Oblique metal cutting is the basic building block for understanding all metal removal processes. However, it presents a 3D assemblage of velocities and forces that is complicated even for the thin shear plane approximation. This Demonstration attempts to collect all the equations given in Sec. 2.5 of . Three shear angle models are incorporated, the minimum energy principle of Merchant (MME), maximum shear stress (on the shear plane) of Krystof (KMSS), and the Armarego–Whitfield (AW) model. The full nomenclature used and the basic relations are available in .
The normal plane
perpendicular to the cutting edge is shown as a green plane, transparent to let you see various angles. The three input angles are the normal rake
, and the rake-face friction
. The five output angles are chip-flow
, normal shear
, oblique shear
, oblique force
between resultant force
, and the normal force
between the projection of
However, two extra relations are derived for the purpose of illustration.
First is the formula for chip thickness ratio
represent the cutting, shear, and chip velocity. Let their magnitudes be
, respectively. The thin shear plane requires that
. It is easy to see from material conservation that
represents the unit-vector normal to the shear plane; then
; the net material accumulating at the shear plane is zero. If the cutting velocity is taken to be of unit magnitude, that is,
Second, a unit vector that lies in the intersection of the shear plane
and the plane
formed by the
is desired. Consider the vector
. This needs to be coplanar with
, which may be written as the scalar vector triple-product equation
is the desired vector. It can be found that
A little more vector algebra can solve for the points on the intersection of the shear plane and the uncut work surface. In this Demonstration,
are shown in red. The lengths of the vectors show relative magnitudes.
With a given friction angle
of the chip sliding on the tool, the direction of the resultant force
can be calculated taking only the two components
along the chip velocity direction and
normal to the tool surface.
can also be computed from the components (given in ) in the directions of the cutting velocity as
, thrust as
, and normal as
. It can be seen in this Demonstration that the two resultant force vectors line up as expected. From the postulated direction of the resultant force vector, two angles
are obtained. The lengths of all vectors show relative magnitudes.
Thus with the three input angles
and five output angles
, the complete force and velocity diagrams (like the Merchant's circle diagram in orthogonal cutting) can be constructed.
The calculation of the two shear angles
from some hypotheses drives the remaining outputs. Unlike orthogonal cutting, there is no single relation connecting the rake, friction, and shear angles. A set of five equations needs to be solved simultaneously under different hypotheses.
The three hypotheses coded in the Demonstration are MME, KMSS, and AW. However, one common approximation used in oblique cutting is Stabler's rule, which says that the chip flow angle is equal to the inclination angle. It is of interest to see which of the hypotheses agree with Stabler's rule.
The MME model involves minimization of the nondimensionalized power
with respect to
gives rise to the equations
This Demonstration plots the five output angles as a "web" plot with a point along the
axis corresponding to
. The baseline is orthogonal cutting shown as the blue curve. When
is changed from 0, all the angles change to form the red curve.
For any setting of
, Stabler's rule
can be compared to the models' results. When
, the MME and AW models converge to the well-known Merchant's model, while KMSS converges to the Lee and Shaffer model.
 Y. Altintas, Manufacturing Automation: Metal Cutting Mechanics, Machine Tool Vibrations, and CNC Design
, 2nd ed., Cambridge: Cambridge University Press, 2012.