Biomedical Engineering Reference
In-Depth Information
is found that lamellae are ten times more closely packed in the high speed
chips than the low speed chips. The purpose of machining is to create surfaces
that are useful, hence surface quality should be an important consideration of
milling, a measure of this is scallop height.
An improvement is seen in the micro high-speed case with a scallop
height of 1.58x10
-11
m compared to 8.9x10
-9
m for macro slow speeds.
Although both values seem insignificant it must be remembered at the micro
and nano scales post process finishing is inappropriate, therefore created
structures must be produced to specification without further processing.
Additionally, owing to the aspect ratio small imperfections become serious
defects at small scales. From the calculations it can be seen that there is an
improvement in the scallop height, which is not the improvement required
when considering the scale order of magnitude has changed by a factor of four.
This is because the current spindle speeds reached are not high enough for
effective machining, if this speed is increased to 1,400,000 rpm then the orders
of magnitude are increased further still.
The experimental results and observations provide an interesting view of
machining bone at the microscale. When one considers the approximations
made in the derivation of the chip curl model, the experimentally measured
results compare well with the calculated chip curl. This indicates that cutting
tool bending contributes significantly to initial chip curl prior to any
significant frictional interactions on the rake face of the cutting tool. The
proposed model describes the initial stages of chip curl quite well. If the
description of chip curl is accurate, then continuous chip formation at the
microscale needs to be re-investigated. If one considers the movement of the
cutting tool, from point A towards point D, we expect the shear plane to
oscillate between AC and HC depending on the amount of energy required to
move the built-up edge into the segment of the subsequent chip. The cycle
begins again when accumulated material is deposited on to the edge of the
cutting tool then on to the subsequent segment of the chip produced during
machining.
6.
C
ONCLUSIONS
The equations of metal cutting can be applied in the high-speed micro
scale environment. The nomographs of Merchant and Zlatin [2] can be applied
confirming that future calculations can be compared to these well-constructed
charts. High strain rates change the mechanism of chip formation thereby
