IPE 441_ Machine Tools and Machining _ Lecture 17 - 20.pdf

Lecture Note on Machine Tools and Machining
1
Lecture 17-20
CNC Machines
Machine Tool Control Systems
At the end of this chapters, students will be able to
• identify the types of systems and controls devices used in computer
numerical control
• list the steps required to produce a part by computer numerical control
• explore advantages and disadvantages of computer numerical control
• xyz
17.1 Introduction
In the 19th
century, Charles Babbage, an English mathematician, invented a
machine that could rapidly and precisely calculate long lists of various
functions, including logarithms. In the same time, a French mechanician, J.
M. Jacquard, introduced a punch card system to direct the operations of a
weaving loom. This use of punched cards led to the development of the early
office machines for the tabulation of data. In the 1930s, Konrad Zuse built a
simple computer that was used to calculate wing designs for the German
aircraft industry. The earliest digital computers used electromechanical on-off
switches or relays. After the huge modification in the computational system,

2
in 1971, the Intel Corporation introduced the microprocessor, a chip that
contained the entire central processing unit (CPU) for a single computer, that
could be programmed to do any number of tasks, from steering a spacecraft
to operating a watch or controlling the new personal computers. Machine tools
with such systems of automatic control can be used with advantage in batch
and mass production. Currently, computers have contributed to the efficient
manufacture of all goods. It appears that the impact of the computer will be
even greater in the years to come.
Computers are now being continued to improve productivity through
computer-aided design (CAD), by which the design of a product can be
researched, fully developed, and tested before production begins. Computer-
assisted manufacturing (CAM) results in less scrap and more reliability
through the computer control of the machining sequence and the cutting
speeds and feeds. Robots, which are computer-controlled, are used by industry
to an increasing extent. Robots can be programmed to paint cars, weld, feed
forges, load and unload machinery, assemble electric motors, and perform
dangerous and boring tasks formerly done by humans.
Numerical control (NC) may be defined as ‘a method of accurately controlling
the operation of a machine tool by a series of coded instructions, consisting of
numbers, letters of the alphabet, and symbols that the machine control unit
(MCU) can understand’. These instructions are converted into electrical
pulses of current, which the machine motors and controls follow to carry out
machining operations on a workpiece. The measuring and recording devices
incorporated into computer numerical control machine tools ensure that the
part being manufactured will be accurate. Computer numerical control (CNC)
and the computer have brought tremendous changes to the metalworking
industry. New machine tools, in combination with CNC, enable industry to
consistently produce parts to the desired accuracies with amazing speed,
quality, efficiency, and repeatability.
17.2 When is a system called Numerically Controlled?

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A machine tool is said to be numerically controlled if it operates in a semi-
automatic or automatic cycle as per instructions transmitted to it in a coded
form. Strictly speaking, the term ‘numerical control’ is a misnomer, because
the coded instructions are expressed not only through numerals, but also
through letters, punctuation marks and other symbols. However, although
‘symbolic control’ would have, perhaps, been a more appropriate name, the
term ‘numerical control’ (NC) has come to be so closely associated with
control through symbols that it is now universally accepted and applied in the
latter sense.
It is obvious that numbers by themselves cannot do any work, leave aside
operating a machine. A comprehensive electrical, electronic and mechanical
processing and transmission system is required to affect the movement of a
slide or cutting tool from information coded on a program medium, such as a
punch card, punch tape, magnetic tape, etc.
With numerical controllers becoming increasingly widespread, requirements
from users were becoming stricter in terms of availability, flexibility, ease of
maintenance, functional scope and ease of use of controllers, and their simple
adaptability to the machine types. These requirements could only be satisfied
by hardwired controllers with great overhead. After 1968, a simpler and more
cost-effective solution for realizing such complex controllers arose with the
development of highly integrated electronic components (IC technology).
Since 1972, one or more computers were used as the central component (first
minicomputers up to about 1977, afterward microcomputers were more
common). This gave rise to computerized numerical control (CNC).
Behind the conceptual idea of numerical controllers based on computers is the
production of large volumes by standardized and reliable computer hardware.
By implementing different system programs, they can then be adapted to
production jobs as well as to special machines. This means the controller

4
software can be upgraded at a later time with additional functions that are
offered to the user as options. No major modifications to hardware are
required here. The significance of numerical control for machine tools has
risen continually due to its increased functionality. Figure 11.1 shows the
requirements nowadays of numerical control inside a machine tool.
Since the end of the 1970s, all numerical controllers contain microprocessors.
Use of the “CNC” term made sense in the transition period to differentiate
between hardwired numerical control and computer control. Today, however,
NC should always be used instead of CNC.
The competitiveness of machine tool manufacturers and users is determined
to a high degree by the performance and price of the controller used. In the
control technology market, so important for this very reason, European
manufacturers were in danger of missing out at the start of the 1990s. Japanese
products dominated the world market for numerical controllers in particular,
both in terms of technology and volumes. Since then, the market (made up of
over 60 European controller manufacturers represented primarily on the
European market) has been consolidated substantially. In 2015, following
numerous acquisitions and mergers, just a few high-profile European
manufacturers are represented on the world market with numerical controllers.
In addition, there are some smaller companies with more of a regional
significance.
Control technology has also benefited from the rapid advance of personal
computers for the office and home user markets. With commercially available
PC-based hardware and software, controller manufacturers have been able to
reduce considerably their investments for launching new products. This has
meant the importance of software has increased significantly compared to

5
hardware. Extending beyond their roles as providers of systems only, control
manufacturers are increasingly using their expertise to offer tailored
customer services. Hardware and software standards, standardized interfaces,
as well as open control systems that are expandable flexibly are increasingly
in demand by users. In particular, the capability of integrating a controller into
networks and master systems took on enormous significance at the turn of the
millennium.
Fig. 11.1 Requirements of numerical control inside a machine tool.
Therefore, the role of a computer in CNC system:
Computer is used for part design using computer-aided design (CAD), testing,
inspection, quality control, planning, inventory control, gathering of data,
work scheduling, warehousing, and many other functions in manufacturing.
The computer is having profound effects on manufacturing techniques and
will continue in the future. Computers fill three major roles in CNC:

6
1. Almost all machine control units (MCUs) include or incorporate a
computer in their operation. These units are generally called computer
numerical controls (CNC).
2. Most of the part programming for CNC machine tools is done with
off-line computer assistance.
3. An increasing number of machine tools are controlled or supervised
by computers that may be in a separate control room or even in another
plant. This is more commonly known as direct numerical control
(DNC).
CNC has grown at an ever-increasing rate, and its use will continue to grow
because of the many advantages that it has to offer industry. Some of the most
important advantages of CNC are:
1. Greater operator safety. CNC systems are generally operated from a
console away from the machining area, which is enclosed on most
machines. Therefore, the operator is exposed less to moving machine
parts and to the cutting tool.
2. Greater operator efficiency. A CNC machine does not require as much
attention as a conventional machine, allowing the operator to perform
other jobs while it is running.
3. Reduction of scrap. Because of the high degree of accuracy of CNC
systems, scrap has been drastically reduced.
4. Reduced lead time for production. The program preparation and setup
time for computer numerically controlled machines is usually short.
Many jigs and fixtures formerly required are not necessary.
5. Fewer chances for human error. The CNC program reduces or
eliminates the need for an operator to take trial cuts, make trial
measurements, make positioning movements, or change tools.
6. Maximum part accuracy and interchange. CNC ensures that all parts
produced will be accurate and of uniform quality.

7
7. Complex machining operations. Complex operations can be
performed quickly and accurately with CNC and electronic measuring
equipment.
8. Lower tooling costs. CNC machines generally use simple holding
fixtures, which reduce the cost of tooling by as much as 70%. Standard
turning and milling tools eliminate the need for special form tools.
9. Increased productivity. Because the CNC system controls all the
machine functions, parts are produced faster and with less setup and
lead time.
10. Minimal spare parts inventory. A large inventory of spare parts is no
longer necessary, since additional parts can be made to the same
accuracy when the same program is used again.
11. Greater machine tool safety. The damage to machine tools as a result
of operator error is virtually eliminated, since there is less operator
intervention.
12. Fewer worker hours for inspection. Because CNC machines produce
parts of uniform quality, less inspection time is required.
17.3 General Functional Description
The main task of NC is to control relative motion between the tool and
workpiece. Path and velocity instructions are specified in the form of an NC
program, called parts program, and contain control information in the form of
alphanumeric characters. An NC program can be created in two different ways
depending on the organizational form. Firstly, NC programs are generated in
production planning using automatic NC programming systems and are
transferred to the controller. In this solution, the data of an NC program are
entered into the controller from a USB stick or network connection. In
addition, programs already read can be modified or corrected later on from the
control panel.

8
The second way to enter simple machining tasks is program creation or
revision by the machine user on the controller itself. Here, graphically aided
methods such as WOP (workshop-oriented programming) provide the ability
to enter a program conveniently and easily in production planning as well as
at the machine itself.
The NC program information entered and the corrections to be made are
decoded in the controller (interpreter) and processed separately according to
geometrical and technological data, as well as switching functions (Fig. 14.2).
Geometrical data are all the information on the tool and workpiece paths to
traverse. These data are ultimately used to create the desired workpiece
geometry. Functions for selecting the tool, spindle speed and cutting speed are
examples of technological information.

9
The switching functions arrive at the “logic controller” of the machine tool as
switching commands. Here they are logically linked with feedback signals
coming from the machine tool and converted to control commands for the
units to be activated. Nowadays, the largest part of logic control is realized
using a PLC (programmable logic controller, refer to 7 Sect. 10.3). The logical
operations comprise to a large extent locking and safety circuits, so that
conflicting actions and actions that can endanger the machine user and
machine (such as “Feed on” before “Spindle on”) cannot be triggered. With
the help of logic control, machine-independent functions of the numerical
controller can be reproduced on (adapted to) machine-specific devices.
For a specified path segment, an interpolator calculates the motion sequence
to be coordinated on the axes by direction and velocity. From this, it generates
the reference variables for the drives of the axes [SAUT87]. The superposition
of individual axis movements means a tool movement is then created along
the programmed workpiece contour (see 7 Sect. 12.1).
The contour to be generated is programmed by specifying the contour end
point coordinates and type of connection between the respective start and end
points as a path condition (e.g., straight line or arc). Controllers of varying
complexity are suitable for realization here depending on the movement
trajectory. In this regard, a distinction is made between point-to-point, line
motion and path controls [DIN96] [KAMP70]. Point-to-point and line motion
controls are no longer in use as autonomous controllers. Sometimes they are
only used as modules inside PLCs.

10
Point-to-point controllers are used for simple positioning tasks such as drilling
and spot welding. When they are used, the tool must not be in contact during
the positioning process because the programmed end point of the tool
movement is traversed on a non-defined path.
For line motion control, the end point of a machining section is traversed in a
straight line and the tool may be in contact when traversing. Traversing is only
possible on axially parallel straight lines with simple line motion controls and
on any straight lines with advanced line motion controls. Line motion controls
are suited, for example, to simple milling and turning processes and to
controlling transfer lines.
Tool and table movements to generate arbitrary contours are only possible
using path controllers. Every numerical controller nowadays has a path
interpolator, making it a continuous path controller. These continuous path
controllers can normally perform movements in straight lines and arcs and
sometimes even spline segments. Movements on individual axes have a strict
functional dependence on one another. Continuous path controllers are used
in the machine tool industry for turning, milling and grinding machines, laser
beam and oxyacetylene cutting systems, nibblers, die sinking and wire-electro
discharge machines, water jet cutting machines, stereolithography machines
and many more.
Before the controller sends the reference axis values to the drives, these values
must be adapted to the current workpiece and tool position. For example, NC
programming is independent of the current clamping position of the blank on
the table of a milling machine. Thus, the current position of the workpiece
zero point in relation to the machine zero point must also be entered and be
taken into account by the controller using a coordinate transformation.

11
17.3 Hardware and Interfaces
Nowadays, standard components of the PC are most powerful components.
However, for special tasks, particular interfaces to external hardware are also
used. The hardware components include:
1. A man-machine control (MMC) module using an integrated industrial
PC and a control panel with thin film translator (TFT) color display
and full numerical control keyboard as a central display and input unit.
2. The machine control panel for operating the machine by hand, for
specifying the type of motion and override values and for defining, by
the machine manufacturer, of individual keyboard assignments (e.g.,
running of particular NC line when a key is pressed).
3. A numerical control unit (NCU) with an integrated NC and PLC. The
multiprocessor NCU module is integrated directly into the digital
converter system and connected to the drive modules. Profibus/
Profinet1
can connect input/output modules for machine control
directly to the NCU.
4. All components are networked together using a serial bus system.
Other controller components can also be connected to the bus.
Internal structure: The hardware of a numerical controller comprises
different modules depending on variant. The classic splits into three hardware
components are:
i. Operating area hardware: for all operation and display functions; not
real-time capable, normally PC-based hardware with Windows
operating systems
ii. NC kernel hardware: for running geometric calculations and path control
in line with the specifications of the NC program; real-time capable,
normally special hardware with a real-time operating system
1
Profibus/Profinet is a standardized, open, digital communications system for all areas of
application in manufacturing and process automation.

12
iii. PLC hardware: for machine adaptation of the controller (logic control);
real-time capable, normally special hardware with firmware for cyclic
sequencing.
The key functional areas of a numerical controller are explained below.
HMC functional area: The HMC (human-machine control) functional area
comprises all of the functions for operation and data management of the
controller. This includes visualization of process and master data,
programming of NC workpiece programs using a text editor or WOP system
and managing different production data.
MC functional area: The motion control functional area comprises NC data
preparation and interpolation of the reference position values for every NC
channel. The multichannel design means different axis groups can perform
movements independently of one another. Each channel represents an
autonomous system with multiple axes and its own NC program as well as its
own NC line preparation and interpolation. The programs on the individual
channels can process in parallel or synchronized to each other.
AC and SC functional areas: The axes control (AC) and spindle control (SC)
functional areas manage one or more groups of axes or spindles. In terms of
the software architecture, they comprise all of the feedback control even if this
is housed in the drive amplifier on separate hardware.
LC functional area: The logic control (LC) functional area forms the interface
between the controller and the machine. The logic controller assumes the
running of machine specific functional processes and logical operations such
as monitoring guard doors and controlling the supply of coolant.
Other functional areas: Additional functions such as the organizational and
planning tasks of a numerical controller can be assigned to other functional
areas, such as

13
• Tool management: for saving and managing all tool data (tool
numbers, geometries, wear information, etc.)
• Order management: for managing production orders and the material
flow of workpiece and tool.
• Control of conveying equipment: for performing tool conveying, tool
changes, palletconveying and pallet changes within one machine tool.
External interfaces: An array of different communication interfaces is
available to integrate a numerical controller into a production-related
environment. A fundamental distinction can be made between interfaces to higher
and lower level systems and to the machine user.
The pronounced proliferation of network technologies in production
means a numerical controller is hardly likely to be operated in a self-
contained system. The interfaces comprise real-time capable connections to
field devices (such as sensors, actuators and other controllers) as well as less
real-time capable interfaces to the company network (diagnostics, order and
re- source planning—refer to 7 Sect. 14.2).
Figure 11.6 shows the functional interfaces of a numerical controller in relation
to the level model of production (refer to 7 Sect. 14.2).
Master systems are computers on the cell or control level. These systems are to
supply the controller with NC programs and tool data, as well as, if required, re-
mote control (program start/stop). The term used for this mode is DNC, that is
deployed for automated production systems as well as flexible manufacturing
systems (FMS) and flexible manufacturing cells (FMC). Refer to 7 Sect. 14.3.
The interface to the machine user is on the same level in the form of graphic
screen, keyboard, handwheels, rotary switches, etc., enabling manual, direct
operation of the controller and machine (see 7 Sect. 11.5). The lower-level
actuator/sensor level is controlled by reference position, velocity and current
values for the drives (amplifiers) and switching functions. It returns the actual
values (from encoders), process data (such as temperature changes) and any

14
error messages.
Now that the functional interfaces of a numerical controller have been
described, the physical interfaces are explained below. . Figure 11.7 shows the
typ- ical peripherals and physical interfaces of a numerical controller [INDR99].
Ethernet and TCP/IP have es- tablished themselves as the standard for
connections to cell/master computers. For older controllers, serial interfaces
(RS-232 and RS-422) were commonly used as point-to-point connections.
Ethernet is also the
17.5 How a Numerical Controller works
The way a numerical controller works is determined by the NC core functionality
and by additional functions. The functions are:
• NC interpreter: The function of the NC interpreter is one of syntax
analyzer that translates the different formats of the NC programs and input
data to a standardized, internally readable form. This means different data
formats such as simple NC programs, can be processed by one and the
same controller.
The NC interpreter passes the position of the path corner points, the type
of motion stipulated (such as straight line or circular motion) and the
necessary feed rate to the geometrical data processing function block for
every NC line.
Switching functions such as tool change and workpiece feed are passed
later to the logic controller (usually realized by a PLC). Of course, these
functions must be synchronized with the geometry data. It must be
guaranteed for example that a tool change (as a switching command to the
PLC) may only be performed once the last geometry block has run to
completion with the previous tool.

15
The prime tasks of a numerical controller are interpretation of the input
information in the form of alpha-numeric characters and preparation of the
corresponding path or switching information for every machine tool axis or
actuator. Once the functional programs are complete, they can be transferred to
the hardware of the numerical controller. This method of programming is
generally known as cross-development.
The main part of the software is made up of the NC functional modules that
essentially embody the controller functionality. system. The operating system
handles memory management, input/output management and synchronization of
the individual processes. A standard operating system suitable for numerical
controllers must support multitasking—because multiple functional modules
(tasks), such as interpreter and interpolator, must be active at the same time.
Geometrical Data Processing: The functions Velocity control, Geometric
transforma tions and Correction calculations are combined into one function
block in . Fig. 11.9.
Each individual function comprises in turn subfunc-tions where the order and
implementation can be realized differently from controller to controller.
To enable NC programming independently of machine type and actual tool
geometry, the controller makes geometrical transformations (see 7 Sect.
12.2).First, a zero point offset is realized here that describesthe position of the
workpiece zero point relative to themachine zero point. This enables the further
transfor-mation between workpiece and machine coordinates. Second, the tool
correction, generally comprising a tool length and tool radius correction, permits
the calculation of the necessary equidistances to the path programmed as a
function of the current tool data. This guarantees independence of NC
programming from the exact length and diameter of the tool used later.
As part of velocity control, the velocities and accelerations of the programmed
feed rate are aligned to the appropriate boundary conditions.

16
For major velocity changes, such as when small radii are circumscribed, their
limited dynamic range means the drives are not able to follow the motions
specified. In these cases, the controller automatically decreases the path
velocity to keep to the workpiece dimen- sion tolerances. Modern-day
controllers have a Look-ahead function to optimize further velocity control.
Using this monitoring function, multiple NC lines (10to 100) are analyzed in
advance to determine whether the velocity profile can be followed or
improved (see 7 Sect. 12.1.2.2).

17
Interpolation: The task of interpolation is to calculate intermedi- ate points
that lie on a path section defined by the NCprogram. This splits the path
movement arising from the simultaneous movement of multiple axes, piece-
by-piece across the individual axes, such that the axeschange their positions
independently. Simple methods such as linear and circular interpolation are
used,as well as more intricate types such as spline interpola- tions (see 7 Sect.
12.1.4) [KIEF99]. The other tasks ofinterpolation are following the velocity
profile specified by velocity control and, for non-Cartesian machines,
kinematic transformation of the coordinates to the indi-vidual machine axes.
Furthermore, external factors, such as correction of the feed or spindle speed
(override), as well asstatic and dynamic displacements, must be factored in or
compensated with compensation calculations in the interpolation process.
Using a rotary switch for the feed correction, the machine user is able to
change theprogrammed path velocity (generally between 0 and 120%). The
compensation calculation calculates the velocity change during the machining
process without allowing geometric deviations to occur on the workpiece. In
addition, any displacements must be compensated continually during the
process. These displacements come from static loads or thermal distortion of the
machine structure, such as thermal extension of the spindle, the reversal error
of the spindle nut, the spindle pitch error or tool wear (see 7 Sect. 12.3).
Axis Control: Depending on the controller design, the individual feed drive units
are supplied with reference position, velocity or current values by geometrical
data processing. The control principle predominantly in use today is cascading
position control. The motor current controller forms the inner control loop using
a P or PI controller. It is superimposed by the speed control loop that receives
its reference variables from the position controller. The controllers used are
generally PI or PID controllers for the speed control loop and P controllers for
the position control loop (see 7 Sect. 3.2).
On some numerical controllers, position control takes place in digital form
internal to the controller, i.e., reference velocity values are relayed to the

18
drives.The velocity and current controllers and also the posi-tion controller
depending on the design, work on the drive side in the servo modules (see 7
Sect. 2.2.2). The described information flow in the numerical controller (i.e.,
from the interpretation of an NC program to output of the reference axis
values) occurs in one NC channel at any one time. With every NC channel,
mul-tiple axes or spindles are operated asynchronously or synchronously in an
axis group. The NC channels canbe divided into operating mode groups (. Fig.
11.5). More NC axes can be defined, for example to control turret, tool
magazine, workpiece/tool pallets, tailstock, quill, cutting-off slide or workpiece
and tool loaders.
Advantages of CNC:
Computer numerically controlled machine tools have many advantages that
can be summarized by
1. Greater flexibility because a wide variety of operations is performed
and product design changes modifications through tape/program
changes are made rapidly.
2. Elimination of templates, models, jigs, and fixtures because the control
system takes over the job of locating the tools.
3. Easier setups by using more simple work holding and locating devices.
4. Reduced production time by using a wider range of speeds and feeds
than conventional machine tools. Additionally, the CNC equipment
moves from one operation to the next faster than the operator that
reduces the total production time.
5. Greater accuracy and uniformity are possible because the same part is

19
produced using the same stored NC program that improves parts
uniformity and interchangeability and reduces scrape and rework.
6. Greater safety because the tape/program is checked out before the
actual production runs, thus allowing less chance of machine damage
that may cause human injuries.
7. Conversion from English to the metric system of units.
The use of CNC has the following disadvantages.
1.It follows programmed instructions that may cause destruction if they are
not properly prepared.
2. CNC cannot add any extra capability to the machine in terms of power of
the original drive motor and the machine table travel.
3. CNC machines cost 5 to 10 times more than conventional machines of the
same working capacity.
4. The skill required to operate is usually high.
5. CNC requires high investments in terms of wages, expensive spare parts,
and special training.

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