ch1 intro nc

7/27/2019 Ch1 Intro NC

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NUMERICAL CONTROL

CHAPTER 1


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What Is Numerical Control?

An automated manufacturing process inwhich a machine tool is controlled;

In an ordered sequenceAlong a predetermined path

To require dimensions

At a specific feed and speed rateDone through a group of alphanumeric

instructions


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A Definition:

Numerical Control is a system in which

actions are controlled by the direct insertion

ofnumerical data at some point. Thesystem must automatically interpret at leastsome portion of the data


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Numerically Controlled Machine Tools

Functionally the same as a conventional machinetool. The difference is in the way in which thevariousmachine functions and slide movements are controlled.

The functions and motions such as;

turning the spindle on and offsetting cutting speeds

setting feed rate

turning coolant on and off

moving tool with respect to work part

are performed by Machine Control Unit (MCU) inNC machine tools.


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MACHINE UNIT

NUMERICAL

CONTROLLER

NUMERICAL

DATA

(NC CODE)

MANUFACTURING

OPERATOR

PROCESSEDPART

Drive Control


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Objectives

1. Increased production

2. Reduced labor costs

3. Make production more economical

4. Make production jobs easier

5. Do production jobs that are impossiblewithout numeric control

6. Increased accuracy on duplicate parts


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Brief History of Machine Tool

Basic machining, as early as 700 B.C.

15th century - Metal machining,

18th century - industrialization, production-type machine tools.

early 20th century - Invention of high speed steel,

First two decades of 20th century - Automated machine controlled bymechanical devices,

Screw machines

Transfer lines

Assembly lines

using cams and preset stops

1955 - John Parsons and US Air Force define a need to develop amachine tool capable of machining complex and close toleranceaircraft parts with the same quality time after time (repeatability).MIT is the subcontractor and builds the machine for the project.


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1959 - MIT announces Automatic Programmed Tools (APT)programming language

1960 - Direct Numerical Control (DNC). This eliminates paper tapepunch programs and allows programmers to send files directly to

machine tools 1968 - Kearney & Trecker machine tool builders market firstmachining center

1970s - CNC machine tools & Distributed Numerical Control

1980s - Graphics based CAM systems introduced. Unix and PCbased systems available

1990s - Price drop in CNC technology

1997 - PC- Windows/NT based Open Modular Architecture Control(OMAC) systems introduced to replace firmware controllers.

Brief History of Machine Tool


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Types of Numerical Control

Conventional Numerical Control (NC)

Direct Numerical Control (DNC) Computer Numerical Control (CNC)


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Conventional Numerical

Control (NC)

Data is sent to the machine tool by means of

punch cards or tapes. The reader at themachine performs no calculations orinterpolations.


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The part cannot produce without a tape being run

through the m/c

The tape has to be run repeatedly depending onthe number of components to be produced.

The modification in the design of components is

not possible, the tape is discarded and new tapewith changed program has to be produced.

Features of NC system


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The control system does not provide the

opportunity to make the changes in speeds and

feeds.

There are always chances of part programming

mistakes on the running tape.

The storage of these input media is also oneproblem.

Features of NC system


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Computer Numerical Control (CNC)

The idea of computer numerical control is to

position a computer right at the machine tool.Most, if not all machine tools that are

numericaly controlled are CNC machine tools.


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Basic Elements of a CNC System

DEFINE GEOMETRY

CREATE TOOL PATH

POST PROCESS TO

CREATE CODE

READ CODE TO

CONTROLLER

INTERPOLATE CODEINTO DRIVE

FINISHED PART

COMPUTER

MACHINE

TOOL

MACHINECONTROL UNIT


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Some ofadditional features available in CNC machine tools are

given below:

1.The part program can be input to controller unit through keyboard

or the tape reader in the control unit can read the paper tape.

2.The part program once entered into the computer memory can be

used again and again.

3.The part program can be edited and optimized at the machine toolitself.

4.The part program can be changed according to the designrequirements of the component produced on the machine tool.

Additional features available inCNC machine


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5. The input information can be reduced to a great extent with the

use of special sub-programs developed for repetitive machining

operations.

6. The CNC systems have facility for providing the part programwithout actually running it on the machine tool.

7. The CNC control unit allows compensation fro any changes in

the dimensions of the cutting tool.

8. The CNC system also provides the facility to obtain informationon machine utilisation, which is useful to the management.

Additional features availablein CNC machine


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Computer Numerical Control (CNC)


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Direct Numeric Control

Is a method where a single computer controlsmany numerical control machine tools. Thesemachine tools may or may not be of a similar nature


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The central computerdownloads complete

programs to the CNC

machines, which can be

workstations or PCs, and

can get the information for

the machine operations.

The speed of the system

is increased, large files can

be handled and the number

of machine tools used is

expanded.

Distributed Numerical Control (DNC)


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Distributive Numerical Control

Because CNC machine tools have the ability to run

multiple programs without being attached to a

centralized computer this enables a centralized

computer to perform many different tasks.Line balancing

Scheduling

Monitoring

Quality


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Execute Machine Control Unit(MCU) or ConsoleSetups

Start and Stop Machines

Load and Unload Workpieces

Maintain High- Level Machine Tool PerformanceStandards

Change NC Inputs as Necessary (As Per Engineering)

Feedback Information to Programmer/Engineer

Role of the Operator


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Advantages

The various Advantages of CNC system are:o Reduces time for delivery of part

o Reduces scrap rate of material

o Reduces tooling costs

o Reduces layout time

o Increases machine and tool life

o Reduces storage problems

o Less setup time

o Reduces actual machining time Allows rapid design

changes in part Less jigs and fixtures are needed


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Advantages of Numerical Control

o Greater operator safety

o Reduced lead time for production

o Fewer chances for human error

o Maximal accuracy andinterchangeability of parts

o Increased productivity

o Minimal spare parts inventory

o Greater machine tool safety

o Fewer man hours for inspection

o Greater machine utilization


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Limitations

o High initial investmentMachine tools cost $30,000 - $1,500,000

o High maintenance requirements

Maintenance personnel must have both mechanicaland electronics expertise

o Not cost-effective for low-level production onsimple parts

As geometric complexity or volume increases CNCbecomes more economical

o Retraining and recruitment of staff

o Newsupport facilities


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Industries Most Affected by

Numerical Controlo Aerospace

o Machinery

o Electricalo Fabrication

o Automotive

o Instrumentationo Mold making


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Chip producing machines:

Drills, Mills, Lathes, Bores, Saws, Etc.

Chipless machining:Flame Cutting, Punches, Wire EDM, Welding,

Non machining:

Paint Spraying, Tube Bending, Assembly, Etc.

APPLICATIONS OF NC MACHINES


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CNC TURNING


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CNC MILLING


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CNC LASER CUTTING


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CNC PLASMA CUTTING


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CNC PRESS


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CNC RAPID PROTOTYPING


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Mass production quantities

Complex geometries

Tight tolerances

Replacement parts

Parts subject to modification

Circumstances best suited to NC


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SAMPLE PRODUCTS

OFCNC MANUFACTURING


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AUTOMOTIVE INDUSTRY

Engine Block


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AUTOMOTIVE INDUSTRY(Contd)

Different Products


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AEROSPACE INDUSTRY

Aircraft Turbine Machined by

5-Axis CNC Milling Machine


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CNC MOLD MAKING


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ELECTRONIC INDUSTRY


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RAPID PROTOTYPING

PRODUCTS

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