Comparison between AM and CNC

1. L8 – CNC Principles & L9 – Machining
Centre

2. CNC Machines
What is a CNC Machine?
Numerical Control Machines.
Programs are input thru input devices like tape or punched
cards.
NC machines control only the position of job relative to
cutting tool.
Feed rate, speed and tools will be selected by the operator.
The instruction from the Tape will be converted
into machine movement by MCU units.

3. Basic components of CNC Machine
Program of instructions is detailed step by step command to
that direct the processing unit.
Machine Control Unit (MCU) is the brain of the NC machine.
Machine Tools or Processing equipment which
performs useful work

4. 102

5. Basic principle of CNC
Each axis consists of a mechanical component, such as a
slide that moves, a servo drive motor that powers the
mechanical movement, and a ball screw to transfer the power
from the servo drive motor to the mechanical component.
These components, along with the computer controls that
govern them, are referred to as an axis drive system.
Additionally, a CNC axis may be either a linear
axis in which movement is in a straight line, or a
rotary axis with motion following a circular path.

6. Basic principle of CNC
The principles of CNC operation.
Movement of X, Y, Z axis are controlled by a motor which
supplies either Alternating current or Direct current.
• Movement of the machine is done by giving commands.
• All the operations are carried out by codes like speed,
feed, depth of cut, etc.
• For each operation separate code is available.
• The warning system is available to save guard the
various operations and components.

7. Categories of CNC M/c
NC applications have two categories
1. Machine tool applications
- drilling, milling, turning & other metal working operations
1. Non-machine tool applications
- assembly, drafting & inspection.
History: John Parson , Frank Stulen in collaboration
with MIT for the USA air force.
NC – predecessor of CNC

8. TYPES OF CNC MACHINE
CNC Machining Centers
CNC Turning Centers
CNC Drilling Machines
EDM Sinker and wire cut
Machines Flame and Laser-Cutting
Machines Water Jet Profilers
CNC Boring machines
CNC Punching machines
CNC Grinding machines
CNC Welding machines
CNC Bending machines

9. CNC Machine Tools

11. CNC Machining Process

12. NC Motion Control
Point-to-Point systems
Also called position systems
System moves to a location and performs an
operation at that location (e.g., drilling)
Also applicable in robotics
Continuous path systems
Also called contouring systems when control of 2
or>2 axe is present
System performs an operation during movement
(e.g., milling and turning)

13. NC Motion Control
Continuous path systems
Point-to-Point systems

14. 1.Control system based
Point-to-point control systems
It cause the tool to maintain continuous contact with the
part as the tool cuts a contour shape. These operations
include milling along any lines at any angle, milling arcs
and lathe turning.

15. Motion Control
Linear control Contouring control
Point-to-point control
(continual)
drilli
ng
milling machines, lathes machining (milling, turning)
point-to-point motion – tool is moved to specific location, path or speed of move
between location is not controlled. (drilling)
Linear control – movement along one of the axes of the machine.
Contouring control – movement can be made in two or more axes
simultaneously.

16. Interpolation Methods
1. Linear interpolation
Straight line between two points in space
1. Circular interpolation
Circular arc defined by starting point, end point,
center or radius, and direction
1. Helical interpolation
Circular plus linear motion
1. Parabolic interpolation
2. Cubic interpolation
Free form curves using higher order equations

17. Circular Interpolation Methods
Approximation of a curved path in NC by a series of straight
line segments, where tolerance is defined on only the inside of
the nominal curve

18. Circular Interpolation Methods
Approximation of a curved path in NC by a series of straight line
segments, where tolerance is defined on both the inside and outside of
the nominal curve

19. 2.Feedback based
Open Loop System
• Uses stepping motor to create
movement. Motors rotate a fixed amount
for each pulse received from the MCU.
The motor sends a signal back indicating
that the movement is completed. No
feedback to check how close the actual
machine movement comes to the exact
movement programmed.
Closed Loop System
• AC, DC, and hydraulic servo motors are
used. The speed of these motors are
variable and controlled by the amount of
current or fluid. The motors are connect
to the spindle and the table. A device
called a resolver continuously monitors
the movement and sends back a single to
MCU to make adjustments.

20. 3.Drives of CNC machine tool
Hydraulic actuator
high power machine tool
Stepping motor
small machine due to limited power and torque
DC motor
- excellent speed regulation, high torque, most
widely used

21. 4.Coordinate System
1.Absolute . In this mode, the desired target position of the tool
for a particular move is given relative to the origin point of the
program.
2.Incremental. In this mode, the next target position for the tool
is given relative to the current tool position.

22. NC Coordinate Systems
Based on the Cartesian Coordinate system
For flat and prismatic (block-like) parts:
Milling and drilling operations
Conventional Cartesian coordinate system
Rotational axes about each linear axis
For rotational parts:
Turning operations
Only x- and z-axes
Z-axis is always
parallel to the axis
of rotation

23. NC Coordinate Systems

24. Features of CNC
1. Storage of more than one part program
-Memory expansions possible
2. Various forms of program input
- initially hardwired, magnetic tapes, floppy. Now RS2 32
3. Program editing at the machine tool
4. Fixed cycles and programming subroutines - Macros
5. Interpolation

25. Features of CNC
6. Positioning features for setup
- ‘Position set’ feature, references the m/c tool axes to a point or
a set of points on the fixture. Operator does not do it.
7. Cutter length and size compensation
- cutter sensor used to calculate exact dimensions of tool and
cutter offset, as they may be different from that originally planned
8. Acceleration and deceleration computations
- to avoid tool marks during direction change
9. Communications interface
- RS-232 standards allow m/c tool to link with
other computer driven devices for
downloading programs,
collecting operational data and
interfacing with peripheral equipment like robots, AS/R, AGVs

26. Machining Centre
Machining centre is a machine tool capable of
performing several different machining
operations on a work part in one setup under
program control capable of milling, drilling,
reaming, tapping, boring, facing, and similar
operations.
Characterizations of an NC machining center:
Automatic tool-changing capability
Automatic work part positioning
–has a
rotary table Pallet shuttle.

27. Machining Centre - types
Three types of machining centres
1. Vertical Machining centre
2. Horizontal Machining centre
3. Universal Machining centre

28. Vertical Machining Centre

29. Machining Centre - types
Vertical
For flat
work pieces
that require
tool access
from the top

33. Machining Centre - types
Horizontal
For cube shaped parts, where access is required all sides Has spindle
on the horizontal axis

38. Universal
Machining
Centre
Spindle axis can be
tilted from
horizontal to vertical
Equivalent to 5-
axis machining