Biomedical Engineering Reference
In-Depth Information
the laminates may indeed control the direction of fracture during the formation
of a chip and will inevitably change the direction and angle of the shear plane
during machining. Therefore, a simple chip curl model that accounts for the
change in laminate direction and tool bending is possible.
3.
C
HIP
F
ORMATION
Chip curvature is a significant parameter in machining operations from
which a continuous chip is produced. In this paper, observations are made on
initial chip curl in the simplified case of orthogonal cutting at the microscale.
The cutting process may be modeled using a simple primary shear plane and
frictional sliding of the chip along the rake face. When the region of chip and
tool interaction at the rake face is treated as a secondary shear zone and the
shear zones are analyzed by means of slip-line field theory, it is predicted that
the chip will curl. Thus, chip curvature may be interpreted as the consequence
of secondary shear. Tight chip curl is usually associated with conditions of
good rake face lubrication [4]. At the beginning of the cut, a transient tight curl
is often observed, the chip radius increasing as the contact area on the rake
face grows to an equilibrium value. Thus, it might be suggested that tight curl
is an integral part of the primary deformation due to friction interactions
between chip and tool edge.
The process of continuous chip formation is not uniquely defined by the
boundary conditions in the steady state and that the radius of curl may depend
on the build-up of deformation at the beginning of the cut [4]. A treatment of
primary chip formation at the microscale is presented, which considers chip
curl as a series of heterogeneous elements in continuous chip formation at the
microscale. The free surface of the chip always displays ‗lamellae', which are
parallel to the cutting edge. The chip is usually considered to form by a regular
series of discrete shear events giving a straight chip made up of small parallel
segments. It is assumed that multiple shear planes are created owing to the
formation of discrete lamellae. However, no account is taken of how bone
material moves passed the tool between shearing events. The following
observations follow on from Doyle, Horne, and Tabor's [4] analysis of
primary chip formation.
Figure 1 shows the instabilities during chip formation (previously defined)
that gives rise to instantaneous chip curl. The shaded range of Figure 1b is the
consequence of a built-up edge that very quickly becomes part of the
segmented chips shown in Figure 1d. This ‗material' provides the means to
