Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)10Precision Machining by
Optical Image Type Tool Measurement System

YOSHIKATSU SATO*1

Due to the globalization of production bases and increasing demand for accuracy in recent
years, machines and applications that can achieve stable accuracy independently from theoperator's skill are necessary. Mitsubishi Heavy Industries, Ltd. (MHI) has been seeking
technologies that can meet such needs. Our uniquely developed optical image type tool
measurement system can automatically optimize and perform warm-up of the machine, toolmeasurement, machining and other processes that were conventionally dependent on the operator'sexperience and intuition. The stable accuracy of the machines and the user friendliness of theapplications that MHI has attained can meet the latest needs in the manufacturing industry, such as
the creation of high-added value, the shortening of delivery time and the stabilization of quality.

|1. Introduction
In the manufacturing industry, which is undergoing globalization, there is growing demand
for the creation of high-added value and the shortening of delivery time in the Japanese market, as
well as quality stabilization and the improvement of productivity in the overseas market. In respect
to processing equipment, not only are basic machine capabilities such as high speed, high accuracy,high stability and high reproducibility increasingly in demand, but so are peripheral applications
such as on-machine measurement.
In responding to these requests, MHI has equipped the μV1 micro milling machine (exteriorview shown in Figure 1 and specifications in Table 1) with our uniquely developed optical image
type tool measurement system to work on various high precision machining and measurementtasks.
This paper describes the features and examples of the system.

Figure 1*1

μV1 micro milling machine

Engineering Manager, Engineering Department, Machine Tool

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)11

Table 1
Travels

Table

SpindleFeedrate

Specifications of μV1 micro milling machine

X-axis x Y-axis x Z-axis
Spindle end to table surface
Working area
Maximum workpiece size
Maximum laden weight (distributed evenly)
Shape ofWidth of T-grooves
upper surface
Pitch
Floor to table surface
Rotation speed
Tapered hole of spindle
Rapid traverseCutting feedratemmmmmmmm
kgmmmmmm
min-1mm/minmm/min

Number of tools-Automatic toolchange device

Machinedimensions

Maximum tool diametermm

450×350×300
150 to 450
500×400
500×500×200
125
14100
850
400 to 40000
HSK-E3215000
1 to 15000
18 or30 (optional)
φ40

Maximum tool length
HeightWidth x length

mmmm
mm

1302355
1920×2054

Weightkg

5500|2. Features of μV1 micro milling machine
One of the inhibitory factors for high precision machining is the thermal displacement of themachine. In particular, the thermal displacement of the spindle, which is the greatest heat source,
accounts for a large portion of such factor. The maximum rotation speed of the spindle of the μV1
is 40,000 min-1, and this machine uses ball bearings, not noncontact bearings such as air bearings,
in order to attain high rigidity. As a result, it is important to reduce and remove the high heatgenerated by friction and pressure during high speed rotation of the ball bearings.
Typical spindles have a coolant path only in the external jacket, which is the casing of thespindle. However, the μV1 has an additional coolant path inside the high-speed rotating spindle, inaddition to our unique method for lubricating the bearing in order to forcibly and quickly remove
the generated heat (Figure 2). In this way, the μV1 reduces thermal displacement to the minimum
level, even at the maximum rotation speed of 40,000 min-1, and attains stability after the saturationof displacement and high reproducibility in repeating starts/stops of spindle rotation (Figure 3).
Such stability and reproducibility establish a foundation for not only machining accuracy, butalso the improvement of on-machine measurement accuracy and reliability.

Figure 2 Spindle coolingsystemFigure 3 Spindle thermal displacement

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)12|3. Features of optical image type tool measurement system
For high precision machining, on-machine measurement including tool measurement andworkpiece measurement is essential and one of the important factors equal to machine accuracy,
because measurement errors directly result in machining error. In the past, however, machine tool
manufacturers simply equipped their products with measuring instrument manufacturer's products,and left their operation to users.
Taking this into consideration, we decided to develop on-machine measurement applications
that are more convenient from a user perspective. The newly developed optical image type tool
measurement system resolves various problems with existing tool measurement devices. Existing
tool measurement devices are classified into contact types where the tool makes contact with asensor and laser types where the tool blocks the laser beam, and both have various features.
However, they have the following problems:(1) Contact type. Tool breakage due to impact caused by contact may occur to tools with smaller diameters,
particularly in the case of tools with a diameter of 1 mm or smaller.
. Measurement errors caused by wear of the measuring surface of the sensor may occur.
. Measurement is performed in a stopped state. Errors such as thermal displacement due torotation after measurement may occur.
(2) Laser type (Figure 4). An error due to a possible difference between the actual end of the tool and the laser beam
radiation point may occur.
. Because measurement uses the blocking rate of the laser beam, an error may occur dependingon the shape of the tool end.
. A tool thinner than the laser beam cannot be measured because it cannot block the laser beam
sufficiently.

Figure 4

Existing laser type tool measurement system

These systems also have an operational problem where a measurement error may occur due
to thermal displacement of the machine described in the previous section. Laser type methods canmeasure the tool during its rotation, but can only perform instantaneous measurement using theon/off signal of the laser beam. Therefore if measurement is performed during ongoing thermaldisplacement, for example, all of the subsequent thermal displacement will result in machiningerror.
To prevent such errors, typically tool measurement is performed after a "sufficient" warm-up
operation. However, how long is "sufficient?" On the other hand, the contact type method needs tostop the spindle for the measurement of a tool. However, how necessary is it to take theinstantaneous thermal displacement into consideration? Like the answers to these questions, theactual operation significantly depends on the operator's skill, experience and intuition.
The optical image type tool measurement system uses a high resolution CCD camera toperform tool measurement where no contact occurs and the tool is measured as an area, not a point.
As a result, problems with existing tool measurement devices are resolved. Figure 5 shows theconfiguration of the optical image type tool measurement system.

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)
13Figure 5

Configuration of the optical image type tool measurement system

A saturation determination function has also been added. This function uses continuous datacollected by the camera to measure changes in the condition of the machine and the tool with acertain time interval, and then detects the saturation and stabilization of their thermal displacement.
Figure 6 shows a schematic view. This function enables the system to automatically optimize andperform warm-up, measurement and machining processes that were dependent on the operator's
skill in the past.

Figure 6

Comparison of existing system and optical image type system|4. Examples of high precision machining
One example where the problems with the existing tool measurement devices as describedabove appear on actual machining is mold machining, and direct machining in particular. In manycases, direct machining uses different tools with different diameters or shapes to cut a mold, andthe machining conditions differ for each tool. In such cases, a difference in machined height
between tools may occur as a result of measurement errors caused by differences in tool shapes, ordifferences in thermal displacement saturation time due to different spindle speeds. Hand finishingor polishing for correction can also take time, or even result in the workpiece being discarded and a
new one created from scratch in the worst case scenario.
Because such machining often takes a long time, the wear of the tool and the thermal
displacement of the machine, tool, workpiece, etc., due to the heat generation of the machine, as
well as changes in peripheral temperature during machining, may cause machining accuracy errors.
We developed and implemented test machining to verify whether or not these problems canbe resolved by an optical image type tool measurement system. Table 2 shows the machining
conditions. In this verification, six different tools with different diameters and shapes were used at
different speeds to form tile shapes on a workpiece targeting a single height. For each tool, tool
length measurement was performed before and during machining so that differences in length
between tools, wear of the tool, and thermal displacement of the spindle were eliminated. In
addition, the saturation determination function was used to automatically stabilize thermal
displacement during machining. The height of the reference plane of the workpiece was measured
several times in machining so that the thermal displacement of the workpiece could automaticallybe eliminated.

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)
14Table 2Tool number
1234
5
6

Machining conditions

Tool typeR3 ball end mill
φ2 flat end mill
R2 ball end mill
φ1 flat end mill
R0.5 ball end mill
φ0.3 flat end mill

Spindle rotation speed (min-1)
16,00020,00026,00032,00040,00040,000The length of the probe used for workpiece height measurement was measured on-machineby the optical image type tool measurement system in each case. In the conventional measurementof workpieces and tools, the length of the probe, the height of the tool length measuring device, andthe distance between the spindle and the table at the machine home position are considered as fixedvalues, and these values are used to calculate the height of the workpiece and the length of the tool.
However, because these dimensions have natural thermal displacement caused by heat generationof the machine and changes in peripheral temperature, measurement errors occur. In order toeliminate these errors relatively, the length of the probe was measured by the tool measuring device
as described above.
The optical image type tool measurement system enables accurate measurement of the probe
length, which was difficult for existing tool measuring devices. Figure 7 shows the probe length,
the workpiece reference plane height, the tool length, and the machine peripheral temperature in
actual workpiece machining. Although it took four days in total to finish machining, this was notcontinuous machining; the machine was turned off after eight to ten hours of machining each day,and then was turned on the next day before machining was resumed. The measured value of theprobe length varied significantly (this value includes thermal displacement of not only the probe,
but also the machine and the tool measuring device) because the change in the peripheral
temperature of the machine was up to 5 degrees. If the conventional method, which considers thisvariation as a fixed value, were used, an error equivalent to this value would have occurred. Of
course, this was a verification test where measurement errors from the conventional method
increased because the machining was interrupted by being intentionally turned off, and themeasurement of the workpiece reference height was performed in each case; therefore this differsfrom the actual machining of a workpiece. However, in the case of the machining of many
workpieces, resetting a workpiece or re-machining, an error in workpiece measurement appears at
each stage, and the ability to eliminate such errors is meaningful.

Figure 7

Measurement result in actual workpiece machining

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)15

Figure 8 shows the measured unevenness of the machined workpiece surface. The height
difference for a straight line passing over all planes machined with the six types of tools is slight;
this verifies the error suppression effect of the optical image type tool measurement system. In fact,
at a user's site, a machine delivered with this system was well received after it attained a machining
accuracy of 2 μm when measurement was performed with this method after automatic changes ofworkpieces.

Figure 8Measured unevenness of the machined workpiece surface|5. Conclusion
Examples of accurate machining using the μV1 micro milling machine equipped with theoptical image type tool measurement system are described above. This system enables the machine
to automatically optimize and perform warm-up, measurement, and machining processes that weredependent on the operator's skill in the past, and can attain accurate machining using multiple tools,
which was conventionally a difficult task.
MHI will continuously seek the further improvement of machine technologies, as well as
applications such as on-machine measurement, in order to meet market needs and make acontribution to the development of the manufacturing industry.