stiffness." This is highly undesirable because if a machine accelerates faster, it needs to be stiffer, in order not to distort under increased mass times acceleration loads. The new geometry was arrived at by mapping the robot design parameters into design criteria - requirements and constraints - and undertaking multi-objective optimisations of parameters such as dimensions, drive and control parameters. The technique involves the creation of what are known as Pareto sets, named after nineteenth century Italian sociologist and economist Vilfredo Pareto, who lived from 1848 to 1923 and is best known for his discovery of the 80-20 rule. The methodology has involved the breaking down of the problem into "islands" and "conflicts", with the main computation inside the "islands". New computational tools have been developed in the form of maps of collisions, dynamics and stiffness. 50 structural concepts and more than 100,000 parametric variants were investigated in the course of the study before arriving at the final optimised geometry. The study was conducted in conjunction with the Czech machine tool company, Kovosvit, which has now launched the design commercially and designated it the 'trijoint 900H'. The machine has a stiffness of 121 N/micron, accelerations of 1.2 to 1.5g, maximum traverse speeds of 80m/min to 100m/min and a first vibration eigen value at 100Hz. It has three working pallets, 630 x 630mm and a 28/32kW spindle motor with a maximum torque of 67/76 Nm and a maximum speed of 24,000 rpm. The control system is a Siemens Sinumerik 840D and the machine measures 6m x 6m x 3.5m and weighs 30 tonnes. The machine design concepts can be applied to the optimisation of any piece of jointed machinery. Trijoint web site Kovosvit Mas Pointers * Multi-objective optimisation has resulted in the development of a machining centre with novel mechanical geometry * Stiffness, acceleration and the first eigen natural vibration frequency have all been doubled as a result