Mitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)
39

Modern Technology that Achieves
Minute Cuts for Carbide and Brittle Materials

YOSHIKATSU SATO*1

In cutting processing, recent years have seen the need to process workpieces of various
materials including carbide, brittle materials and other difficult-to-machine materials, as well aseasy-to-machine ones including copper and graphite, with high accuracy and high efficiency.
Mitsubishi Heavy Industries, Ltd. (MHI) has satisfied such needs with the μV1 micro millingmachine, our independently-developed optical image type tool measuring system, as well as byenhancing machining technology, enabling different existing processing machines used for
different workpiece materials and/or different processes to be integrated into a single unit.

|1. Introduction
In an attempt at differentiation from the products produced in emerging countries, mainly incontinuously growing Southeast Asia, domestic manufacturers are pressing for the production ofhigher-precision and higher value-added products for quick delivery at low cost. With thesesituations, where domestic manufacturers are placed in the background, it is notable at present thatdurable high-precision dies are in growing demand for the long-term stable production ofhigh-precision and high-quality products and, as a material for dies, cemented carbide, which isexcellent in heat and wear resistance, is being employed.
Since cemented carbide is characteristically very hard but brittle, electrical dischargemachining or grinding rather than cutting has been used for machining it thus far. Recently,
however, the development of cutting tools has advanced, and direct machining of cemented carbide
dies by cutting is coming to a practical level in terms of quality, quickness of delivery and cost. On
the other hand, in electrical discharge machining as well, electrode materials and the machining
methods used were further developed, gradually bringing low-cost, but high-efficiency machining
into reality.
It is necessary in machining to process with high accuracy, high efficiency
difficult-to-machine materials such as cemented carbide, as well as easy-to-machine materials suchas for electrodes used in electrical discharge machining. To meet the needs of covering bothmaterial extremes, MHI has thus far exerted efforts for high-accuracy, high-efficiency machiningof various materials, using the “μV1” micro milling machine (the main specifications of which are
shown in Table 1) and this paper introduces actual examples of such efforts and the technologies tomaterialize them.

*1

Staff Manager, Engineering Department, Machine Tool Division, Machinery, Equipment & InfrastructureMitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)40

Table 1
TravelTable

SpindleFeed rate
Automatictool changer
Machinedimensions

Main specifications of μV1 micro milling machineX/Y/Z axis
Spindle end to table top
Working surface
Maximum workpiece size
Maximum loading capacity(equal distribution)
Table top shape
Floor to table top
Rotation speed
Spindle taper hole
Rapid traverse rate
Cutting feed rate
Number of tools stored
Maximum tool diameter
Maximum tool length
HeightWidth x depth
Weight450 x 350 x 300 mm
150 to 450 mm
500 x 400 mm
500 x 500 x 200 mm
125 kgT groove width 14 x 3 grooves, 100 mm-pitch
850 mm
400 to 40000 min-1
HSK-E32
15000 mm/min
1 to 15000 mm/min
18pieces, 30 pieces (option)
Φ40 mm130 mm
2260 mm
1920 x 2065 mm
5500 kg|2. Carbide/brittle material machining
2.1

Problem with carbide/brittle material machining

Some tools for cutting cemented carbide mainly use a diamond coat or sintered diamond, but
the cuts of any tool are on a level of several μm to submicron-level, particularly in terms offinishing. Such ultra-minute-cut machining raises the problem that, if error factors such asmachinery positioning inaccuracy, thermal displacement inaccuracy and vibration are significant,the tool cannot cut into the material or that the tool cuts in too deeply causing breakage.
At the same time, since cutting conditions are intensified in rough processing for higher
efficiency of machining, cutting loads increase to cause chatter marks and tool damage if themachine or spindle is less rigid. The process machine used for rough processing must therefore bedifferent from that for finishing, causing a heavier burden of processes and equipment.
Moreover, unless the position of the machining point at the tool end, as well as thermal
displacement, vibration and other behaviors can be grasped correctly, machining becomes difficult,which also makes on-machine measurement technology important. A typical on-machine tool
measuring device is of a contact type, which lets the tool directly contact the sensor for detection,
or a laser type, in which the tool interrupts laser light for detection.
If the timing of measurement is inappropriate in laser type measurement, a machining error
may sometimes occur, which is also naturally seen in contact type measurement where the tool isbrought to a halt for measurement. This is because in the case of laser type measurement, whichallows for instantaneous measurement while the tool turns, it is impossible to determine whether
the thermal displacement in the spindle or the tool has converged, and unless the timing ofmeasurement is right, thermal displacement can take place after measurement.

2.2

The μV1 and optical image type tool measuring system-based solution

The μV1 uses ball bearings in its spindle and makes high rigidity compatible with low fast
vibration characteristics by means of its own special oil lubrication and spindle internal cooling,
thereby realizing low thermal displacement, long-term stability and repetitive reproducibility. In
addition, a sideway is employed on the machine body to provide high rigidity and high attenuation
characteristics, resulting in highly rigid equipment that is a high-precision process machine but
capable of rough processing.
Regarding on-machine measuring technology, MHI developed its own “optical image type
tool measuring system” using a CCD camera, enabling the thermal displacement of the spindle andtool to be measured in real time while allowing the tool to turn at the speed of rotation with which
the tool is machined.(1)

2.3
(1)

Example case of machining
Carbide drilling and profilingSuperfine cemented carbide was subjected to drilling and profiling. Table 2 shows thecutting conditions and Figure 1 is a workpiece photo. The achieved efficiency of machining
was as high as 4 minutes for per hole for drilling and 24 minutes per shape for profiling.

Mitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)41

Table 2
Process

Tool

Drilling
Profiling

Figure 1(2)Cutting conditions for carbide drilling and profilingΦ1.0 drill
Φ1R0.5 ball end millSpindle
rotation speed(min-1)
20000
30000

Pick feed pitch (mm)Feed rate(mm/min)

XY－0.258300

Z0.00040.05

Carbide drilled and profiled workpiece

Carbide punch and die machining
Superfine cemented carbide was machined into punch and die shapes. Table 3 shows thecutting conditions and Figure 2 is a workpiece photo. In total, punch profiling took 4 hours and40 minutes, while die shaping required 6 hours and 30 minutes. Nevertheless, good fitting
accuracy could be obtained without tool wear.
Table 3Process
Very roughprocessing
Rough processing
Rough processing
Semi-finishingSemi-finishing
Finishing
Finishing
Finishing
Finishing

Cutting conditions for carbide punch and die machining
Spindlerotation speed(min-1)

Feed rate(mm/min)Φ6R3 ball end mill

10000

Φ1R0.5 ball end mill
Φ0.6R0.3 ball end mill
Φ0.3R0.15 ball end mill
Φ0.2R0.1 ball end mill
Φ3R1.5 ball end mill
Φ1R0.05 radius end mill
Φ0.2R0.2 ball end mill
Φ0.8R0.4 ball end mill

30000
30000
30000
30000
27500
30000
30000
30000

Tool

Pick feed pitch (mm)
XYXY

200

0.4

0.1200150
100
100200200200200

0.2
0.1
0.03
0.01
0.08
0.1
0.01
0.01

0.05
0.02
0.01
0.01
0.005
0.01
0.01
0.01

Figure 2 Carbide punch- and die workpieces(3)

SiC ceramic grooving
SiC ceramic, which is the third hardest material after diamond and boron carbide, wasMitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)
42

cut. Table 4 shows the cutting conditions and Figure 3 is a workpiece photo. Although Z-axial
cuts were as ultra-minute as 0.001 or 0.002 mm, the resulting machining was favorably freefrom any damage in shape.
Table 4

Spindlerotation speed(min-1)

Feed rate(mm/min)

Φ1 flat end mill40000

Φ1 flat end millProcess
Outer ring roughprocessing
Outer ring finishing
Center hole roughprocessing
Center hole finishing
Blade shape roughprocessing
Blade shape finishing

Figure 3

Cutting conditions for SiC ceramic grooving
Tool

Pick feed pitch (mm)
XYZ

100

0.50.002

40000

100－0.002

Φ4 flat end mill

40000

100－0.001

Φ1 flat end mill

40000

100

0.030.002

Φ1 flat end mill

40000

100

0.030.002

Φ1 flat end mill

40000

1000.03

0.002

Grooved SiC ceramic workpiece|3. Copper/graphite machining
3.1

Problem with copper/graphite machining

Copper or graphite is mainly used as material for electrodes in electrical dischargemachining. Generally, copper and graphite are better in terms of machinability than steel material,
and accordingly the cutting conditions can be intensified.
With today's need for smaller/higher-functioning products, the die and the electrode shape
has also been further miniaturized and its complication has been increased.
When such a miniaturized/complicated shape is machined at a high speed, the shape beingmachined often collapses if the machine itself or the feed system has poor rigidity or if the machineis improperly controlled. To prevent this, the cutting conditions can be relaxed for slow machining,
but when a complicated shape is machined three-dimensionally, the production process suffersfrom issues such as increased machining time, since machining is carried out by making fine cuts
while the position of the workpiece is changed.

3.2

μV1-based solution

The μV1 has, as noted above, a high-rigidity structure in its spindle and in the machine body.
In addition, the employment of large-diameter narrow-pitch ball screws and a strong support
system in the feed axes results in a high-rigidity, high-response feed axes system. Furthermore,
based on the high-speed, high-accuracy NC control function, the use of MHI’s proprietarycontrol-tuned “HGP2 Control” has realized a mechanical behavior of the shape being machined
never collapsing even during high-speed fine/complicated profiling.
Although more machines have adopted a linear motor to drive the feed axes rather than ballscrews, linear motors are good at running for long distances at high speed, but weak in stopping on
a dime or remaining halted, and this indicates that the movements for detailed/complicated shapes
while machining take time in contrast to their image. This is well exemplified with cases where aheavy load is lifted by hand or, using a jack. It is found quicker to simply lift it by hand, but it ismuch easier to use a jack for a quick accurate vertical motion.

Mitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)
433.3(1)

Example case of machining
Connector copper electrode machining
An electrode (small fin-shaped) used for a connector die was machined. Table 5 shows
the cutting conditions and Figure 4 is a workpiece photo. Its profile is square by nature, butmachining with HGP2 control turned off has rounded the corners to collapse the shape asshown in Figure 4(a). In contrast, the HGP2 control-on state has brought about high-speed,
high-accuracy machining even without shape collapse (Figure 4(b)).
Table 5
Process

Cutting conditions for connector copper electrode machiningΦ6 flat end mill

Spindlerotation speed(min-1)8000

Φ0.8 flat end mill
Φ0.4R0.2 ball end mill
Φ0.4R0.2 ball end mill

Tool

Rough processing
Fin roughprocessing
Semi-finishing
Finishing

Feed rate(mm/min)

Pick feed pitch (mm)

1000

XY
3.0

Z
0.5

30000

1000

－0.1

40000
40000

1200
1500

0.05
0.01

0.05
0.05

Figure 4 Workpiece machined into connecter copper electrode(2)Bevel gear die-purpose graphite electrode machining
As a rule, graphite electrodes, unlike copper ones, have advantages such as highmachinability and no burrs, and they allow the flow of large amounts of current for electricaldischarge machining, while at the same time there are also risks including the occurrence ofabnormal friction and surfaces that are too rough. Recently, “copper graphite” orgraphite-impregnated copper has attracted attention, the risks and costs of which were mitigatedwhile taking advantage of graphite’s advantages
Bevel gear die-purpose electrodes were machined using copper graphite. Table 6 shows
the cutting conditions. Then actually prepared electrodes were used to practice electricaldischarge machining of cemented carbide dies and after that, the μV1 was used to finish thework. In Figure 6, a machined copper graphite electrode appears at the top, cemented carbide
subjected to electrical discharge machining with a copper graphite electrode is shown at thelower right, and the same workpiece further subjected to finish cutting with the μV1 is at thelower left. The number of man-hours required could be reduced by about 13% from the level ofexisting cases where electrical discharge machining was followed by polishing.
Table 6Cutting conditions for bevel gear die-purpose graphite electrode machining

Process

Tool

Rough processing
Semi-finishing
FinishingΦ6R1 radius end mill
Φ3R0.5 radius end mill
Φ1.5R0.5 radius end mill

Spindlerotation speed(min-1)
8000
30000
31000

Feed rate(mm/min)
2500
2000
1000

Pick feed pitch (mm)XY
2.0
1.00.5Z
0.5
0.10.03

Mitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)
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Figure 5 Copper graphite electrode workpiece (above) andbevel gear die (lower left: finished, lower right: unfinished)|4. Conclusion
Example cases of high-accuracy, high-efficiency machining of cemented carbide, brittle
materials, copper, and graphite with the μV1 micro milling machine were introduced. In terms of
the recently notable machining of cemented carbide in particular, the μV1’s concept of maintaining
high accuracy stably for a long time and the technology of its optical image type tool measuringsystem to accurately measure tool end match such machining needs well. Moreover, roughprocessing through to finishing can be undertaken by one μV1 unit for workpieces made of a wide
range of materials, from those referred to as difficult-to-machine materials to easy-to-machine
ones. MHI considers that those customers having thus far used different machines for different
workpiece materials, as well as for different processes, can advantageously curtail equipment andcosts.
In the future, MHI will endeavor to further upgrade the machine tool, its options andmachining technology in an effort to meet market needs, thereby contributing to the manufacturingindustry, even in some small way.

Reference
1. Sato, Y. et al., Precision Machining by Optical Image Type Tool Measurement System, Mitsubishi HeavyIndustries Technical Review Vol. 50 No. 1 (2012) pp. 10-15