Online proceedings - EDA Publishing Association

11-13
May 2011, Aix-en-Provence, France
(machining with the side or with the tip), coolant
and tool wear.
The machining parameters were therefore chosen as
follows: For the spindle rotation speed and the feed
rate, the values chosen for the experimental tests were
varied around the recommended values as shown in
Table III.
• For the depth of cut, most of the time a value of
0.02 mm was chosen, even though a lower value
was sometimes tested (especially for the
finishing operations). Using smaller depth of cut
would probably improve the surface roughness,
but it would also increase the time of machining.
• For the tool positioning, both the side and the tip
were used.
• Air and water-based coolants were used. White
spirit was used once only as it was not
environment-friendly.
• For evaluating the influence of tool wear, some
operations were performed twice, using the same
machining parameters, first with a used tool, then
with an unused tool.
TABLE III.
PARAMETERS RECOMMENDED BY TOOL MANUFACTURER
Tool
Spindle rotation Feed rate Depth of
diameter
speed (rpm) (mm/min) cut (mm)
(mm)
0.5 18,000 1.0 0.02
1.0 40,000 1.0 0.02
3.0 60,000 1.0 0.02
3.5 60,000 1.0 0.02
After grinding the sapphire samples, the surface
quality was measured. The best way to analyse the
surface quality is to measure the depth of sub-surface
damage of the surface machined. However, the
surface roughness of a machined area can be
correlated with its sub-surface damage [10]. Since a
lot of samples had to be measured, and because it is
easier to measure a surface roughness than a depth of
sub-surface damage, the surface average roughness
Sa was measured to assess the surface quality of the
machined areas.
The machine used to measure the average roughness
(Sa) was a Talysurf CCI 6000 (white light
interferometer). The lowest value of Sa recorded was
65nm. A magnification of x50 was chosen, which
resulted in a 360µm x 360µm window analysis. The
fractures and the dimensions of the features machined
were measured using the SEM (Scanning Electron
Microscope). This was also a good method to analyse
the aesthetic appearance of the features.
The results appeared to be completely different using
the side or the tip of the tool. Indeed, a better surface
roughness was usually achieved using the tip of the
tool. However, machining with the side of the tool
resulted in a homogenous result, while machining
with the tip of the tool resulted in a nonhomogeneous
result with more fractures. This is
explained for two main reasons:
• Firstly, when machining with the tip of the tool,
the contact area between the tool and the
workpiece is a disk, in which the cutting speed is
not uniform. Indeed, the centre of the tool is not
rotating, and the cutting speed increases with the
radial position up to a maximum value on the
edge of the tool. The middle part of the tool is
therefore not cutting sapphire but rubbing on
sapphire, which most probably chips off some
material. When machining with the side of the
tool, the contact area is theoretically a line where
the cutting speed is uniform.
• Secondly, when machining with the tip of the
tool, since the contact area is a disk, machined
material can get trapped between the tool and the
workpiece, fracturing the sample. This problem
does not occur when machining with the side of
the tool, where the machined material can easily
be removed from the working area.
A comparison between the two machining methods is
shown in Fig. 9. It can be seen that the side-machined
surface has no fractures with a better aesthetic
appearance than the tip-machined surface, but it also
has a higher average roughness than the tip-machined
surface. The average roughness of 810nm for the
side-machined surface is mainly the result of the tool
profile that has shaped the surface. Although the tipmachined
surface has a better average roughness of
690nm, it has a higher light scatter and a resultant
lower aesthetic quality.
Fig. 8. 0.5mm diameter hole diamond machined in sapphire [7].
Using a 0.5mm diameter D76 pintool as a drill, some
well-defined holes (Fig. 8) were also fabricated by
drilling half-way through the disk, turning it over and
registering a second machining operation with the
first operation. This prevented edge fractures.
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