met237 computer numerical control - almandeel.net · direct numerical control [dnc]. 7. several dnc...

MET 237 Computer Numerical Control

Dr. Ali Almandeel

Summer 2016

Introduction

PAAET MET 237 | Dept. of Manufacturing Engineering Technology

1 | Introduction to CNC

CNC: Is the process of manufacturing machined parts. The process iscontrolled by a computerized controller called Machine Control Unit (MCU).The MCU generates, stores, and processes CNC programs. The MCU usesmotors to drive each axis of a machine toll and regulates its direction ,speed, and the amount of time each motor rotates.

Figure 1-1 Components of modern CNC systems. Source: introduction to Computer Numerical Control.


1 | Introduction to CNC

NC: Numerical Control were used in industry for more than 40 years. And it’s a method of automatically operating a manufacturing machine based on a code of letters, numbers, and special characters. A complete set of codes for executing an operation is called a program. These codes are translated into corresponding electrical signals for input to motors which runs the machine.

Figure 1-2 Components of traditional NC systems. Source: introduction to Computer Numerical Control.


1-2 | CNC Vs. NC

Advantages of CNC over NC:1. Reduction in hardware for adding functions.2. New functions can be programmed into the MCU as software.3. The CNC program can be written, stored, and executed at the CNC.4. Any portion of an entered CNC program can be played back and edited.5. Many Different programs can be stored in the MCU.6. Several CNC machine can be linked together to main computer. This known as

Direct Numerical Control [DNC].7. Several DNC systems can be networked to form Distributive Numerical Control

system.8. The CNC program can be input from zip, floppy disks, flash, portable external

drive, or can be downloaded from the network.

Figure 1-4 Distributive numerical control. Source:introduction to Computer Numerical Control.

Figure 1-3 DNC. Source: introduction to Computer Numerical Control.


2 | Modern Machine Tool Controls

Types of machine motion controllers in the MCU:1. Fanuc.2. Allen-Bradley.3. GE.4. Okuma.5. Bendix.6. Mazak and others.

The physical appearance of these controllers is similar and each responds to a slightly different set of codes.



Types of system control:

1. Point-to-Point Tool Movement:The tool move to a point on the part and execute an operation at that point only.The tool is not in continues contact with the part while it is being moved to aworking location.

Operations: drilling, reaming, boring, tapping and punching.

Figure 2-1 Point-to-point tool movement. Source:introduction to Computer Numerical Control.



Types of system control:

2. Continuous Path Tool Movement:The tool maintain continuous contact with the part as the tool cuts a contourshape. The continuous path controllers output motion by interpolating each point.Interpolation is a mathematical method of approximating the true or exact positions required to follow a precalculated path.

Operations: milling along lines at any angle, milling arcs, and lathe turning.

Figure 2-2 Continuous tool movement. Figure 2-3 Interpolation used for continuous path movement.



Loop Systems For Controlling Tool Movement [ CNC Machine Movement ] :A loop system sends electrical signals to drive motor controllers and receives some form of electrical feedback from the motor controllers. Tolerance to which a part can be cut depends on the loop system type.

1. Open Loop System.2. Closed Loop System.

Figure 2-4 Open loop system. Figure 2-5 Closed loop system.



1. Open Loop System:Utilize stepping motors to create machine movements. Stepping motors rotate afixed amount, usually 1.8° for each pulse received. The motors are connected to themachine table lead screw and spindle. The motor controller send signals backindicating the end of motion. The feedback is not used to check the accuracy ofmachine movement with the exact movement programmed. Backlash is increaseddue to friction generated from the lead screw. Backlash can cause positioningerrors when reversing the motion.

Figure 2-6 Continuous tool movement.Figure 2-7 (a) Stepper motors, (b) Cross-section of a stepper motor.

(a) (b)



2. Closed Loop System:Utilize servo motors to create machine movements. Motor types include AC servos,DC servos, and hydraulic servos. Hydraulic servos are the most powerful and usedIn large CNC machines. The speed of AC or DC servo is variable and depends on theamount of current passing through it. The feedback from servo is sent to the MCU. The unit compares the motion command from MCU and the voltage feedback[motor speed] from the tachometer and outputs back to the motor the differencebetween the two values or error.

Figure 2-6 Closed loop system. Figure 2-7 AC servo motor.


2 -1| Backlash

Controlling Backlash:

All ball screws have some “slop” or backlash at assembly. This backlash causeserrors when the screw reverses direction and the nut lags behind. Modern machinetool laser calibration equipment is used to precisely measure the amount of pitcherror and backlash in CNC positioning system. The data is input into a backlashcompensation program installed in the MCU every 3-6 months.

As the machine wears, the value of backlash increases.

Figure 2-8 Ball screw. Figure 2-8 Lead screw.


3| CNC Fundamentals

Axis and Motion. CNC Milling Fundamentals. CNC Turning Fundamentals.

Cartesian Coordinates:The location of CNC tool at any time is controlled by a system of XYZ coordinates.

Figure 3 The Cartesian coordinates system.


3-1| Axis & Motion

CNC Machine Axis Of Motion.

CNC machines execute machining operations by some form of linear motion orrotary motion. Such movement vary from machine to machine according tomanufacturer design. For example, the table can move in the XY-axis motion andthe spindle in the Z-axis motion. The programmer need not to be concernedwhether it is the spindle or the table moves.

The machine axis will be defined in terms of spindle movement.

Figure 3-1 The right-hand rule for linear motion. Figure 3-2 Machine axis for a 3-axis vertical CNC machine.


3-1| Axis & Motion

EIA and ISO Standards.1. ISO 6983. [ International Standardization Organization ]2. EIA RS274. [ Electronic Industries Association ]

The main standards for NC provide simple programming instructions to allow amachine tool to carry out a particular operation.

Standards include: machine axes should follow the right-hand rule, spindle movesalong the Z axis….etc.

Figure 3-3 The right-hand rule rotary motion. Figure 3-4 The six possible machine axes for a vertical CNC machine.


3-1| Axis & Motion Tool Positioning Modes.1. Absolute Positioning.2. Incremental Positioning.

CNC machines can be programmed to locate tool positions in absolute,incremental, or mixed modes ( incremental & absolute).

Figure 3-5 Absolute positioning.

Point X Y

1 1 -2

2 -2 -6

3 3 -4

4 4 5

5 2 2

6 -2 1

7 -5 6

8 -4 -3

Point X Y

1 1 -2

2 -3 -4

3 5 2

4 1 9

5 -2 -3

6 -4 -1

7 -3 5

8 1 -9

Figure 3-6 Incremental positioning.


3-2| CNC Milling Fundamentals Machine Home and Part Origin.

Part Origin is usually defined at the lower left corner on the top of the stock ofeach part.

Machine Home it is a point on the actual machine set once by the machinemanufacturer.

Figure 3-7 Machine home and part origin locations. Figure 3-8 Edge finding.


3-2| CNC Milling Fundamentals Cutting Speeds and Feeds.1. Cutting Speed.

Cutting speeds in hole and milling operations are defined as follow:

English Units:

Metric Units:

High speed can lead to burning of the tool’s cutting edges.Low speed can cause the tool to wear excessively or break.

Figure 3-5 Hole/Milling: Spindle rpm

and cutting speed.

12

:

Cutting Speed sfpm

Tool Diameter in

Where

sf Surface feet Per Mi upm n te

Spindle =rpm

1000

:

Cutting Speed mpm

Tool Diameter mm

Wh

mpm

ere

Meters Per Minute

Sp indle rpm =


3-2| CNC Milling Fundamentals Cutting Speeds and Feeds.2. Tool Feed.

Tool feeds in hole operations are defined as follow:

English Units:

Metric Units:

Figure 3-5 Hole/Milling: Spindle rpm

and cutting speed.

:

Whe

ipm

ipr

ip

re

m

Tool =Penetration rate (ipr) rpm

InchesPer Revolution

Inches

feed rate

Per Minute

:

Where

mmp

p

s r

mm m =Penetration rate (s) rpm

Millimeters Per Revol

Tool feed rate

ution


3-2| CNC Milling Fundamentals Cutting Speeds and Feeds.2. Tool Feed.

Tool feeds in milling operations are defined as follow:

English Units:

Metric Units:Figure 3-6 Tool feed.

ipm

ip

Number of cu

t

ipm

tter teeth =ipt rpm

Where:

Inches of material cut

Tool feed ra

Per Tooth

Inches Per Mi

te

nute

Numbmmpm

mmpt

mm

er of cutter

p

teet

m

h =mmpt rpm

Where:

Millimeters of material cut

To

Per Tooth

Millimeter

ol

s P

feed ra

er M

te

inute


3-3| CNC Lathe Fundamentals Locations Via Cartesian Coordinates.

Figure 3-7 Machine home and part origin locations.

Figure 3-8 Programming by diameter (absolute positioning).

Figure 3-9 Programming by diameter (incremental positioning).


3-3| CNC Lathe Fundamentals Machine Home, Machine Origin “Tool Change”, and Program Zero.

Machine Home the position of the turret when the machine’s axes are zeroed out.It is set once by the machine manufacturer.

Machine Origin is determined at setup.

Program Zero is a point from which all dimensions are defined in the partprogram.

Figure 3-10 Important locations for CNC lathes.


3-3| CNC Lathe Fundamentals Cutting Speeds, Feeds and Depth of Cut.1. Cutting Speed.

Cutting speeds in drilling, tapping, and reaming operations are defined as follow:

English Units:

Metric Units:

Figure 3-11 Tool feed.

12

:

Cutting Speed sfpm

Tool Diameter in

Where

sf Surface feet Per Mi upm n te

Spindle =rpm

1000

:

Cutting Speed mpm

Tool Diameter mm

Wh

mpm

ere

Meters Per Minute

Sp indle rpm =


3-3| CNC Lathe Fundamentals Cutting Speeds, Feeds and Depth of Cut.2. Depth of Cut.

Depth of cut in lathe operations are defined as follow:

Figure 3-12 Turning: spindle rpm, material cutting speed, cut diameter, depth of cut and tool feed.

2

:

Where

DOC

Uncut Diameter DOCCut d =

Dept

iameter

h Of Cut


4| Mathematics For CNC Programming Determining Sides Of Right Triangles.

Determining Angles Of Right Triangles. Figure 4 A right triangle.

90 180A B

Inside Angle Formula

sin

cos

tan

A

A

A

A

C

B

C

A

B

Example

1

1

1

sin

cos

tan

A

A

A

A

C

B

C

A

B

2 2 2

sin

cos

tan

Side opposite

Hypotenuse

Side adjacent

Hypotenuse

Side opposite

Side adjacent

Pythagorean Formula

C A B


4| Mathematics For CNC Programming Useful Angle Concepts.

Figure 4-1 A circle contains 360ᵒ.

Figure 4-2 Vertical angles are equal.

Figure 4-3 Alternate interior angles are equal.


5| Word Address Programming Programming Language Terminology.

Programming Character is an alphanumeric character or punctuation mark.

Example: N G ;

Address is a letter that describes the meaning of the numerical value following theaddress.

Example:

Word is an address followed by a number.


5| Word Address Programming Programming Language Terminology.

Blocks is a complete line of information to the CNC machine. It compose of oneword or an arrangement of words.

Example:

Program is a sequence of blocks that describe in detail the motions of CNC machine to manufacture a part.

Example:


5| Word Address Programming Arrangement Of Address In A Block.

The order of address in a block can very. The following sequence is normally used:

Figure 5 -1 Addresses arranged in a block.


5| Word Address Programming Program And Sequence Number (O, N Codes).

Program Number (O) programs are stored in the MCU memory by programnumber. Program numbers range from O1 to O9999.

Sequence Number (N) an optional tag coded at the beginning of a block. It is usedso that operators can locate specific lines of a program. These numbers range from N1 to N9999.

Figure 5 -2 Program number and sequence numbers.


5| Word Address Programming Preparatory Functions (G Codes).

Preparatory Functions (G Codes) prepares the control system for the informationthat is too follow in the block. And is designated by the address G followed by oneOr two digits.

Figure 5 -3 Effect of modality.


5| Word Address Programming Preparatory Functions (G Codes).

Figure 5 -4 Word address blocks example.


5| Word Address Programming Dimension Words (X, Y, Z…Codes).

Dimension words specify the movement of the programming axes.



5| Word Address Programming Feed Rate (F Code).

Feed rate is the rate at which the cutting tool moves along programming axis. Andspecified by the numerical value following the address F.

F code is modal and remains in effect in a program for all following toolmovements. It can be changed by entering a new F command.

Example:F5 [feed rate of 5 ipm]



5| Word Address Programming Spindle Speed (S Code).

S Code is modal and remains in effect in a program for all following toolmovements. It can be changed by entering a new S command or cancelled by aspindle-off (M5) M code.

S address can take a maximum value of four digits following the address.

Example:S1600 [spindle speed at 1600 rpm]

Spindle speed should be specified before entering blocks containing cuttingcommands.


5| Word Address Programming Miscellaneous Machine Functions (M Codes).

M Codes specify functions not related to dimensional or axis movements. Theydirect the controller to immediately excite the machine function indicated.

Numbers following the address M call for functions such as : spindle on/off,coolant on/off, program stop/automatic tool change, program end, and so on.

M codes are classified into two groups:

Type A: Those executed with the start of axis movements in block.Type B: Those executed after the completion of axis movement in a block.

Example:

Type A: M03 [Turns spindle on clockwise]Type B: M02 [Program end]


5| Word Address Programming Miscellaneous Machine Functions (M Codes).

M Codes specify functions not related to dimensional or axis movements. Theydirect the controller to immediately excite the machine function indicated.

Figure 5 -6 Frequently used M codes.


5| Word Address Programming Automatic Tool Change (M06 Code).

M06 Code directs the CNC machine to change to the new tool. The number of thenew tool is identified by the T word.

The spindle must be at safe Z distance to execute a tool change. Because,retracting to a safe Height will ensure that the tool does not strike the part orfixture when moving in the XY plane.

Figure 5 -7 Frequently used M codes.


5| Word Address Programming Tool Length Offset & Cutter Radius Compensation (H, D Codes).

H Code is used to specify where the values of the tool length offset and the toolposition offset are located.

D code indicates where the value of the cutter radius compensation for a tool islocated.

A tow digit number ranging from 01 to 99 is used with H or D address.



5| Word Address Programming Excersice-1 [Absolute coordinates].

Hole A: N01 G90 G00 X50 Y20 Hole B: N01 G90 G00 X50 Y50 Hole C: N01 G90 G00 X50 Y80 Hole D: N01 G90 G00 X_Y_ Hole E: N01 G90 G00 X_ Y_ Hole F: N01 G90 G00 X_ Y_ Hole G: N01 G90 G00 X_ Y_ Hole H: N01 G90 G00 X_Y_ Hole I: N01 G90 G00 X_ Y_


5| Word Address Programming Excersice-2 [Incremental coordinates].

Hole A: N01 G91 G00 X50 Y20 Hole B: N01 G91 G00 X0 Y30 Hole C: N01 G91 G00 X0 Y30 Hole D: N01 G91 G00 X_Y_ Hole E: N01 G91 G00 X_ Y_ Hole F: N01 G91 G00 X_ Y_ Hole G: N01 G91 G00 X_ Y_ Hole H: N01 G91 G00 X_Y_ Hole I: N01 G91 G00 X_ Y_

met237 computer numerical control - almandeel.net · direct numerical control [dnc]. 7. several dnc...

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