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Machine Tool Characterization
 Machine tool performance is directly linked to the parametric errors within a machine. Parametric testing isolates and identifies each of the error sources.
Time spent documenting a machine's geometry and performance through parametric testing, during initial installation or prior to moving it from one location to another, can save a great deal of time and money. Subsequent problems can easily be traced back to changes that occurred since the testing was performed.
Base line characterization is the systematic approach to acquiring and documenting machine tool parametric errors. Using our proprietary V Double E "Volumetric Error Estimator" program, we can easily and rapidly use the data collected to calculate the Volumetric Performance of a machine. This program is initially used to calculate the volumetric performance of a machine using the manufacturer's specification for the machine. This value is then input as the machine's "Standard". Then, as the machine is calibrated the OEM specs are replaced by the measured results. The VEE program will recalculate the Volumetric Error Estimator as each new value is entered and provide a VEE deviation from standard.
Our V Double E program uses the conventional definition of 3D volumetric positioning error as the root mean square of the displacement and geometry errors. For a better and more complete understanding of this method for calculating 3D volumetric performance, you can refer to the in-depth article and case study prepared by Mr. Charles Wang, President of Optodyne, Inc. at http://www.optodyne.com/opnew4/techart23.html.
Click on the link below to view a sample Volumetric Error Estimator report calculated for a 5 Axis White Sundstrand Omni-Mill.
Volumetric Error Estimator (VEE) in PDF Format (2.1MB)
Each linear motion in a machine contains 7 parametric errors:
1. Axial linear positioning
2. Axial straightness horizontal direction
3. Axial straightness vertical direction
4. Roll error
5. Pitch error
6. Yaw error
7. Axial squareness
In addition to the 7 basic parametric errors associated with linear motion, there are others that can be extrapolated from the data acquired during testing.
From axial linear positioning we obtain repeatability, average and maximum reversal error.
From axial straightness we obtain translational lost motion.
From roll, pitch and yaw we obtain angular lost motion.
The 7 basic errors associated with linear motion simply represent the starting point in base line characterization of a machine tool. However, most of the positioning problems in machine tools can be traced to one of these basic parametric errors.
Performing additional tests helps to further isolate and identify performance problems such as:
1. Periodic error testing of the positioning and feedback device
2. Servo performance
3. Contouring performance using the Telescopic Ball Bar
4. Laser diagonal performance
5. Rotary positioning accuracy, periodic error, repeatability and reversal errors
6. Off axis/Trunion positioning accuracy, periodic error, repeatability and reversal errors
7. Tool change repeatability
8. Pallet change repeatability
At MD Calibrations, we perform base line characterizations on machine tools of all makes and models. Our summary report (PDF, 2.9MB) includes all parametric results, as well as setup information in compliance with ISO documentation requirements. By fully documenting the setup procedure, we ensure repeatability and reproducibility of the test results.
After calibration, the VEE program can again be used to track machine performance. The Volumetric Error Estimator for the calibrated and optimized machine is input for the "Standard" value. The next time the machine is calibrated, the VEE value will provide the user with an easily understood number identifying whether the machines performance is improving or degrading.
A full base line characterization typically takes 5-7 hours per axis. A typical 4-axis machine can be characterized in 2-3 days. We can’t tell you how often we’ve heard maintenance mechanics mumble, "I wish I had known what this error was prior to the crash!"
Most machine tools represent a significant investment for a company. Performing base line characterization is an insurance policy on that investment. If something goes wrong, the machine can be corrected and put back into production as quickly as possible.
Contact MD Calibrations today for updated pricing and availability.
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