21-04-2014, 12:49 PM
Integration of new programming capabilities into a CNC milling system
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Abstract
This paper proposes a series of machine codes selected for integrating advanced programming capabilities into the control of a
modern CNC system. The new programming capabilities were developed and tested in the framework of a PC-based milling
machine controller. Namely, tool-motion along space curves, cutter offsetting for free-form curves and two machining cycles for
revolved (external or internal) surfaces with free-form profiles, constitute the new characteristics proposed to be integrated into the
system of a CNC milling machine. Based on recently developed algorithms whose mathematical description, formulation and
verification are available in respective referred published articles, this paper describes how the new functions are properly integrated
into a CNC milling system. In this direction, a new class of machine codes for the specification of each of the functions is proposed,
while certain topics arised in practice are extensively further discussed. The selected machine codes, together with their
complementary requisite data, needed to be introduced in the NC program, are exemplified via certain examples and actual
machining tests are presented for each of the cases.
Introduction
The development and incorporation of tool path
generators into CNC systems, based on efficient and
accurate curve tracing methods, capable to satisfy the
increasing industrial demand for machining complex-
shape parts is an important goal in the field of
computer-aided manufacturing.
In solid modeling, for example, the generated edges at
which adjacent faces of a solid model intersect are three-
dimensional (3D) curves. In the machining stage
efficient interpolators are needed to drive the cutting-
tool along similar space paths. Tool-radius offsetting of
free-form curves is another case in point. Offset tracing
typically arises in the generation of CAM data from
CAD, with the cutter radius being the offset distance. In
both cases
G-code programming language
The operation of a CNC machine tool is controlled by
a program written in the G-code programming language
called NC or part program. An NC program contains
an ordered sequence of blocks, being essentially
commands specifying in detail the cutter motions to be
executed and the auxiliary operations (e.g., spindle on/
off, spindle speed and feedrate) to be realized by the
CNC machine in order to machine a specified part. That
is, an NC program is executed sequentially, one
command at a time. When a program is executed, the
control will encounter the first command in the
program, execute it, then go on to the second command.
The control executes each command in the same order
encountered. A block is made up of words. Characters,
and numerical digits are the elements that constitute a
word.
Motion along space curves
As a result of the increasing industrial demand for
intricate shapes, the proportion of parts, which are not
covered by the standard CNC linear and circular
motions parallel to the coordinate planes, is certainly
growing. In the direction of satisfying this demand, an
efficient and accurate method for developing a class of
precise interpolation algorithms which can drive the
cutter of a CNC machine along 3D trajectories is
described in [11,13]. The latter work constitutes an
expansion of the method to the field of boundary
machining. In both mentioned papers, the method
engages a pair of primitive shapes and uses their implicit
or parametric definition to interpolate their intersection
curve. Since a large number of space curves may be
obtained as intersections of various shapes taken into
pairs, the method constitutes an efficient tool for
developing a series of real-time space curve interpola-
tors. In the present work, however, we are limited to
dealing with the representative case of space circular
interpolation, while the tool is available for the reader
who wishes to verify specific forms of curves.
CONCLUSION
The main contribution of this paper has been to
enhance the feature-generating potentials of a CNC
milling machine. In this direction, a series of new G-
codes for communicating space circular paths, cutter
offsets along free-form curves and revolved surface
machining to the CNC machine are proposed. Each of
the new programming capabilities is treated separately
and details for proper integration are provided. The key
requirement in all three cases is the incorporation of the
mentioned real-time interpolation algorithms in the
machine’s controller. The machining tests implemented
on the PC-based CNC milling machine confirm the
adequacy of these algorithms in real cutting conditions,
while the use of the proposed codes leads to the
development of more concise NC programs compared
to those obtained with the conventional curve-segmen-
tation approach.