Queensborough Community College
NSF Tech ASCEND
Computer Numerical Control (CNC)
Component
Student Manual
Prepared by Prof. Joseph Goldenberg, MET&DD Department
2003 Edition
Table of Contents
Introduction
3
Why learn Computer Numerical Control (CNC)?
4
CNC definition, its concepts, advantages and financial rewards
5
Planning for writing a CNC program
7
Modern machine tool controls - the heart of CNC
9
How axis motion is commanded - understanding coordinate systems
10
Understanding absolute versus incremental motion
11
Assigning program zero
11
Telling the machine what to do - the CNC program
12
Word address programming
13
Decimal point programming
15
Needs to know the machine
15
Directions of motion (axes)
17
The reference point for each axis
18
Accessories to the machine
18
Programmable functions
18
Spindle control
19
Automatic tool changer (machining center)
20
Coolant control
20
Understanding interpolation
20
Other interpolation types
21
The three most basic types of motion
22
Rapid motion (positioning)
22
Straight line motion (linear interpolation)
23
Circular motion (circular interpolation)
23
Example of programming for CNC milling
24
Tooling for Hole producing and Milling operations
26
Hands-on project to be machined on a Machining Center
29
Tooling for Turning operations
36
Hands-on project to be machined on a Turning Center
38
Conclusion
42
2
Introduction
Have you ever wondered who figures out how to manufacture new products so
everyone can get their hands on them? That’s the job of manufacturing engineers and
manufacturing engineering technologists.
When a new product is designed, manufacturing professionals figure out how to
customize existing manufacturing processes to meet the new demands. They work in
exciting high-tech environments, using computer-aided design (CAD), computer-aided
manufacturing (CAM), robotics, lasers, and Computer Numerical Controls (CNC) to
manufacture all the products that we need and enjoy.
Manufacturing professionals also help improve the quality of life for people all over
the world by improving manufacturing processes. They transform plans and
specifications into quality products. Today’s high-tech manufacturing environment
requires the knowledge of skilled professionals
Manufacturing Engineering Technology is a varied and challenging field that is
becoming increasingly important because of new production methods. Manufacturing
Engineering Technicians work with engineers to design experiments, plan production
methods, find better ways to manufacture products, and in troubleshooting, inspection,
and quality control.
Computer Integrated Manufacturing (CIM) is becoming a key tool for competitive
manufacturing in today’s global market. Key element of CIM: flexible manufacturing
systems (FMS), and its component, flexible manufacturing cell (FMC), are being
implemented.
3
Why learn CNC?
Everyone involved in the manufacturing environment should be well aware of what
is possible with sophisticated, computer controlled, machine tools.
The design engineer, for example, must possess enough knowledge of CNC to
perfect dimensioning and tolerancing techniques for workpieces to be machined on CNC
machines. Manufacturing engineers direct and coordinate the processes for making things
- from the beginning to the end. The tool engineer must understand CNC in order to
design fixtures and cutting tools for use with CNC machines. Quality control people
should understand the CNC machine tools used within their company in order to plan
quality control and statistical process control accordingly. Production control personnel
should be abreast of their company's CNC technology in order to make realistic
production schedules. Managers, foremen, and team leaders should understand CNC well
enough to communicate intelligently with fellow workers.
It goes without saying that CNC programmers, setup people, operators, and others
working directly with the CNC equipment must have an extremely good understanding of
this technology. To a large extend, the computer has revolutionized manufacturing
technology. Industries are finding that the new manufacturing technology demands welltrained personnel
In this course, we will explore the basics of CNC, showing you what is involved
with using these sophisticated machine tools. While our main focus will be on the two
most popular forms of CNC machine tools (machining centers and turning centers), the
concepts can be applied to virtually any kind of CNC machine. Our goal is to explain to
you how CNC works and what role it plays in improving our lives.
4
CNC definition, it’s concepts, advantages and financial rewards
Numerical Control has been used in manufacturing for more than 50 years.
Simply put, Numerical Control is a method of automatically operating a manufacturing
machine based on a code of letters, numbers, and special characters (Introduction to
Computer Numerical Control (CNC) J. Valentino, J. Goldenberg, Prentice Hall, 2003).
At the beginning, these machines were controlled by a tape, which carried to the
machine the codes. A traditional NC system was composed of:
•
tape punch
•
tape reader
•
controller
•
NC machine
NC systems offered some of the following advantages over manual methods of
production:
•
better control of tool motion under optimum cutting conditions
•
improved product quality and repeatability
•
reduced tooling costs, tool wear, job set-up time
•
reduced time to manufacture the product
•
reduced scrap
•
better production planning and placement of machining in the hands of engineering.
A Computer Numerical Control machine is a numerical control machine with an
on board computer. This computer is often referred to as the Machine Control Unit
(MCU). The machine functions are encoded into the computer at the time of
manufacture. They are not erased when the CNC machine is turned off (similar to the
operating system of a desktop computer).
CNC opens up new possibilities and advantages not offered by the older NC
machines:
5
• reduction of hardware necessary to add machine functions
• CNC programs can be written, stored, and executed directly at the CNC machine
• any portion of an entered CNC program can be played back and displayed at will
• many different CNC programs can be stored in the MCU
• several CNC machines could be linked together to a main computer
• programs written via a main computer can be downloaded to any machine in the
network. This is called Direct Numerical Control (DNC)
• several DNC systems can be networked into a larger distributive system
The manufacturing community turns to CNC technology for the following
benefits:
• savings on direct labor
• savings in operator training costs (versus experienced machinists)
• savings in savings shop supervisory costs
• savings due to tighter, more predictable scheduling
• savings in real estate (since fewer CNC machines are needed)
• savings in power consumption (minimum of motor idle time, while using a CNC
machine tool)
• savings from improved cost estimating and pricing
• savings due to elimination of costly jig: their design, manufacture and storage
• savings in special tooling design, manufacture and documentation
• reduced inspection time due to CNC’s ability to produce better quality parts.
A complete set of coded instructions for executing an operation is called a
program. The program is translated into corresponding electrical signals for input to
motors that run the machine. Numerically controlled machines can be programmed
manually or by use of CAD/CAM (Computer Aided design/Computer Aided
Manufacturing) technology. The approach taken in this manual will be in the form of
manual programming.
6
Planning for writing a CNC program
The first step to be taken before writing any CNC program is to thoroughly study
the part drawing. The drawing indicates what the part looks like, its material, precision,
surface finish, material treatment (if any), and many other requirements. Important
factors in the decision to utilize CNC operations in the part’s manufacture include the
quantity of parts to be machined, the part quality, the tooling costs, the holding device
design and manufacturing and the cost running the CNC machine.
Based on the requirements of the part drawing a method of manufacturing has to
be written. Among other things it includes the order and the ways of machining the part.
Ordinarily it is the programmer’s responsibility to develop the order of operations
(operation sheets), which later will be used in writing the program.
Next step is to decide on a CNC machine to be used. The programmer selects a
machine based on its ability to optimize the cutting operations required to manufacture
the part. Availability as well as the capacity of the CNC machine must be considered.
It is the programmer’s job to decide how the part is to be held during machining.
Acting on a sketch or a formal drawing, the setup person carries out the task of securing
the part to the table of the CNC machine. It is important that the part does not move or
become disengaged from the machine table during the machining process.
The programmer continues with the job of deciding on the machining sequence
and the tooling needed. In general, machining should proceed in the following order:
•
rough turning
•
finish turning
•
rough boring
•
finish boring
•
drilling and tapping
•
finish reaming
The cutting sequence, together with the required cutting tools, is documented.
The holding devices and the cutting tools must be available at the time the job is
scheduled to run.
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Based on the material of the part, tooling, finish requirements, the programmer
decides on cutting parameters: spindle speed and feed rate. These values he uses later
when he writes the program.
After all the planning is completed the programmer is ready to write the program.
Based on the decisions he made while planning the job the programmer establishes:
• programming origins in relations to the machine origins for X, Y, Z
coordinates
• mode of programming: absolute or incremental
• units of programming (inch or metric)
• tool change position
After finishing writing the program the programmer will transfer the program,
together with the setup instructions, to the machine.
A person (setup man) who is responsible for setting up and, sometimes, for the
operation of the machine, will act upon the instructions of the programmer and set up the
job. He will:
•
load the program into the
control
•
prove-out the program
•
do partial inspection
•
set the tools
•
adjust cutting parameters
•
set the holding device
•
report any problems to the
•
get “tool offsets”
programmer
The programmer will actively participate in debugging the program for any
discrepancy and make the required adjustments needed for the job to run trouble free. A
completed program is ready for production only if it is capable of producing quality parts
in the time projected. Programs that meet these criteria are a direct reflection of the
programmer’s professionalism.
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Modern machine tool controls - the heart of CNC
The most basic function of any CNC machine is automatic, precise, and consistent
motion control. Rather than applying completely mechanical devices to cause motion as
is required on most conventional machine tools, CNC machines allow motion control in a
revolutionary manner. All forms of CNC equipment have two or more directions of
motion, called axes. These axes can be precisely and automatically positioned along their
lengths of travel. The two most common axis types are linear (driven along a straight
path) and rotary (driven along a circular path).
Instead of causing motion by turning cranks and handwheels, as is required on
conventional machine tools, CNC machines allow motions to be commanded through
programmed commands. Generally speaking, the motion type (rapid, linear, and circular),
the axes to move, the amount of motion and the motion rate (feedrate) are programmable
with almost all CNC machine tools.
Accurate positioning is accomplished by the operator counting the number of
revolutions made on the handwheel plus the graduations on the dial. The drive motor is
rotated a corresponding amount, which in turn drives the ball screw, causing linear
motion of the axis
A CNC command executed within the control (commonly through a program)
tells the drive motor to rotate a precise number of times. The rotation of the drive motor
in turn rotates the ball screw. And the ball screw causes drives the linear axis. A feedback
device at the opposite end of the ball screw allows the control to confirm that the
commanded number of rotations has taken place.
Though a rather crude analogy, the same basic linear motion can be found on a
common table vise. As you rotate the vise crank, you rotate a lead screw that, in turn,
drives the movable jaw on the vise. By comparison, a linear axis on a CNC machine tool
is extremely precise. The number of revolutions of the axis drive motor precisely controls
the amount of linear motion along the axis.
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How axis motion is commanded - understanding coordinate systems
The two most popular coordinate systems used with CNC machines are the
rectangular coordinate system and the polar coordinate system. By far, the most popular
of these two is the rectangular coordinate system, and we'll use it for all discussions made
during this presentation.
Let's relate the coordinate system to the CNC axis motion. Each linear axis of a
CNC machine's rectangular coordinate system is broken into increments of measurement.
In the inch mode, the smallest increment is usually 0.0001 inch. In the metric mode, the
smallest increment is 0.001 millimeter. (By the way, for rotary axes the increment is
0.001 degrees). Each axis within the CNC machine's coordinate system must start
somewhere. For CNC purposes, this origin point is commonly called the program zero
point (also called work zero, part zero, and program origin).
The program zero point establishes the point of reference for motion commands in
a CNC program. This allows the programmer to specify movements from a common
location. If program zero is chosen wisely, usually coordinates needed for the program
can be taken directly from the print.
With this technique, if the programmer wishes the tool to be sent to a position one
inch to the right of the program zero point, X1.0 is commanded. If the programmer
wishes the tool to move to a position one inch above the program zero point, Y1.0 is
commanded. The control will automatically determine how many times to rotate each
axis drive motor and ball screw to make the axis reach the commanded destination point.
This lets the programmer command axis motion in a very logical manner.
With the examples given so far, all points happened to be up and to the right of
the program zero point. This area up and to the right of the program zero point is called a
quadrant (in this case, quadrant number one). It is not uncommon on CNC machines that
end points needed within the program fall in other quadrants. When this happens, at least
one of the coordinates must be specified as minus.
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Understanding absolute versus incremental motion
All discussions to this point assume that the absolute mode of programming is
used. The most common CNC word used to designate the absolute mode is G90. In the
absolute mode, the end points for all motions will be specified from the program zero
point. For beginners, this is usually the best and easiest method of specifying end points
for motion commands. However, there is another way of specifying end points for axis
motion.
In the incremental mode (commonly specified by G91), end points for motions
are specified from the tool's current position, not from program zero. With this method of
commanding motion, the programmer must always be asking "How far should I move the
tool?" While there are times when the incremental mode can be very helpful, generally
speaking, this is the more cumbersome and difficult method of specifying motion and
beginners should concentrate on using the absolute mode.
Be careful when making motion commands. Beginners have the tendency to think
incrementally. If working in the absolute mode (as beginners should), the programmer
should always be asking "To what position should the tool be moved?" This position is
relative to program zero, NOT from the tools current position.
Aside from making it very easy to determine the current position for any
command, another benefit of working in the absolute mode has to do with mistakes made
during motion commands. In the absolute mode, if a motion mistake is made in one
command of the program, only one movement will be incorrect. On the other hand, if a
mistake is made during incremental movements, all motions from the point of the mistake
will also be incorrect.
Assigning program zero
Keep in mind that the CNC control must be told the location of the program zero
point by one means or another. How this is done varies dramatically from one CNC
machine and control to another. One (older) method is to assign program zero in the
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program. With this method, the programmer tells the control how far it is from the
program zero point to the starting position of the machine. This is commonly done with a
G92 (or G50) command at least at the beginning of the program and possibly at the
beginning of each tool.
Another, newer and better way to assign program zero is through some form of
offset. Commonly machining center control manufacturers call offsets used to assign
program zero fixture offsets. Turning center manufacturers commonly call offsets used to
assign program zero for each tool geometry offsets. More on how program zero can be
assigned will be presented during key concept number four.
Telling the machine what to do - the CNC program
Almost all current CNC controls use a word address format for programming.
(The only exceptions to this are certain conversational controls.) By word address format,
we mean that the CNC program is made up of sentence-like commands. Each command
is made up of CNC words. Each CNC word has a letter address and a numerical value.
The letter address (X, Y, Z, etc.) tells the control the kind of word and the numerical
value tells the control the value of the word. Used like words and sentences in the English
language, words in a CNC command tell the CNC machine what it is we wish to do at the
present time.
A CNC programmer must be able to visualize the machining operations that are
to be performed during the execution of the program. Then, in step by step order, the
programmer will give a set of commands that makes the machine behave accordingly.
Without visualization ability, the programmer will not be able to develop the movements
in the program correctly. An experienced programmer should be able to easily visualize
any machining operation taking place.
Each instruction given within a CNC program is made up of one command. A
CNC command is made up of CNC words. The CNC machine will execute a CNC
program explicitly. If there is a mistake in the program, the CNC machine will not
behave correctly.
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Program example:
O0001 (Program number)
N005 G54 G90 S1200 M03 (Select coordinate system, absolute mode, and turn
spindle on CW at 1200 RPM)
N010 G00 X-.25 Y-.25 (Rapid to X-.25 Y-.25 location)
N015 G43 H01 Z.1 M08 (Instate tool length compensation, rapid in Z to
clearance position above surface to drill, turn on coolant)
N020 G01 Z-.25 F10. (Feed into the part Z-.25 at 10 inches per minute)
N025 Y3.25 (Feed tool to Y3.25 location)
N030 X4.25 (Feed tool to X4.25 location)
N035 Y-.25 (Feed tool to Y-.25 location)
N040 X-.25 (Feed tool to X-.25 location)
N045 G00 Z.1 M09 (Rapid out of part to Z.1, turn off coolant)
N050 G91 G28 Z0 (Return to reference position in Z)
N055 M30 (End of program command)
While the words and commands in this program probably do not make much
sense to you (yet), remember that we are stressing the sequential order by which the CNC
program will be executed. The control will first read, interpret and execute the very first
command in the program. Only then will it go on to the next command. Read, interpret,
execute. Then on to the next command. The control will continue to execute the program
in sequential order for the balance of the program.
Word address programming
As stated programs are made up of commands and commands are made up of
words. Each word has a letter address and a numerical value. The letter address tells the
control the word type. CNC control manufacturers do vary with regard to how they
determine word names (letter addresses) and their meanings. The beginning CNC
programmer must reference the control manufacturer's programming manual to determine
13
the word names and meanings. Here is a brief list of some of the word types and their
common letter address specifications:
O - Program number (Used for
program identification)
R - Radius designation
F - Feedrate designation
N - Sequence number (Used for line
identification)
S - Spindle speed designation
G - Preparatory function
H - Tool length offset designation
X - X axis designation
D - Tool radius offset designation
Y - Y axis designation
T - Tool Designation
Z - Z axis designation
M - Miscellaneous function
As you can see, many of the letter addresses are chosen in a rather logical manner
(T for tool, S for spindle, F for feedrate, etc.). A few require memorizing.
There are two letter addresses (G and M) which allow special functions to be
designated. The preparatory function (G) specifies is commonly used to set modes. We
already introduced absolute mode, specified by G90 and incremental mode, specified by
G91. These are but two of the preparatory functions used. You must reference your
control manufacturer's manual to find the list of preparatory functions for your particular
machine.
Like preparatory functions, miscellaneous functions (M words) allow a variety of
special functions. Miscellaneous functions are typically used as programmable switches
(like spindle on/off, coolant on/off, and so on). They are also used to allow programming
of many other programmable functions of the CNC machine tool.
To a beginner, all of this may seem like CNC programming requires a great deal
of memorization. But rest assured that there are only about 30-40 different words used
with CNC programming. If you can think of learning CNC manual programming as like
learning a foreign language that has only 40 words, it shouldn't seem too difficult.
14
Decimal point programming
Certain letter addresses (CNC words) allow the specification of real numbers
(numbers that require portions of a whole number). Examples include X axis designator
(X), Y axis designator (Y), and radius designator (R). Almost all current model CNC
controls allow a decimal point to be used within the specification of each letter address
requiring real numbers. For example, X3.0625 can be used to specify a position along the
X axis.
On the other hand, some letter addresses are used to specify integer numbers.
Examples include the spindle speed designator (S), the tool station designator (T),
sequence numbers (N), preparatory functions (G), and miscellaneous functions (M). For
these word types, most controls do NOT allow a decimal point to be used. The beginning
programmer must reference the CNC control manufacturer's programming manual to find
out which words allow the use of a decimal point.
Needs to know the machine
Many forms of CNC machines are designed to enhance or replace what is
currently being done with more conventional machines. The first goal of any CNC
beginner should be to understand the basic machining practice that goes into using the
CNC machine tool. The more the beginning CNC user knows about basic machining
practice, the easier it will be to adapt to CNC.
Think of it this way. If you already know basic machining practice as it relates to
the CNC machine you will be working with, you already know what it is you want the
machine to do. It will be a relatively simple matter of learning how to tell the CNC
machine what it is you want it to do (learning to program). This is why machinists make
the best CNC programmers, operators, and setup personnel. Machinists already know
what it is the machine will be doing. It will be a relatively simple matter of adapting what
they already know to the CNC machine.
15
For example, a beginner to CNC turning centers should understand the basic
machining practice related to turning operations like rough and finish turning, rough and
finish boring, grooving, threading, and necking. Since this form of CNC machine can
perform multiple operations in a single program (as many CNC machines can), the
beginner should also know the basics of how to process workpieces machined by turning
so a sequence of machining operations can be developed for workpieces to be machined.
From a programmer's standpoint, as you begin to learn about any new CNC
machine, you should concentrate on four basic areas. First, you should understand the
machine's most basic components. Second, you should become comfortable with your
machine's directions of motion (axes). Third, you should become familiar with any
accessories equipped with the machine. And fourth, you should find out what
programmable functions are included with the machine and learn how they are
programmed.
While you do not have to be a machine designer to work with CNC equipment, it
is important to know how your CNC machine is constructed. Understanding your
machine's construction will help you to gauge the limits of what is possible with your
machine. Just as the race car driver should understand the basics of suspension systems,
breaking systems, and the workings of internal combustion engines (among other things)
in order to get the most out of a given car, so must the CNC programmer understand the
basic workings of the CNC machine in order to get the most from the CNC machine tool.
For a universal style slant bed turning center, for example, the programmer should
know the most basic machine components, including bed, way system, headstock &
spindle, turret construction, tailstock, and work holding device. Information regarding the
machine's construction including assembly drawings is usually published right in the
machine tool builder's manual. As you read the machine tool builder's manual, here are
some of the machine capacity and construction questions to which you should find
answers.
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•
What is the machine's maximum RPM?
•
How many spindle ranges does the machine have (and what are the cut-off points
for each range?
•
What is the spindle and axis drive motor horsepower?
•
What is the maximum travel distance in each axis?
•
How many tools can the machine hold?
•
What way construction does the machine incorporate (usually square ways,
dovetail, and/or linear bearing ways)?
•
What is the machine's rapid rate (fastest traverse rate)?
•
What is the machine's fastest cutting feedrate?
These are but a few of the questions you should be asking yourself as you begin
working with any new CNC machine. Truly, the more you know about your machine's
capacity and construction, the easier it will be to get comfortable with the machine.
Directions of motion (axes)
The CNC programmer MUST know the programmable motion directions (axes)
available for the CNC machine tool. The axes names will vary from one machine tool
type to the next. They are always referred to with a letter address. Common axis names
are X, Y, Z, U, V, and W for linear axes and A, C, and C for rotary axes. However, the
beginning programmer should confirm these axis designations and directions (plus and
minus) in the machine tool builder's manual since not all machine tool builders conform
to the axis names we show.
Whenever a programmer wishes to command movement in one or more axes, the
letter address corresponding to the moving axes as well as the destination in each axis are
specified. X5.25, for example tells the machine to move the X axis to a position of 5.25
inches from the program zero point in X (assuming the absolute mode of programming is
used.
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The reference point for each axis
Most CNC machines utilize a very accurate position along each axis as a starting
point or reference point for the axis. Some control manufacturers call this position the
zero return position. Others call it the grid zero position. Yet others call it the home
position. Regardless of what it is called, the reference position is required by many
controls to give the control an accurate point of reference. CNC controls that utilize a
reference point for each axis require that the machine be manually sent to its reference
point in each axis as part of the power up procedure. Once this is completed, the control
will be in sync with the machine's position.
Accessories to the machine
Many CNC machine tools are equipped with accessories designed to enhance
what the basic machine tool can do. Some of these accessories may be made and
supported by the machine tool builder. These accessories should be well documented in
the machine tool builder's manual. Other accessories may be made by an after-market
manufacturer, in which case a separate manual may be involved.
Examples of CNC accessories include probing systems, tool length measuring
devices, post process gaging systems, automatic pallet changers, adaptive control
systems, bar feeders for turning centers, live tooling and C axis for turning centers, and
automation systems. Truly, the list of potential accessory devices goes on and on.
Programmable functions
The programmer must also know what functions of the CNC machine are
programmable (as well as the commands related to programmable functions). With low
cost CNC equipment, often times many machine functions must be manually activated.
With some CNC milling machines, for example, about the only programmable function is
axis motion. Just about everything else may have to be activated by the operator. With
this type of machine, the spindle speed and direction, coolant and tool changes may have
to be activated manually by the operator.
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With full blown CNC equipment, on the other hand, almost everything is
programmable and the operator may only be required to load and remove workpieces.
Once the cycle is activated, the operator may be freed to do other company functions.
Programmable functions will vary dramatically from one machine to the next. The
actual programming commands needed vary from builder to builder. Be sure to check the
M codes list (miscellaneous functions) given in the machine tool builder's manual to find
out more about what other functions may be programmable on your particular machine.
M codes are commonly used by the machine tool builder to give the user programmable
ON/OFF switches for machine functions. In any case, you must know what you have
available for activating within your CNC programs.
For turning centers, for example, you may find that the tailstock and tailstock quill
is programmable. The chuck jaw open and close may be programmable. If the machine
has more than one spindle range, commonly the spindle range selection is programmable.
And if the machine has a bar feeder, it will be programmable. You may even find that
your machine's chip conveyor can be turned on and off through programmed commands.
All of this, of course, is important information to the CNC programmer
Reference the machine tool builder's manual to find out what functions of your
machine are programmable. To give you some examples of how many programmable
functions are handled, here is a list a few of the most common programmable functions
along with their related programming words
Spindle control
An "S" word is used to specify the spindle speed (in RPM for machining centers).
M03 is used to turn the spindle on in a clockwise (forward) manner. M04 turns the
spindle on in a counter clockwise manner. M05 turns the spindle off. Note that turning
centers also have a feature called constant surface speed which allows spindle speed to
also be specified in surface feet per minute (or meters per minute)
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Automatic tool changer (machining center)
A "T" word is used to tell the machine which tool station is to be placed in the
spindle. On most machines, an M06 tells the machine to actually make the tool change.
[H4]Tool change (on turning centers) A four digit "T" word is used to command tool
changes on most turning centers. The first two digits of the T word specify the turret
station number and the second two digits specify the offset number to be used with the
tool. T0101, for example specifies tool station number one with offset number one.
Coolant control
M08 is used to turn on flood coolant. If available M07 is used to turn on mist
coolant. M09 turns off the coolant.
Key concept number three: You Must Understand the Motion Types Available on Your
CNC Machine
Understanding interpolation
To effectively command motion on most CNC machines requires more than just
specifying end points for positioning movements. CNC control manufacturers try to make
it as easy as possible to make movement commands within the program. For those styles
of motion that are commonly needed, they give the CNC user interpolation types.
For example, you wish to move only one linear axis in a command. Say you wish
to move the X axis to a position one inch to the right of program zero. In this case, the
command X1. would be given (assuming the absolute mode is instated). The machine
would move along a perfectly straight line during this movement (since only one axis is
moving). Now let's say you wish to include a Y axis movement to a position one inch
above program zero in Y (with the X movement). For the control to move along a
perfectly straight line to get to the programmed end point, it must perfectly synchronize
the X and Y axis movements. Also, if machining is to occur during the motion, a motion
rate (feedrate) must also be specified. This requires linear interpolation.
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During linear interpolation commands, the control will precisely and
automatically calculate a series of very tiny single axis departures, keeping the tool as
close to the programmed linear path as possible. With today's CNC machine tools, it will
appear that the machine is forming a perfectly straight line motion. However, the CNC
control is actually doing during linear interpolation (approximation). The step size is
equal to the machine's resolution, usually 0.0001 in or 0.001 mm.
In similar fashion, many applications for CNC machine tools require that the
machine be able to form circular motions. Applications for circular motions include
forming radii on turned workpieces between faces and turns and milling radii on contours
of machining center workpieces. This kind of motion requires circular interpolation. As
with linear interpolation, the control will do its best to generate as close to a circular path
as possible.
Other interpolation types
Depending on the machine's application, you may find that you have other
interpolation types available. CNC control manufacturers try to make it as easy as
possible to program their controls. If an application requires a special kind of movement,
the control manufacturer can give the applicable interpolation type. For example, many
machining center users perform thread milling operations on their machines. During
thread milling, the machine must move in a circular manner along two axes (usually X
and Y) at the same time a third axis (usually Z) moves in a linear manner. This allows the
helix of the thread to be properly machined. This motion resembles a spiraling motion
(though the radius of the spiral remains constant).
Knowing that their customers need this type of motion for thread milling, CNC
machining center control manufacturers offer the feature helical interpolation. With this
feature, the user can easily command the motions necessary for thread milling.
21
The three most basic types of motion
While a particular CNC machine may have more motion types (depending on
your application), let's concentrate on becoming familiar with the three most common
types of motion. These three motion types are available on almost all forms of CNC
equipment. After briefly introducing each type of motion, we'll show an example
program that stresses the use of all three.
These motion types share two things in common. First, they are all modal. This
means they remain in effect until changed. If for example, several motions of the same
kind are to be given consecutively, the corresponding G code need only be specified in
the first command. Second, the end point of the motion is specified in each motion
command. The current position of the machine will be taken as the starting point.
Rapid motion (positioning)
This motion type (as the name implies) is used to command motion at the
machine's fastest possible rate. It is used to minimize non-productive time during the
machining cycle. Common uses for rapid motion include positioning the tool to and from
cutting positions, moving to clear clamps and other obstructions, and in general, any noncutting motion during the program.
You must check in the machine tool builder's manual to determine a machine's
rapid rate. Usually this rate is extremely fast (some machines boast rapid rates of well
over 1400 IPM!), meaning the operator must be cautious when verifying programs during
rapid motion commands. Fortunately, there is a way for the operator to override the rapid
rate during program verification.
The command almost all CNC machines use to command rapid motion is G00.
Within the G00 Command, the end point for the motion is given. Control manufacturers
vary with regard to what actually happens if more than one axis is included in the rapid
motion command. With most controls, the machine will move as fast as possible in all
axes commanded. In this case, one axis will probably reach its destination point before
22
the other/s. With this kind of rapid command, straight line movement will NOT occur
during rapid and the programmer must be very careful if there are obstructions to avoid.
With other controls, straight line motion will occur, even during rapid motion commands.
Straight line motion (linear interpolation)
This motion type allows the programmer to command perfectly straight line
movements as discussed earlier during our discussion of linear interpolation. This motion
type also allows the programmer to specify the motion rate (feedrate) to be used during
the movement. Straight line motion can be used any time a straight cutting movement is
required, including when drilling, turning a straight diameter, face or taper, and when
milling straight surfaces. The method by which feedrate is programmed varies from one
machine type to the next. Generally speaking, machining centers only allow the feedrate
to be specific in per minute format (inches or millimeters per minute). Turning centers
also allow feedrate to be specified in per revolution format (inches or millimeters per
revolution).
A G01 word is commonly used to specify straight line motion. Within the G01,
the programmer will include the desired end point in each axis.
Circular motion (circular interpolation)
This motion type causes the machine to make movements in the form of a circular
path. As discussed earlier during our presentation of circular interpolation, this motion
type is used to generate radii during machining. All feedrate related points made during
our discussion of straight line motion still apply.
Two G codes are used with circular motion. G02 is commonly used to specify
clockwise motion while G03 is used to specify counter clockwise motion. To evaluate
which to use, you simply view the movement from the same perspective the machine will
view the motion. For example, if making a circular motion in XY on a machining center,
simply view the motion from the spindle's vantage point. If making a circular motion in
23
XZ on a turning center, simply view the motion from above the spindle. In most cases,
this is as simple as viewing the print from above.
Additionally, circular motion requires that, by one means or another, the
programmer specifies the radius of the arc to be generated. With newer CNC controls this
is handled by a simple "R" word. The R word within the circular command simply tells
the control the radius of the arc being commanded. With older controls, directional
vectors (specified by I, J, and K) tell the control the location of the arc's center point.
Since controls vary with regard to how directional vectors are programmed, and since the
R word is becoming more and more popular for radius designation, our examples will
show the use of the R word. If you wish to learn more about directional vectors, you must
reference your control manufacturer's manual.
Programming example for milling.
4.000
1.000 R
.500 R
3.000
1.000 R
.250 R
.250
24
In this particular example, we are milling around the outside of a workpiece
contour. Notice that we are using a half inch diameter end mill for machining the contour
and we are programming the very center of the end mill. While you may not understand
all commands given in this program, concentrate on understanding what is happening in
the motion commands (G00, G01, and G02/G03). With study, you should be able to see
what is happening. Messages in parentheses are provided to document what is happening
in each command.
Program
O1234 (Program number)
N005 G54 G90 S1000 M03 (Select coordinate system, absolute mode, start
spindle CW at 1000 RPM)
N010 G00 X-.25 Y-.35 (Rapid to first position)
N015 G43 H01 Z.1 (Instate tool length compensation, rapid tool down to .1
from work surface)
N020 G01 Z-.26 F10.0 (cut at feed rate 10” to -.26 into the part)
N025 Y2.0 (move tool linearly to the start of 1” radius)
N030 G02 X1.0 Y.25 R1.25 (CW circular motion to cut 1” radius)
N035 G01 X3.5 (linear motion to .5 radius)
N040 G02 X4.25 Y2.5 R.75 (CW circular motion to cut .5 radius)
N045 G01 Y.25 (linear motion to .250 radius)
N050 G02 X3.75 Y-.25 R.5 (CW circular motion to cut .25 radius)
N055 G01 X.75 (linear motion past a line tangent to 1” radius)
N060 Y0 (linear motion tangent to 1” radius)
N065 G03 X0 Y.75 R1.0 CCW motion to cut 1” radius)
N070 G01 X-.1 (linear motion to clear the part)
N075 G00 Z.1 (rapid away from workpiece in Z)
N080 G91 G28 Z0 (go to the machine's reference point in Z)
N085 M30 (End of program)
25
Keep in mind that CNC controls do vary with regard to limitations with motion
types. For example, some controls have strict rules governing how much of a full circle
you are allowed to make within one circular command. Some require directional vectors
for circular motion commands instead of allowing the R word. Some even incorporate
automatic corner rounding and chamfering, minimizing the number of motion commands
that must be given. Though you must be prepared for variations, and you must reference
your control manufacturer's programming manual to find out more about your machine's
motion commands, at least this presentation has shown you the basics of motion
commands and you should be able to adapt to your particular machine and control with
relative ease.
Tooling for Hole producing and Milling operations
A programmer must be thoroughly knowledgeable in the machining operations
performed on a CNC machine. Tooling for CNC is tied to machining operations
planning.
The most important tool used in drilling is a twist drill. This cutting tool has two
helical flutes or grooves cut around the center called a web. The flutes act as cutting
edges. They also serve as channels for admitting lubricant (or coolant) and carry out the
cut chips. The web gives the tool strength.
Twist drills are made in a wide range of sizes and from materials extending from
carbon steel to solid carbides. Usually they produce holes with low size and position
accuracy. They have also a tendency to deflect.
To reduce deflection and increase position accuracy a tool called a center drill is
being used. Another tool for this purpose is called a spot drill. The center drill is a short
drill with a pilot on the front. The spot drill looks like a regular drill but with a very short
body.
26
A very popular tool used for producing medium to large holes is a spade drill. It
consists of blade (made out of high speed steel or carbide) and a blade holder. Blade
holders are designed to hold a range of different size drills.
A drill which was designed specifically for CNC operations is a indexable insert
drill. They offer all the advantages of a spade drill plus the ability of indexing (rotating)
the cutting edges of the inserts. There a holes provided thru the body of the holder to cool
the inserts and to flush out the chips. These tools are capable of drilling at rates 5 to 10
times faster that of twist drills or spade drills.
Better hole location, size, straightness and surface finish can be accomplished by
boring. Boring operation follows drilling. A boring tool is a single point tool which
rotates around its own center of rotation.
27
Another way of producing a good size in a hole is by using a reamer. A reamer is
a cylindrical tool with straight or helical cutting edges located on the periphery. A reamer
can be made out of high speed steel, carbide tipped or solid carbide. Reamers produce
extremely accurate holes, but do not correct problems related to the location and
straightness of the hole. Sometimes reamers are used in conjunction with a boring tool.
When a hole requires threading a tool called a tap is used. The shape of the thread
is on the periphery of the tap. Taps, depending on the application, could be straight flute,
spiral flute or no flute at all.
When it comes to machining a profile on a milling machine the most popular tool
used is an end mill. End mills come in a wide variety of sizes, flute shapes and are made
from different materials. They come in two, three, four or more flute selection. Another
way to distinguish end mills is as center cutting or non center cutting. If a lot of material
is removed at once, a roughing end mill is usually used, followed by a finishing end mill.
Smaller size tools are made single end or double end. To produce different surface
configurations there are tapered, ball nose and corner rounding end mills.
28
If a large surface needs to be machined a tool called face or shell mill is used.
These tools usually cut mostly with the bottom of the tool.
It’s hard to describe all tools available to the programmer in such condensed
manual. But it can not be over emphasized the importance of the knowledge of tooling.
The more a programmer knows about tooling the better job can he do in programming.
Choosing the right tools for the job can make the job a money maker or money can be
lost.
Hands-on project to be machined on a Machining Center
29
M1
(****)
(****)
N300(1/4",END,MILL,CONTOUR,OUT
SIDE)
T3M6
G0G90X.2982Y.7655S4500M3
G43H3Z.05
/M8
G1Z-.11F10.
X.3864Y.7499
G2X1.125Y0.R.75
X.3864Y-.7499R.75
G1X.2982Y-.7655
G0Z.1
X-.2982
G1Z-.11
X-.3864Y-.7499
G2X-1.125Y0.R.75
X-.3864Y.7499R.75
G1X-.2982Y.7655
G0Z.1
(**)
(1/4",END,MILL,POCKETING)
X.3843Y-.4279
G1Z-.02
X-.3843
G2X-.7174Y-.2567R.428
G1X.7174
G3X.7944Y-.0856R.428
G1X-.7944
G2X-.803Y0.R.428
X-.7944Y.0856R.428
G1X.7944
G3X.7174Y.2567R.428
G1X-.7174
%
O1()
(DATE=DD-MM-YY - 17-04-04
TIME=HH:MM - 19:57)
(GEOMETRY=DOGTAG1A)
(PROGRAM NAME - DOGTAG1A.NC)
(OPERATION#1)
(ZERO,POSITION)
(X,Y=CENTERLINE,OF,MATERIAL)
(Z=TOP,OF,MATERIAL)
(TOOL,LIST,AT,END)
G0G40G80G90
G54
N100(#2,CENTERDRILL
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G0G90X-.846Y0S3000M3
G43H1Z.05
/M8
G98G81Z-.15R.05F5.
G80
M5
M9
G91G28Z0
M1
(****)
(****)
N200(1/8",DRILL)
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G0G90X-.846Y0S4000M3
G43H2Z.05
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G98G81Z-.1806R.05F6.
G80
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G1X-.375
G0Z.1
M5
M9
G91G28Z0
M1
(****)
(****)
N400(1/32",END,MILL)
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31
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32
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33
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34
X-.4695
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G0Z.05
M5
M9
G91G28Z0
G28Y0
(TOOL,LIST)
(#01=#2,CENTERDRILL)
(#02=1/8",DRILL)
(#03=1/4",END,MILL)
(#04=1/32",DRILL)
M30
%
X-.3395Y-.3071
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G1Z-.03
35
Tooling for Turning operations
The basic difference between milling and turning is that in milling the cutter is
being rotated and the part is being feed into the path of the cutter, in turning the part is
being rotated and the tool is being feed along or perpendicular the center of rotation. The
most basic lathe (turning) operations are: facing, turning, grooving, parting, drilling,
boring, reaming threading. Because of the nature of turning some additional tools are
being used.
Facing operation involves cutting the end of the material such that the resulting end is
perpendicular to the centerline of the rotation. Turning involves removal of material from
the outside diameter of rotating material. Different profile shapes can be created
including, cylinders, tapers, contours, shoulders. The toll shapes for facing and turning
are very similar, with facing tools having less side clearance.
Grooving requires the tool to be fed into the work in a direction perpendicular to
the work centerline. The cutting edge of the tool is on its end.
36
Parting involves cutting off the part from the main bar stock. This operation is
done with a cutoff tool that has some clearance on the sides while the cutting is done with
the front of the tool. The tool is fed into the part perpendicular to the center of rotation.
Threading on a CNC lathe is done with a tool which has a 60º included angle
insert. This operation involves cutting of helical grooves on the outside or inside surface
of a cylinder or cone.
It is important to mention the materials used in turning. Cemented carbides are
formed by using tungsten carbide sintered in a cobalt matrix. Some grades contain
titanium carbide, tantalum carbide or some other materials as additives. To increase the
wear resistance, cemented carbides are coated. Coating materials include titanium carbide
or aluminum oxide. Both coatings offer excellent performance on steel, cast irons and
nonferrous materials.
37
Another type material used is ceramic. It is a very hard material formed without
metallic bonding. It displays exceptional resistance to wear and heat load. The most
popular material used in ceramics is aluminum oxide.
Where everything else fails we turn to diamonds. There are two types of diamond
cutting materials. The first is a single crystal natural diamond. It has an outstanding wear
resistance but low shock resistance. The other type diamond consists of smaller synthetic
diamond crystals fused together at high temperatures and pressure. This material displays
good resistance to shock loading. Diamond tools offer substantial improvement over
carbides.
Hands-on project to be machined on a Turning Center
G0G28U0
G28W0
N100( VNMG332,ROUGH,FRONT )
G0G96S2500
%
:0011(REVISED)
(TECHASCEND LATHE PROJECT)
(KEY,CHAIN)
(MATERIAL,DIA.,=1.000")
(PART,STICK,OUT,3.550",FROM,SPIN
DLE)
(1.000",DIA.,STOCK)
(DISK,#1)
G50S3500
T0101M3
/M8
G0Z.005
38
G0Z.05
X-.0625
G1Z.01
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G0Z.1
M9
M5
G28U0
G28W0
T0100
M01
(****)
(****)
N300(VNMG331,FINISH,FRONT)
G0G96S2500
G50S3500
T0303M3
/M8
G0Z0
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G0Z.1
M9
M5
G28U0
G28W0
T0300
M0(WITH,SPECIAL,CUP,BRING,OUT,C
ENTER)
(****)
(****)
N900(SPECIAL,"V",TOOL,IN,CENTER)
G0G96S2500
G50S3500
T0909M3
/M8
G0Z-1.0255
X1.15
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39
G2X.471Z-1.9029R.2087
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40
M5
G28U0
G28W0
T0900
G0G97S1500
M0(REMOVE,CENTER,AND,POLISH,P
ART)
(****)
(****)
N500G0G97S1000
T0505M3
/M8
G0Z-3.15
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G1X0.19F0.003
G0X0.45
G1Z-2.8114F0.02
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G3X-0.006Z-3.125R0.128F.001
G1X1.1F.02
M9
M5
G0G28U0
G28W0
T0500
(TOOL,LIST)
(#01=VNMG332)
(ROUGH,O/D)
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G3X.7087Z-2.471R.2913F.003
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G3X.98Z-2.2188R.2813
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G1Z-2.8438
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G0X1.1
M9
(#03=VNMG331)
(FINISH,O/D)
(#05=1/8",ISCAR,PARTING,TOOL,WITH
ANGLE)
(#09=SPECIAL,"V",TOOL,IN,CENTER,.
031",RAD.)
(ROUGH,&,FINISH,BACK)
M30
%
41
Conclusion
In this program you have learned the basic concepts of NC and CNC technology;
traditional and contemporary CNC hardware configurations; advantages and
disadvantages of the new technology.
Topics included Computer Numerical Control (CNC) concepts, modern machine
tool controls, an overview of CNC shop activities, cutting tool selection and many other
important for CNC programming topics. You were introduced to Word Address
programming.
We told you that a CNC Programmer must be able to:
9 do process planning;
9
do scheduling;
9
design tooling;
9
create work holding devices;
9
write documentation;
9
to handle troubleshooting technical problems that happen;
9
learn any skills that are needed.
He must have:
9
CNC machine tool programming experience;
9
Computer language programming experience;
9
CAD/CAM programming software experience.
He must be:
9
Creative and effective problem-solving skills;
9
Highly motivated with the ability to work independently and in a team;
9
Detailed oriented;
9
Quick learner.
We, at the Department of Mechanical Engineering Technology at Queensborough
Community College, tried to open your minds to the latest developments happening to
our technology as we speak. If you decide to learn more about CNC, CAD/CAM or
Mechanical Engineering Technology we will be delighted to see you as our students.
And last, but not the least: have a POSITIVE ATTITUTE and GOOD LUCK in
whatever you decide to do in your life.
42