Active Vibration Control of a Turning Process Using Magnetostrictive Actuation and Modified Rate Feedback
 
 

Timothy J. Sturos | MS | 1994

In the manufacturing sector there is a never ending quest to improve productivity and product quality. With this quest comes the challenge to better understand the manufac-turing processes and increase the performance of process equipment. In this spirit, a unique approach to the control of machining process dynamics has been investigated.

The specific machining process is turning and is shown in Figure 1. 1. Briefly stated, turning is a material removal process which creates cylindrical parts. Typically the work piece is fixed in a lathe at one end by the spindle chuck and at the other end by a tail stock. Under certain cutting conditions, thick or short work pieces may not require a tail stock. The tool is secured by the tool post and carnage assembly. The work piece is rotated by the spindle drive assembly, while the tool is moved by the feed mechanism. The amount of material removed per revolution is governed by the feed and radial engagement of the tool with the work (depth of cut.) The rate at which the material can be removed is dictated by the strength of the tool structure, the available power from the spindle drive motor, and more subtly, by the overall stability of the tool-process system.

Turning, being one the most common machining processes, has many applications in industry. Some typical applications include the turning of shafts, pins, drums, rollers, and disks. A few less obvious applications include the production of engine and compressor pistons, and stethoscope housings and the finishing of gears. In certain cases, using special fixtures and/or machines, non-circular objects such as high performance pistons and special rollers may be turned. Some objects, although round, may include slots or key ways which give rise to a turning condition called interrupted cutting.

The turning process has been studied throughout the years with the major parameters affecting turning process well defined. Although there is a gamut of variables which affect the turning process, it is generally agreed that the depth of cut, feed and cutting velocity are a few of the most significant. Other parameters such as tool geometry, tool condition, and work piece material also greatly influence the performance of the process.
 

Often of primary interest, are the finished part's dimensional accuracy and surface qualities, which can be required in the face of very demanding productivity goals. Dimensional accuracy is important for components which are used in precision mechanisms, high performance machines and injection molds, to name two examples. Surface quality is important where aesthetics, wear resistance, paint and coating adhesion and lubrication
retention is concerned. In a turned part, surface quality and dimensional accuracy are directly affected by the relative radial displacement between tool and work piece which arises during cutting.

Were the process perfect, there would be very little if any perturbations to cause the tool to displace from the desired position; furthermore, the cutting forces would be constant. Hence, maintaining a constant depth of cut would be a trivial task. The only compensation required would be for the static deflection of the tool due to the constant resultant cutting force. In reality, the force exerted on the tool varies due to part geometry (i.e.; interrupted cutting), non-homogeneity of the work material, built up edge, tool breakage, and other non-stationary effects. Since these changes are often sudden, pulses are generated, exciting the system over a relatively wide bandwidth. If the machine tool struc-ture is excited at one or more of it's modes, significant vibration (and possibly instability) can occur, causing deviations in the depth of cut, among other things. Furthermore, the tool structure and the process are coupled, suggesting that a disturbance in one causes a distur-bance in the other.

To achieve desired surface and dimensional qualities in the presence of the mentioned disturbances, the appropriate cutting conditions are selected either by careful study or by trial and error. Ideally, for a given machine tool, the operator can choose the appropriate machine settings to maximize production. In reality, fixed machine settings are often chosen to guarantee process stability for a critical point in the operation, i.e., the worst case scenario, resulting in conservative cutting conditions for the remaining portion of the production cycle. This approach is at the expense of optimum productivity.

To meet demanding production goals, even in the presence of the mentioned process disturbances, there has been interest in automatic process parameter adjustments to keep the production machine operating at or near optimum performance, without the intervention of a programmer or operator. Thus, the active/adaptive manipulation of machining parameters to achieve improved process stability, dimensional accuracy and surface quality has long been of interest to material removal and machine tool researchers. Some examples of active control include varying the speed of the spindle to improve process stability, automatically adjusting the feed to regulate cutting forces, and actively manipulating the radial engagement of the tool with the work to improve dimensional accuracy. Researchers at Michigan Technological University have joined the effort to improve the turning process using automatic process manipulation and have developed an active prototype tool holder to abate tool vibration with the goal of improving surface texture.

If you have any comments or suggestions please e-mail jwsuther@mtu.edu.