Part programming for nc machines

Presented by
Ms.DORATHI.K
Asst Prof
SVEC

Introduction to Computer Aided Manufacturing - CAM
Computer Aided Manufacturing involves the use of computer programs
specifically designed to create the geometry and tool paths needed for
parts to be machined. These tool paths can then be automatically
processed into a program specific for the CNC machine to be used.

A Definition:A Definition:
• Numerical Control is a system
in which actions are controlled by
the direct insertion of numerical
data at some point.
• In other words, Programmable
automation in which the
mechanical actions of a ‘machine
tool’ are controlled by a program
containing coded alphanumeric3

Definition of Numerical Control (NC)
• Numerical Control (NC) is a form of programmable automation in
which the mechanical actions of a machine tool or other
equipment are controlled by a program containing coded
instructions (alphanumeric data)
• The collection of all instructions (or program of instruction)
necessary to machine a part is called an NC program, CNC
program, or a part program.
• The person who prepares this program is called a part programmer.

Basic Components of NC System
• An NC system consists of three basic components:
1. A program of instructions,
2. A machine control unit, and
3. Processing equipment.

MACHINE UNIT
NUMERICAL
CONTROLLER
NUMERICAL
DATA
(NC CODE)
MANUFACTURING
OPERATOR
PROCESSED
PART
Drive Control
6

MCU
Machine
Tool
CLU
DPU
MCU - Machine control unit
CLU - Control-loops unit
DPU - Data processing unit
Hardware Configuration of NC Machine

 NC machine tool has a main unit, which is known as
Machine Control Unit.
 It consists of some electronic hardware that reads the NC
programme, interprets it and conversely translates it for
mechanical actions of the machine tool.
A typical Machine Control Unit may consist of the following units :
 Input or Reader Unit
 Memory
 Processor
 Output Channels
 Control Panel
 Feedback Channels
8

9
Machine Tool
•Machine tool is the main components of a numerical
control system, which executes the operations.
•It may consist of worktable, cutting tools, jigs and fixtures,
motors for driving spindle and coolant and lubricating
system.
•The latest development in the numerical control machine
tool is the versatile machining center.
•This is a single machine capable of doing a number of
operations such as milling, boring, drilling, reaming, and
tapping by Automatic Tool Changer (ATC) under the control
of tool selection instruction.

NC Technology
• The NC system uses a fixed logical functions, those that are built-in and
permanently wired within the control unit.
• These functions can not be changed by the programmer or the machine
tool operator.
• The system can interpret a part program, but it does not allow any
changes to the program.
• NC system requires the use of punched tapes for input of the program
instructions.

Advantages and Disadvantages of NC
• The advantages generally attributed to NC, with emphasis on machine tool
applications, are the following:
1. Non-productive time is reduced (fewer setups, less setup time, reduced
work piece handling time, and automatic tool changes).
2. Greater accuracy and repeatability.
3. More-complex part geometries are possible.
4. Simplified tooling and work holding.
5. Operator skill-level requirements are reduced.
6. Inspection requirements are reduced.
• The disadvantages of NC include the following:
1. Higher investment cost.
2. Higher maintenance effort.
3. Part programming.

Motion Control Systems for NC
• Motion control systems for NC can be divided into two types:
1. Point-to-point systems.
2. Continuous systems.
(1) Point-to-point systems (positioning systems)
• These systems move the worktable to a programmed location without regard
for the path taken to get to that location.
• Once the move has been completed, some processing action is
accomplished by the workhead at the location, such as drilling or punching a
hole.

Motion Control Systems for NC
(2) Continuous Path Systems
• Generally refer to systems that are capable of continuous simultaneous
control of two or more axes.
• This provides control of the tool trajectory relative to the workpart.
• The tool performs the process while the worktable is moving, thus
enabling the system to generate angular surfaces, 2D curves, and 3D
contours.

Types of Continuous paths
• Straight-Cut
When continuous path control is utilized to move the tool parallel to only
one of the major axes of the machine tool worktable.
• Contouring
When continuous path control is used for simultaneous control of two or
more axes in machining operations.

 Conventional Numerical Control (NC)
 Computer Numerical Control (CNC)
 Direct Numerical Control (DNC)
 Adaptive Control (AC)
24

 Computer Numerical Control is a numerical
control system that utilizes a dedicated stored
programmed. Computer is used to perform
some or all basic function. Numerical control
is a programmable automation in which
process is controlled by Numbers, Letters, and
symbols. (Computer +NC= CNC).

 Part program:
A part program is a series of coded instructions
required to produce a part. It controls the
movement of the machine tool and on/off
control of auxiliary functions such as spindle
rotation and coolant. The coded instructions are
composed of letters, numbers and symbols.

Program input device:
The program input device is the means for part
program to be entered into the CNC control.
Three commonly used program input devices are
punch tape reader, magnetic tape reader, and
computer via RS-232-C communication.

Machine Control Unit:

The machine control unit (MCU) is the heart of a CNC system. It
is used to perform the following functions:

To read the coded instructions.
 To decode the coded instructions.
 To implement interpolations (linear, circular, and helical) to
generate axis motion commands.
 To feed the axis motion commands to the amplifier circuits for
driving the axis mechanisms.
 To receive the feedback signals of position and speed for each
drive axis.
 To implement auxiliary control functions such as coolant or
spindle on/off and tool change

 Drive System:
A drive system consists of amplifier circuits,
drive motors, and ball lead-screws. The MCU
feeds the control signals (position and speed) of
each axis to the amplifier circuits. The control
signals are augmented to actuate drive motors
which in turn rotate the ball lead-screws to
position the machine table.

Machine Tool:
CNC controls are used to control various types of
machine tools. Regardless of which type of
machine tool is controlled, it always has a slide
table and a spindle to control of position and
speed. The machine table is controlled in the X
and Y axes, while the spindle runs along the Z
axis.

 Feed Back System:
The feedback system is also referred to as
the measuring system. It uses position and
speed transducers to continuously monitor
the position at which the cutting tool is
located at any particular instant. The MCU
uses the difference between reference
signals and feedback signals to generate the
control signals for correcting position and
speed errors.

 Machine Tool Control
 In Process Compensation
 Improved programming and operating features
 Diagnostics

 Machine tool control
 Hybrid CNC –Hard-wired logic circuits for
functions like feed rate generation , circular
interpolation etc. in addition to computer
 Mass production of circuits and less
expensive computer
 Straight CNC –Computer to perform all NC
functions

 In-process compensation
 Dynamic correction of machine tool motion for
changes or errors that occur during processing
 Adjustment of errors sensed by in-process inspection
probes and gauges
 Recomputation of axis positions when an inspection
probe is used to locate a datum reference on the
work part
 Offset adjustments for tool radius and length
 Adaptive control adjustments to sped and feed
 Computation of predicted tool life and selection of
alternate tooling when indicated.

 Improved programming and operating
features
 Use of tape and tape reader only once
 On-line editing of part programs at the machine
 Special canned cycles.
 Graphic display of tool path to verify the tape
 Various types of interpolation: circular, parabolic, cubic
 Support of various units. Conversion from one unit to
another unit.
 Use of specially written subroutines or macros
 Manual data input (MDI)
 Several part programs in bulk can be stored

 Diagnostics
 Equipped with diagnostic capability to assist in
maintaining and repairing the system
 Identification of reason for downtime
 Indication of imminent failure of certain
component
 Redundancy of components

 High accuracy in manufacturing
Short production time
Simpler fixturing
Contour machining (2 to 5 –axis machining)
Reduced human error
High repeatability and precision e.g. Aircraft parts
Volume of production is very high
Complex contours/surfaces need to be machined.
E.g. Turbines
Flexibility in job change, automatic tool settings,
less scrap
More safe, higher productivity, better quality
Less paper work, faster prototype production,
reduction in lead times

1. Costly setup
2. Skilled operators
3. Computers programming knowledge required
4. Maintenance is difficult

 DNC is a manufacturing system in which a
number of machines are controlled by a
computer through direct- connection and in
real time.
 DNC is a system connecting a set of NC
machines to a common memory for part
program or machine program storage with
provision for on- demand distribution of
data to machines.

 Central computer
 Bulk memory which stores the NC part programs
 Telecommunication lines
 Machine Tools.

 A central computer connected to a number of
machine tools and control them
 Part program of all machine tools are stored in the
memory of the central computer and transmitted on
direct transmission lines on demand
 Two way information flow take place in real time
 Various machine tools can communicate with the
computer in real time
 Programs in full or segment can be transferred to NC
machines
 Computer can be used for program editing
 No tape readers are used
 No limitation for the number or size of programs
 stored

 The configuration of the DNC system can be
divided into:
 1.DNC system without satellite computer.
 2.DNC system with satellite computer.
 Satellite computers are minicomputers and
they serve to take some of the burden off
central computer. Each satellites controls
several machine tools.

 There are two alternative system configurations
by which the communication link is established
between the control computer and the machine
tool.
 Behind the Tape Reader (BTR) system.
 Special Machine Control Unit.

 The computer is linked directly to the regular NC
controller unit.
 Except for the source of the command instructions,
the operation of the system is very similar to
conventional NC.
 The controller unit uses two temporary storage
buffers to receive blocks of instructions from the
DNC computer and convert them into machine
actions.
 One buffer is receiving a block of data, the other is
providing control instructions to machine tool.
 Cost is very less

 Replace the regular controller unit with a special
machine control unit.
 The special control unit is designed to facilitate
communication between the machine tool and
the computer.
 The special MCU configuration achieve a superior
balance between accuracy of the interpolation
and fast metal removal rates than is generally
possible with the BTR system

Behind the Tape Reader (BTR)
system Special Machine Control Unit

 The functions which a DNC system is
designed to perform:
 NC without punched tape.
 NC part program storage.
 Data collection, processing, and reporting.
 Communication

 The program storage subsystem must be structured
to satisfy several purposes:
 The program must be made available for
downloading to the NC machine tools.
 The subsystem must allow for new programs to be
entered, old programs to be deleted, and existing
programs to be edited.
 The storage subsystem must be structured to
perform certain data processing and management
functions, such as file security, displays of
programs, and manipulation of data

 The purpose of this functions is to "monitor"
production of the factory.
The data concerned are:
Tool usage
Machine utilization
 Production piece counts
 These data must be processed by the DNC
computer, and reports are prepared to provide
management with information necessary for
running the plant.

 Communication Network" is required to
accomplish the previous functions of DNC.
 The essential communication links in DNC are
between the following components of the
system:
Central computer and machine tools
Central computer and NC part programmer
terminal
Central computer and bulk memory

 Elimination of punched tapes and tape
readers
 Convenient storage of NC part programs in
computer files
 Greater computational capability and
flexibility
 Reporting of shop performance.
 Convenient editing and diagnostic features

 DNC concepts represents a first step in the
development of production plants which will
be managed by computer systems. This
establishes the framework for the evolution
of computer automated factories

 “to adapt” means to change a behavior to conform
to new circumstances.
 An adaptive controller
a controller that can modify its behavior in
response to the changes in dynamics of the
processes and the disturbances acting on the
process.
 A self-correcting form of optimal control

 In machining, it includes automatic adjustment of cutting
parameters like speeds, feeds, depth of cut, etc.

Adaptive controller performs 3 functions
1. Identification
- Identifies the current value of performance index
- Functions continuously to be dynamic
1. Decision
- decide what changes have to be made to improve
system performance
3. Modification
- implement the decision

 Increased production rate
 Increased tool life
 Greater part protection
 Increases machine life
 Less operator intervention
 Easier part programming

 Industrial surveys in 1960's showed smaller
machine components requiring several operations
tool long time to complete
Part sent to several machines before finished
There was much "operator intervention" during
machining process
 In late 1960s and early 70s, begin to design
machine that would perform several operations
and do 90% of machining on one machine

CNC machine centre is advance manufacturing
machine tool which performs wide range of
machining operation with accuracy and
inspection free product with good quality
surface finishing.
Operations done on drilling, milling and lathe
can be performed on CNC machine centre.
A computer-controlled machine tool capable of
many types of cutting operations on multiple
surfaces and directions on a work piece

78-65
 Horizontal Machining Center
 vertical Machining Center
 universal Machining Center

78-66
 Traveling-column
 One or usually two tables where work mounted
 Column and cutter move toward work on one table while operator
changes work piece on other table
 Fixed-column
 Equipped with pallet (removable table)
 After work piece machined, pallet and work piece moved off
receiver onto shuttle; shuttle rotated, bringing new pallet into
position for shuttle and finished work pallet into position for
unloading

HMC (Horizontal Machining Center)

78-68
 Saddle-type construction with sliding bedways that
use a sliding vertical head instead of quill
movement
 Generally used to machine flat parts held in vise or
simple fixture
 Versatility increased by addition of rotary
accessories

 These are similar to horizontal machining centers but
with the spindle axis capable of tilting from
horizontal to the vertical position continuously under
computer control.
 This machine centers consists of 5 or more axis.
 Such machines facilities access to the top surface of
work piece mounted on a horizontal machining centre
so that all the five sides of a components can be
machined in a single set up.
 Has additional accessories such as indexible pallets
and rotary-tilt tables

 Spindle axis can be
tilted from
horizontal to
vertical
 Equivalent to 5-
axis machining

77-73
 In mid-1960, 40% all metal-cutting operations
performed on lathes
 Not very efficient
 Research led to development of numerically
controlled turning centers and chucking lathes
 Could produce round work almost any contour
automatically and efficiently

77-74
1. CNC chucking center
• Holds part in some form of jaw chuck
• Some have dual spindles (work both ends)
1. CNC universal turning center
• Can use continuous bar feed system to machine and
cut off parts from bar
• Some have dual tool turrets
1. Combination turning/milling center
• Uses combination of turning tools

77-75
 Designed to machine work held in chuck
 Variety of sizes from 8 to 36 in. in diameter
 Four-axis chucking center has two turrets
 Separate sides; each machine work at same time
 Seven-tool upper turret
 Seven-tool lower turret
 Two-axis model has one or two turrets
 Will drive only one turret at a time

 CNC machining centers are usually designed with
features to reduce non productive time.
 Automatic tool changer :- The tools are
contained in a storage unit that is integrated
with the machine tool. When a cutter needs to
be changed, the tool drum rotates to the proper
position and an automatic tool changer (ATC)
operating under program control, exchanges the
tool in the spindle for the tool in the tool storage
unit. Capacities of tool storage unit commonly
range from 16 to 80 cutting tools. •

 Automatic work part positioner:-
 Many horizontal and vertical machining
centers have the capability to orient the
work part relative to the spindle. This is
accomplished by means of a rotary table on
which work part is fixtured. The table can be
oriented at any angle about a vertical axis to
permit the cutting tool to access almost the
entire surface of the part in a single setup.

 Machining centers are often equipped with two
(or more) separate pallets that can be
presented to the cutting tool using an automatic
pallet changer.
 While machining is performed with one pallet in
position at the machine, the other pallet is in a
safe location away from the spindle. In this
location, the operator can unload the finished
part and then fixture the raw work part for next
cycle.

 The part program is a sequence of
instructions, which describe the work, which
has to be done on a part, in the form
required by a computer under the control of
a numerical control computer program.
 It is the task of preparing a program sheet
from a drawing sheet.

 All data is fed into the numerical control
system using a standardized format.
 Programming is where all the machining data
are compiled and where the data are
translated into a language which can be
understood by the control system of the
machine tool.

 (a) Machining sequence classification of
process, tool start up point, cutting depth,
tool path, etc.
 (b) Cutting conditions, spindle speed, feed
rate, coolant, etc.
 (c) Selection of cutting tools.

 Determine the startup procedure, which includes
the extraction of dimensional data from part
drawings.
 Select the tool and determine the tool offset.
 Set up the zero position for the work piece.
 Select the speed and rotation of the spindle.
 Set up the tool motions according to the profile
required.
 Return the cutting tool to the reference point after
completion of work.
 End the program by stopping the spindle and
coolant.

 (a) Manual part programming,
 (b) Computer aided part programming

The programmer first prepares the program
manuscript in a standard format.
 Manuscripts are typed with a device known as flexo
writer, which is also used to type the program
instructions.
After the program is typed, the punched tape is
prepared on the flexo writer.
Complex shaped components require tedious
calculations.
This type of programming is carried out for simple
machining parts produced on point-to-point machine
tool.

(a) Knowledge about various manufacturing
processes and machines.
(b) Sequence of operations to be performed
for a given component.
(c) Knowledge of the selection of cutting
parameters.
(d) Editing the part program according to the
design changes.
(e) Knowledge about the codes and functions
used in part programs.

If five complete words are not included in each block, the
machine control unit (MCU) will not recognize the
information; therefore the control unit will not be
activated

The most common codes used when
programming NC machines tools are G-codes
(preparatory functions), and M codes
(miscellaneous functions).
Other codes such as F, S, D, and T are used
for machine functions such as feed, speed,
cutter diameter offset, tool number, etc.
G-codes are sometimes called cycle codes
because they refer to some action occurring
on the X, Y, and/or Z-axis of a machine tool.

 G00 Rapid positioning
 G01 Linear interpolation
 G02 Circular interpolation
clockwise (CW)
 G03 Circular interpolation
counterclockwise (CCW)
 G20 Inch input (in.)
 G21 Metric input (mm)
 G24 Radius programming
 G28 Return to reference point
 G29 Return from reference
point
 G32 Thread cutting
 G40 Cutter compensation
cancel
 G41 Cutter compensation left
 G42 Cutter compensation right
 G43 Tool length compensation
positive (+) direction
minus (-) direction
cancels
 G 53 Zero offset or M/c
reference
 G54 Settable zero offset
 G84 canned turn cycle
 G90 Absolute programming
 G91 Incremental programming

 M00 Program stop
 M02 End of program
 M03 Spindle start (forward
CW)
 M04 Spindle start (reverse
CCW)
 M05 Spindle stop
 M06 Tool change
 M08 Coolant on
 M09 Coolant off
 M10 Chuck - clamping
 M11 Chuck - unclamping
 M12 Tailstock spindle out
 M13 Tailstock spindle in
 M17 Tool post rotation
normal
 M18 Tool post rotation
reverse
 M30 End of tape and
rewind or main program
end
 M98 Transfer to
subprogram
 M99 End of subprogram

 % – Main Programme (1
to 9999)
 L – Sub program (1 to
999)/Home position
 N – Sequence of block
number.
 Lf – Block end (EOB)
means “; or *”
 T – Tool number or Tool
station number
 D – Tool offset
 S – Spindle speed
 F – Feed
 M – Switching function
 G – Transverse
commands
 R – Parameters
 I, J, K – Circle
parameters
 B/U/R – Radius
 X/Y/Z – Axis
coordinates
 P – Passes

If the complex-shaped component requires
calculations to produce the component are done
by the programming software contained in the
computer.
The programmer communicates with this system
through the system language, which is based on
words.


There are various programming languages
developed in the recent past, such as
 APT (Automatically Programmed Tools),
 ADAPT, AUTOSPOT, COMPAT-II, 2CL, ROMANCE,
SPLIT is used for writing a computer programme,
which has English like statements.
 A translator known as compiler program is used to
translate it in a form acceptable to MCU

 The APT language was the product of MIT development work
on NC programming systems.
 Its development began in June 1956, and it was first used in
production around 1959.
 Today it is the most widely used language in NC part
programming.
 Although first intended as a contouring language , modern
versions of APT can be used for both positioning and
continuous path programming.

The four types of statements in the APT language are:
1. Geometry Statements: which define primitive elements such as points,
lines, circles, planes, cones and spheres. They are also sometimes called
definition statements
2. Motion Statements: which describe the tool path in relation to the part
geometry
3. Postprocessor Statements: which give specific machine tool code
information as well as feeds and speeds
4. Auxiliary Statements: which give part and tool tolerances

 The general form of geometry statements is:
symbol = geometry type / descriptive data
An example of such statement is:
P1 = POINT/100.0, 200.0, 300.0
 The statement is made up of three sections.
 The first is the symbol use to identify the geometric element. Normally the
alphabet “P” is used for defining a Point, “C” for Circle, “L” for Line and
“Pl” for Plane.
 The second section of the geometry statement is an APT vocabulary word
that identifies the type of geometry element. e.g. POINT, LINE, CIRCLE,
PLANE, etc.
 The third section of the geometry statement is the descriptive data

P2 = POINT / INTOF,L1,L2
P2
L2
L1

P3 = POINT / XSMALL, INTOF, L1, C1
Or P3 = POINT / YSMALL, INTOF, L1, C1
P4 = POINT / XLARGE, INTOF, L1, C1
Or P4 = POINT / YLARGE, INTOF, L1, C1
P3
C1
L1
P4
Y
X

P3 = POINT / XSMALL, INTOF,C1,C2
Or P3 = POINT / YLARGE, INTOF, C1,C2
P4 = POINT / XLARGE, INTOF, C1,C2
Or P4 = POINT / YSMALL, INTOF, C1,C2
P3
P4
C1
C2
Y
X

L3 = LINE/P3,PARLEL,L2
L3
P3
L2

L3 = LINE/P1,PERPTO,L2
L3
L2
P1

C2 = CIRCLE/CENTER,P1,P2
P1
C2
P2

C3 = CIRCLE/P1,P2,P3
C3
P2
P3
P1

 APT motion statements have a general format:
motion command / descriptive data
e.g. GOTO / P1
 At the beginning of the motion statements tool must be given a starting
point
FROM / P0
Or FROM / -2, -2, 0
 The FROM is an APT vocabulary word which indicates that this is the
initial point from which others will be referenced. The FROM statement
occurs only once at the start of the motion sequence.
 In APT there are two basic types of motion statements:
a) Point to Point motion
b) Contouring motion

 There are only two basic point to point motion commands
GOTO:
The GOTO statement instructs the tool to go to a particular point location
specified in the descriptive data.
e.g. GOTO / P2
GOTO / 2, 7, 0
GODLTA:
The GODLTA command specifies an incremental move for the tool.
e,g, GODLTA / 2, 7, 0 instructs the tool to move from its
present position to 2 units in x-direction, 7 units in y-direction and 0 units
in z-direction

 Modifier words, such as TO, ON, PAST or TANTO, are used to govern the
position of the tool in relation to the check surface.

 Motion statements, GOLFT (go to the left), GOFWD (go forward) and
GORGT(go to the right), are also used to control the cutter motion.

3. POST PROCESSOR STATEMENT
Post processor command for a particular tool are :
MACHIN/ : use to specify the machine tool and call the post processor for the tool.
e.g.- MACHIN/DRILL,3
COOLNT/ : allow the coolant fluid to be turned on or off.
e.g.- COOLNT/OFF
COOLNT /ON
COOLNT/FLOOD
COOLNT/MIST
FEDRAT/: specifies the feed rate for moving the tool along the part surface in
inches per minutes:
e.g.- FEDRAT/4.5
SPINDL/: gives the spindle rotation speed in revolution per minute.
e.g.- SPINDL/ 850
FINI: end of program

4. AUXILIARY STATEMENTS
• It give part and tool tolerances.
• The statement used for this are
INTOL/0.0015
OUTTOL/0.001

 G00 X40
 Y0
 Z5
 G01 Z-0.5 F40
 X40 Y40
 X-40
 Y40
 X-40
 Y-40
 X40
 Y-40
 X40 Y0
 Z5
 G00 X0 Y0
 M30

Part programming for nc machines

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Part programming for nc machines