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Machine Tool Characterization

 


*The C4 Service Plan _ The Four C’s of Machine Tool Performance

Machine Tools today are complicated and expensive investments for any company. When purchasing a new or used machine it is a good idea to have a plan. Your plan should include how you intend to protect your investment in order to obtain positive return on investment. At MD Calibrations we embrace the “Cradle to Grave” philosophy. The overall objective is to maintain maximum performance and minimum downtime. This is achieved through applying C4, “The Four C’s of Machine Tool Performance”.

C1 _ Characterization

When a machine is delivered and installed, the machines performance has usually been documented back at the factory or at a previous facility. The machine is placed on a new foundation in a completely different environment. The machines performance is entirely different than what it was previously. The geometry (Roll, Pitch, Yaw, Squareness, and Straightness) have all changed. Each of these parametric errors has a tolerance assigned to it. Take angular errors roll, pitch and yaw. Say we assign a tolerance of +/- 5 Seconds. Say at the factory the X axis pitch was adjusted to +5 seconds. On the new foundation it is then adjusted to within -5 seconds. This machine is then technically within OEM specifications. But what does this change have on the machines performance and what can be done about it. Especially when you’re overall concern is the machines Volumetric Performance. Quite often, here at MD Calibrations, we witness situations like this where the machine has been installed and the geometry adjusted then the end user begins the arduous task of debugging the machines performance problems by trying to manufacture their parts. This results in lots of head scratching and frustration.

Upon completion of an installation, we at MD Calibrations recommend a full 3D Characterization be performed. In compliance with National and International Standards such as ANSI/ASME B5.54, ANSI/ASME B5.57 and ISO-230 this testing is performed after the installation has been completed, all covers and guards have been installed and the machine is ready for production. The machines geometry is measured and documented in compliance with the standards. Testing must be performed without making any “ON THE FLY” adjustments to the machine. Any geometry adjustments made during the machine testing may or may not make changes to geometry that has already been documented. The whole idea of a 3D Characterization is to document the machine which will be placed into production. 3D Characterization of a 5 axis machine would consist of the following tests;

X axis Roll, Pitch and Yaw
X axis Straightness (XY and XZ)
XY Squareness
X Positioning
X Periodic Error

Y axis Roll, Pitch and Yaw
Y axis Straightness (YZ and YX)
YZ Squareness
Y Positioning
Y Periodic Error

Z axis Roll, Pitch and Yaw
Z axis Straightness (ZX and ZY)
ZX Squareness
Z Positioning
Z Periodic Error

A axis Parallel to Y axis
A axis Positioning
A axis Periodic Error

C axis Parallel to Z axis
C axis squareness to Y axis
C axis Positioning
C axis Periodic Error

Upon completion of documenting the machines parametric errors, we use our VEE form (MD Calibrations Volumetric Error Estimator) to sum the errors and determine the machines 3D Performance using the method of square root sum of the squares. Initially this form is used to clearly document the machines as found condition for 3D performance. If all geometry errors documented are within OEM specifications we proceed to the next step. The machines linear and rotary axis positioning errors are optimized.

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 to discuss your machine tool calibration needs and to find out how we can meet them.

 

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