Nc Machine

Numerical Control Machine

Aim : To study about NUMERICAL CONTROL(NC) Machine Tools.

Introduction:
Numerical Control (NC) refers to the method of
controlling the manufacturing operation by means of directly inserted coded
numerical instructions into the machine tool. It is important to realize that
NC is not a machining method, rather, it is a concept of machine control.
Although the most popular applications of NC are in machining, NC can be
applied to many other operations, including welding, sheet metalworking,
riveting, etc.

History:
The invention of numerical control has been due to the
pioneering works of John T. Parsons in the year 1940, when he tried to
generate a curve automatically by milling cutters by providing coordinate
motions. In the late 1940s Parsons conceived the method of using punched
cards containing coordinate position system to control a machine tool. The
machine directed to move in small increments and generate the desired
finish. In the year, 1948, Parons demonstrated this concept to the US Air
Force, who sponsored the series of project at laboratories of Massachusetts
Institute of Technology (MIT). After lots of research MIT was able to
demonstrate first NC prototype in the year 1952 and in the next year they
were able to prove the potential applications of the NC.

Concept:
PRINCIPLE OF NUMERICAL CONTROL
“a system in which actions are controlled by direct insertion of numerical
data at some point. The system must automatically interpret this data.”
Numerical control, popularly known as the NC is very
commonly used in the machine tools. Numerical control is defined as the form
of programmable automation, in which the process is controlled by the
number, letters, and symbols. In case of the machine tools this programmable
automation is used for the operation of the machines.

In numerical control method the numbers form the
basic program instructions for different types of jobs; hence the name

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numerical control is given to this type of programming. When the type of job
changes, the program instructions of the job also change. It is easier to write
the new instructions for each job, hence NC provides lots of flexibility in its
use.

Program Machine
Instructions Control Unit

Transformation
Process

Power

BASIC BLOCK DIAGRAM

The NC technology can be applied to wide variety of
operations like drafting, assembly, inspection, sheet metal working, etc. But
it is more prominently used for various metal machining processes like
turning, drilling, milling, shaping etc. Due to NC all the machining
operations can be performed at the fast rate resulting in bulk manufacturing
becoming quite cheaper.

NC Coordinate Systems:

For flat and prismatic (blocklike) parts:
 Milling and drilling operations
 Conventional Cartesian coordinate system
 Rotational axes about each linear axis


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For rotational parts:
 Turning Operation
 Only x and zaxes

Components of the Numerical Control System:

There are three important components of the numerical control or NC
system. These are:

1) Program of instructions

2) Controller unit, also called as the machine control unit (MCU) and

3) Machine tool

All these have been shown in the figure below and also described in the
subsequent sections.

1) Program of Instructions

The typical desktop program gives the instructions to the computers to
perform certain functions. The program of instructions of the NC machine is
the stepbystep set of instructions that tells the machines what it has to do.
operations.

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One can also input the instructions directly into the controller unit manually,
this method is called as manual data input (MDI), which is used for very
simple jobs. Then there is direct numerical control method (DNC) in which
the machines are controlled by the computers by direct link omitting the tape
reader.

Program Instruction MCU Machine Tool

2) Controller Unit or Machine Controller Unit (MCU)

The controller unit is most vital parts part of the NC and CNC machines. The
controller unit is made of the electronics components. It reads and interprets
the program of instructions and converts them in the mechanical actions of
the machine tool. Thus the controller unit forms an important link between
the program and the machine tool. The control unit operates the machines as
per the set of instructions given to it.

The typical control unit comprises of tape reader, a date buffer, signal output
channels to the machine tools, feedback channel from the machine tool, and
the sequence control to coordinate the overall machining operation.

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Control Systems of NC Machine:
(1) OpenLoop Control
(2) ClosedLoop Control

(1)OpenLoop Control
 Stepper motor system
 Current pulses sent from control unit to motor
Advantages
 Less complex, Less costly, and lower maintenance costs
Limitations
 Control unit “assumes” desired position is achieved
 No positioning compensation
 Typically, a lower torque motor

(2)ClosedLoop Control
 Variable DC motors Servos
 Positioning sensors Resolvers
 Feedback to control unit
 Position information compared to target location
 Location errors corrected

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Advantages
 DC motors have the ability to reverse instantly to adjust for
position error
 Error compensation allows for greater positional accuracy (.0001”)
 DC motors have higher torque ranges vs.. stepper motors
Limitations
 Cost

3) Machine Tool

It is the machine tool that performs the actual machining operations. The
machine tool can be any machine like lathe, drilling machine, milling
machine etc. The machine tool is the controlled part of the NC system..

Motion control :
(1) Point to Point (PTP)
(2) Continuous (Contouring) Path

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(1) Point to Point (PTP)
 To move the machine table or spindle to a specified position so that
machining operations may be performed at that point.
 Path taken to reach the specific point is not defined.
 Movement from one point to the next is nonmachining, it is made as
rapidly as possible.

(2) Continuous (Contouring) Path
 To control two or more axes simultaneously to get desired shape.
 To control not only the destinations, but also the paths through which the
tool reaches these destinations.
 In the process of machining, the tool contacts the workpiece.

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Role of the Operator:
 Execute Machine Control Unit (MCU) or Console Setups
 Start and Stop Machines
 Load and Unload Workpieces
 Maintain High Level Machine Tool Performance Standards
 Change NC Inputs as Necessary (Per Engineering)
 “Feedback” Information to Programmer/Engineer

Two ways information is fed into an NC machine:
1. Auxiliary Operations:
Tool change, spindle reversal, tool on/off, coolant on/off, spindle speeds
(RPM), spindle feeds (IPM)
2. Geometrical Machine Movements:
a) Translation – X , Y , Z
b) Rotation – about X , Y , Z axis

Any movement under control of NC input is called an axis.

 2 axis machine: X,Y control (usually lathe)
 3 axis machine: X,Y,Z control
 4 axis machine: X,Y,Z, one rotational control
 5 axis machine: X,Y,Z, two rotational control

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Applications of NC:
 Batch and High Volume production
 Repeat and/or Repetitive orders
 Complex part geometries
 Mundane operations
 Many separate operations on one part
Costs:
 High investment cost
 High maintenance effort
 Need for skilled programmers
 High utilization required

Benefits:
 Cycle time reduction
 Nonproductive time reduction
 Greater accuracy and repeatability
 Lower scrap rates
 Reduced parts inventory and floor space
 Operator skilllevel reduced Greater operator efficiency
 Greater operator safety
 Reduction of scrap

NC used to:
1) Position cutter (move table)
2) Change tooling
3) Adjust coolant flow (flood/miston/off)
4) Adjust spindle speeds
5) Perform operations at a point (plunge, tap, bore, etc.)

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Nc Machine

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