High speed machining provides dynamic accuracy to machine tools

To evaluate the capability of a machine tool for high-speed machining (HSM), it is more important to consider acceleration than velocity, according to Control Engineering Poland.

03/18/2014


Figure 1 shows the Avia X-5 machine tool. Courtesy: Poznan University of Technology, Institute of Mechanical Technology; Control Engineering PolandNumerical control (NC) machine tools used for high-speed machining (HSM) must be concerned with dynamic accuracy. Investigations were performed on a type Avia X5 machining center designed for conventional machining and HSM, looking at dynamic accuracy (circular deviations, in accordance with the ISO 230/4 standard) in three planes, with various feed rates by means of a quick diagnostic test type QC 10. Results have been verified and analyzed.

1. Demands on machine tools for HSM

HSM technology requires applying new, alternative solutions related to machines and their components. Very important characteristics of HSM machine tools are the main spindle, abilities of the control feed drives, rigidity and accuracy of the machine tool, and the control system's data processing speed. Accuracy of HSM results from the speed and capabilities of the drive system for the NC axes, which provide proper velocity and acceleration. Machining 3D-surfaces, common for industrial applications, demands tool movement interpolated in at least 3 axes. In that case, acceleration of the moving parts can reach up to 100 m/s2. The smaller the curvature radius and the higher the speed at a given point in the path, the higher value of the required acceleration. For this reason, to evaluate the ability of a machine tool for HSM, it is more important to consider acceleration than velocity.

Figure 2 shows the diagnostic test for numerical control (NC) machine tool. Courtesy: Poznan University of Technology, Institute of Mechanical Technology; Control Engineering PolandFigure 3 shows how measurements are taken for 360 degrees in both directions. Courtesy: Poznan University of Technology, Institute of Mechanical Technology; Control Engineering Poland

Machine tools with high accelerations can achieve a higher value for the mean efficient feedrate, which is similar to the programmed feedrate. This results in shorter machining time, more precise machining surfaces (fewer undercutting areas), and increased tool durability.

2. Machine tool example

The Avia X-5 is a vertical 5-axis machining center. Its stable, diagonally ribbed table design and robust bed and column structure guarantee constant precision even under high loads.

The Figure 4 photo provides a view of the diagnostic test in the Y-Z plane. Courtesy: Poznan University of Technology, Institute of Mechanical Technology; Control Engineering Poland

The TNC i530 Heidenhain control was applied. The shape of the Avia X-5 machine center is shown in Figure 1.

3. Dynamic testing Diagnostic device

Quick Test QC-10 can evaluate the accuracy of servodrives and determine geometric and dynamic errors in CNC servomotors. QC-10 allows measuring circularity of feed motion during a circular interpolated movement of the CNC axes, according to ISO-230 (Figure 2). The measurements look at the current radius and present results graphically. If the machine has no errors, the graph is supposed to be a circle. Motion system faults distort the circle shape and the graph analysis shows.

The QC-10 software can analyze test results by giving values of individual errors as well as their influence on circularity and positional tolerance values. The measurement should be performed for 360 degrees, in both directions (clockwise and counter-clockwise, Figure 3).

Measurements were carried out in X-Y, Y-Z, and Z-X planes by feedrate vf = 500, 1000, 5000, and 10,000 mm/min. Radius of the performed motion was R = 150 mm.

Figure 5 is a graph of test results in the X-Y plane by feed motion, where a (top) vf = 1000 mm/min, b (bottom) vf = 10000 mm/min. Note the deviation from the circle. Courtesy: Poznan University of Technology, Institute of Mechanical Technology; Control EFigure 6 is a graph of test results in the Z-X plane by feed motion: a (top) vf = 1000 mm/min, b (bottom) vf = 10,000 mm/min. Courtesy: Poznan University of Technology, Institute of Mechanical Technology; Control Engineering Poland

Figure 4 shows a view of a diagnostic test in the Y-Z plane. Figure 5 presents the result of a diagnostic test in the X-Y plane by vf = 1000 mm/min and vf = 10,000 mm/min, and Figure 6 presents the result of a diagnostic test in the Z-X plane.

4. High-velocity servodrives

The bar chart in Figure 7 shows the circularity of motion for Avia X-5. Courtesy: Poznan University of Technology, Institute of Mechanical Technology; Control Engineering PolandThe research confirmed high positioning accuracy of the machining center. Dynamic tests by means of QC-10 confirmed high dynamic accuracy of the servodrives during circular interpolated movement (Figure 7). The circularity tolerance value by vf = 10,000 mm/min didn't exceed 30 mm. It proves that the servo drives work properly even with high velocity. The results show that static and dynamic accuracy of the Avia X-5 servodrives are sufficient for HSM.

- Wojciech Ptaszynski and Roman Staniek are faculty members at the Poznan University of Technology, Institute of Mechanical Technology. Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering, mhoske@cfemedia.com.

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Key concepts

  • Numerical control (NC) machine tools used for high-speed machining (HSM) must be concerned with dynamic accuracy; acceleration is more critical than velocity.
  • Very important characteristics of HSM machine tools are main spindle, abilities of the control feed drives, rigidity and accuracy of the machine tool, and the control system's data processing speed.
  • Testing can verify critical parameters, confirming position accuracy.


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