Modern Machine Shop

MAY 2018

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MMS MAY 2018 68 MEASUREMENT AND INSPECTION TABLE 1 TEST RESULTS MAXIMUM ERROR BEFORE VEC MAXIMUM ERROR AFTER VEC Machine 1 0.424 mm 0.196 mm Machine 2 1.0 mm 0.039 mm Machine 3 0.162 mm 0.041 mm Machine 4 0.02076 in. 0.00354 in. Machine 5 0.01206 in. 0.00360 in. This table, provided by API, shows five dif ferent types of machine tools to which VEC was applied. The largest single error in any parameter of the machine served as a benchmark in these tests. assembly accommodations such as make-to-fit, drill-at-assembly, shim-as-needed and rework. API's VEC process is essentially a method to create a volumetric map that shows the position of the tool tip in hundreds of points in all possi- ble poses of which its arrangement of linear and rotary axes is capable. Creating this map is an alternative to how a typical three-axis machine used the traditional 21-error-parameter method to measure machine axis errors. In that example, measurements were taken one at a time along the X, Y and Z axes to detect errors in six parameters (linear position, vertical straightness, horizontal straightness, pitch, yaw and roll). Each error parameter required a different setup, and each required its own measurement process. In addi- tion to these 18 error parameters, three more error parameters were checked to determine squareness of the X, Y and Z axes to each other (X to Y, Y to Z and Z to X) for a total of 21 error parameters. The company notes that even with the most sophisticated laser, acquiring this data is a long process that does not count time to check roll error in the Z axis, which must be determined by some other means. API's process consists of a precision laser tracker with an interferometer capability. The interferometer feature is the same technology that has been used with traditional laser systems for years to calibrate machine tools. Coupling the interferometry technology with a laser tracker enables it to track the machine's tool-tip location during extensive axial movements throughout the entire volume of the machine, the company says. To measure and track the tool-tip position accurately, API developed a spindle-mounted tar- geting system called an active target. The active target is described as a compact, motorized-target system that enables the laser tracker to accurately measure the machine's tool-tip position from the laser tracker's position. This capability is significant because the VEC process intentionally exercises the spindle in real time through all the machine's axes during the measurement routine. By using the laser tracker and active target, all 21 error parameters for 200 to 400 randomly selected points within the machine's work enve- lope can be measured. These points represent all possible machine poses, including the rotary axes. The company says that each point is an accurate ref lection of the machine's kinematic errors. A software algorithm extracts error sources and builds an extremely accurate volu- metric map. Based on this map, compensatory values are added or subtracted to the appropriate axis positions as an adjustment to offset the error. In addition to the measuring routine, the API solution includes proprietary calibration software that uses a polynomial-equation-based kinematic model of the machine to map errors through- out its work envelope. In order to compensate for machine error in real time, the API software resides on the machine's controller or an indus- trial computer interfaced to it. In simple terms, the software uses the mea- surements from the laser tracker and active target, then develops compensation values to reduce machine errors. Once the machine is put back in production, the API software runs in the background to monitor the programmed tool path and apply real-time compensations automat- ically to keep the tool tip at the desired location. The company reports that in a variety of field tests, this method improved machine tool accuracy by fourfold or better. The table above, provided by API, shows five dif ferent types of machine tools to which VEC was applied. The largest single error in any parameter of the machine served as a benchmark in these tests. According to the company, a large machine tool can be calibrated and get excellent results using this method in a few hours instead of a week or more. | Automated Precision Inc. | 800-537-2720

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