DEPARTMENT OF MANUFACTURING AND MATERIALS

WORKSHOP TECHNOLOGY MME 1103

CNC MACHINING

MANUFACTURING AND MATERIALS DEPARTMENT MME1103 Workshop Technology Manual

CNC Machining

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Apparatus: Tools : Turning tools Milling tools

Machine: CNC Turning Center CNC Machine Center

Introduction:

The abbreviation CNC stands for computer numerical control, and refers specifically to a computer "controller" that reads G-code instructions and drives the machine tool, a powered mechanical device typically used to fabricate metal components by the selective removal of metal. CNC does numerically directed interpolation of a cutting tool in the work envelope of a machine. The operating parameters of the CNC can be altered via software load program.

NC was developed in the late 1940s and early 1950s by John T. Parsons in collaboration with the MIT Servomechanisms Laboratory. CNC was preceded by NC (Numerically Controlled) machines, which were hard wired and their operating parameters could not be changed. The first CNC systems used NC style hardware, and the computer was used for the tool compensation calculations and sometimes for editing.

Punched tape continued to be used as a medium for transferring G-codes into the controller for many decades after 1950, until it was eventually superseded by RS232 cables, floppy disks, and finally standard computer network cables. The files containing the G-codes to be interpreted by the controller are usually saved under the .NC extension. Most shops have their own saving format that matches their ISO certification requirements.

The introduction of CNC machines radically changed the manufacturing industry. Curves are as easy to cut as straight lines, complex 3-D structures are relatively easy to produce, and the number of machining steps that required human action have been dramatically reduced.

With the increased automation of manufacturing processes with CNC machining, considerable improvements in consistency and quality have been achieved. CNC automation reduced the frequency of errors and provided CNC operators with time to perform additional tasks. CNC automation also allows for more flexibility in the way parts are held in the manufacturing process and the time required to change the machine to produce different components.

In a production environment, a series of CNC machines may be combined into one station, commonly called a "cell", to progressively machine a part requiring several operations. CNC machines today are controlled directly from files created by CAM software packages, so that a part or assembly can go directly from design to manufacturing without the need of producing a drafted paper drawing of the manufactured component. In a sense, the CNC machines represent a special segment of

MANUFACTURING AND MATERIALS DEPARTMENT MME1103 Workshop Technology Manual

CNC Machining

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industrial robot systems, as they are programmable to perform many kinds of machining operations (within their designed physical limits, like other robotic systems). CNC machines can run over night and over weekends without operator intervention. Error detection features have been developed, giving CNC machines the ability to call the operator's mobile phone if it detects that a tool has broken. While the machine is awaiting replacement on the tool, it would run other parts it is already loaded with up to that tool and wait for the operator. The ever changing intelligence of CNC controllers has dramatically increased job shop cell production. Some machines might even make 1000 parts on a weekend with no operator, checking each part with lasers and sensors.

Figure 1: CNC Turning Center

MANUFACTURING AND MATERIALS DEPARTMENT MME1103 Workshop Technology Manual

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Figure 2: CNC Milling Machine

Example of NC part programming (2D contouring)

Figure 3: Sample Part

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We assume that the machining is a contouring operation along the outer boundary of a simple part, whose nominal geometry is as shown on Fig.3.

The tool size is 0.25 inch, and the feed rate of 6 inch per minute is used.

The cutting speed is required to be 300 rpm. To simplify the program, we ignore the Z-axis motions, and that the home position of the tool is at the correct height, centered on the point located in the machine tool coordinates as (2, 2).

In order to specify the geometry of the motion, we need to compute the location of 5 points from p1 to p5 (later, we shall see that additional points will be needed).

The cutting will proceed along the contour: p0 p1 p2 p3 p4 p5 p1.

// Block 1: start program, use absolute coordinates, spindle speed in rpm, feed in inch/min, select tool no 1001, turn coolant ON, use up milling (CCW in this case)

N010 G70 G90 G94 G97 M04[EOB]

NOTE: we could use M14 instead of M04 and M08 that we shall use in the next block.

// Block 2: cutting on XY plane, set feed, spindle speed,

N020 G17 F6.0 S300 T1001 M08 [EOB]

// Block 3: move to first point in a straight line (linear interpolation). We now need to compute the coordinates of p1, as shown in figure below:

Figure 4 : Calculating p1 coordinate

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N030 G01 X3.875 Y3.698 [EOB]

// Block 4: move to p2 in straight line. coordinates of point p2: x = same as p1; y = 4 + 5 + 0.125.

N040 G01 X3.875 Y9.125 [EOB]

// Block 5: move to p3. coordinates calculated as shown in figure below:

Figure 5 : Calculating p3 coordinate

N050 G01 X5.635 Y9.125 [EOB]

// blocks 6, 7, 8: now we need to cut along a circular arc.

Most NC controllers cannot cut along a full circle directly -- they need to be programmed once FOR EACH QUADRANT of the arc. In our case, the circular path of the tool goes through tree quadrants, so we need to find two additional points, p31, and p32.

Programming for circular interpolation (moving the tool along a circular arc):

Circular arcs are machined only in main planes. The plane is selected by the preparatory functions (G17, G18, G19).

The direction of the tool along the arc must be specified

(G02, G03, G20 G21, G30, G31).

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Four dimension words are needed per block. Two dimension words specify the distance to the end of the arc from current position. Two circular dimension words specify the distance to the arc center. Usually, I, J, K specify the distances parallel to the X, Y, Z axes, respectively.

The I, J, K values are absolute values (unless there is ambiguity).

The following figure shows the computations of the next three blocks of motion:

Figure 6 : Circular movement from p3 to p4

The corresponding blocks are:

*NOTE: EOB stands for End Of Block

N060 G03 X5.625 Y9.0 I0.866 J0.125 [EOB]

N070 G03 X6.5 Y8.125 I0.875 J0.0 [EOB]

N080 G03 X7.375 Y9.0 I0.0 J0.875 [EOB]

// block 9: the next move is to point p4 (coordinates computed similar to p3).

N090 G03 X7.366 Y9.125 I0.875 J0.00[EOB]

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// blocks 10, 11, 12: all linear interpolation. Computations for the point p5 are as follows:

p4: y = 9.125; x = 9 + 0.125 tan 67.5 = 9.302

N100 G01 X9.302 [EOB]

NOTE: here we did not specify the Y coordinate, so it will be kept constant !

N110 G01 X3.875 Y3.698 [EOB]

N120 G01 X2.0 Y2.0 M30 [EOB]

CNC TURNING

Table 1: Basic codes/ Basic Keys G-

code Function M-code Function

G00 Positioning (Rapid Traverse) M00 Program STOP G01 Linear Interpolation (Cutting

Feed) M01 Program STOP optional

G02 Circular Interpolation/Helical Interpolation CW

M03 Spindle START CW

G03 Circular Interpolation/Helical Interpolation CCW

M04 Spindle START CCW

G17 XY plane selection M05 Spindle STOP G18 ZX plane selection M08 Coolant ON G19 YZ plane selection M09 Coolant OF G20 Input in inch M30 END of program G21 Input in mm M41 Spindle gear range 1-LOW

(45-1500RPM) G28 Auto Return to reference point M42 Spindle gear range 1-

HIGH(135-4500RPM) G54-59

Working coordinate system (1-6 selection)

Other codes

G70 Finishing cycle N Block Number G71 Stock Removal in turning F Feed (mm/min or inch/min) G72 Stock Removal in Facing T Tool G73 Pattern repeating ; End of block [EOB] G74 End face peck drilling U Incremental coordinate for x-

axis G92 Maximum Spindle speed W Incremental coordinate for z-

axis G96 Constant Surface speed control

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ABBREVIATION KEY SYMBOL DESCRIPTION

HOME (ZERO RETURN)

Axis Home Selection : Both axes move to home position

JOG

JOG feed selection: Mode is step feed

MPG

MANUAL PULSE GENERATOR: To use Handwheel

AUTO

AUTO Mode Selection: Mode is automatic operation

SINGLE BLOCK

SINGLE BLOCK program execution for test operation.

EDIT

EDIT Mode Selection: for editing program

MPG x1

MANUAL PULSE GENERATOR: electronics handwheel, low feed rate

MPG x10

MANUAL PULSE GENERATOR: electronics handwheel, medium feed rate

MPG x100

MANUAL PULSE GENERATOR: electronics handwheel, high feed rate

CYCLE START

Automatic operation is started

CYCLE STOP

Automatic operation is stopped

MANUFACTURING AND MATERIALS DEPARTMENT MME1103 Workshop Technology Manual

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CLNT ON

COOLANT ON: Manual selection of coolant ON

CLNT OFF

COOLANT OFF: Manual selection of coolant OFF

POS

POSITION: Shows all coordinates

OFFSET SETTING

TOOL OFFSET SETTING: Write mode selection, manually input tool offset data

PROG

PROGRAM: To open existing program, load new program, edit program

CANCEL

To cancel input data *same as ‘backspace’ key in computer keyboard

INPUT

To enter data

INSERT

To insert every single code into the program

DELETE

To delete code, data, block, program

ALTER

To change program number, input data

RESET

To terminate the current operation

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Example for CNC Turning Program O1234; Program name N10 G28 U0 W0; Go to Reference point (Machine Zero position) N20 G92 S1200 M41 M03; Max. spindle speed, 1200 RPM, Low gear, Spindle

CW

N30 G96 S400 T0404; Constant surface speed, 400 RPM, Tool no. 4 with tool offset no. 4

N40 G00 X2.6 Z0 M08; Rapid positioning(without cutting) to position X=2.6, Z=0 in mm, coolant ON

N50 G01 X0.74 F0.5; Facing process tool will move to position X=0.74 with feed 0.5mm/min

N60 G00 X3. Z5. M09; Rapid positioning(without cutting) to position X=3, Z5 in mm, coolant OFF

N70 G28 U0 W0; Go to Reference point (Machine Zero position) N80 M30; End of program

MDI MANUAL DATA INPUT

MANUFACTURING AND MATERIALS DEPARTMENT MME1103 Workshop Technology Manual

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CNC MILLING Table 2: Basic Codes/ Basic Keys G-code Function M-code Function

G00 Positioning (Rapid Traverse) M00 Program STOP

G01 Linear Interpolation (Cutting Feed) M01 Program STOP optional

G02 Circular Interpolation/Helical Interpolation CW M03 Spindle START CW

G03 Circular Interpolation/Helical Interpolation CCW M04 Spindle START CCW

G17 XY plane selection M05 Spindle STOP G18 ZX plane selection M06 Tool Change G19 YZ plane selection M08 Coolant ON G30 Return to reference point Z0 M09 Coolant OF

G40 Cutter Diameter compensation OFF M30 END of program

G41 Cutter Diameter compensation LEFT

G42 Cutter Diameter compensation RIGHT

Other codes

G43 Tool length compensation +ve direction N Sequence Number

G44 Tool length compensation -ve direction F Feed (mm/min or

inch/min)

G49 Tool length compensation CANCEL T Tool number

G54-59 Working coordinate system (1-6 selection) ; End of block

G70 or G20 Input in inch U Incremental coordinate

for x-axis G71 or G21 Input in mm V Incremental coordinate

for y-axis

G90 Absolute Dimension Input W Incremental coordinate for z-axis

G91 Incremental Dimension Input R Radius

G92 Position Set I,J,K Axis Interpolation Parameter

G96 Constant Surface speed control H Fixture Offset Selector (Tool length)

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ABBREVIATION SYMBOL DESCRIPTION POS POSITION:

Shows all coordinates PROG PROGRAM:

To open existing program, load new program, edit program

OFFSET SETTING TOOL OFFSET SETTING: Write mode selection, manually input tool offset data

CAN To cancel input data *same as ‘backspace’ key in computer keyboard

INPUT To enter data ALTER To change program number,

input data INSERT To insert every single code

into the program DELETE To delete code, data, block,

program RESET To terminate the current

operation MDI MANUAL DATA INPUT

*NOTE: CNC milling and turning machine have the same basic keys. Refer to Table 1. Example for CNC Milling Program O1234; Name of program N10 G21 G40 G49; Metric programming, tool compensation cancel N20 G91 G30 Z0; Incremental programming, Return to zero position

for z N30 T02 M06; Change tool no. 2 N40 M03 S1000; Spindle rotates clockwise 1000RPM N50 G43 H5 Z2.; Tool length compensation +ve direction up to 2 mm

N60 G55 G90 G00 X0 Y0; Working coordinate G55, Absolute programming, Tool at X=0 Y=0 position

N70 G01 Z-5. F10; Cutting in Z-direction until it reaches z = –5mm with feed 10mm/min

N80 G01 X0 Y50. F100; Cutting to Y-direction to Y=50 with 100mm/min N90 Y100.; Cutting to Y-direction Y=100 N100 X100.; Cutting to X-direction X=100 N110 X0; Cutting to X-direction X=0 N120 G30 Z0 Return to zero position for z

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N130 M30; End of program Question CNC Turning Center 1. List the steps of operating the machine starting with;

i. ON/OFF the machine ii. Reference set-up iii. Tool offset set-up

2. Write a program on turning and facing process. 3. Give 3 advantages of using CNC turning machine compare to conventional machine.

CNC Machine Center 1. List the steps of operating the machine starting with;

iv. ON/OFF the machine v. Reference set-up vi. Tool offset set-up

2. Write a program on facing process. 3. Give 3 advantages of using CNC milling machine compare to conventional machine.