Online proceedings - EDA Publishing Association

11-13 May 2011, Aix-en-Provence, France
III. LASER MACHINING
The details of the four different lasers used for
micromachining sapphire are summarised in Table II.
The diode-pumped solid state (DPSS) laser is a
Trumpf TruMicro 5350 picosecond laser with
maximum pulse energy of 50µJ, operating at 343nm;
the most energetic of the laser wavelengths tested.
Holes were trepanned as shown in Fig. 4. An
additional observation that can be drawn from the
figure is that the dimensions of the successfully
drilled holes are different on both sides,
demonstrating tapering. Hole H1 has a diameter of
2.5 mm on the laser entrance side but only 2.1 mm on
the exit side showing significant sidewall tapering.
The most successful laser was the mode-locked
Lumera Laser GmbH Nd:YVO 4 picosecond pulse
duration laser operated at MEC in the uv range
(355nm) as a third harmonic from the 1064nm
wavelength. This laser has a maximum power of 2W
and maximum pulse energy of 20µJ. Holes and a
curved profile are shown in Fig.5. Less taper on the
cut profiles was noted in comparison to the DPSS
laser.
The Oxford Lasers nanosecond pulse duration copper
vapour laser (CVL) operates in the visible region of
the electromagnetic spectrum with the intensity of the
green wavelength (511nm) twice that of the yellow
wavelength (578nm). Some machining was effected
but the resolution was poor and fracturing at edges
was prevalent in small holes of Φ = 0.1mm.
However, it should be noted that even frequency
doubled CVL radiation of 255nm wavelength has
only been used successfully in the past to scribe
sapphire to a maximum thickness of 90µm [9].
The ytterbium-doped glass fibre laser has the highest
repetition frequency and the longest wavelength (in
the near ir) of the lasers investigated. The absorption
of the 1064nm wavelength is therefore extremely low
and the machining is ineffective over an acceptable
maximum processing period of a few minutes.
IV. DIAMOND MACHINING
Typical sapphire components may require flats,
chamfers and grooves to be machined and these
features cannot be fabricated using laser technology.
Diamond machining was therefore tested to
determine whether it was a viable manufacturing
technique. It should be noted that the surfaces
produced needed to be extremely smooth and
optically transparent to meet the aesthetic
specifications in addition to those related to
dimensions and tolerances. Diamond machining was
carried out on a Kern Evo Machining Centre fitted
with a prototype high speed Westwind air-bearing
spindle capable of 350,000 rpm.
Electroplated diamond pin tools (D76 and D126)
were used for machining (Fig. 6). Phenolic resinbonded
pin tools (D25 and D07) were utilised to
reduce Sa but proved unsatisfactory due to high bond
wear rate.
Fig. 6. D76 electroplated diamond pin tool (Φ= 1mm)
To machine a flat step, either the side or the tip of the
diamond pin tool was used. In both cases, the
sapphire disk was waxed onto a holder after an
essential preheating to melt the wax. Then, the disk
and the holder were mounted in the machine (on a
pallet or in a vice); the surface of the disk being
either horizontal (to use the tip of the tool) or vertical
(to use the end of the tool).
To machine a chamfer, the sapphire sample was
waxed onto an aluminium set square holder. Firstly, a
flat step was machined on the sample, then the holder
was rotated by 45° and mounted on the pallet
(without having to unwax the sapphire sample) and
the chamfer was machined as shown in Fig.7.
Fig. 7. Sapphire disk mounted on aluminium set square holder to
machine a 45º chamfer using the tip of the tool [7].
Machining parameters included; spindle rotation
speed, feed rate, depth of cut, tool positioning used
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