chapter 13 cnc programming

8/10/2019 Chapter 13 Cnc Programming

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CAD/CAM Principles and

Applications by P N Rao, 2nd Ed

1

CAD/CAM Principles andApplications

Ch 13 CNC Programming


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Objectives Understand the fundamentals of part programming in terms of the

various steps needed to be taken for completing a successful CNC partprogram.

Comprehend the elements of manual part programming methods usingword address format and the ISO G-coding systems.

Learn the part programming fundamentals related to the use of variousword addresses.

Appreciate different preparatory (G codes) and miscellaneous functions(M codes) as used in CNC part programs.

Write and prove sample part programs for CNC machining centres inplanar milling operations using the word address format.

Understand the concept of canned or fixed cycles for the hole makingoperations

Learn the use of cutter diameter and length compensation while usingwith multiple cutting tools.


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Fig. 13.1 The stepsinvolved in thedevelopment of a

proven part program

in NC machining

Study part print

Identify machinetool

Identify operationsand tooling

Determine feeds,speeds, depths,power

Prepare setupsheet with worksetting and toolingassembly

Develop partprogram

Simulate toolpath

Dry run

Cut the first part

Proven part

Machinetooldatabace

Cuttingtooldatabase

Machinabilitydatabase

Capabilityof cuttingtools

Inspect the part


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Fig. 13.2 A typical component for NC

machining

100

20

10

10

6

R5

40

20

40

10

15 15

15

85


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Fig. 13.3 Part for NC machining shown with axes system at the

centre

6

R5

40

X

X

Z

Y35

10

30

50

50

20

20


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Fig. 13.5 Tool path of the part for proving the

NC part program


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Fig. 13.6 Shaded 3D image of the tool cutting the part for providing

more realistic proving of the NC part program (Courtesy Virtual Gibbs Inc.)


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Fig. 13.7 Setup sheet for a machining centre part


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13.2 Manual PartProgramming Methods

ISO standards for coding Co-ordinate function

Feed function Speed function

Tool function G Codes

M Codes


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Fig. 13.8 Absolute (G90) and incremental (G91)

systems

X

Y

5

25 355

10

15A

B

C

O


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Fig. 13.9 XY plane selection for vertical axis milling

machinesZ

X

Y

G17


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Fig. 13.10 XY plane selection for horizontal axis

milling machines

ZX

Y

G17


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Fig. 13.11 XZ plane selection for horizontal axis milling

machinesZ

X

Y

G18


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Fig. 13.12 YZ plane selection for horizontal axis

milling machinesZ

X

YG19


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Fig. 13.13 Setting the workpiece on the machine table


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Fig. 13.14 Positioning, preparatory function G00

B

A

150

45

30

A

C

20X

Y


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Fig. 13.15 Positioning, preparatory function G00 in 3

dimensions

Z

X

YA

B

50

90

40

70

9045


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Fig. 13.16 Linear interpolation, preparatory function

G01

20

D

E

110

30

100

X

Y


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Fig. 13.18 Circular interpolation in XY plane using G17

plane selection

A

B

Z

X

Y


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Fig. 13.19 Circular interpolation in XZ plane using G18

plane selection

A

B

Z

X

Y


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Fig. 13.20 Example

Z

X

Y

A25

25

50

75

65

R10

R10

R35

X

125

100

B

C D

10

E


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Fig. 13.21 Simulation of the above part program for

component shown in Fig. 13.20


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Fig. 13.22 Component for example 13.2

80

2

X

Y

Z 18.757

20

98

20

98

18.757

80

X5


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Fig. 13.24 Offset Tool path for machining contours

that are not parallel to the principal axes

R

X

Y

Y1

Y2

X1

X2

R

Part contour

Cutter centre path


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Fig. 13.26 Tool length compensation

Z

X

1 2 3 4TOOL

L4

L3

L

2


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Fig. 13.27 Typical motions embedded in G81 canned

cycle

RAPID

FEED

RAPID

Clearance plane

Hole

Depth

Breakthrough

distance


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Fig. 13.29 Component for NC program in example

13.3

820 10

Cutting tools:

2 Centre drill3 Twist drill 6 mm4 Twist drill 8.5 mm5 Tap M10

X

Y

80

95

115

16

40

60

84

34

84

20

40

601 End mill 40 mm

100

130


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Fig. 13.31 Cutter radius compensation

0 10 30 70 90

10

20

40

60

80

12

Programmed Path

Offset Path of Cutter


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Fig. 13.32 Example showing the cutter radius

compensation using the G codes G42

12

34

5

6

Set point

G40

140

110

60

90

X

YAG42


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Fig. 13.33 Example showing the cutter radius

compensation using the G codes G41 and G42

B

A

C

DE

FG

HJ

K

L

200

200

100

60

20

120

41

42


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Fig. 13.34 Example for contour programming using

the cutter radius compensation

22.5

20

10

33

77.503

91.645

R15

10

R10

R15

4.393 53.713124.853

134.799

1525.607

39.571

70.432

84.574

46.642

R10

120


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Summary Part programming is a systematic activity and following the

steps in proper sequence will help in developing efficient part

programs Selection of the part coordinate axes system with respect to the

part geometry help in reducing the mathematical calculations

that are needed to write the part program

ISO standard for part programming utilises all the 26 characters

of English alphabet as word addresses in a part program.

Preparatory functions or G-codes control the geometrical nature

of the data input as well as operation of some of the data in aprogram. A large number of these codes are standardised by

ISO.


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CAD/CAM Principles and 40

Summary Motion commands are the most important part of the G-codes

used to control the motion in rapid (G00), linear interpolation

(G01) and Circular interpolation (G02 and G03). Miscellaneous functions or M-codes are used to specify the

machine tool functions in the program. Less number of codesare standardised in view of these being machine specific.

For programs involving multiple tools, tool length compensationcan be activated by the use of tool offset registers.

Canned or fixed cycles offer the facility to reduce the bulk ofprogram by canning the often repeated command into a single

G-code. Examples are for the drilling of holes (G81). In milling of contours large amount of complex calculations are

often required to take multiple cuts. This can be reduced by theuse of cutter diameter compensation (G41 and G42).

chapter 13 cnc programming

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