When the manufactured workpiece exceeds the allowable tolerance, or although the tolerance is not exceeded, but the maximum value of the tolerance is approached, the source of the problem should be discovered during the inspection of the workpiece.
After eliminating the possibility of tool wear, workpiece programming errors, fixture changes and raw materials, the rest is to determine if the CNC machine is the source of the problem. The first thing to be clear is whether the tool moves to the correct position according to the programmed command of the workpiece. For example, a tool mounted on a three-axis machining center is programmed to a series of X, Y, Z coordinate positions. Because the machine is three-axis, each axis must be tested.
When evaluating a single coordinate axis, the core issue is the complexity of the work. There may be six errors for each axis: the position error of the line displacement, the straightness error in two orthogonal directions perpendicular to the axis, and the three angular errors of pitch, yaw, and roll. In addition, an additional device is needed to detect the mutual perpendicularity between the three axes of the machine tool. Therefore, there are 21 possible sources of error for a machine with three axes.
According to China Machine Tool Network|Machine35.com, for the long time, the measurement of machine tools uses step gauges, rulers, square irons and indicator tables. The use of these tools for testing requires skilled and experienced inspectors, and even then, large computational errors are inevitable. Since a single measurement device is required for each measurement error, time is the biggest obstacle to complete 21 error measurements. Due to production and time constraints, the number of error items detected is often limited. The production shop often only attempts to determine and correct the main error source without comprehensive testing of the machine tool. For example, the commonly used diagnostic tool retractable club ruler is very effective for determining dynamic error and providing relative coordinate motion information. It can also measure backlash, creep, scale mismatch and servo lag error, but it can not provide except for the axis. Reliable measurement of other geometrical elements of the machine tool other than orthogonality.
Using a laser interferometer to evaluate a machine eliminates the problems that can occur when using other methods to measure machine axes. Laser interferometers are seen as the standard for precision length measurement and for establishing line displacement accuracy. Using some specialized optical components, it is possible to measure two straightness directions and two errors in three angular errors: pitch error and deflection error.
In order to meet the needs of precision measurement of machine tool motion, various laser measurement systems have been developed. Although many different measurement systems can be applied, most rely on one of the three measurement schemes to achieve the same measurement. One solution is to measure the diagonal of the workspace, the accuracy of which depends on the repeatability of the machine being tested, but this method does not provide individual errors for each axis, and these errors help to determine the appropriate mechanical corrections that may be required. (such as the correction of the verticality). The other two programs focus on direct measurement of machine axes. Additional axes are measured by other systems while measuring one coordinate axis with some systems. The latter example is a 6-dimensional laser measurement system of API (Automated Precision Inc.). It can measure 5 or 6 errors of one coordinate at the same time, which can reduce the measurement time by 80%. In addition, these errors are also measured while providing a relationship to each other.
After testing with the API system, the analysis shows that perhaps one axis is much less accurate than the other two axes. Often, using this information is sufficient to determine which axis needs to be corrected. If the error of a single coordinate axis is smaller than the workpiece error to be machined, the operator should still determine whether the machine can process a qualified workpiece, because the error will increase according to geometric synthesis (synthesized in three-dimensional angle), so it is necessary See how the error of one axis affects the other two axes. Typically, the Y-axis overlaps the X-axis on a CNC machine. If the X-axis has a straightness error in the direction of the Y-axis, the error is superimposed (added or subtracted) onto the linear displacement error of the Y-axis, and during the measurement, it is impossible to find these errors because of the measurement. Only one axis is moved at a time. Furthermore, it is very complicated to analyze each of the possible superposition effects of the 21 errors.
Error modal analysis software makes this analysis very easy. The software provides a spatial error graph that highlights the combined results of the 21 individual errors measured at each location of the machine's effective workspace. The software makes it easy to determine if the machine is capable of machining workpieces within a certain tolerance range.
Correction step
Once it is determined that the CNC machine tool is the cause of the workpiece tolerance change, the machine tool error must be corrected. In order to determine the most effective correction method, the repeatability of the machine must be assessed. Repeatability is a measure of machine stability, and machine tools move to a command position based on their stability. For example, if the tool receives a command to move to X=5, Y=5 and Z=0, but it moves to X=4.950, Y=4.950 and Z=0 every time, the machine is highly stable. Machine tool, but not a precision machine tool. When a machine repeats an error, or with a small change, it is convenient to correct the error by adjusting the position of the command.
In this example, the operator can command the tool to X = 5.050, at which point the tool will reach the desired position very close on the X-axis. For a machine tool, changing the program by simply adjusting the command position is not the best correction method. Therefore, many controllers currently allow adjustment of the position software of the encoder to correct these errors, which is commonly referred to as "pitch compensation" because the common method of moving the coordinate axes is to drive the screw nut drive pair with a motor. The position is determined by the counting pulse on the optical disc in the encoder. The encoder can emit a large number of pulses per revolution, the encoder rotates one turn, and the machine moves a pitch.
If an error is repeated, it can be corrected by the controller; if an error does not repeat or the change exceeds the desired tolerance value, the mechanical system or electrical part of the machine must be repaired.
Most machine tool controllers provide the operator with the ability to adjust the position of the line displacement to correct the stroke error - the pitch error. In addition, many new controllers provide straightness correction and coordinate orthogonality correction in two orthogonal directions, and some controllers have the ability to correct all 21 errors.
Error corrections are often based on a three-dimensional grid approach, which is a set of points in the machine's effective space. A correction value is given for each of the specific X, Y, and Z points in the grid. For each point, combining 21 individual errors into a single correction requires the creation of error models and calculations that often exceed the capabilities of the machine tool technician. 3-D error models and correction software are now available as well as these grids.
If a controller cannot compensate for all errors in the machine and the separate line displacement compensation does not provide the desired result, the correction can still be achieved by simply transforming the information provided to the controller by the scale (encoder) system. This needs to be done using a second controller, which will transform the machine scale information provided to the original controller of the machine based on the error model analysis software, in any case the same result.
Using a laser system to evaluate CNC machines can be a quick and comprehensive analysis that can be time consuming and uneconomical if done in other ways. Analysis of data with error model software reduces the complexity of the process and allows for the continuous production of qualified workpieces.
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