Data processing: generic control systems or specific application – Specific application – apparatus or process – Product assembly or manufacturing
Reexamination Certificate
1997-06-03
2004-01-20
Picard, Leo (Department: 2125)
Data processing: generic control systems or specific application
Specific application, apparatus or process
Product assembly or manufacturing
C700S176000, C700S195000
Reexamination Certificate
active
06681145
ABSTRACT:
TECHNICAL FIELD
The present invention relates to machine control, and, more particularly, to a method using 3-dimensional laser measurement of the true position of a machine tool to augment the accuracy and control of a machine. The invention is especially useful in the accurate machining, inspecting, or both of a part based upon a digital definition of the part. A preferred method, apparatus, and related software provide end point control of the machine tool to place holes and other features accurately on aerospace structural detail parts.
BACKGROUND OF THE INVENTION
Machine tools exhibit dimensional positioning errors that are difficult to minimize. The primary contributors to these positioning errors are: (1) expansion and contraction of the machine structure and the workpiece (i.e., the part) because of thermal changes in the factory during machining, and (2) mechanical misalignments of and between individual axes of the machine. The accuracy of the machine is often so uncertain that post-machining inspection of the dimensions of the parts must be made using an independent measuring method. Such inspection requires special tools and skilled workers as well as significant factory space. It slows the production process. Failing inspection, parts must be reworked or scrapped. Post-production inspection, rework, and scrap are the result of poor design or manufacturing processes. The method of the present invention addresses the root cause for errors and, thereby, reduces the need for post-production inspection and the costs of poor quality.
A. Machine Error Control
National standards and “best practices” exist for determining and correcting NC machine geometry errors. (See ANSI/ASME B89.1.12M-1985, ANSI B89.6.2-1973, AMSE B.54-1992) These “best practices” constitute the currently accepted methods for achieving machine accuracy. We will discuss the standards and “best practices” briefly.
1. Thermally Controlled Environment
The machine is held at a constant temperature, e.g., 68° F., in an air-conditioned factory. Errors arising from temperature variations are reduced, but this method does not solve the thermal error problem entirely. Three main drawbacks are:
(i) The cost of controlling the environment is high and sometimes exceeds the cost of acquiring the machine.
(ii) Thermal effects induced by the machine itself ( e.g. motor heat from driving under load, and spindle heating due to friction) still can cause machine distortion
(iii) Mechanical misalignment of axes remains uncorrected. Mechanical alignments change over time as the machine experiences normal and abnormal wear. They are essentially unpredictable, unavoidable, and difficult to control.
2. Machine Calibration
Three-axis machines have 21 error parameters in addition to the errors introduced with the machine spindle. The errors are linearity in each axis (
3
), straightness in each axis (
6
), squareness between each axis pair (
3
), and pitch, yaw, and roll in and between each axis (
9
). Machine calibration measures some or all of these 21 error parameters, then makes physical or software adjustments to the parameters which are out of tolerance. Once each error is identified, quantified, and minimized, the combination of errors are summed using the root mean squares algorithm to gain an estimate for the machine's overall working tolerance. Machine calibration is inadequate for two reasons. First, the method requires extensive machine downtime to measure and to adjust the error parameters. The difficulty in the measurement and adjustment is compounded by the fact that thermal variation causes dimensional changes from shift to shift and day to day. Second, because of constant readjustment of the machine, the changes mean that the final set of data is not a single “snapshot” of the machine errors, but are a series of snapshots each of a different parameter, at a different time, as the machine changes. The root cause of inaccuracy is not fixed, but simply is accommodated between readjustments. Production is a compromise and drift occurs in the produced parts as the machine tool changes.
3. Linear Interferometry of Each Machine Axis
The X, Y, and Z axes of a machine are each equipped with a linear interferometer as an accurate positional encoder. The method allows real-time compensation for thermal growth and shrinkage, but is inadequate for at least three reasons. First, it cannot be applied to rotary axes. Second, it does not compensate for mechanical misalignments between axes. Third, it does not address the interaction between axes as thermal changes occur.
4. Volumetric Look-up Table
This method accurately measures performance of the machine in a specified dimensional envelope. The accurate performance measurements are made using an independent, highly accurate measurement machine to determine the difference between the measured data and the commanded machine position. The collection of all such errors constitutes or can be used to generate an error map. A complete error map is used in two ways. First, the error map may be used as a look-up table to determine a simple position correction to the machine when in that vicinity. Second, polynomial equations can be calculated from the error map to interpolate error corrections over the entire measured envelope. The machine command for a position is adjusted with the polynomial equations. Look-up tables are inadequate primarily because the tables are valid for only one machine temperature. At other temperatures, the machine will be larger or smaller or have a slightly different geometry. There is no guarantee that a machine will behave isometrically and return to its original geometry as temperature changes occur. So, after a laborious data collection exercise leading to an empirical table or set of equations to adjust the position of the machine based upon its history of performance, the root cause(s) for inaccuracy will still continue to degrade the effectiveness of the error map. The error map is inherently inaccurate whenever the machine has changed. As the machine continues to wear and age, variations from the measured offsets of the original error map occur. As a result, errors in part construction may increase. Frequent recalibration is necessary to continue to have an accurate correct error map.
5. Combination of Methods
Certain combinations of these methods can be used to overcome weaknesses in the individual methods, but the net effect remains: (1) long downtime of the machine to measure its true position; (2) expensive testing; and (3) only temporary, corrective results. The root cause for the inaccuracies still remains. For instance, a combination of a thermally controlled environment with machine calibration can result in an accurate machine for a period of time. The cost of controlling the environment combined with the cost of machine downtime for checking and readjusting the machine can be expensive.
6. Thermal Compensation
The axes of the machine are equipped with thermal probes. The temperature measured by each probe is used to calculate independent from the other axes the theoretical expansion of that machine axis. The expansion factors are used to compensate the feedback to the controller, thus eliminating the expansion and contraction of the machine positioning capability. A newer but similar technique called “real time error correction” also uses thermal probes, but attempts to provide a 3D “error model” of the nonlinear thermal behavior of the machine structure. The error map reflects interdependence between axes, such as buckling or warping, caused by heating. Compensation is made with a complicated algorithm that is accurate only for the tested/measured envelope of variation and, then, only as the machine remains repeatable. This error model is established by gathering actual 3D machine position and corresponding temperature data over a range of temperatures, which can require significant machine downtime. It can also be difficult to place the machine in the desired thermal status. While the purpose of this technique is to avoid the costs a
Greenwood Thomas A.
Pastusak Thomas W.
Garland Steven R.
Hammar John C.
Picard Leo
The Boeing Company
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