Mitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)106

High-Speed & High-Quality Laser Drilling Technology
Using a Prism Rotator

SANEYUKI GOYA*1TOSHIYA WATANABE*2

JUNPEI YAMAGUCHI*3 TSUGUMARU YAMASHITA*4RYU SUZUKI*5

The laser head for processing or the processing stage in the conventional laser processing
method is mechanically rotated for drilling. The processing speed depends upon these structures’acceleration/deceleration, which is limited by their masses and rigidities. Therefore, the laser
processing method has so far had limitations in enhancing the processing speed. Since processing
speed is limited, mitigation of the laser’s thermal influences affecting processing quality has also
been limited. In coping with the problems, a laser optical system, with a mass much smaller thanthat of the laser head or drilling stage was rotated quickly, thereby realizing much fasterprocessing with processing quality maintained equal to or better than that of conventional laser
processing.

|1. Introduction
Laser machining is quite popular as a technology for fast processing. This was because lasers
can melt and evaporate the work piece efficiently as being so directional and light-harvesting as toconcentrate energy in a micro area, resulting in an increase in processing speed. On the other hand,in some cases, the high energy density prevents a sufficient processing quality from being obtained.
This is because the irradiated laser’s excessive heat input into the processing object causes anaffected layer in the processed area under the influence of heat, and such a heat-affected layer
prevents the specifications required for the work piece, including material strength, from being
satisfied. In the laser processing field, therefore, heat influence-free processing, or the so-called
non-thermal processing technology, is under active development.
In drilling, quick laser mobility based on factors such as the fast movement of agalvanometer mirror to increase processing speed enables processing to be faster than by the laser
head’s mechanical rotation, thus serving as an extremely powerful tool for drilling when no assist
gas is required(1)-(4). But at present, since laser cutting requires an assist gas supply in many cases,
fast processing technology using a tool such as the galvanometer mirror is sometimes applicable
only to quite limited areas.
As a challenging method to make both quick laser mobility and assist gas supply compatible,
this research tried to improve processing speed and quality by polarizing the optical path of thelaser through prisms and then by rotating these prisms quickly, while the laser head kept fixed.
This paper reports on the verification results concerning the higher speed and better
processing quality likely to be achieved by allowing a polarizing prism to rotate quickly.

|2. Principle of processing
Figure 1 shows a conventional laser drilling method. In conventional laser drilling, since alaser head set is rotated (or a work piece is allowed to circulate around the laser head) for*1 Yokohama Research & Development Center, Technology & Innovation Headquarters*2 Chief Staff Manager, Yokohama Research & Development Center, Technology & Innovation Headquarters*3 Engineering Department, Machine Tool Division, Machinery, Equipment & Infrastructure*4 Chief Staff Manager, Solution Technology Center, Machine Tool Division, Machinery, Equipment & Infrastructure*5 Engineering Department Mitsubishi Heavy Industries Aero Engines Ltd.

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processing, the mechanical movement speed of the laser head (or stage) determines the processing
speed, thus making it difficult to shorten processing time from the level of seconds even for a small
hole with a submillimeter-order diameter.
Hence, in order to realize a processing speed comparable to that of a galvanometer mirror
system, only the opposing pair of prisms was rotated with the laser head kept fixed as shown in
Figure 2 for this development, in an attempt to embody an optical system that allows laser light torevolve quickly.

Figure 1

Conventional drilling method

Figure 2 Conceptual diagram ofmachining head for fast drillingThis embodies a quick motion approximate to that of the galvanometer mirror optical system
by using the prism’s polarization angle with the optical axis to adjust the processing diameter andallowing the polarizing optical system including prisms to turn quickly. In addition, by keeping theprism’s turning radius equal to or shorter than the inside diameter of the nozzle outlet, assist gas
could always be fed in the direction vertical to the drilling axis and one of the defects due to fast
processing by the galvanometer mirror system was thus successfully compensated for.
Figure 3 shows the principle of polarization through a prism. When two prisms of the same
shape are placed so as to oppose each other as shown in Figure 3 (a), each prism’s polarization
angle is offset by the other’s and the laser goes straight ahead through the center of the condenser
lens to be directly placed on the work as if it were not passing through any prisms. On the other
hand, if these prisms are rotated in the direction opposite to each other as shown in Figure 3 (b), thehorizontal polarization angle is cancelled and the vertical one is aligned, permitting emitted light tocurve upward as in the figure, and as a result, the focal point on the work also moves upward fromthe center. If the prisms are rotated with their polarization angles sustained, a hole is drilled along
the locus of a circle with a radius equal to the length of travel by the focal point from the work
center.

(a) When prisms were placedin the direction opposite to each otherFigure 3 Principle of polarization through prisms(b) When prisms are rotatedin the direction opposite to each otherMitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)
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Figure 4 shows the laser intensity distribution in the image formation area obtained fromoptical tracking simulation for different prism polarization angles. Even if a prism causes the image
formation position to move on the work, the laser intensity distribution is found to remainunchanged.

Figure 4

Simulation results of image formation|3. Processing work verification
Metal processing conditions for drilling (material: Inconel, plate thickness: 1 mm, holediameter: 0.7 mm), were optimized, aiming to achieve an improvement in both processing quality
and processing speed by using fiber laser.
Figure 5 is the photographed surface of a hole drilled under optimized laser processing
conditions and Figure 6 is a cross-sectional photo of the same. Table 1 compares the present
studied drilling method with a conventional one for the relative difference in processing time andquality. As shown in Figures 5, 6 and Table 1, although even conventional methods have met thespecified processing quality, further prospective improvement of processing quality wasdemonstrated, together with the possibility of processing speed being shortened to 1/9.

Figure 5 Processed surface view after
being made appropriate

Figure 6 Cross-sectional view of processed holeafter being made appropriate

Table 1 Comparison with conventional method for processing time/quality
Conventional method
The present reported development

Drilling time1
Up to 1/9

Processing quality
Good
Excellent|4. Equipment composition
The equipment composition used for recent demonstration experiments is as follows (seeFigure 7): For first allowing an arbitrary diameter to be drilled and then realizing fast processing, it
was necessary to rotate two opposing prisms quickly and change phases without steps. Hence, eachof the two prisms (Prism 1 and Prism 2) was controlled by either of two independent direct-drive
servo motors (Motor 1 and Motor 2). If these two prisms are synchronized and kept turning
continuously, the acceleration/deceleration time of prism rotation can be reduced to zero whenidentical hole diameters are processed, thus enabling processing time to be further shortened. In
addition, a laser-interferometric measurement sensor and a drilling point checking camera wereincorporated into a single optical system to precisely measure the focal position and shorten thenozzle’s center location setup time.

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Furthermore, by installing a curved-axis motor to tilt the nozzle and a swiveling table on theside of the work, it became possible to drill a three-dimensional shape with a laser.

Figure 7

Machining head composition|5. Conclusion
By allowing a prism unit to turn alone to polarize laser light with the laser head fixed, anoptical system capable of turning quickly was developed. The laser drilling technology using aprism rotator was demonstrated to improve processing quality and shorten processing time to 1/9
compared with conventional methods.

Reference
1.

Miyamoto, Journal of The Japan Society for Precision Engineering, Vol. 75 No.1 (2009) p. 66.