Mitsubishi Heavy Industries Technical Review Vol. 57 No. 3 (September 2020)
1Improvement and Stabilization of Spatial Accuracy ofEntire Processing Area Using Three-Dimensional
Spatial Error Correction System for
Double-Column Milling Machines

KOJI IWATA*1

KATSUHIRO FUJIMOTO*2

SHINGO NAKAMURA*3

The improvement of machining precision for double-column milling machines is inincreasing demand, especially for such as automobile die and mold machining. Whileconventionally the priority lies in the precision attainable within a limited machining area and therelative error compared to the adjoining machined areas, demand in recent years calls for
improving overall machining precision. In double-column milling machines, the error caused bychanges in the level of the machine foundations over time, etc., is not insignificant. This report
describes the “three-dimensional spatial error correction system,” which can maintain spatial
accuracy throughout the machine’s entire stroke range over a long period of time, successfullyensuring the required machining precision in a stable manner.

|1. Introduction
In machine tools, spatial error can occur as a result of fluctuating temperatures in the factory
environment or changes in the level of the foundations on which the machine is installed. Spatialerror here denotes the error in positioning the tip of the spindle in a space defined by the threecoordinate axes of X, Y and Z, and involves three position errors and three orientation errors.
Generally speaking, error due to temperature fluctuations is compensated for using technology tocorrect thermal displacements of the machine. Mitsubishi Heavy Industries Machine Tool Co., Ltd.
has further reduced the spatial error caused by environmental temperature change by employing theinternal spindle cooling system to prevent the occurrence of error and the thermo-stabilizer column
to minimize displacement due to temperature changes. On the other hand, in the case of a change inthe level of the machine foundations, the resulting spatial error can be minimized by enhancing thestructural rigidity of the machine body for smaller machines. However, when it comes to facilities
for large parts exceeding 2 m in size, such as dies/molds and semiconductor production equipment,
the effect of such changes is not insignificant and compromises machining precision, leading topart quality issues such as defective accuracy or requiring returning to a previous processing stage
for adjustment or modification.

|2. Three-dimensional spatial error correction system
Changes in the factory environment or the level of the machine foundations over time canresult in errors in positioning the tip of the spindle in the X, Y, Z-coordinate space. The
three-dimensional spatial error correction system as shown in Figure 1 was developed to make itpossible for the machine itself to automatically correct such geometric deviations.
*1Engineering Manager, Customer Support Center, Customer Support & Sales Headquarters, Mitsubishi Heavy Industries*2Engineering Manager, Machine Tool Designing Section, Engineering Headquarters , Mitsubishi Heavy Industries Machine*3Value Chain Industrial Technology Group, Value Chain Headquarters, Mitsubishi Heavy Industries Machine Tool Co., Ltd.

Machine Tool Co., Ltd.
Tool Co., Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 57 No. 3 (September 2020)
22.1

Measurement system

Automatic measurement units (touch probes) are used to measure true spheres on thestandard gauges installed along the X, Y and Z axes around the table as shown in Figure 2. The
detection and correction of spatial error of the machine results in greater form accuracy (forexample, in terms of straightness and squareness) and better dimensional accuracy, thereby
realizing high-precision machining. Being made of a special material free from thermal
deformation, the standard gauges are mounted on the table using a floating support structure andtherefore are completely unaffected by external factors such as changes in temperature or themachine foundations. Moreover, as protection against cutting chip and fluid coming from themachine tool, the standard gauges are normally kept inside a protective cover that automatically
opens at the time of measurement to allow access to the gauges.

Figure 1 Spatial error correction system

2.2

Figure 2 Measurement using
master gauge

Correction system

The correction system as shown in Figure 3 is comprised of the arithmetic logic unit tocompute how much correction is necessary and the three-dimensional spatial error correction unitthat can reflect the computation results in the computerized numerical control (CNC) unit’s
instructions for positioning. Based on the spatial error detected by the measurement system, thearithmetic logic unit for correction computation calculates the deviation of position using reference
grid points in the three-dimensional coordinate system that is constructed in such a way as to cover
the machine’s entire stroke range for the X, Y and Z axes as shown in Figure 4. The
three-dimensional spatial error correction unit then proportions the amount of correction between
grid points based on the positioning error of the grid points, before reflecting this in the CNC unit’spositioning instructions.

Figure 32.3Correction system Figure 4

Positioning error of grid points

Results

With the three-dimensional spatial error correction system, it has become possible to correct
the machine precision (e.g., straightness and squareness), which is adversely affected by changes in
the level of the machine foundations. One example of this includes a straightness of 18-25 μm
before correction, being improved to 3-5 μm after correction. Regularly using this system makes itpossible to maintain the machine’s milling precision over a long period of time as shown inFigure 5.

Mitsubishi Heavy Industries Technical Review Vol. 57 No. 3 (September 2020)
3Figure 5

Example results of correction for changes in level of machine foundations|3. Effectiveness
Here we present an application case of a customer who manufactures dies and molds. The
three-dimensional spatial error correction system was installed on a large precision machine
MVR30Fx. Over the three years since its introduction in 2017, the required machining precision interms of spatial accuracy has been maintained in a stable manner throughout the machine’s entire
stroke range. In a practical sense, the need for upper/lower die clearance adjustment, which wasconventionally an indispensable modification process, has been eliminated as shown in Figure 6.
Consequently, when manufacturing dies that are 2.5 m long, 40 hours per die have been saved.
Moreover, the problem of compromised form accuracy of the finished product after themodification process has also been resolved.

Figure 6Images showing sliding section for upper/lower die clearance adjustment|4. Conclusion
The aforementioned application example pertains to the newly introduction of afully-automatic correction system that can be embedded in the machine. The outcome has beenoutstanding for the customer, who required high-precision performance. To enable easy application
to existing machines, we are also developing a portable system that can be carried by the operator
him/herself, and we are planning to make it available in our maintenance services. By offering thistechnology to customers looking to improve the level and stability of required machining precision,
we will continue to help solve the problems arising at manufacturing sites.