Computer Integrated Manufacturing Notes

7/28/2019 Computer Integrated Manufacturing Notes

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COMPUTER INTEGRATED MANUFACTURING

(Theory & Practice)

Subject Code: 10IM64 CIE Marks: 100 + 50

Hrs/Week: L T P SEE Marks: 100 + 50

3 0 2

Credits: 04 SEE : 3 HrsCourse Learning Objective:

Unit I 06 Hrs

Introduction to CAD/CAM: Evolution of CAD/CAM, Need for CAD System, CAD/CAM in

Automation, benefits & advantage of CAD/CAM.

NC, CNC & DNC Technologies: Co-ordinate systems, basic motion control system.

Application of NC System, Advantages and limitations of NC Machines. Need for CNC, CNC

system, Functions and advantages of CNC System. Components of DNC system, Functions and

advantages of DNC System

Unit II 06 HrsCNC Machines: Horizontal machining center(HMC), Vertical machining center(VMC),

Universal machining center(UMC), Turn mill centers, Vertical turning centers, High speedMachining center, Machine control Unit(MCU)

CNC Programming: Steps in Part Programming, NC manual part programming, G & M codes

for turning and milling, Canned Cycles, Problems on Milling and turning using G & M codes.

Unit III 06 Hrs

Automation: Introduction, Types of Automation, Organization & information processing in

manufacturing, Production concepts, Automation Strategies.

High Volume Production System: Automated flow lines, work part Transport, Transfer

Mechanism, and Buffer Storage.

Unit IV 06 Hrs

Automated Assembly System: Types, Parts feeding Devices, Analysis of Single Station

Assembly Machine, Analysis of Multi station Assembly machine, Automated Material

handling System, Automated guided vehicle system.

Unit V 06 hrs

Computerized Manufacturing Planning System: Computer Aided Process Planning,

retrieval types, Generative type.

Flexible Manufacturing Systems: Definition, FMS workstations, Materials handling &

storage system, Computer control, Applications & benefits

Shop Floor Control: Factory Data Collection System, Bar code technology, bar code symbol,bar code reader.

Unit VI

(Laboratory Work)

Part I

1. Creation of Orthographic views.

2. Conversion of Isometric view to Orthographic Views.

3. Conversion of orthographic views to Isometric View.


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Part II

4. Analysis of Simple & Compound bars Subjected to Axial Loads.

5. Analysis of Trusses subjected to point loads.

6. Analysis of Beams Subjected to concentrate & UDL loads.

7. Analysis of Shafts subjected to twisting moment.

Part - III

8. Turning & Milling operation on CNC Train Software.9. Demonstration of Flexible manufacturing system.

Suggested Software Packages: Auto-CAD, ANSYS, CADEM

Note: A minimum of 10 experiments to be conducted covering the entire syllabus in Unit VI.

Reference Books:

1. Mikell. P. Grover & E.W. Zimmer, CAD / CAM, by PHI, New Delhi 2003, ISBN: 0131101307

2. Ibrahim Zeid, CAD / CAM, McGraw Hill. 2000, ISBN 0070728577.

3. Mikell.P.Groover, Automation, Production system and Computer Integrated Manufacturing, PHI

New Delhi, 2007, ISBN 0132393212

Scheme of Continuous Internal Evaluation:

CIE consists of Three Tests each for 45 marks (15 marks for Quiz + 30 marks for descriptive) out of

which best of two will be considered. In addition there will be one seminar on new topics / model

presentation etc. for 10 marks.

Scheme of Semester End Examination:

The question paper consists of Part A and Part B. Part A will be for 20 marks covering the complete

syllabus and is compulsory. Part B will be for 80 marks and shall consist of five questions carrying 16

marks each. All five from Part B will have internal choice and one of the two have to be answered

compulsorily.

Scheme of Semester End Evaluation (SEE) (Laboratory Work, Unit VI): 50 Marks

Question I : 20 marks

Question II : 20 marks

Viva Voce : 10 marks


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AN OVERVIEW OF CNC MACHINES

( 1 ) Historical Perspective

The word NC which stands for numerical control refer to control of a machine or a process using

symbolic codes consisting of characters and numerals. The word CNC came into existence in

seventies when microprocessors and microcomputers replaced integrated circuit IC based

controls used for NC machines. The development of numerical control owes much to the United

States air force. The concept of NC was proposed in the late 1940s by John Parsons who

recommended a method of automatic machine control that would guide a milling cutter to

produce a curvilinear motion in order to generate smooth profiles on the work-pieces. In 1949,

the U.S Air Force awarded Parsons a contract to develop new type of machine tool that would be

able to speed up production methods.

Parsons sub-contracted the Massachusetts Institute of Technology (MIT) to develop a practical

implementation of his concept. Scientists and engineers at M.I.T built a control system for a two

axis milling machine that used a perforated paper tape as the input media. This prototype was

produced by retrofitting a conventional tracer mill with numerical control servomechanisms for

the three axes of the machine. By 1955, these machines were available to industries with some

small modifications.

The machine tool builders gradually began developing their own projects to introducecommercial NC units. Also, certain industry users, especially airframe builders, worked to devise

numerical control machines to satisfy their own particular production needs. The Air force

continued its encouragement of NC development by sponsoring additional research at MIT to

design a part programming language that could be used in controlling N.C. machines.

In a short period of time, all the major machine tool manufacturers were producing some

machines with NC, but it was not until late 1970s that computer-based NC became widely used.

NC matured as an automation technology when electronics industry developed new products. At

first, miniature electronic tubes were developed, but the controls were big, bulky, and not very

reliable. Then solid-state circuitry and eventually modular or integrated circuits were developed.

The control unit became smaller, more reliable, and less expensive.

(2) Computer Numerical Control

Computer numerical control (CNC) is the numerical control system in which a dedicated

computer is built into the control to perform basic and advanced NC functions. CNC controls are

also referred to as soft-wired NC systems because most of their control functions are

implemented by the control software programs. CNC is a computer assisted process to control


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general purpose machines from instructions generated by a processor and stored in a memory

system. It is a specific form of control system where position is the principal controlled variable.

All numerical control machines manufactured since the seventies are of CNC type. The computer

allows for the following: storage of additional programs, program editing, running of program

from memory, machine and control diagnostics, special routines, inch/metric,

incremental/absolute switchability.

CNC machines can be used as stand alone units or in a network of machines such as flexible

machine centres. The controller uses a permanent resident program called an executive program

to process the codes into the electrical pulses that control the machine. In any CNC machine,

executive program resides in ROM and all the NC codes in RAM. The information in ROM is

written into the electronic chips and cannot be erased and they become active whenever the

machine is on. The contents in RAM are lost when the controller is turned off. Some use special

type of RAM called CMOS memory, which retains its contents even when the power is turned

off.

CNC milling machine

( 1.3 ) Direct Numerical Control

In a Direct Numerical Control system (DNC), a mainframe computer is used to coordinate the

simultaneous operations of a number NC machines as shown in the figures 21.2 & 21.3. Themain tasks performed by the computer are to program and edit part programs as well as


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download part programs to NC machines. Machine tool controllers have limited memory and a

part program may contain few thousands of blocks.So the program is stored in a separate

computer and sent directly to the machine, one block at a time.First DNC system developed was Molins System 24 in 1967 by Cincinnati Milacron and General

Electric. They are now referred to as flexible manufacturing systems (FMS). The computers that

were used at those times were quite expensive.

DNC system

DNC system


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Manually operated Milling Machine

Computer Controlled Milling Machine

Some of the dominant advantages of the CNC machines are: CNC machines can be used continuously and only need to be switched off for occasional

maintenance.

These machines require less skilled people to operate unlike manual lathes / milling

machines etc.

CNC machines can be updated by improving the software used to drive the machines.

Training for the use of CNC machines can be done through the use of 'virtual software'.

The manufacturing process can be simulated virtually and no need to make a prototype or

a model. This saves time and money.

Once programmed, these machines can be left and do not require any human

intervention, except for work loading and unloading.

These machines can manufacture several components to the required accuracy without

any fatigue as in the case of manually operated machines.

Savings in time that could be achieved with the CNC machines are quite significant.


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Some of the disadvantages of the CNC machines are:

CNC machines are generally more expensive than manually operated machines.

The CNC machine operator only needs basic training and skills, enough to superviseseveral machines.

Increase in electrical maintenance, high initial investment and high per hour operating

costs than the traditional systems.

Fewer workers are required to operate CNC machines compared to manually operated machines.

Investment in CNC machines can lead to unemployment.

( 5 ) Applications of NC/CNC machine tools

CNC was initially applied to metal working machinery: Mills, Drills, boring machines, punchpresses etc and now expanded to robotics, grinders, welding machinery, EDM's, flame cutters andalso for inspection equipment etc. The machines controlled by CNC can be classified into the

following categories: CNC mills and machining centres.

CNC lathes and turning centers

CNC EDM

CNC grinding machines

CNC cutting machines (laser, plasma, electron, or flame)

CNC fabrication machines (sheet metal punch press, bending machine, or press brake)

CNC welding machines

CNC coordinate measuring machines

CNC Coordinate Measuring Machines:

A coordinate measuring machine is a dimensional measuring device, designed to move the

measuring probe to determine the coordinates along the surface of the work piece. Apart from

dimensional measurement, these machines are also used for profile measurement, angularity,digitizing or imaging.

A CMM consists of four main components: the machine, measuring probe, control system and

the measuring software. The control system in a CMM performs the function of a live interactionbetween various machine drives, displacement transducers, probing systems and the peripheral

devices. Control systems can be classified according to the following groups of CMMs.

1. Manually driven CMMs

2. Motorized CMMs with automatic probing systems

3. Direct computer controlled (DCC) CMMs


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4. CMMs linked with CAD, CAM and FMS etc.

The first two methods are very common and self explanatory. In the case of DCC CMMs, thecomputer control is responsible for the movement of the slides, readout from displacement

transducers and data communication. CMM are of different configurations-fixed bridge, moving

bridge, cantilever arm figure 21.5(a), horizontal arm and gantry type CMM as shown in figure

21.5(b).

Cantilever type CMM

Gantry type CMM

(6 ) CNC welding machines:


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4 axis CNC Tig welding machine

The salient features of CNC welding machines are:

Superior quality and weld precision.

These machines are also equipped with rotary tables.

Weld moves, welding feed rate, wire feed, torch heights & welding current can be

programmed.

CNC welding machines are used for laser welding, welding of plastics, submerged arc

welding, wire welding machines, butt welding, flash butt welding etc.

These machines are generally used in automobile work shops

Cost of these machines will be twice than the conventional welding machines.

CNC EDM & WEDM machines:

EDM is a nontraditional machining method primarily used to machine hard metals that

could not be machined by traditional machining methods. Material removal will be taking place

by a series of electric arcs discharging across the gap between the electrode and the work piece.

There are two main types- ram EDM & wire cut EDM. In wire-cut EDM, a thin wire is fed

through the work piece and is constantly fed from a spool and is held between upper and lower

guides. These guides move in the x-y plane and are precisely controlled by the CNC. Wire feed

rate is also controlled by the CNC.

Ram EDM Wire cut EDM


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CLASSIFICATION OF CNC MACHINE TOOLS

(1) Based on the motion type ' Point-to-point & Contouring systems

There are two main types of machine tools and the control systems required for use with them

differ because of the basic differences in the functions of the machines to be controlled. They are

known as point-to-point and contouring controls.

(1.1) Point-to-point systems

Some machine tools for example drilling, boring and tapping machines etc, require the

cutter and the work piece to be placed at a certain fixed relative positions at which they must

remain while the cutter does its work. These machines are known as point-to-point machines as

shown in figure 22.1 (a) and the control equipment for use with them are known as point-to-point

control equipment. Feed rates need not to be programmed. In theses machine tools, each axis is

driven separately. In a point-to-point control system, the dimensional information that must be

given to the machine tool will be a series of required position of the two slides. Servo systems

can be used to move the slides and no attempt is made to move the slide until the cutter has been

retracted back.

(1.2) Contouring systems (Continuous path systems)

Other type of machine tools involves motion of work piece with respect to the cutter

while cutting operation is taking place. These machine tools include milling, routing machines

etc. and are known as contouring machines as shown in figure 22.1 (b) and the controls required

for their control are known as contouring control.

Contouring machines can also be used as point-to-point machines, but it will be

uneconomical to use them unless the work piece also requires having a contouring operation to

be performed on it. These machines require simultaneous control of axes. In contouring

machines, relative positions of the work piece and the tool should be continuously controlled.

The control system must be able to accept information regarding velocities and positions of themachines slides. Feed rates should be programmed.


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Point-to-point Contouring Systems

Contouring systems

22.2 Based on the control loops ' Open loop & Closed loop systems

22.2.1 Open loop systems:

Programmed instructions are fed into the controller through an input device. Theseinstructions are then converted to electrical pulses (signals) by the controller and sent to the

servo amplifier to energize the servo motors. The primary drawback of the open-loop system is

that there is no feedback system to check whether the program position and velocity has been

achieved. If the system performance is affected by load, temperature, humidity, or lubricationthen the actual output could deviate from the desired output. For these reasons the open -loop

system is generally used in point-to-point systems where the accuracy requirements are not

critical. Very few continuous-path systems utilize open-loop control.


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Figure 22.2 (a) Open loop control system Figure 22.2 (b) Closed loop control system

Courtesy: http://jjjtrain.kanabco.com/vms/Media/glossary_o/cnc_opencloseloop.gif

Courtesy: http://jjjtrain.kanabco.com/vms/Media/glossary_o/cnc_opencloseloop.gif

Figure 22.2 (c) Open loop system

22.2.1 Closed loop systems:

The closed-loop system has a feedback subsystem to monitor the actual output and

correct any discrepancy from the programmed input. These systems use position and velocity

feed back. The feedback system could be either analog or digital. The analog systems measure

the variation of physical variables such as position and velocity in terms of voltage levels. Digitalsystems monitor output variations by means of electrical pulses. To control the dynamic behavior

and the final position of the machine slides, a variety of position transducers are employed.

Majority of CNC systems operate on servo mechanism, a closed loop principle. If a discrepancyis revealed between where the machine element should be and where it actually is, the sensing

device signals the driving unit to make an adjustment, bringing the movable component to therequired location.

Closed-loop systems are very powerful and accurate because they are capable of

monitoring operating conditions through feedback subsystems and automatically compensatingfor any variations in real-time.


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Figure 22.2 (d) Closed loop system

(3 ) Based on the number of axes ' 2, 3, 4 & 5 axes CNC machines.

( 3.1) 2& 3 axes CNC machines:

CNC lathes will be coming under 2 axes machines. There will be two axes along whichmotion takes place. The saddle will be moving longitudinally on the bed (Z-axis) and the cross

slide moves transversely on the saddle (along X-axis). In 3-axes machines, there will be one

more axis, perpendicular to the above two axes. By the simultaneous control of all the 3 axes,

complex surfaces can be machined.

( 3.2 ) 4 & 5 axes CNC machines:

4 and 5 axes CNC machines provide multi-axis machining capabilities beyond thestandard 3-axis CNC tool path movements. A 5-axis milling centre includes the three X, Y, Z

axes, the A axis which is rotary tilting of the spindle and the B-axis, which can be a rotary index

table.


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Figure 22.3 Five axes CNC machine

Importance of higher axes machining :

Reduced cycle time by machining complex components using a single setup. In addition

to time savings, improved accuracy can also be achieved as positioning errors between setups are

eliminated.

Improved surface finish and tool life by tilting the tool to maintain optimum tool to part

contact all the times.

Improved access to under cuts and deep pockets. By tilting the tool, the tool can be madenormal to the work surface and the errors may be reduced as the major component ofcutting force will be along the tool axis.

Higher axes machining has been widely used for machining sculptures surfaces in

aerospace and automobile industry.

(3.3) Turning centre:

Traditional centre lathes have horizontal beds. The saddle moves longitudinally and thecross slide moves transversely. Although the tools can be clearly seen, the operator must lean

over the tool post to position them accurately. Concentration of chips may be creating a heatsource and there may be temperature gradients in the machine tool. Keeping the above points inview, developments in the structure of the turning centres lead to the positioning the saddle and

the cross slide behind the spindle on a slant bed as shown in the figure 22.4. Chips fall freely

because of slant bed configuration which is more ergonomically acceptable from operator's pointof view.


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Figure 22.4 Slant bed turning centre

22.4 Based on the power supply ' Electric, Hydraulic & Pneumatic systems

Mechanical power unit refers to a device which transforms some form of energy to mechanicalpower which may be used for driving slides, saddles or gantries forming a part of machine tool.

The input power may be of electrical, hydraulic or pneumatic.


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22.4.1 Electric systems:

Electric motors may be used for controlling both positioning and contouring machines.

They may be either a.c. or d.c. motor and the torque and direction of rotation need to be

controlled. The speed of a d.c. motor can be controlled by varying either the field or the armature

supply. The clutch-controlled motor can either be an a.c. or d.c. motor. They are generally usedfor small machine tools because of heat losses in the clutches. Split field motors are the simplest

form of motors and can be controlled in a manner according to the machine tool. These are smalland generally run at high maximum speeds and so require reduction gears of high ratio.

Separately excited motors are used with control systems for driving the slides of large machine

tools.

22.4.2 Hydraulic systems:

These hydraulic systems may be used with positioning and contouring machine tools of allsizes. These systems may be either in the form of rams or motors. Hydraulic motors are smaller

than electric motors of equivalent power. There are several types of hydraulic motors. Theadvantage of using hydraulic motors is that they can be very small and have considerable torque.

This means that they may be incorporated in servosystems which require having a rapid response.

( 1 ) Different components related to CNC machine tools

Any CNC machine tool essentially consists of the following parts:

( 1.1 ) Part program:

A part program is a series of coded instructions required to produce a part. It controls the

movement of the machine tool and on/off control of auxiliary functions such as spindle rotationand coolant. The coded instructions are composed of letters, numbers and symbols.

( 1.2 ) Program input device:

The program input device is the means for part program to be entered into the CNC control.Three commonly used program input devices are punch tape reader, magnetic tape reader, and

computer via RS-232-C communication.

( 1.3 ) Machine Control Unit:

The machine control unit (MCU) is the heart of a CNC system. It is used to perform thefollowing functions:

To read the coded instructions.

To decode the coded instructions.

To implement interpolations (linear, circular, and helical) to generate axis motion

commands.


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To feed the axis motion commands to the amplifier circuits for driving the axis

mechanisms.

To receive the feedback signals of position and speed for each drive axis.

To implement auxiliary control functions such as coolant or spindle on/off and toolchange.

( 1.4 ) Drive System:

A drive system consists of amplifier circuits, drive motors, and ball lead-screws. The MCU feeds

the control signals (position and speed) of each axis to the amplifier circuits. The control signals

are augmented to actuate drive motors which in turn rotate the ball lead-screws to position themachine table.

( 1.5 ) Machine Tool:

CNC controls are used to control various types of machine tools. Regardless of which type ofmachine tool is controlled, it always has a slide table and a spindle to control of position and

speed. The machine table is controlled in the X and Y axes, while the spindle runs along the Zaxis.

( 1.6 ) Feed Back System:

The feedback system is also referred to as the measuring system. It uses position and speed

transducers to continuously monitor the position at which the cutting tool is located at any

particular instant. The MCU uses the difference between reference signals and feedback signalsto generate the control signals for correcting position and speed errors.

( 2 ) Machine axes designation

Machine axes are designated according to the "right-hand rule", When the thumb of right hand

points in the direction of the positive X axis, the index finger points toward the positive Y axis,

and the middle finger toward the positive Z axis. Figure 10 shows the right-hand rule applied to

vertical machines, while Figure 23.1 applies to horizontal machines.


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Figure 23.1: Right hand rule for vertical and horizontal machine

(1) Tool changing arrangements

There are two types of tool changing arrangements: manual and automatic. Machiningcentres incorporate automatic tool changer (ATC). It is the automatic tool changing

capability that distinguishes CNC machining centres from CNC milling machines.

(1.1) Manual tool changing arrangement:

Tool changing time belongs to non-productive time. So, it should be kept as minimum as

possible. Also the tool must be located rigidly and accurately in the spindle to assure propermachining and should maintain the same relation with the work piece each time. This is known as

the repeatability of the tool. CNC milling machines have some type of quick tool changing

systems, which generally comprises of a quick release chuck. The chuck is a different toolholding mechanism that will be inside the spindle and is operated either hydraulically or

pneumatically. The tool holder which fits into the chuck can be released by pressing a button

which releases the hydraulically operated chuck. The advantage of manual tool changing is thateach tool can be checked manually before loading the tools and there will be no limitation on the

number of tools from which selection can be made.

(1.2) Automatic tool changing arrangement

Tooling used with an automatic tool changer should be easy to center in the spindle, each for the

tool changer to grab the tool holder and the tool changer should safely disengage the tool holderafter it is secured properly. Figure 27.1 shows a tool holder used with ATC. The tool changer

grips the tool at point A and places it in a position aligned with the spindle. The tool changer will

then insert the tool holder into the spindle. A split bushing in the spindle will enclose the portionB. Tool changer releases the tool holder. Tool holder is drawn inside the spindle and is tightened.


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Figure 27.1: Tool holder28. CNC WORK HOLDING DEVICES

With the advent of CNC technology, machining cycle times were drastically reduced and the

desire to combine greater accuracy with higher productivity has led to the reappraisal of workholding technology. Loading or unloading of the work will be the non-productive time which

needs to be minimized. So the work is usually loaded on a special work holder away from the

machine and then transferred it to the machine table. The work should be located precisely andsecured properly and should be well supported.

28.1 Turning center work holding methods:

Machining operations on turning centers or CNC lathes are carried out mostly for axi-symmetrical components. Surfaces are generated by the simultaneous motions of X and Z axes.

For any work holding device used on a turning centre there is a direct "trade off" between part

accuracy and the flexibility of work holding device used.


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Work holding

methodsAdvantages Disadvantages

Automatic Jaw &

chuck changing

Adaptable for a range of work-

piece shapes and sizes

High cost of jaw/chuck changing

automation. Resulting in a more

complex & higher cost machine tool

Indexing chucks

Figure 28.1

Very quick loading andunloading of the workpiece can

be achieved. Reasonable range of

work piece sizes can be loadedautomatically

Expensive optional equipment. Bar-

feeders cannot be incorporated.

Short/medium length parts only can be

incorporated. Heavy chucks.

Pneumatic/Magnetic

chucks

Figure 28.3

Simple in design and relativelyinexpensive. Part automation is

possible. No part distortion is

caused due to clamping force

Limited to a range of flat parts with littleoverhang. Bar-feeders cannot be

incorporated. Parts on magnetic chucks

must be ferrous. Heavy cuts must be

avoided.

Automatic Chucks

with soft jaws

Adaptable to automation. Heavycuts can be taken. Individual

parts can be small or large in

diameter

Jaws must be changed manually &

bared, so slow part change-overs. A

range of jaw blanks required.

Expanding mandrels& collets

Figure 28.2

Long & short parts of reasonably

large size accommodated.Automation can be incorporated.

Clamping forces do not distort

part. Simple in design

Limitation on part shape. Heavy cutsshould be avoided.

Dedicated Chucks Excellent restraint & location of

a wide range of individual &

Expensive & can only be financially

justified with either large runs or when


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irregular -shaped parts can beobtained.

extremely complex & accurate parts arerequired. Tool making facilities

required. Large storage space.


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CNC PART PROGRAMMING

(1) Programming fundamentals

Machining involves an important aspect of relative movement between cutting tool and

work piece. In machine tools this is accomplished by either moving the tool with respect towork piece or vice versa. In order to define relative motion of two objects, reference

directions are required to be defined. These reference directions depend on type of machine

tool and are defined by considering an imaginary coordinate system on the machine tool. Aprogram defining motion of tool / workpiece in this coordinate system is known as a part

program. Lathe and Milling machines are taken for case study but other machine tools like

CNC grinding; CNC Hobbing, CNC filament winding machine, etc. can also be dealt with inthe same manner.

1.1) Reference Points

Part programming requires establishment of some reference points. Three reference points areeither set by manufacturer or user.

a) Machine Origin

The machine origin is a fixed point set by the machine tool builder. Usually it cannot bechanged. Any tool movement is measured from this point. The controller always remembers

tool distance from the machine origin.

b) Program Origin

It is also called home position of the tool. Program origin is point from where the tool starts for

its motion while executing a program and returns back at the end of the cycle. This can be any

point within the workspace of the tool which is sufficiently away from the part. In case ofCNC lathe it is a point where tool change is carried out.

c) Part Origin

The part origin can be set at any point inside the machine's electronic grid system. Establishing

the part origin is also known as zero shift, work shift, floating zero or datum. Usually part

origin needs to be defined for each new setup. Zero shifting allows the relocation of the part.Sometimes the part accuracy is affected by the location of the part origin. Figure 29.1 and 29.2

shows the reference points on a lathe and milling machine.


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Figure 29.1. Reference points and axis on a lathe

Figure 29.2. Reference points and axis on a Milling Machine

(1.2 ) Axis Designation

An object in space can have six degrees of freedom with respect to an imaginary Cartesian

coordinate system. Three of them are liner movements and other three are rotary. Machining of

simple part does not require all degrees of freedom. With the increase in degrees of freedom,complexity of hardware and programming increases. Number of degree of freedom defines axis

of machine.

Axes interpolation means simultaneous movement of two or more different axes to generate

required contour.


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For typical lathe machine degree of freedom is 2 and so it called 2 axis machines. For typical

milling machine degree of freedom is , which means that two axes can be interpolated at a

time and third remains independent. Typical direction for the lathe and milling machine is as

shown in figure 12 and figure 13.

(1.2) Setting up of Origin

In case of CNC machine tool rotation of the reference axis is not possible. Origin can setby selecting three reference planes X, Y and Z. Planes can be set by touching tool on the surfaces

of the work piece and setting that surfaces as X=x, Y=y and Z=z.

(1.3) Coding Systems

The programmer and the operator must use a coding system to represent information,

which the controller can interpret and execute. A frequently used coding system is the Binary-

Coded Decimal or BCD system. This system is also known as the EIA Code set because it wasdeveloped by Electronics Industries Association. The newer coding system is ASCII and it has

become the ISO code set because of its wide acceptance.

(2) CNC Code Syntax

The CNC machine uses a set of rules to enter, edit, receive and output data. These rules areknown as CNC Syntax, Programming format, or tape format. The format specifies the order

and arrangement of information entered. This is an area where controls differ widely. There

are rules for the maximum and minimum numerical values and word lengths and can be

entered, and the arrangement of the characters and word is important. The most commonCNC format is the word address format and the other two formats are fixed sequential block

address format and tab sequential format, which are obsolete. The instruction block consists

of one or more words. A word consists of an address followed by numerals. For the address,one of the letters from A to Z is used. The address defines the meaning of the number that

follows. In other words, the address determines what the number stands for. For example it

may be an instruction to move the tool along the X axis, or to select a particular tool.

Most controllers allow suppressing the leading zeros when entering data. This is known as

leading zero suppression. When this method is used, the machine control reads the numbers fromright to left, allowing the zeros to the left of the significant digit to be omitted. Some controls


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allow entering data without using the trailing zeros. Consequently it is called trailing zero

suppression. The machine control reads from left to right, and zeros to the right of the significantdigit may be omitted.

(3) Types of CNC codes

(3.1) Preparatory codes

The term "preparatory" in NC means that it "prepares" the control system to be ready forimplementing the information that follows in the next block of instructions. A preparatory

function is designated in a program by the word address G followed by two digits. Preparatory

functions are also called G-codes and they specify the control mode of the operation.

(3.2) Miscellaneous codes

Miscellaneous functions use the address letter M followed by two digits. They perform a group of

instructions such as coolant on/off, spindle on/off, tool change, program stop, or program end.They are often referred to as machine functions orM-functions. Some of the M codes are given

below.

In principle, all codes are either modal or non-modal. Modal code stays in effect until cancelled

by another code in the same group. The control remembers modal codes. This gives theprogrammer an opportunity to save programming time. Non-modal code stays in effect only forthe block in which it is programmed. Afterwards, its function is turned off automatically. For

instance G04 is a non-modal code to program a dwell. After one second, which is say, the

programmed dwell time in one particular case, this function is cancelled. To perform dwell in thenext blocks, this code has to be reprogrammed. The control does not memorize the non-modal

code, so it is called as one shot codes. One-shot commands are non-modal. Commands known as

"canned cycles" (a controller's internal set of preprogrammed subroutines for generatingcommonly machined features such as internal pockets and drilled holes) are non-modal and only

function during the call.

On some older controllers, cutter positioning (axis) commands (e.g., G00, G01, G02, G03, &G04) are non-modal requiring a new positioning command to be entered each time the cutter (or

axis) is moved to another location.

M00 Unconditional stop

M02 End of program

M03 Spindle clockwiseM04 Spindle counterclockwise

M05 Spindle stop

M06 Tool change (see Note below)

M30 End of program


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Command group G-code Function and Command Statement Illustration

Tool motion

G00 Rapid traverseG00 Xx Yy Zz

G01Linear interpolation

G01 Xx Yy Zz Ff

G02

Circular Interpolation in clock-wise

direction

G02 Xx Yy Ii JjG02 Xx Zz Ii Kk

G02 Yy Zz Jj Kk

G03

Circular interpolation in counter-clockwise direction

G03 Xx Yy Ii Jj

G03 Xx Zz Ii KkG03 Yy Zz Jj Kk


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Plane Selection

G17XY - Plane selection

G18 ZX - Plane selection

G19 YZ - plane selection


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Unit Selection

G20 or

G70 Inch unit selection

G21 orG71

Metric unit selection


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Command groupG-

code

Function and

Command

Statement

Illustration

Offset and

compensation

G40

Cutter diameter

compensation

cancel

G41

G42

Cutter diametercompensation left

Cutter diameter

compensation

right


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Tool motion

G00Rapid traverse

G00 Xx Zz

G01Linear interpolation

G01 Xx Zz

G02

Circular Interpolation in clock-wise

direction

G02 Xx Zz Ii Kk

(or)G02 Xx Zz Rr

G03

Circular interpolation in counter-

clockwise directionG03 Xx Zz Ii Kk

(or)

G03 Yy Zz Rr


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Example Program

A contour illustrated in figure 29.3 is to be machined using a CNC milling machine. The details

of the codes and programs used are given below.

Example:

Figure 29.3 An illustrative example

O5678 Program number

N02 G21 Metric programming

N03 M03 S1000 Spindle start clockwise with 1000rpm

N04 G00 X0 Y0 Rapid motion towards (0,0)N05 G00 Z-10.0 Rapid motion towards Z=-10 plane

N06 G01 X50.0 Linear interpolation

N07 G01 Y20.0 Linear interpolation

N08 G02 X25.0 Y45.0

R25.0Circular interpolation clockwise(cw)

N09 G03 X-25.0

Y45.0 R25.0Circular interpolation counter clockwise(ccw)

N10 G02 X-50.0

Y20.0 R25.0Circular interpolation clockwise(cw)

N11 G01 Y0.0 Linear interpolation

N12 G01 X0.0 Linear interpolation

N13 G00 Z10.0 Rapid motion towards Z=10 planeN14 M05 M09 Spindle stop and program end


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30. CNC Part Programming II

In the previous section, fundamentals of programming as well basic motion commands formilling and turning have been discussed. This section gives an overview of G codes used for

changing the programming mode, applying transformations etc.,

30.1 Programming modes

Programming mode should be specified when it needs to be changed from absolute toincremental and vice versa. There are two programming modes, absolute and incremental and is

discussed below.

30.1.1 Absolute programming (G90)

In absolute programming, all measurements are made from the part origin established by the

programmer and set up by the operator. Any programmed coordinate has the absolute value in

respect to the absolute coordinate system zero point. The machine control uses the part origin asthe reference point in order to position the tool during program execution (Figure 30.1).

30.1.2 Relative programming (G91)

In incremental programming, the tool movement is measured from the last tool position. Theprogrammed movement is based on the change in position between two successive points. The

coordinate value is always incremented according to the preceding tool location. The programmer

enters the relative distance between current location and the next point ( Figure 30.2).

30.2 Spindle control


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The spindle speed is programmed by the letter 'S' followed by four digit number, such as S1000.

There are two ways to define speed.1. Revolutions per minute (RPM)

2. Constant surface speed

The spindle speed in revolutions per minute is also known as constant rpm or direct rpm. Thechange in tool position does not affect the rpm commanded. It means that the spindle RPM will

remain constant until another RPM is programmed. Constant surface speed is almost exclusively

used on lathes. The RPM changes according to diameter being cut. The smaller the diameter, themore RPM is achieved; the bigger the diameter, the less RPM is commanded. This is changed

automatically by the machine speed control unit while the tool is changing positions. This is the

reason that, this spindle speed mode is known as diameter speed.

30.3 Loops and Unconditional jump (G25)The unconditional jump is used to repeat a set of statements a number of times.

Example: N10

In the above example, the program statements from N70 to N100 are repeated once when thestatement N160 is executed. Usually the G25 is used after a mirror statement. Illustrative

example geometry and its program are given below (Figure 30.3).

Example:

Fig. 30.3 example for programming loops


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30.4 Mirroring

The mirroring command is used when features of components shares symmetry about one or

more axes and are also dimensionally identical. By using this code components can be machinedusing a single set of data and length of programs can be reduced.

G10 cancellation of mirroring image

G11 Mirror image on X axis

G12 Mirror image on Y axisG13 Mirror image on Z axis

Example:

Fig. 30.4 Illustrative Example for mirroring


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30.5 Shifting origin

G92 code is used to temporarily shift the origin to the reference point specified.

Example: G92 X-100 Y-80

In the above statement the x and y values gives the present values of original origin after shiftingit. This is illustrated through an example (Figure 30.5).


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Example:

Fig. 30.5 Illustrative Example for shifting origin

30.6 Scaling

Scaling function is used to program geometrically similar components with varying sizes.

Syntax: G72 Kk, where k is the scaling factor.

The scaling command can be cancelled by using the statement G72 K1.0.

Example:


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Fig. 30.6 Illustrative Example for scaling


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30.7 Pattern rotation

Pattern rotation is used to obtain a pattern of similar features. G73 code is used to rotate the

feature to form a pattern.Syntax G73 Aa, where 'a' is the angle of rotation. This command is cumulative, and the angle

gets added up on time the program is executed. So all the rotational angle parameters should be

cancelled using the code G73.

The unconditional jump code G25 is used in conjunction with this code to achieve the desired

rotation.

The following example (Figure 30.7) depicts the case of a pattern which needs to be programmedthrough G73.

Example:

Fig. 30.7 Illustrative Example for Pattern rotation


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30.8 Tool selection

Tool selection is accomplished using 'T' function followed by a four digit number where, firsttwo digits are used to call the particular tool and last two digits are used to represent tool offset

in the program. The tool offset is used to correct the values entered in the coordinate system

preset block. This can be done quickly on the machine without actually changing the values in

the program.

Using the tool offsets, it is easy to set up the tools and to make adjustments

Feed rate control

Cutting operations may be programmed using two basic feed rate modes:

1. Feed rate per spindle revolution

2. Feed rate per time

The feed rate per spindle revolution depends on the RPM programmed.


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31.1 Tool radius Compensation

The programmed point on the part is the command point. It is the destination point of the tool.

The point on the tool that is used for programming is the tool reference point. These points mayor may not coincide, depending on the type of tool used and machining operation being

performed. When drilling, tapping, reaming, countersinking or boring on the machining center,

the tool is programmed to the position of the hole or bore center - this is the command point.When milling a contour, the tool radius center is used as the reference point on the tool while

writing the program, but the part is actually cut by the point on the cutter periphery. This point is

at 'r' distance from the tool center. This means that the programmer should shift the tool centeraway from the part in order to perform the cutting by the tool cutting edge. The shift amount

depends upon the part geometry and tool radius. This technique is known as tool radius

compensation or cutter radius compensation.

In case of machining with a single point cutting tool, the nose radius of the tool tip is required tobe accounted for, as programs are being written assuming zero nose radius. The tool nose radius

center is not only the reference point that can be used for programming contours. On the tool

there is a point known as imaginary tool tip, which is at the intersection of the lines tangent to thetool nose radius.

Cutter compensation allows programming the geometry and not the toolpath. It also allows

adjusting the size of the part, based on the tool radius used to cut part. This is useful when cutterof the proper diameter is not found. This is best explained in the Figure 31.1.

Figure 31.1. Cutter diameter compensation

The information on the diameter of the tool, which the control system uses to calculate the

required compensation, must be input into the control unit's memory before the operation. Tooldiameter compensation is activated by the relevant preparatory functions (G codes) as shown in

Figure 31.2.

Compensation for tool radius can be of either right or left side compensation. This can be

determined by direction of tool motion. If you are on the tool path facing direction of tool path


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and if tool is on your left and workpiece is on your right side then use G41 (left side

compensation). For, reverse use other code G42 (Right side compensation). Both the codes aremodal in nature and remain active in the program until it is cancelled by using another code, G40.

Offset Direction = Left (G41)

Offset Direction = Right (G42)

Offset Direction = Off (G40)

31.2 Ramp on/off motion

When activating cutter radius compensation, it must be ensured that the slides will first make a

non-cutting move to enable the correct tool and workpiece relation to be established. A similar

move is necessary prior to cancellation of the radius compensation. These non-cutting moves arereferred as "ramp on" and "ramp off" respectively. Figure 31.3 shows the ramp on motion for

different angles of approach.


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Figure 31.3. Ramp up motion

31.3 Subroutines

Any frequently programmed order of instruction or unchanging sequences can benefit bybecoming a subprogram. Typical applications for subprogram applications in CNC programming

are

Repetitive machining motions

Functions relating to tool change

Hole patterns

Grooves and threads

Machine warm-up routines


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Pallet changing

Special functions and others

Structurally, subprograms are similar to standard programs. They use the same syntax rules. The

benefits of subroutines involve the reduction in length of program, and reduction in program

errors. There is a definition statement and subroutine call function.

Standard sub-routineN10

N20

N30

.N70 G22 N5

N80

N90.

N100 G24

.N160 G20 N5

In the above example G22 statement defines the start block of the sub-routine and G24 marks the

end of the sub-routine statement. The subroutine is called by another code G20 identified by the

label N5.

Parametric subroutine..

..

..G23 N18

G01 X P0 Y P1

..

..

G21 N18 P0=k10 P1=k20

In the above example G23 starts the subprogram label and starts the definition, and the

parameters P0 , P1 are defined for values of x and y. The G21 statement is used to call thesubroutine and to assign the values to the parameters.


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31.4 Canned Cycles

A canned cycle is a preprogrammed sequence of events / motions of tool / spindle stored inmemory of controller. Every canned cycle has a format. Canned cycle is modal in nature and

remains activated until cancelled. Canned cycles are a great resource to make manual

programming easier. Often underutilized, canned cycles save time and effort.

31.4.1 Machining a Rectangular pocket

This cycle assumes the cutter is initially placed over the center of the pocket and at some

clearance distance (typically 0.100 inch) above the top of the pocket. Then the cycle will take

over from that point, plunging the cutter down to the "peck depth" and feeding the cutter aroundthe pocket in ever increasing increments until the final size is attained. The process is repeated

until the desired total depth is attained. Then the cutter is returned to the center of the pocket atthe clearance height as shown in figure 31.4

Figure 31.4. Pocket machining


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The overall length and width of the pocket, rather than the distance of cutter motion, areprogrammed into this cycle.

The syntax is : G87 Xx Yy Zz Ii Jj Kk Bb Cc Dd Hh Ll Ss (This g code is entirely controller

specific and the syntax may vary between controller to controller).Description:x,y - Center of the part

z - Distance of the reference plane from top of part

i - Pocket depthj,k - Half dimensions of the target geometry (pocket)

b - Step depth

c - Step over

d - Distance of the reference plane from top of parth - Feed for finish pass

l - Finishing allowance

s - Speed

For machining a circular pocket, the same syntax with code G88 is used.

31.4.2 Turning Cycles

The G80 command will make the tool move in a series of rectangular paths cutting materialaxially until the tool tip reaches target point P1 where the cycle ends as shown in figure 31.5.

Cutting movements will be at the cutting feed rate. All other movements will be at rapid traverse

rate.

Figure 31.5 Turning cycle (Straight cutting)

The syntax is G80 Xx Zz Ff

31.4.3 Roughing Cycle

In roughing cycle, the final finishing cycle profile is used to perform the roughing operation for

the higher material removal rate. The syntax for the roughing cycle is given below.


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G81 Pp Qq Uu Ww Dd Ff Ss

31.5 The APT Programming LanguageThe APT (Automatically Programmed Tool)programming language was developed in early 1960s to assist engineers in defining the proper

instructions and calculations for NC part programming. A great strength of APT is its ability to

perform precise calculations for complicated tool paths when contouring on a three dimensionalsurface in a multiaxis programming mode. Now APT has become obsolete. Please click here to

know more about APT. Automatic generation of NC code is dealt in this page

Part Programming for machining curved surfaces

With increased demand for aesthetic form and functional surface shape in many industrial

products, the need for a method of CNC machining of curved and intricate surfaces is required.One of the most important features in determining CNC machining efficiency and productivity is

cutter path motion. This cutter motion on compound-curvature surfaces determines the time for

machining as well as determines the surface roughness (or cusp height or scellop height). Surfaceroughness always exists because of the lack of geometry matching between cutter and surface.

( 1 ) Representation of Curves

Description of geometry of curves and surfaces may be done in several ways. It can be both

analytical and parametric. It can be described mathematically by nonparametric or parametric

equations. Nonparametric equations can be explicit or implicit. For a nonparametric curve, thecoordinates y and z of a point on the curve are expressed as two separate functions of third

coordinate x as the independent variable.


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CNC MACHINES:

Most CNC milling machines (also called machining centers) are computer controlled

vertical mills with the ability to move the spindle vertically along the Z-axis. This extra degree offreedom permits their use in die sinking, engraving applications, and 2.5D surfaces such as relief

sculptures. When combined with the use ofconical tools or aball nose cutter, it also significantlyimproves milling precision without impacting speed, providing a cost-efficient alternative to

most flat-surface hand-engravingwork.

Five-axis machining center with

Rotating table and computer interface

CNC machines can exist in virtually any of the forms of manual machinery, like horizontal mills.

The most advanced CNC milling-machines, the multiaxis machine, add two more axes in

addition to the three normal axes (XYZ). Horizontal milling machines also have a C or Q axis,

allowing the horizontally mounted work piece to be rotated, essentially allowing asymmetric andeccentricturning. The fifthaxis (B axis) controls the tilt of the tool itself. When all of these axes

are used in conjunction with each other, extremely complicated geometries, even organic

geometries such as a human head can be made with relative ease with these machines. But the

skill to program such geometries is beyond that of most operators. Therefore, 5-axis millingmachines are practically always programmed withCAM.

With the declining price of computers and open source CNC software, the entry price of CNC

machines has plummeted.
http://en.wikipedia.org/wiki/CNChttp://en.wikipedia.org/wiki/2.5D_(machining)http://en.wikipedia.org/wiki/Reliefhttp://en.wikipedia.org/wiki/Cone_(geometry)http://en.wikipedia.org/wiki/Milling_cutter#Ball_nose_cutterhttp://en.wikipedia.org/wiki/Milling_cutter#Ball_nose_cutterhttp://en.wikipedia.org/wiki/Engravinghttp://en.wikipedia.org/wiki/Engravinghttp://en.wikipedia.org/wiki/Computer_interfacehttp://en.wikipedia.org/wiki/CNChttp://en.wikipedia.org/wiki/Multiaxis_machininghttp://en.wikipedia.org/wiki/Eccentric_(mechanism)http://en.wikipedia.org/wiki/Eccentric_(mechanism)http://en.wikipedia.org/wiki/Turninghttp://en.wikipedia.org/wiki/Turninghttp://en.wikipedia.org/wiki/Axis_of_rotationhttp://en.wikipedia.org/wiki/Axis_of_rotationhttp://en.wikipedia.org/wiki/Computer-aided_manufacturinghttp://en.wikipedia.org/wiki/Computer-aided_manufacturinghttp://en.wikipedia.org/wiki/CNC#DIY.2C_hobby.2C_and_personal_CNChttp://en.wikipedia.org/wiki/2.5D_(machining)http://en.wikipedia.org/wiki/Reliefhttp://en.wikipedia.org/wiki/Cone_(geometry)http://en.wikipedia.org/wiki/Milling_cutter#Ball_nose_cutterhttp://en.wikipedia.org/wiki/Engravinghttp://en.wikipedia.org/wiki/Computer_interfacehttp://en.wikipedia.org/wiki/CNChttp://en.wikipedia.org/wiki/Multiaxis_machininghttp://en.wikipedia.org/wiki/Eccentric_(mechanism)http://en.wikipedia.org/wiki/Turninghttp://en.wikipedia.org/wiki/Axis_of_rotationhttp://en.wikipedia.org/wiki/Computer-aided_manufacturinghttp://en.wikipedia.org/wiki/CNC#DIY.2C_hobby.2C_and_personal_CNChttp://en.wikipedia.org/wiki/CNC


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Horizontal machining center (HMC)

A horizontal mill has the same sort ofxy table, but the cutters are mounted on ahorizontal arbor (see Arbor milling) across the table. Many horizontal mills also feature a built-in

rotary table that allows milling at various angles; this feature is called a universal table. While

end mills and the other types of tools available to a vertical mill may be used in a horizontal mill,their real advantage lies in arbor-mounted cutters, called side and face mills, which have a cross

section rather like a circular saw, but are generally wider and smaller in diameter. Because the

cutters have good support from the arbor and have a larger cross-sectional area than an end mill,quite heavy cuts can be taken enabling rapid material removal rates. These are used to mill

grooves and slots. Plain mills are used to shape flat surfaces. Several cutters may be ganged

together on the arbor to mill a complex shape of slots and planes. Special cutters can also cut

grooves, bevels, radii, or indeed any section desired. These specialty cutters tend to beexpensive. Simplex mills have one spindle, and duplex mills have two. It is also easier to cut

gears on a horizontal mill. Some horizontal milling machines are equipped with a power-take-off

provision on the table. This allows the table feed to be synchronized to a rotary fixture, enablingthe milling of spiral features such as hypoid gears
http://en.wikipedia.org/wiki/Arbor_millinghttp://en.wikipedia.org/wiki/Gear_cuttinghttp://en.wikipedia.org/wiki/Gear_cuttinghttp://en.wikipedia.org/wiki/Hypoidhttp://en.wikipedia.org/wiki/Arbor_millinghttp://en.wikipedia.org/wiki/Gear_cuttinghttp://en.wikipedia.org/wiki/Gear_cuttinghttp://en.wikipedia.org/wiki/Hypoid


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Vertical Machining Center (VMC):

Computer Integrated Manufacturing Notes

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