Petr Kolar

Case Study 2.2: Clamping of Thin-Walled Curved Workpieces

Thin-walled curved workpieces are typical for structural parts of airplanes. The issue in the workpiece clamping and subsequent machining is the changeable workpiece stiffness during material removal. The fixture forces and the cutting forces deform the workpiece with dependence on the part decreasing static stiffness that causes large local surface location errors of the part. As a result, the workpiece wall thickness is out of tolerance and consequently the part’s weight is also out of tolerance. The proposed solution is based on new fixtures with integrated support and clamping function. The workpiece is clamped using a vacuum. A suitable thickness measurement sensor was integrated into the machine tool. The new fixture elements are autonomous and plug-and-produce ready, with integrated safety by monitoring the minimal workpiece clamping force. The fixture control enables fully automated operation using the specific control software. The machining process was optimized in terms of tool path strategy and cutting conditions to avoid chatter during machining and shorten the production time. The proposed manufacturing process leads to a shortening of the production time with the requested surface quality. The presented manufacturing procedure is beneficial from the productivity and cost point of view. A group of fixtures, including the necessary harness and control, offers a universal possibility for replacing a set of six specific fixtures designed as a mould with part negative shape.

Case Study 3.1: Fixture System for Workpiece Adjustment and Clamping with/without its Pre-deformation

The Case Study is focused on the improvement of the productivity and accuracy of large workpieces production thought the shortening of the setup and clamping time. Current situation in the workpiece adjustment is based on extremely time demanding manual setting, when the operator has to find optimal zero point of the workpiece manually—scribing operation and to fix the workpiece. The main goal of the Case Study is the development of a low time consuming clamping system with low requirements for the operator. This objective was achieved by the development and testing of a comprehensive solution that automates the workpiece adjustment process by active fixture units with centralized control and uses a machine tool touch probe for workpiece current state automatic inspection. This approach is more effective than manual setting and also reduces the risk of errors. In addition, the developed solution allows the workpiece automatic clamping in an adjusted position and with a pre-deformation if necessary.

Experimental Investigations into Machining of FRP Material

Machining of fibre-reinforced thermosets is becoming a very popular technology today. Nevertheless, machinability of these materials is rather different from conventional materials such as metals since hard and abrasive fibres are combined with relatively soft resin with low glass transition temperature. Special attention has to be given to workpiece quality because delamination and burning of machined surface can occur. An experimental investigation into machinability of a polymeric and cellulose fibre-reinforced resin material was carried out. Milling operations were inspected with respect to process temperature, cutting forces and machined surface quality. The effect of cutting conditions on the mentioned aspects was determined. Standard and tailored cutting tools were used in the investigation. It was observed that surface quality is strongly dependent on tool geometry, milling strategy, fibre orientation and feed. On the other hand, cutting forces are relatively low and dependent on tool geometry and feed. The modified cutting tool with more positive tool geometry showed better results compared to the conventional one.

Influence of Machine Tool Covers on Feed Drives

Machine tool covers are important parts of the machine. From the point of view of feed drives, a cover is an additional multi-body system that influences the dynamic properties of the feed drive and the positioning accuracy of the machine. The advantages of covers connected to the machine table with flexible elements are shown on simulation and experimental results. A mathematical model of the cover and its connection to the machine table is described. Optimization of the stiffness and damping ratio for the connection, using a model of the cover, is suggested. The optimal connection parameters cause decreasing of maximum reaction force acting from the cover to the feed drive. This phenomenon is presented on simulation example and also on the experiment results.Copyright