This document contains:
Numerical Control intro, BASIC COMPONENTS OF AN NC SYSTEM, NC PROCEDURE, NC COORDINATE SYSTEMS, NC MOTION CONTROL SYSTEMS, Point-to-point NC, Straight-cut NC, Contouring NC, APPLICATIONS OF NUMERICAL CONTROL, Advantages of NC, Disadvantages of NC, NC part programming, THE P...
Many of the achievements in computer-aided design and manufacturing have a
common origin in numerical control (abbreviated NC). The conceptual framework
established during the development of numerical control is still undergoing further
refinement and enhancement in today's CAD/CAM technology.
Numerical control can be defined as a form of programmable automation in which the
process is controlled by numbers, letters, and symbols. In NC, the numbers form a program
of instructions designed for a particular work part or job. When the job changes, the program
of instructions is changed. This capability to change the program for each new job is what
gives NC its flexibility. It is much easier to write new programs than to make major changes
in the production equipment.
NC technology has been applied to a wide variety of operations, including drafting,
assembly, inspection, sheet metal press working, and spot welding. However, numerical
control finds its principal applications in metal machining processes. The machined work
parts are designed in various sizes and shapes, and most machined parts produced in industry
today are made in small to medium-size batches. To produce each part, a sequence of drilling
operations may be required, or a series of turning or milling operations. The suitability of NC
for these kinds of jobs is the reason for the tremendous growth of numerical control in the
metalworking industry over the last 25 years.
BASIC COMPONENTS OF AN NC SYSTEM
An operational numerical control system consists of the following three basic components:
1. Program of instructions
2. Controller unit, also called a machine control unit (MCU)
3. Machine tool or other controlled process
,Program of instructions
The program of instructions is the detailed step-by-step set of directions which tell the
machine tool what to do. It is coded in numerical or symbolic form on some type of input
medium that can be interpreted by the controller unit. The most common input medium today
is l-in.-wide punched tape. Over the years, other forms of input media have been used,
including punched cards, magnetic tape, and even 35-mm motion picture film.
There are two other methods of input to the NC system which should be mentioned. The
first is by manual entry of instructional data to the controller unit. This method is called
manual data input, abbreviated MDI, and is appropriate only or relatively simple jobs where
the order will not be repeated.
The second other method of input is by means of a direct link with a computer. This is called
direct numerical control, or DNC.
The program of instructions is prepared by someone called a part programmer. The
programmer's job is to provide a set of detailed instructions by which the sequence of
processing steps is to be performed. For a machining operation, the processing steps involve
the relative movement between the cutting tool and the workpiece.
Controller unit
The second basic component of the NC system is the controller unit. This consist of the
electronics and hardware that read and interpret the program of instruction and convert it into
mechanical actions of the machine tool. The typical elements of a conventional NC controller
unit include the tape reader, a data buffer, signal output channels to the machine tool,
feedback channels from the machine tool, and the sequence controls to coordinate the overall
operation of the foregoing elements. It should be noted that nearly all modern NC systems
today are sold with microcomputer as the controller unit. This type of NC is called computer
numerical control (CNC
The tape reader is an electromechanical device for winding and reading the punched tape
containing the program of instructions. The data contained on the tape are read into the data
buffer. The purpose of this device is to store the input instructions in logical blocks of
information. A block of information usually represents one complete step in the sequence of
processing elements. Another element of the NC system, which may be physically part of the
controller unit or part of the machine tool, is the control panel. The control panel or control
console contains the dials and switches by which the machine operator runs the NC system.
,Machine tool or other controlled process
The third basic component of an NC system is the machine tool or other controlled
process. It is the part of the NC system which performs useful work. In the most common
example of an NC system, one designed to perform machining operations, the machine tool
consists of the worktable and spindle as well as the motors and controls necessary to drive
them. It also includes the cutting tools, work fixtures, and other auxiliary equipment needed
in the machining operation.
THE NC PROCEDURE
To utilize numerical control in manufacturing, the following steps must be
accomplished;
1. Process planning.
The engineering drawing of the work part must be interpreted in terms of the
manufacturing processes to be used. This step is referred to as process planning and it is
concerned with the preparation of a route sheet. The route sheet is a listing of the sequence of
operations which must be performed on the work part. It is called a route sheet because it also
lists the machines through which the part must be routed in order to accomplish the sequence
of operations.
2. Part programming.
A part programmer plans the process for the portions of the job to be accomplished by
NC. Part programmers are knowledgeable about the machining process and they have been
trained to program for numerical control. They are responsible for planning the sequence of
machining steps to be performed by NC and to document these in a special format. There are
two ways to program for NC:
Manual part programming
Computer-assisted part programming
In manual part programming, the machining instructions are prepared on a form called a
part program manuscript. The manuscript is a listing of the relative cutter/workpiece
positions which must be followed to machine the part. In computer-assisted part
programming, much of the tedious computational work required in manual part
programming is transferred to the computer. This is especially appropriate for complex work-
piece geometries and jobs with many machining steps
3. Tape preparation.
A punched tape is prepared from the part programmer's NC process plan. In manual part
, programming, the punched tape is prepared directly from the part program manuscript on a
typewriter like device equipped with tape punching capability. In computer-assisted part
programming, the computer interprets the list of part programming instructions, performs
the necessary calculations to convert this into a detailed set of machine tool motion
commands, and then controls a tape punch device to prepare the tape for the specific NC
machine.
4. Tape verification.
After the punched tape has been prepared, a method is usually provided for checking the
accuracy of the tape. Sometimes the tape is checked by running it through a computer
program which plots the various tool movements (or table movements) on paper..In this way,
major errors in the tape can be discovered. The "acid test" of the tape involves trying it out
on the machine tool to make the part. A foam or plastic material is sometimes uses this tryout
5. Production
The final step in the NC procedure is to use the NC in production. This involves ordering the
raw work parts, specifying and preparing tooling and any special fixturing that may be
required, and setting up the machine tool for the job. The machine tool operator's function
during product to load the raw work part in the machine and establish the starting position of
cutting tool relative to the workpiece. The NC system then takes over machines the part
according to the instructions on tape.
NC COORDINATE SYSTEMS
Two axes, x and y, are defined in the plane of the table, as shown in Fig. The z axis is
perpendicular to this plane and movement in the z direction is controlled by the vertical
motion of the spindle. The positive and negative directions of motion of tool relative to table
along these axes. NC drill presses are classified as either two-axis or three-axis machine
depending on whether or not they have the capability to control the z axis.
For turning operations, two axes are normally all that are required to command the movement
of the tool relative to the rotating workpiece. The z axis is the axis of rotation of the workpart,
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