16-10-2012, 12:34 PM
NUMERICAL CONTROL AND INDUSTRIAL ROBOTICS
NUMERICAL CONTROL.pdf (Size: 1.83 MB / Downloads: 167)
Numerical Control
A form of programmable automation in which the
mechanical actions of a piece of equipment are
controlled by a program containing coded
alphanumeric data
•The data represent relative positions between a
workhead and a workpart
Workhead = tool or other processing element
Workpart = object being processed
•NC operating principle is to control the motion of the
workhead relative to the workpart and to control the
sequence in which the motions are carried out
Components of a Numerical Control System
•Part program - the detailed set of commands to be
followed by the processing equipment
•Machine control unit (MCU) - microcomputer that
stores and executes the program by converting each
command into actions by the processing equipment,
one command at a time
•Processing equipment - accomplishes the sequence
of processing steps to transform starting workpart
into completed part
NC Coordinate System
•Consists of three linear axes (x, y, z) of Cartesian
coordinate system, plus three rotational axes (a, b, c)
Rotational axes are used to orient workpart or
workhead to access different surfaces for
machining
Most NC systems do not require all six axes
NC Positioning System
Converts the coordinates specified in the NC part
program into relative positions and velocities between
tool and workpart during processing
Leadscrew pitch p - table is moved a distance
equal to the pitch for each revolution
Table velocity (e.g., feed rate in machining) is set
by the RPM of leadscrew
•To provide x-y capability, a single-axis system is
piggybacked on top of a second perpendicular axis
Statistical Distribution of Mechanical Errors
•When a positioning system is directed to move to a
given control point, the capability to move to that
point is limited by mechanical errors
Errors are due to a variety of inaccuracies and
imperfections, such as play between leadscrew
and worktable, gear backlash, and deflection of
machine components
• It is assumed that the errors form an unbiased normal
distribution with mean = 0 and that the standard
deviation is constant over axis range
Computer-Assisted Part Programming
• Uses a high-level programming language
Suited to programming of more complex jobs
First NC part programming language was APT =
Automatically Programmed Tooling
In APT, part programming is divided into two
basic steps:
1. Definition of part geometry
2. Specification of tool path and operation
sequence
CAD/CAM-Assisted Part Programming
Takes computer-assisted part programming further by
using a CAD/CAM system to interact with
programmer as part program is being prepared
• In conventional use of APT, program is written and
then entered into the computer for processing
Programming errors may not be detected until
computer processing
•With CAD/CAM, programmer receives immediate
visual verification as each statement is entered
Errors can be corrected immediately rather than
after entire program has been written