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10.2.1.3 The Tooling and the Fixture System
Fixture system and tooling are the essential parts of the machining system. They also play significant roles in the machine tool design, because they are at the end of the machine tool-machining loop. The deformation of the tooling and the fixture system both in static and dynamic circumstances will entirely be copied to the workpiece surface and hence influence the workpiece form and dimensional accuracy as well as its surface texture and topography.
In contrast to the machine tool dynamics, the dynamics of tooling and fixture could significantly, depending on the location of the cutting tool with respect to the workpiece, owing to its localized structure and geometry of the workpiece [12]. In precision machine tool design, it is very important if designers can take this varying dynamics into account in spite of the possible difficulty, because this will be helpful to accurately evaluate the machine dynamics and errors budget. In practice, the dynamics change by the location of the cutting tool with respect to the workpiece can be dealt with by putting a larger safe bandwidth of the machine tool, and the speed of spindle can be limited as designed to decrease this dynamic change.
10.2.1.4 Control and Sensor System
Computer numerical control (CNC) was introduced into the machine tools industry in the early 1970s and since then many companies started to develop their own control systems for machine tools. The control sub-system includes motors, amplifiers, switches and the controlled sequence and time. High speed multi-axis CNC controllers are essential for the efficient control of, not only servo drives in high precision position loop synchronism for contouring, but also thermal and
geometrical error compensation, optimized tool setting and direct entry of the equation of shapes [13].
From the dynamics viewpoint, stiffness in the control system indicates the capability to hold a position when dynamic forces try to move it. Therefore, a proper design of the control system and its algorithms can lead to a high servo-stiffness and hence improve machining precision through the machine tools.
10.2.1.5 The Metrology and the Inspection System
The metrology and the inspection systems are the basis for the qualifying assurance of precision machining and enabling the technology to be widely applied in industry. On the other hand, higher level accuracy assurance in metrology and inspection system is also a drive for precision machines towards a higher precision requested for the future engineering industry. Fast and accurate positioning of the cutting tools towards the workpiece surface and monitoring of the tool conditions visually by the operator should be integrated into the inspection system especially for on-line operation purposes.
10.2.1.6 The Machine Tool Performance Evaluation
The overall objective of the design of machine tool sub-systems discussed above is to achieve required machine performances. The performances are evaluated normally in the following aspects:
• Accuracy
• Kinematics
• Static performances
• Dynamics performances
• Strength performances
• Thermal performances
• Noise
• Vibration
These machine performances are collectively reflected on the tool-workpiece loop in terms of stiffness, thermal stability, static and dynamics as shown in Figure 10.1. The following sections will focus on the tool-workpiece loops, in relation to the machine dynamics in particular.
10.2.2 Machine Tool Loops and the Dynamics of Machine Tools
From a machining viewpoint, the main function of a machine tool is to accurately and repeatedly control the point of contact between the cutting tool and the uncut material - the “machining interface”. This interface is normally better defined as tool-workpiece loops. Figure 10.3 shows a typical machine tool-workpiece loop. The position loop is the relative position between the workpiece and the cutting tools which directly contributes to the precision of a machine tool and directly leads to the machining errors.