E252 Learning Module I--CNC Fundamentals and Terminology

8/9/2019 E252 Learning Module I--CNC Fundamentals and Terminology

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ENT 252 Learning Module I-CNCFundamentals and Terminology 1

Computer Aided

Manufacturing II

Introductions

ExpectationsPreparation for lecture, lab, etc.


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Computer Aided

Manufacturing II

ENT 252 CAM II

Extension of CAM IEmphasis on generating part programs


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Computer Aided Manufacturing II

Review syllabus

Textbook

CIMTEXT.doc available on web Prerequisite sign off

Term Project

Using the course web page as a resource Feedback and discussion

Lets begin


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What is Computer Aided Manufacturing?

CNC Machines

Robots

Material HandlingAutomation

Automatic part program generation

Paperless factory

Others


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Development of NC/CNC

1957 MIT development of first machine

Retrofit

Replace hand cranks with motors

Control motors with electronics

Eventually led to development of numerically

controlled machine tools

Programs stored on punched paper tape

Special purpose controllers designed for sole

purpose of running NC machines


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NC/CNC Development Continued

Programmer manually programs part

Punches data into PPT

Outputs a manuscript

Used typewriter-like device

Controller connected to motor which is

connected to lead screw (threaded rod).

Follower (nut), connected to machine slide,rides on lead screw resulting in linear motion.


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So What is CNC? By early 1970s, INTEL had developed first

microprocessor (computer on a chip)

Think about technological developments leading up

to this time What are some of the significantdevelopments?

Well, once OEMs had computers on a chip theyreplaced thousands of wires with a single chipresulting in Computer Numerical Control (CNC)

Computer Numerical Control told potentialcustomers that their control was computer basedand not hard wired.


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CNC Continued

Today the terms NC and CNC have become

interchangeable.

CNC offers shop floor editing of programs (for

programs errors, tool wear, etc.), more

methods of program storage, calculations,

canned routines, and variable-basedprograms.


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CNC Continued

CNC Machines are best suited for small to

medium lot sizes

Explain what this means

What are the alternative methods of

manufacturing?

When is CNC well-suited for one part?

What about continuous runs? What are advantage/disadvantage of CNC over

other other manufacturing choices?


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CNC Part Program Development

CNC part programming may be

accomplished in a variety of ways, such as:

Manual part programming

Computer assisted part programming

CAD/CAM

Shop Floor Parametric


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Manual Part Programming Manual part programming is the oldest

method and still quite popular. This

technique requires the programmer to

examine a part drawing then manually

calculate all tool paths. This information is

recorded on a manuscript. A punched

tape is prepared directly from themanuscript.


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Computer Assisted Part Programming

High level language to describe

geometry and tool path

Computer makes all key calculations Tool offsets and compensations

Partial arcs

Part geometry And more...


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CAD/CAM

CAD used to describe geometry

CAM software used to define tool path

relative to CAD geometry

Becoming very popular

Tough to get pure turn-key system


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Conversational/Shop Floor Programming

Interactive method of generating part programs

The CNC programmer or machine tool operatoranswers questions and provides data about toolpaths when prompted by a conversational

programming software system. These answers to questions and associated data

are translated into a CNC program for a particularmachine tool.

Conversational systems reside on the machine tool

controller This method of programming is generally restricted

to relatively simple geometry


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Parametric Programming

Highest level and versatile method of

programming

Good for part family programming

Use variables to describe part features

Values assigned to variables define part


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Programming Terminology and

Formats NC/CNC Part Programs consist of:

BLOCKS or lines of code

Blocks consist of WORDS or commands (e.g.,

feedrate command)

Words consist of CHARACTERS orBYTES (G, X,

any alpha-numeric character)

Characters/Bytes consist of bits (0 or1; hole or no

hole; magnetized spot or not magnetized spot)


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A Sample BLOCK

The following are sample Blocks or Lines

of CNC code:

N00 G01 X+1000 Z-3000 F30 H0

N5M3G1X-1.5Z+3.42F3.5S1200

N10M6T2


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Programming Formats

Fixed Block 0001+0100-01000040

Notice only numeric data with a fixed number of charactersper word and block

Tab Sequential

00 01 +0100 -01000 040 Notice only numeric data with a fixed number of TABS per

block

Word Address

N1G3X-3.5Y+4.0F3.5

Letters precede numbers designating each word.

No fixed structure


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Program Storage and Coding

Storage

Punched tape, disk, hard drive, network drives

www.doit.ort.org

Coding American Standard Code for Information Interchange

(ASCII) is used today

7 bit code with 8th bit for special stuff

Each character is assigned a unique code of1s and 0s. A is 01000001


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Consider Codes

1 bit code Example: 0=A;1=B

2 bit code

Example 00=A, 01=B, 10=C, 11=D BAD is 01 00 11

4 bit code Example: 0000=A, 0001=B, and so on

How many characters can we code with 4 bits? Now consider 7 bits, 8 bits, or10 bits


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More on Codes

Number of Characters (combinations) is:

2N where N is number of bits

ASCII Code

7 bits (27) = 128 characters

8 bits (28) = 256 characters

All keys on keyboard have assigned an ASCII

code


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Consider Number Systems (bases)

Number systems

Consider93510

9 hundreds, 3 tens, 5 ones

9 x 102 + 3 x 101 + 5 x 100 = 93510

Now consider 2548

2 x 82 + 5 x 81 + 4 x 80 = 128+40+4=17210

What about Base 2? 101102 = 2210 What about any base?


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Consider Codes as Numbers

Firstgiven a base such as 8.

How many unique characters in any one column?

What is largest valued character in any one

column?

Now consider the ASCII code as a number?

A is 010000012 = 6510

B is 010000102 = 6610

Work ASCII Code Worksheet


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Bits and Combinations (characters)

With ASCII code using 7 bits we can code

how many characters?

With 8 bits we can code how many?

What about WENDYs hamburgers?

Take it to 10 bits 210 = __________?

210 is K in computer jargon


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More on how CNC Machines work

Programs are fed to control by PPT, disk,LAN, etc.

Controller interprets numeric data and

converts it into electrical signals which drivethe axes motors.

Axes motors can be DC, AC, or morecommonly Stepper Motors. These motors

drive the lead screw. Follower (nut), connected to machine slide,

rides on lead screw resulting in linear motion.


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Feedback

Usually feedback devices are connected to eachaxis.

Feedback devices are used to improve positioning

and are also used to verify that the axes are actuallymoving. If an axis is against a positive stop, thecontrol may say move but the axis cant. A feedbackdevice detects no motion and shuts down thecontrol.

Typical feedback devices are primarily resolvers(AC) and encoders (digital). Lasers, andpotentiometers may also be used.


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Determining Rotary Position-

Mechanical EncoderMechanical Absolute EncodersGray

code wheel shown to left. Gray code

is used because only one bit

transitions at a time. A metal disc

containing a set of concentric rings ofopenings is affixed to an insulating

disc, which is rigidly fixed to the shaft.

A row of sliding contacts is fixed to a

stationary object so that each contact

wipes against the metal disc at a

different distance from the shaft. As

the disc rotates with the shaft, some

of the contacts touch metal, while

others fall in the gaps where the

metal has been cut out.


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Determining Rotary PositionOptical

EncoderOptical Absolute Encoders (Optical

tachometer as shown to left

measures RPM only. Multiple tracks

of patterned holes required for

position as shown below,)The optical encoder's disc is made of

glass with transparent and opaque

areas. A light source and photo

detector array reads the optical

pattern that results from the disc's

position at any one time.


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ENT 252 Learning Module I-CNC

Fundamentals and Terminology 29

Determining RPM

Optical Absolute EncodersOptical

tachometer as shown to left measuresRPM only. Multiple tracks of patterned

holes required for position as shown

previously.


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More on CNC Operation

Stepper motors work off of pulses (e.g., 0 to

24V). Each pulse causes a specific degrees

of rotation. For example, a 7.5 degree

stepper rotates 7.5 degrees per pulse.

The pitch of the lead screw needs to be

coordinated with the controller to affect

precise linear motion. What is Pitch?


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Still More on CNC Operation

If a stepper motor is used, the number of

pulses from a stepper motor determines the

revolutions made by the lead screw. This in-

turn, determines linear distance.

The rate or frequency of pulses determines

the rate of axis travel.

Consider the stepper motor worksheet


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First steps toward programming

CNC Programs can be written in either

Incremental (relative) or Absolute mode.

With Incremental, the command issued is

the DISTANCE the programmer wants the

axes to move.

With Absolute, the command issued is the

COORDINATE the programmer wants theaxes to move to.


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Programming modes continued

The Incremental and Absolute modes can be

intermixed in a single program.

The command to set Incremental is G91 and

the command to set Absolute is G90 or G92.

All of these commands are modal which

means once set, they are retained from block

to block (line to line) of CNC code. More on commands later.


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Consider an example

X

Y

0 2 4 6

5

A

B C

D

Move Incremental AbsoluteA-B X=2, Y=5 X=2, Y=5

B-C X=2, Y=0 X=4, Y=5C-D X=2, Y=-5 X=6, Y=0D-A (you finish)

0


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Example continued

In the example, note the pattern/relationshipbetween Incremental and Absolute.

What is the relationship?

Also, note that in Incremental, if the programreturns to starting point, then X=0 and Y=0

In Absolute, there is no sum check but if thelast coordinate is the starting coordinate thenthe program returns to starting point.

Work the ABS/INC Worksheet.


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Beginning to Program

NC/CNC Programs are made up ofBLOCKS or

lines of code. Each block is a complete command

and the machine tool executes one block at a time.

Blocks are made up of WORDS. Words are theactual instruction set and are used to establish

motion pattern, feedrates, coolant and spindle

control, etc.

Words are made up of alpha-numericCHARACTERS or bytes.

Characters (or bytes) are made up ofBITS.


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Its in the Code

The language of NC/CNC Machines is G-

Code. G-codes are used to provide path

definition to the control. For example, G01 is

feed linear, G02 is arc CW, G03 is arc CCW,and so on.

Generally, G codes are two characters and

can range from G00 through G99. Not all 100codes are used and not all G codes are

standardized which presents challenges.


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A Typical CNC Block (Command)

N00 G01 X-1.00 Y-.750 F3.0

In this command there are 5 Words (N, G, X, Zand F) N is the line or block number,. G is the motion or Preparatory Function

command.

X & Z are movement/coordinate data

F is a feedrate command

Search the web for a list of G codes. Whatdid you learn from this?


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CNC Machines and Programming

Fundamentals

Most machine tools have a fixed machinezero/home location.

CNC machines must be initialized or

zeroed prior to operating the machine In addition to a machine zero, programmers

will usually also specify a program zero. Thisis usually a location on the part from which

dimensions are referenced. Checking partsafter they have been machined should alsobe done using these same reference points.


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Fundamentals continued

Program zero and machine zero are generally not thesame point. For example, machine zero on a lathe is atthe centerline of the spindle against the spindle surfaceplate (to which a chuck or collet is installed). When a

lathe is initialized, the tool will automatically move to apre-set location a specific distance in X and Z frommachine zero. Program zero on the other hand, will likelybe on the part surface at the face or tailstock end andusually on the part centerline.

CNC programs will begin at a set point. This is the pointat which the tool is located in reference to the part whenthe start button is pressed. This is usually referencedfrom program zero. (see last three slides)


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Programming Fundamentals continued

It is the programmers responsibility to

communicate to the CNC machine operator

set point locations, tooling information, and

special setup instructions.

It is the machine tool operators responsibility

to make sure the machine is setup properly

and particularly that the tool is properlyreferenced from the set point.


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Fundamentals

CNC programs begin at a set point and

should return to that set point

Why?

The Z axis on a machine tool is always along

the centerline of the spindle. (-Z is tool

movement away from the spindle motor; +Z

is toward the spindle motor) On a lathe, the X axis is the tool moving

toward (-) or away from (+) the part centerline


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Fundamentals continued

On a vertical mill (as the operator faces it), X

is effective tool motion left (-) and right (+);Y

is toward (-) and away from (+) the operator.

On a horizontal mill (as the operator faces it),

X is effective tool motion left (-) and right (+);

Y is down (-) and up (+).

Why is the term effective tool motion usedwhen describing the mills above?


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Mill ProgrammingReference Points




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Mill ProgrammingZero Offset




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Mill ProgrammingCoordinate

System

Documents

E252 Learning Module I--CNC Fundamentals and Terminology