Introduction
to Numerical Control
Numerical
Control: is a form of programmable
automation in which the process is controlled by numbers, letters and symbols.
They constitute instructions designed for a particular job. The program can be
changed to suit other job. This capability to change the program is known as NC
flexibility.
Applications
of Numerical Control are wide ranged:
i)
Drafting ii)
Assembly iii) Inspection
iv) Sheet Metal Press working
v) Spot
Welding vi)
Metal Machining
Historical
Background:
John T
Parson started pioneering works in Numerical Control in late 1940s. He designed
some punched cards with coordinate position data to control machine tool. The
cards directed machine to move in small increments, generating desired surface
of airfoil. After 1948 US Air Force sponsored series of research works in NC at
Servomechanism laboratory of Massachusetts Institute of Technology, USA.
First prototype NC machine was a modified Milling Machine. First demonstration of such machine with position servomechanism was made in 1952.Different programming languages were developed for NC machines. Automatically Programmed Tool (APT) language became a mean to communicate instructions to machine tools in simple English Like statements.
Components of NC system:
Program of Instructions is detailed step-by-step set of directions, which tell the machine tool what to do. It is in the alphanumerical form or in some other pattern interpreted by the controller unit. Most common medium is the 1" wide punched tape. Other input units are punched cards or magnetic tape.
Input to
NC system can be supplied manually. This refers to manual data input (MDI) and
is suitable for small jobs. Another mode of supplying input is use of computer
and it is called Direct Numerical Control (DNC).
Part programmer prepares program instructions. A set of detailed instructions indicating sequence of processing steps, make a program. For machining process, steps include relative movement between cutting tool and work piece.
Controller unit consists of electronics and hardware that read and interpret the program and convert it into mechanical actions of machine tool. Typical components of controller unit are:
a) Tape Reader
b) Data Buffer
c) Signal Output Channels to Machine Tool
d) Feedback Channel From Machine Tool
Modern NC
systems have microcomputers (Microprocessors) as control units. Such systems are
Computer Numerically Controlled (CNC) systems.
Tape
Reader is an electromechanical device that winds and reads the punched tape. The
data contained in punched tape are fed into data buffer that stores input
instructions in logical blocks of information. A block of information represents
one complete step in sequence of processing elements. Signal output channels are
connected to servomotors and other controls in machine tool. To ensure correct
functioning feed back are sent to the controller.
Control panel (control console) consisting of dials and switches is often attached to controller unit or machine tool. The NC system can be run with help of switches in control panel. Features like data display, job status, and other measurements parameters of machining are incorporated in control panel.
Machine tool (Process): It is the part performing actual work. Machine tool consists of worktable and spindle as well as motors necessary to drive them. It also includes cutting tools, work fixtures and other auxiliary equipments needed in machining operations.