Introduction to CNC and CAD/CAM

Question 1

Definition of automation in manufacturing

Answer 1

The process of enabling machines to follow a predetermined sequence of operations
with little or no human labor
using specialized equipment that perform and control manufacturing processes.

An automation system is able to

a. monitor the operations
b. make decisions about process parameters
c. control all aspects of machining

Question 2

Automation application in manifacturing (5)

Answer 2

• Manufacturing processes
• Material handling and movement
• Inspection
• Assembly
• Packaging

Question 3

Primary goals of automation (6)

Answer 3

• Integrate devices, machines and systems to improve product quality, reduce cycle times, reduce human effort
• Improve productivity by reducing costs through better control of the production
• Improve quality
• Reduce human involvement
• Improve safety
• Optimize the use of floor space

Question 4

Definition of Numerical Control

Answer 4

A method of controlling the movements of machine components by inserting coded intruction in the form of numbers and letters.

Question 5

Definition of Computer Numerical Control

Answer 5

The realization of NC by a computer installed on the machine

Question 6

Types of control strategies

Answer 6

• Open loop: signals are sent to the positioning system, but the position is not checked
• Closed loop: the control signal sent to the positioning system is based on an error signal between the desired position and the actual measured position; it allows rejection to disturbances

Question 7

Types of tool path control

Answer 7

1. point to point:
   - axis are moved independently
   - path is defined to minimize time
   - used for drilling
2. continuous path (contouring):
   - axis are moved at the same time
   - motion and operation are simultaneous
   - accurate control of velocity and position is needed
   - complex trajectories can be performed

Question 8

Definition of interpolation

Answer 8

Determination of points between known points on a desired path, according to a specific Mathematical function

Question 9

Types of interpolation

Answer 9

• Linear interpolation: straight line from start to end
  reducing the distance between points any path can be approximated by a sequence of lines
• circular interpolation: circle of a given radius around a given centre, between two points

Question 10

Definition of part program

Answer 10

Ordered set of instructions in a coded format used to control the motion of a cnc machine tool

Question 11

Possible commands of a part program

Answer 11

A block of a part program can contain

• NC commands
• call to subprogram
• setting of variables and conditional call

Question 12

Composition of a block

Answer 12

several words, starting from the block number and ending with the end of block code
each word is composed by an address and a number

Question 13

G addresses

Answer 13

Represent preparatory functions.
They are used for controlling axis, in particular:
1. Movement of the tool
2. Setting local coordinate system
3. Interpolation
4. Miscellaneous: tool compensation, safety check, skip

Question 14

Types of addresses

Answer 14

• Modal: effective also in the following blocks, until a cancel block is used
• Non-modal: effective just in the block they are called in

Question 15

Motion addresses

Answer 15

• Translational axis: X,Y,Z for motion on linear axis or U,V,W for motion on linear secondary axis
• Rotary axis: A,B,C

Question 16

Convention for machine axis

Answer 16

Z: spindle rotation axis, positive in the direction with the tool moving away from the workpiece
X: parallel to the working table, direction orthogonal to Z and Y
Y: normal to the XZ plane, positive with the tool moving away from the piece

Question 17

Utility functions

Answer 17

• Subprogram: fixed routine blocks, called from the main program, to simplify its structure
• Macros: register a series of commands or functions as a single command

Question 18

Coordinates systems

Answer 18

• Machine coordinate system: defined by setting a fixed point of the machine as the zero
• Workpiece coordinate system: defined by setting a point of the workpiece as the zero
• Local coordinate system: defined wrt a workpiece coordinate system

Question 19

How to select part zero

Answer 19

• Technical drawing
• Convenieence of setup and operation
• Safety of working conditions

Question 20

Words to command displacements

Answer 20

• G90: absolute coordinate system
• G91: incremental coordinate system

Note that
in turning, absolute quotes are on the diameter, incremental ones are on the radius

Question 21

Rapid positioning word

Answer 21

G00: it moves the specified axis rapidly to a defined position

Question 22

Linear interpolation function

Answer 22

G01: to command the axes to move the tool along a line with specified feedrate

Question 23

Circular interpolation functions

Answer 23

G02: clockwise direction
G03: Counter clockwise direction
defined from the headstock

1. initial point coordinates
2. Distance between start point and centre: I,J,K addresses
   or
   Radius: address R
3. final point
   or
   angle range

Question 24

Feed function

Answer 24

F code

• feed per minute G94
• feed per revolution G95

Question 25

Exact stop

Answer 25

To control the feed at the corner between two blocks, exact stop (G09 or G61 modal) can be used: the tool is stopped, then a position check is made with this command a right angle is obtained

Question 26

Tool function

Answer 26

T code Txxxx - First two digits used for selecting the tool in the specified position, activating the relative geometry offset - Second two digits to select the wear offset in the specified register number the wear offset can be adjusted on the basis of measurements on the machined parts

Question 27

Work offset

Answer 27

Relationship between part zero and machine zero. It allows the programmer to program a part away from the CNC machine

Question 28

Tool length offset

Answer 28

It corrects the difference between programmed tool length and its actual length. Three methods: 1) preset: tools are presetted away from the CNC machine, in line with the work offset 2) touch-off method: preset as distance between tip and piece 3) master tool: touch-off method for the master tool, then other tools are measured from the master reference

Question 29

Tool compensation

Answer 29

G41: radius compensation to the left of the tool movement G42: radius compensation to the right G40: no compensation

Question 30

Spindle function

Answer 30

``` S code (modal) - Constant surface speed (G96): it changes the spindle speed to obtain always the same cutting speed ```

Question 31

How to create a part program

Answer 31

- For very simple parts, an operator can write the iso code directly on the CNC machine - For more complex parts, starting from a CAD model of the part, the operator can design the tool path using a CAM program, which then provides an APT code; a post processor automatically generates the ISO code from the APT code

Question 32

Problems in tool path generation

Answer 32

- tool path approximation: the tool path is always approximated by straight lines, and this gives errors - tool compensation: the real contact point is not the one controlled by the APT code; this gives an error that should be compensated - some material may remain uncut - some features may be impossible to obtain - some part may be cut accidentally In general, a simulation tool is required to predict the possible problems that can be solved at a design stage, and to predict the quality of the final surface

Question 33

STEP-NC: need

Answer 33

The low level standard of interface (G & M code) between CAM and CNC used in the last 50 years is now considered as an obstacle for the spreading of global, collaborative and intelligent manufacturing allowed by modern communication technologies. This old standard has several problems: 1. a lot of information is lost in the passage from CAM to CNC; CNC is simply an executing system, unaware of the motion being executed 2. it's very hard for machine operators to understand the code and eventually modify it 3. the G & M code is dependent on the specific machine tool builder 4. there is no information feedback from the CNC level to the CAM office For these reasons, a great effort has been done in the last 20 years to define a new inerface standard adequate to the needs of information-based manufacturing systems

Question 34

STEP-NC: very general contents

Answer 34

STEP-NC is a new model of data transfer between CAD/CAM systems and CNC machines. It includes high level information based on an object oriented description about what-to-make (geometry) and how-to-make (process plan). It uses STEP geometries and it is vendor neutral It allows interface with both conventional CNC and new intelligent CNC capable of doing automatic decisions

Question 35

STEP-NC: data model

Answer 35

The standard data model is made of - Manufacturing_feature: it contains geometrical data of the feature - Machining_operation: it contains machining data for milling or drilling, like tool, machining strategy, parameters, ausiliary functions