By Winfried Weiland
Web site: www.blum-novotest.com
Miniaturization of tools, faster machining speeds, and contact-sensitive coatings provide a challenge to tool monitoring in micro manufacturing. Noncontact laser-measuring systems are used exclusively to detect and compensate for all factors influencing tools at cutting speed. An important aspect to consider when choosing the right tool-monitoring system is to ensure that it can demonstrate maximum absolute accuracy, even in extreme machining conditions.
The accuracy of a laser-measuring system is defined by its repeatability and its absolute accuracy. For the user, absolute measuring accuracy is more important, because it defines how precisely workpieces are machined. Laser-measuring systems use a light barrier that operates as a simple switch. When a rotating tool breaks the beam, a skip signal is generated and transmitted to the control to record the machine’s axis positions. Software then compares a calibrated reference value to the tool’s measured values and calculates the tool’s length and radius, automatically entering any deviation into the tool table.
A skip signal is generated only after a certain percentage of the beam is shaded. For this reason, when measuring tools with varying characteristics (e.g., diameter, shape, cutting radius, etc.), the beam is shaded in differing ways. This can result in inaccuracies, particularly in laser systems with simpler optics. Different cutting-edge geometries protrude to unequal depths into the beam before the corresponding shading percentage is reached. A beam with a smaller diameter minimizes the shading error for every cutting edge. Large beam diameters have a correspondingly higher shading error. When using multiple tools, deviations will occur on the workpiece resulting from how different tool contours/cutting edges affect the workpiece. Systems with larger beam diameters will measure small tools to be longer than they actually are, because the tool needed to protrude farther through the beam to cause a skip signal.
High-quality optics precisely focus the laser beam, generating a beam with an extremely homogeneous light intensity. This guarantees maximum precision and absolute accuracy from one tool to another. Another advantage of a focused beam is its ability to measure extremely small tools as small as 5 µm in diameter.
Fluctuations in ambient temperature, tool wear, and changes in length of the machine spindle at different speeds influence machining accuracy greatly in the micron range. Laser-measuring systems for in-process tool setting can accurately account for all of these factors. However, if the machine doesn’t have the appropriate positioning accuracy, even the most precise laser-measuring system has its limits. For this reason, it’s important to develop methods to test machine accuracy on the workpiece and deliver proof of the absolute accuracy of the laser-measuring system itself.
To test the positioning accuracy of a machine, a stepped profile, "micron staircase," is cut into the workpiece along the X axis. The height difference from step to step in the Z axis is 1 µm. The positioning accuracy is then tested by milling across the micron steps along the Y axis. This is done multiple times, each time adjusting the depth of the cut by 1 µm. A microscope is then used to identify the exact micron step where the qualified cut was made. On a high-quality machine, a uniform perpendicular offset cut profile is produced. This gives the user information on the quality of the machine’s positioning accuracy.
A modified test can also be used to demonstrate the absolute accuracy from tool to tool of the laser measuring system. The basic requirement for this is the preceding machine accuracy test. In this test, several tools with very different characteristics (e.g. tool diameter/shape) are measured using the laser system immediately before the cutting process. When the measurement is complete, a defined X dimension is cut in the center of the stepped structure (from Y+ to Y-). Finally, the microscope is then used to determine the micron step on which the first qualified cut was produced for each tool. The position of the first cut provides micron accurate reference for answering the question of how precisely a tool has been measured. If there is a positive variation, the tool has been measured as too long and vice versa. It is possible to measure with absolute accuracy in the 0.5-µm range if only tools with similar characteristics are used. ME
This article was first published in the September 2011 edition of Manufacturing Engineering magazine. Click here for PDF.