Reliable Accuracy in CNC Machining Needs a Geometrical Calibration Plan
It is a simple reality that any machined part will not be geometrically perfect. Tolerances are allowed, assuming there will be errors due to multiple setups, clamping distortion, flexibility under machining forces, release of internal stresses, hard spots, temperature changes, tool size errors, datum accuracy and many other causes.
Considering the CNC machine itself, a modern machine control will typically display its position as precisely as 0.001mm, but this can overly influence our perception of the real accuracy of the machine, as these hidden errors are inherently transferred to the parts being machined.
All machine tools are themselves built to accuracy tolerances specified by the manufacturer. On smaller machines these tolerances are controlled primarily in the machine construction and assembly phases prior to delivery, while larger machines which are inherently flexible structures, have to be painstakingly adjusted by skilled installation or alignment technicians to achieve specified geometrical accuracy on the customer’s foundation.
As a part of the ongoing trend to implement better systems for controlling product variation, the establishment and monitoring of the geometrical calibration of CNC machining systems is increasingly being focused upon by buyers of machined parts, seeking to attenuate the risks and costs of inferior product performance or product failure occurring in the field or at later production phases.
For both high production and custom machine shops, this trend translates to the need to establish and document the calibration of the geometrical errors in new machines, and to implement a plan to control the calibration status of production machines within their quality system.
There are two separate aspects of machine calibration, and both are essential to machining good parts. These processes are referred to as axis calibration and geometrical calibration.
Axis calibration refers to the measurement and adjustment of the repeatability and accuracy of machine movements along each axis. This task may be undertaken with reference artifacts such as calibrated length bars, or with a laser interferometer. The measuring instrument is used firstly to check that the axis position repeats to the same point after repeated moves and with approaches from each direction. Typical errors detected here include control instability, slide clearances and backlash in axis drives.
After good repeatability has been established, the axis positioning accuracy can be checked and corrected. Normally, the axis will be checked at many points over its full travel distance. The span (accuracy over full travel) may be adjusted in different ways depending on the drive system. This may include adjustment to the axis scale factor in the control parameters, or adjustment to the tension on a linear scale. Machine accuracy over any part of its travel may be further improved by adjustment to the axis compensation table to linearise its motion. In this process, the difference between machine position readings and interferometer readings at many positions along the axis are entered into a look-up table in the control parameters, which the machine then uses to dynamically adjust its actual axis position.
Axis calibration is a relatively fast process that may be completed within several hours on most machines, and if machines have not been regularly calibrated in the past, it is a cost effective means to improve machining accuracy mostly by simple parameter adjustments.
However, even great axis calibrations will not detect or correct the errors in the geometry of a machine that can also cause machining errors. Geometrical errors refer to the straightness of each axis, and the squareness (or parallelism) of every axis to every other axis. In a 3-axis machine there are 9 primary geometrical features that need to be controlled, and to do so requires a separate type of calibration process to measure and where necessary correct these features.
Similarly to axis calibration, there are two main methods used to check machine geometry. Traditionally, a combination of a highly sensitive machinists level and precise artifacts including parallels, cylinders and granite squares have been used, but today laser measurement techniques are increasingly being applied. The traditional methods have been well perfected by highly skilled technicians over many years, but as a general observation it is clear that the tests require meticulous skilled attention and are time consuming, particularly if adjustments need to be made. Furthermore, size limitations of the reference artifacts lead to measurements being taken over sample distances instead of the full machine travel.
Laser geometrical calibration is by no means a new technique, and has been used by leading machine builders around the world for many years. First implemented by the adaption of straightness and squareness optics on interferometer lasers, the process was found by many users to offer accuracy advantages but remained very time consuming to set up even in a limited range of positions for each axis.
Today, ultra precision multi-plane rotating beam lasers, such as the L-743 Triple-Scan Laser System pioneered by Hamar Laser Instruments, have been found by machine builders and installers to offer a more effective alternative technology that is simpler to set up, provides highly accurate geometrical measurements over full “planes” of machine travel in one setup, and displays direct live values of the adjustment required at any point on the machine.
Whichever measuring technology is selected, it is increasing evident that it is necessary to periodically measure and correct machine geometry errors, both to be able to provide validation data to customer, and to address the reality that geometrical variation is likely to occur over time.
CNC Machine tools generate very high machining forces on a continuous basis, taking heavy cuts on tough materials. In addition, the nature of the process is susceptible to occasional collisions in the unfortunate case of programming or part positioning errors. Furthermore, in many cases the parts being machines are large and very heavy, applying considerable loads to the machine structure, even without being knocked or dropped. And the accuracy of larger machines is as dependent on the machine foundation as much as the machine base itself, so ground movement also becomes a contributing error source.
Apart from all these contributing factors, it is often the case that machined features are not (and often cannot be) independently measured after machining is complete, so machine accuracy becomes the single point of control of part quality.
This becomes even more critical in machines where the powerful capabilities of in-machine measurement of a CNC Touch Probe, such as the Blum TC50 System, are implemented. This process advancement provides documented part quality data without the additional production costs of off-line CMM measurement, and in many cases allows precise metrology data to be recorded that was simply not previously available, especially for large parts. A key element of implementing in-machine measurement is control of the machine axis calibration and geometrical calibration.
In summary, planning for the dual requirements of periodic machine axis calibration and machine geometry calibration is becoming an imperative element of the quality plan for component machining operations. Improved control of these machine features offers real benefits in customer quality assurance, improved product quality and the implementation of in-machine measurement techniques with traceable calibration references.
For more information on this topic, please don’t hesitate to call us at ISTecnik on 1300 699 176.