ch 2 nc machine

NC/CNC Machine tool

Prepared by

Prof. Chirag Patel Department of Mechanical

EngineeringPacific School Of Engineering,

Surat

NC Machine Tool

Prof.Chirag Patel 3

• Numerically controlled (NC) Machine tool are the machine tool , of which the various function are controlled by Letters, numbers and symbols • Nc machine tools run on programme fed to it, without human operator • Nc programme consist set of instruction or statement for controlling the

motion of the drives of the machine , the motion of cutting tool Function of NC Machine

starting and stopping of the Machin tool Controlling the spindle speed Positioning the tool at desired location and guiding it along the desired path Controlling the feed rate Changing the tool

NC Machine Tool

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Prof.Chirag Patel 4

Elements of NC Machine

Part Drawing

Machine tool

Programme Tape Part program

Machine Control Unit (MCU )

Controller Tape Reader

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(1)Part Drawing and Part Programming • Part programme is set of step by step instruction to the machine tool for

caring out the operation.• Part program is written by using part Drawing and cutting process parameter• The set of instruction includes

Instruction position of machine tool spindle with respect to the work table Instruction for cutting operation Instruction for selection of spindle speed, cutting tool etc.

Method of Part Programing I. Manual Part programming II. Computer aided Part programming

Basic Elements of NC Machine

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(2) Programme Tape • Part programme is entered on the programme tape• The tape is a long wound roll of the one inch with.• The programme is entered on the tape in the form of punched

holes ,holes punched with the help of punching machine.• Now a day punchline tape is replaced with magnetic cassetes , floppy

disc , pen drive , hard drive


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(3)Machine control unit (MCU) • MCU is heart of NC/CNC machine which consist of some electronics

hard wear that read the NC programme , interpret it and conversely translates it for mechanical action of machine tool

• A typical MCU consist following units Input or tape reader memory processor output channels control panel feedback channels

• MCU performs Following functions • read coded instruction • Decode coded instruction• implement interpolation (linier ,circular, and Helical )v to generate axis motion

commands • feed axis motion command to amplified circuits for driving the axis mechanism • Received the signals of position and speed for each drive axis• implement auxiliary control function such as coolant or spindle on/off , and tool

change .


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(4) Machine Tool• The Machine tool is operated by MCU • following function may automated by Machine tool

starting and stopping of the machine tool spindle controlling the spindle speed Positioning of tool tip at desires locations and guiding it along the desired path by

automatic control of the slide motion Controlling the feed rate Changing the tool


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Main component of NC Machin tool

I. work Table II. Machine tool Spindle III. Drive unit IV. Machine control unit V. Feedback mechanism VI. Manual Control VII.Electrically operated control

equipment or magnetic box


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Prof.Chirag Patel 106/29/2016

Nc Machine

According to control loop

feedback system

Open loop type

Close loop type

According to type of tool

motion

Finite positioning

control

Point to point

Straight cut

Continuous path control

Two axis contouring

Two and half axes

contouring

Three axes contouring

Multi-axis Contouring

According to Program method

Absolute programming

Incremental programming

According to type of controller

Nc based controlling

system

Cnc based controlling

system


(a) Open loop type Nc MachineThis type pf control have only motion control without feed back

mechanism The open loop system do not have feedback mechanismEx: drilling Machine

1.According to control loop feedback system


(B) Close loop type Nc MachineThis type pf control have motion control with feed back mechanism The close loop system have feedback mechanism which can accurately controlling

the drive system.It drive system by comparing with input information until require position achieved Feed back is measured by monitoring device to determine the drive displacement.

1.According to control loop feedback system


(A) Finite Position Control NC machine

1) Point to point NC Machine

• PTP is simplest of all control mode

• In this type movement of cutting tool

moves from one “Predefined position” to

another Predefined Position , while path

along which this tool moves irevenant.

• Specially used in local position base

operation i.e: drilling ,punching or

welding

2. According to type of tool motion

A

y

Xo

B


(A) Finite Position Control NC

machine

(2)Straight cut NC machine • It is extension of point to point control mode

with provision of straight line cut or machining

• This straight line cut is obtained by controlling

the movement of tool with controlled feed

rate in one of the axis direction at time

• Ex: face milling ,pocket milling , stepped

turning



(B) Finite Position Control NC machine

2) Continuous path control NC Machine

• Used for continuous simulation and coordinate motion of cutting tool and workpiece along

different AXES

• Such control enable for machining of different contorting profile and curved surface

• Due to simultaneously controlling relative position pass trough the desired countering path of

the final shape of require product

Types of Continuous path control NC Machine

I. Two axes contouring

II. Two and half axes contouring

III. Three axes contouring

IV. Multi – axes countering



Two axes contouring • In this method tool is controlled

simultaneously in only two axes (X and Y )


Two and half axes contouring

• In this method the plane of machining is inclined to the plane of motion of tool (i.e. X –Y plane)

yz

x

Two axis counter milling

z

x

y

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Three Axes Contorting

• In this method tool is controlled simultaneously in only three axes (X ,Y and Z )

• In this mode different type of interpolation techniques are used to getting correct tool movement along path

• Application : turning and machining center , flame cutting ,welding , spark- erosion , sawing machine ,


Multi - axes contouring

• In this method the tool orientation can be changed with the tool motion .

• Ex : A six axes machine tool is capable of moving the tool simultaneously along each primary axis and also rotate about each primary axis at same time

y

x


Why programming needed ?• For preparing programme ,it is necessary to specify the path tool

movement .• Therefore a geometric data of a component is converted in to

machine understandable programme.• Which gives specific path to tool on workpiece.• Programing is the bridge between machine and humans.Types of Programming method

a. Absolute programming NC machine system b. Incremental Programming NC Machine system

3. According to Programming Method

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Absolute programming NC machine system • In absolute Programing system each point is always start from same specific point.• In this method first point (ZERO POINT or REFRENCE POINT ) is fixed.• All Process Start from same reference point


34

1020

10

25

40

10

Point X Coordinator Y Coordinator

P1 10 10

P2 25 20

P3 40 34

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Incremental programming NC machine system • In incremental Programing system each point is specified using the path differential from

the preceding point position.• In this method first point (ZERO POINT or REFRENCE POINT ) is last point of previous path.• Process Start from different reference point


Point X Coordinator Y Coordinator

P1 10 10

P2 15 10

P3 15 14

14

10

10 1510

15


1. Process Planning The decision are made about selection of manufacturing operation , their Sequence ,machine

Requirement and tooling requirements. With the use of part drawing , data about machine tooling ,etc.

2. Part Programming for converting the sequence sheet of various machine operation, understandable syntaxes and various control commands for machines so that machine perform exactly the way program

3. Coding of part program on suitable medium part program converted

into machine readable coding using tape punching machine

4. Part Program Testing And Validation The accuracy of part programming is tested and modified by using the dry run on NC Machine , and then print is taken of tool path for validation

5. NC Machining With Tool Pre-setting once testing and validation of part programme is done operator adjust NC Machine with required tool for final batch production

Steps in NC Manufacturing


Part Drawing

Part Program Listing

Part Programming

Part Program And Set Up sheet

Coding of part program on suitable medium

Process Planning

Sequence Sheet

Part Program Testing and Validation

Part Program Listing

NC Machinig

Finished Part

Data Of Machine Tooling

Tool FileProgramming Manual of NC

Machine

Coding Device Part Program processor

Coding Device Part Program processor

Tool SettingSet up Sheet, part drawing


I. Greater machine useII. Complex machine operation

III. High degree of accuracy IV. Less inspection requiredV. Reduction of scrap and

wastageVI. Reduction lead time for

productionVII. Increase productivity

VIII.Lower tooling costIX. Reduction of human errorX. Greater operation safetyXI. Greater operator efficiency XII. Greater machine tool

efficiency XIII.Greater machine tool safety XIV.Reduced space requirement

Advantage of NC Machine


I. High initial costII. High maintenance cost III. High operation cost IV. Limited life of punching tapes V. Limited consuming punching tape preparation

Application NC lathe and turningNC Milling .boring , drilling and Machining center NC welding and Arc cutting machineNC sawing Machine NC pipe bending ,grinding ,inspection and Gauging

Limitation of NC Machine