Evaluating and Correcting CNC Machines with Laser Inspection System

When the manufactured part exceeds the allowable tolerances, or although it does not exceed the tolerance, but it has approached the maximum value of the tolerance, the root cause of the problem should be discovered during the processing of the inspection work.

After eliminating the possibility of tool wear, part programming errors, fixture changes, and raw materials, the rest is to determine whether the CNC machine is the source of the problem. The first thing to be clear is whether the tool has moved to the correct position according to the programming command of the workpiece. For example, a tool mounted on a three-axis machining center is programmed into a series of X, Y, and Z coordinate positions. Because the machine tool is three-axis, each axis must be tested.

When evaluating individual axes, the core problem is the complexity of the work. Each axis may have six errors: position error of the line displacement, straightness error in two orthogonal directions perpendicular to the axis, and three angular errors in pitch, yaw, and roll. In addition, an additional device is required to detect the perpendicularity between the three axes of the machine tool. Therefore, there are 21 possible sources of error for a machine tool with three axes.

Machine Measurement

For a long time, the measurement of machine tools has used step gauges, rulers, square irons and indicator tables. The use of these tools for inspection requires skilled and experienced inspection personnel. Even so, large calculation errors are inevitable. Since each measuring an error requires a separate measuring device, to complete 21 error measurements, time becomes the biggest obstacle. Due to production and time constraints, the number of detected error items is often limited, and the production shop often only tries to determine and correct the main error sources without performing a full inspection of the machine tool. For example, the commonly used diagnostic tool is a scalable club ruler. It is very effective for determining dynamic errors and providing relative coordinate axis motion information. It can also measure backlash, crawling, scale mismatch, and servo lag errors, but it cannot provide axes other than axes. Reliable measurement of other geometric elements of machine tools other than orthogonality.

Using a laser interferometer to evaluate a machine tool eliminates the problems that arise when using other methods to measure machine axes. Laser interferometers are considered as a standard for accurate length measurements and for establishing linear displacement accuracy. Using some dedicated optical components, it is possible to measure the straightness in two orthogonal directions and two of the three angular errors: pitch error and deflection error.

In order to meet the needs of precise measurement of machine movements, various laser measurement systems have been developed. Although many different measurement systems can be used, most rely on one of three measurement schemes to achieve the same measurement results. One solution is to measure the diagonal of the workspace. The accuracy depends on the repeatability of the machine being evaluated. However, this method cannot provide individual errors for each axis. These errors help determine the appropriate mechanical correction that may be required. (such as the correction of the verticality). The other two programs mainly focus on the direct measurement of machine axes. While measuring a coordinate axis with some systems, additional coordinate axes are measured by other systems. The latter example is the 6-dimensional laser measurement system of the API (Automated Precision Inc). It can measure 5 or 6 errors in one axis simultaneously, which can reduce the measurement time by 80%. In addition, these errors are measured while providing their 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. In general, using this information is sufficient to determine which axis needs to be corrected. If the error of a single axis is smaller than the error of the workpiece to be machined, but the operator should still determine whether the machine can process the qualified workpiece, because the error will increase according to the geometric synthesis (synthesized in three-dimensional angle), so it must be Learn how the error of one axis affects the other two axes. It is typical that the Y-axis overlaps with the X-axis on a CNC machine. If the X-axis has a straightness error in the direction of the Y-axis, the measurement error is superimposed (added or subtracted) to the Y-axis linear displacement error, and these errors cannot be found during the measurement because Only one axis is moved at a time. Furthermore, it is very complicated to analyze every possible overlapping effect of evaluating 21 errors.

Error modal analysis software makes this analysis very easy. The software can provide a spatial error graph that indicates the combined effects of 21 individual errors measured at each location of the machine's effective workspace. The software can easily 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 evaluated. Repeatability is a measure of the stability of a machine tool. The machine tool moves to a command position based on its 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 four times each time, the machine is highly stable. Machine, but not a precision machine. When a machine tool repeats one error or has a little change, the error can be easily corrected 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 very closely approach the desired position on the X-axis. For a machine tool, changing the program by simply adjusting the command position is not the best method of correction. Therefore, many current controllers allow the position of the encoder to be adjusted to correct these errors. This is commonly referred to as "pitch compensation," since the common method of moving a coordinate axis is to use a motor to drive the spindle nut driver. The position is determined by the count pulses 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 one pitch.

If an error is repeated, it can be corrected by the controller; if an error is not repeated or the change exceeds the desired tolerance, the machine system or the electrical part of the machine must be repaired.

Most machine controllers provide the operator with the ability to adjust the position of the line displacement to correct the travel error—that is, the pitch error. In addition, many new controllers can provide straightness correction and orthogonality corrections in two orthogonal directions. Some controllers have the ability to correct all 21 errors.

Error correction is often based on a three-dimensional grid, which is a set of points in the machine's effective space. A correction value is given for each specific X, Y, and Z point in the grid. For each point, the synthesis of 21 individual errors into a correction requires the establishment of error models and calculations that often exceed the capabilities of the machine operator. 3-dimensional error models and correction software and these grids are now available.

If one controller cannot compensate for all the errors of the machine tool and a separate line displacement compensation cannot provide the desired result, its correction can still be achieved as long as the information provided by the scale (encoder) system to the controller is converted. This needs to be implemented 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, regardless of how the results are the same.

The use of laser systems to evaluate CNC machines can be used for rapid and comprehensive analysis. If these tasks are performed in other ways, they will be time consuming and uneconomical. The use of error model software to analyze data reduces the complexity of the process and produces consistently qualified parts.

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