In the early days of 5-axis profiling for aerospace structures, the physical limitations of machinery was halting progress in the continuous improvement of machining processes. In order to achieve greater chip removal rates, faster cycle times and throughput, machinery manufacturers began equipping their machinery with higher speed spindles for machining aluminum workpieces. The thought process being that increased rotational speed of the cutter would provide faster removal of material and equip the front-runners of manufacturing with a new competitive edge.
The necessary consequence of these higher speed spindles was a change in the configuration of the machining axes. Conventionally, these profiling machine tools were built with a combination of A and B axes. While sufficient for lower RPM spindles, these configurations would not provide the higher feedrates that are necessary once the spindle speed is increased. As a result machine tool manufacturers began building their machines with A and C (or B and C) configurations [See Figure 1]. The C axis being a rotational axis that is collinear with the spindle axis. The aim of enhancing productivity was now on its way to being achieved.
Figure 1 – Change in the Configuration of Rotational Axes
With all new technological advances comes the onset of new technological problems. The difference in the expectation and observation of results appears quickly. No longer were the machine tool mechanics the limitation of productivity. The new limitation was the CNC control. Prior to the introduction of A/C machines, the control system was dependent on two foundations: 1) the processing time available to determine the machine tool path according to the CAM data was sufficient and 2) the angular position of the A and B axes were reliable feedback mechanisms. With faster feedrates now a reality and large C-Axis rotational movements necessary for achieving part geometry, these two foundations upon which the control system was built are invalid.
These invalidities result in physical manifestations of computer processing flaws. Consider the processing time of the CAM data. To approximate the geometry of a contoured surface, CAM outputs a finite series of line segments. The greater the number the segments, the more accurate the approximation of the contour. The CNC control then receives these line segments and attempts to generate a mechanical path as close to the real contour as possible. In the case of machining contoured surfaces, the control system has a tendency to speed up and slow down repeatedly because it is attempting to generate a toolpath as fast as possible. If the control is provided with few line segments that give a poor approximation of the contoured surface, this speeding up and slowing down of the feedrate is exacerbated. The result is a series of witness marks along the part. Manufacturers must then hand-polish the part after machining.
The other lost foundation is the use of the position of the rotational axis as a means of a control feedback. The range of angular motion on an A/B machine, for example, may only be 40 degrees for the A axis and 35 degrees for the B axis. On the other hand, the C axis on an A/C or B/C machine may rotate 280 degrees in a single move. These large and rapid changes in motion also result in a slowing of the feedrate when machining a contoured surface. Inaccuracies, poor surface finish and slow cycle times are the result.
SNK has developed the DCS-V Aero Edition Software to overcome these technological problems. There are three functions of the software that solve the problems of inaccuracies, poor surface finish and slow cycle time:
1. Speed Smoothing Function
Speed Smoothing Function
2. Shape Smoothing Function
3. Zero Tolerance Function
To resolve the issue of using the rotational position of the axes as a feedback mechanism, SNK has replaced it with the definition of the tool vector. That is, the X, Y, Z components of the tool axis vector is used as a feedback instead of the angular position of the A and C axes. Here is the reason why: there are many cases where the C-Axis rotates considerable when the tool axis is nearly parallel with the Z-Axis. The change in the tool axis vector, therefore, is hardly changed when compared to the C-Axis. This proves to be a much better solution resulting in the reduction of unnecessary machine acceleration and deceleration, thus helping to shorten machine time and improve the quality of machined surfaces.
Shape Smoothing Function
To resolve the issue of poor CAM data, the shape smoothing function reduces shock to the machine by smoothly interpolating the straight lines used to approximate the real contour. This removes the polygonal patterns that appear on machined surfaces and require hand polishing to remove [See Figure 2].
Zero Tolerance Function
Figure 2 – Removal of Polygonal Patterns Using Shape Smoothing
The Zero Tolerance Function is the least intuitive of the three. When machining a contour, the previous control versions have a tendency to create an inner radius error. That is, there is a difference in the desired tool path and the actual tool path [See Figure 3].
Figure 3 – Inner Radius Error
From a mathematical standpoint, the error between the two curves is proportional to the square of the feedrate divided by the inner radius:
To minimize the error, either the feedrate must be drastically reduced, or the inner radius must be very large. Because the inner radius is a function of the part and cannot be changed, the feedrate must be reduced.
The Zero Tolerance Function has the user input a predetermined acceptable error. We recommend 10 – 150 microns. The control then calculates the changes in the part shape, determines the inner radius and then determines the feedrate that keeps the error within the predetermined value. Furthermore, the DCS Software performs the calculation in real time.
When advancing into new technology, it is important to be aware of the inevitable technological problems that will arise. Ignoring these problems would compromise invention, inhibit the realization of potential and deprive users of making the most of what they have. SNK has adapted to the marketplace’s need for higher productivity both mechanically and digitally. When considering implementing new technology for your organization, take the time to investigate all facets therein and select a supplier that is up to the challenge.
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