Laboratory of Fabrication Processes

Department of Mechanical Engineering

Practice
CNC Milling
04
THEORETICAL FRAMEWORK

Numerical control applied to machine tools and other equipment, can be used in a very
wide range of processes, and in components having different characteristics. It is
flexible, because while it can be used in high production quantities, it is also of great
value in the production of small and medium size batches, almost eliminating the need of
assemblies and accessories, which represent high costs.

The computer is a powerful tool in the program preparation for numerical control (NC),
since they are capable of generating simulation graphs, therefore, it is of great value for
making the most of a machine tool. This kind of control is given the name of
computerized, hence the initials CNC.

The application of CNC to a machine tool make it possible for the CNC system to
perform the operations usually made by an operator. These operations can be
categorized in to groups:

Primary operation: It is the movement of the machine tool holder carriages to

keep an interaction between the cutting tool and the workpiece, which will allow
the creation of the desired geometry of the component, with the precision
required.

Secondary operation: Supporting operations that are necessary for the normal
functioning of the machine. Common examples of secondary operations are:

a) Spindle, start/stop/backwards.
b) Cutting fluid, ON/OFF.
c) Selected spindle speed.
d) Desired carriage forward speed.
e) Split/rotate circular table.
f) Change cutting tools.
 Laboratory of Fabrication Processes
Department of Mechanical Engineering

Numerical Control machine tools have a control panel. This panel works as an interface
between the machine and the user, and by means of it, the numerical control program is
introduced. This program is a set of instructions that are transduced to voltage signals,
which activate the movement of the carriages through a control board. The sequence of
the program follows a logic according to the operation to be performed and the cutting
tool available for that operation. The movement of the cutting tool produce machined
surfaces. The group of those surfaces constitute the machined piece. Machining process
control is graphically represented in Figure 1:

Figure 1. Basic diagram of the control process of a CNC machine tool.

From the previous statements we can conclude that numerical control is a manufacturing
language. The language structure and its semantics have been defined according to the
traditional machined surface generation using conventional machines.

Programming language

The steps to follow to numerical control programming are similar to those stablished in
manufacturing.

1) Understanding of the piece definition sketch, which must include:

a) Dimensional data
b) Allowable dimensional and shape tolerances
c) Surface finish of the piece
d) Material of the piece
 Laboratory of Fabrication Processes
Department of Mechanical Engineering

The former is represented in Figure 2:

Figure 2: Data flux from constructive sketching to finished piece.

From analyzing this sketch, the programmer gets the set of surfaces which are to be
machined, the dimensions of the piece prior to machining and the cutting tools required
during the process.

2) Definition of the operation elements, including among others the following:

a) The group of surfaces to be machined along the process.

b) Cutting tools.
c) Cutting parameters1.
d) Dimensions of the piece prior to machining.
e) Dimensions and tolerances of the finished piece.

3) Write the NC program and introduce it to the memory of the machine.

4) Beginning if the piece manufacturing. Tools must be properly fastened and previously
calibrated. Before defining the piece origin, the cutting tool should be rotated in order to
verify that it is properly installed, for which a specific function of the machine is used
(typically the M03 S1000).

1
The cutting parameters can be found in electronic catalogs in the following sites:
http://www.coromant.sandvik.com/ or http://www.kennametal.com/
 Laboratory of Fabrication Processes
Department of Mechanical Engineering

When the position to which the tool is to be moved has been programmed, the CNC
system moves the tool to this position using the coordinates included in the dimensional
term of the block. For the specific machine that we are studying, three types of
coordinate systems are defined:

a) The machine coordinate system.

b) The working coordinate system.
c) The reference coordinate system.

Machine coordinate system

The origin of this coordinate system is known as machine zero. The machine coordinate
system is stablished when the machine is turned on and the tool is taken to the
reference point.

Reference coordinate system

When the machine is turned on, the operation of taking the tool to the reference point is
the first task that must be done. Once the tool is positioned at this point, the reference
system of the machine is stablished.

Working coordinate system

This coordinate system is stablished using the function G54 from the reference point
which is to be defined. In the Millitronics machine, the piece origin is defined in each axis
(X, Y, and Z) from the point at which the tool is in contact with the piece in each axis.
Such data are stored in the coordinate system of the NC.

Figure 3: Communication process between the NC program generator and the machine tool controller.

Finally, before starting to work on the machining, the piece is mounted and the
following procedure is completed:

a) The working piece is mounted in the machine spindle and is properly

fastened.
 Laboratory of Fabrication Processes
Department of Mechanical Engineering
b) The workpiece is rotated using the programming mode (MDI).

c) The cutting tool is moved until it grazes one of the surfaces perpendicular to an axis.
At that point, the position of the tool axis along the considered axis is stablished.
d) The program generated is extracted (typically from the software WorkNC) and is
introduced into the CNC processor using a 3½-inch floppy disky, as shown in Figure 3.
e) Finally, the machining is started.
Examples of pieces that can be fabricated using Numerical Control (Figures 4 and 5):

Figure 4: Examples of CNC models for milling complex pieces with sculpted surfaces.

Figure 5: Examples of pieces machined in a CNC lathe.

 Laboratory of Fabrication Processes
Department of Mechanical Engineering

OBJETIVES
1) The student will understand the safety rules regarding this practice.
2) El alumno conocerá los diferentes tipos de sujeción de herramienta para husillos de
máquinas modernas, la geometría del filo de insertos de WC y algunos principios de
desgaste de herramienta.
3) The student will get to know the basic CNC machine tools, including lathes and
milling machines.
4) The student will get to know the basic operation of a CNC lathe and will understand the
connection between the process parameters and the machine programming (speed,
forward movement and cutting depth).
5) The student will operate a CNC lathe in order to fabricate a simple piece from a rod.
6) The student will use conventional measuring devices (line gauge or ruler, caliper and
micrometer) to determine the dimensions of the piece produced.

SAFETY
In order to use the measuring devices during this practice, it is necessary for the student to
take the following precautions:
ATENTION! MOTIVE
Do not apply excessive force on the Doing so could cause permanent deformation
measuring devices. of the instrument.
Clean the piece and contact surface Get an accurate measurement.
between the measuring device and the
piece.

Keep clean the measuring device being Keep the device well calibrated.
used.
The measuring devices should not be left on This could damage the instrument
surfaces full of chip, grease, or dirt. permanently.

In order to use the machine tools, students must adopt the following precautions:

ATENTION! MOTIVE
Wear safety glasses whenever using a Risk of eye damage due to chip generation is
machine. high.
Never wear loose clothes or hair, or rings Clothes or jewelry can be trapped in rotating
and watches when using any kind of elements of machines, causing important
machine. physical damage.
Keep the floor clear around the machine, This elements make the floor slippery,
free of chip, oil and cutting fluid. increasing the change of an accident to
occur.
 Laboratory of Fabrication Processes
Department of Mechanical Engineering

Please, follow the next procedures when operating a machine tool:

ATENTION! MOTIVE
Never try to operate a machine tool without Personal and machine harm risk are high.
previous knowledge of its functioning.

Do not bend over machines. Keep always Clothes or jewelry can be trapped by rotating
straight and make sure your face and eyes elements causing physical damage.
are away from chips that come out. Moreover, the risk of a chip drives into an
eye is high.
Never try to mount, measure or fasten the
workpiece until the machine is completely Machine inertia is high, thus a rotating
still. component (even at relatively low speeds)
At any moment keep hands, brushes and can easily dislocate an articulation,
clothes away from moving parts of the damage the skin, or even pull out a finger.
machine tool.
Before performing a cut operation, make
sure the piece and cutting tool are properly
mounted and firmly fastened.
Every piece worked in a machine tool must A piece or tool that comes out of the
be properly fastened, using adequate machine is a highly dangerous projectile.
fastening devices. Never try to fasten a piece
with your hands.
Never leave keys or accessories mounted in
mechanisms that are going to rotate.
Use always a brush to remove chip; never do Chip is sharp and may also be hot.
this with your hands.
Never try to shift the speed of a machine tool Ungear an in-movement transmission is
when it is operating. harmful to the mechanism; those are
unsynchronized transmissions.
Never hit the bed of a machine.
Bed is a very important part of the
Chip must be brush-removed at the end of machine, hits and scratches make it less
each operation. precise.
 Laboratory of Fabrication Processes
Department of Mechanical Engineering

MATERIAL, TOOLS AND EQUIPMENT

1) Material
a) Workpiece material (1018 steel)
2) Tools
a) Micrometer and caliper
b) Milling machine tools
c) Cutting tools
3) Equipment and machinery
a) Workstation with software WorkNC
b) CNC Milling machine
4) Safety equipment
a) Safety glasses

PROCEDURE
1) Print and read the practice before the laboratory begins. Prepare the pre-report
2) Be in the laboratory 5 minutes before the time stablished, wearing proper clothes
and shoes, with no jewelry and with the printed practice stapled and pre-report
completed (pages 9 to 14).
3) Go to the computer lab to begin the practice.
4) The instructor will apply a quiz at the beginning of the practice, which will evaluate
your comprehension about the theoretical framework, and it will also be used as
attendance list.
5) The instructor will explain the general functioning of the software for
generating CNC code and the procedure to transfer the program to the
machine that will be used during this practice.
6) The group will go to the CNC lathe in the laboratory basement, where the
general functioning of the machine will be explained.
7) The instructor will perform the machining process according to the program
generated, changing twice the cutting conditions, corresponding to roughing, and
roughing + finish.
8) After that, students will proceed to measure the finished product in order to
verify its dimensions.
 Laboratory of Fabrication Processes
Department of Mechanical Engineering

School of Engineering
Department of Mechanical
Engineering
Laboratory of Fabrication
Processes

PRACTICE PRE-REPORT
1) Acknowledgement of receipt
Capturing my handwritten signature, I acknowledge receipt of the safety guidelines and
procedures necessary to carry out the Practice 2: Machining in conventional lathe, of the
Laboratory of Fabrication Processes.

Full name:

Course ID: Group:

Signature: CNC Milling

_________________________________________ Date:
Practice 2
_____________

Name

ID

Group

Instructor

Id:
 Laboratory of Fabrication Processes
Department of Mechanical Engineering

1) Define the most common programming codes used in NC (G-Code, M-Code)

and give some examples of each one referring to their usage in the
programming language.

Bibliography consulted to learn about CNC programming code:

2) Mention the main advantages and disadvantages of using CNC machines

instead of conventional machines.
 Laboratory of Fabrication Processes
Department of Mechanical Engineering

Bibliography consulted to learn about CNC machines:

 Laboratory of Fabrication Processes
Department of Mechanical Engineering

3) Classify the kinds of CNC machines according to the geometries that can be
generated:

Bibliography consulted to learn about CNC machines:

4) Describe the typical elements of a tool for CNC milling:

Bibliography consulted to learn about CNC machines:

 Laboratory of Fabrication Processes
Department of Mechanical Engineering

CNC Milling
Practice 4

Name
ID
Grou
p
Instruct
or

School of Engineering
Department of Mechanical
Engineering
Laboratory of Fabrication
Processes

PRACTICE REPORT

1) Read carefully the specifications of the part to be worked in the lathe (according to the advice
given by the instructor).
 Laboratory of Fabrication Processes
Department of Mechanical Engineering

Illustration 1. Piece to be machined in CNC milling machine.

 Laboratory of Fabrication Processes
Department of Mechanical Engineering

1) Verify/determine in the program the cutting conditions for the machining.

Name of the Date:

piece:
Cutting speed: m/ Forward movement: mm/
min rev

2) Measurement of the machined piece: Make all the measurements required and write them
down in the following sketch:
 Laboratory of Fabrication Processes
Department of Mechanical Engineering
3) Conclusions: Describe, from a technical perspective, the experience of machining the
piece using a CNC milling machine.

DELIVERY OF PRACTICE PRE-REPORT AND REPORT

Make sure your pre-report and report have all the identification data and that you have
answered every part of them, before giving them to the instructor. Then, go to the
storehouse to return the materials, tools and accessories, and make sure the working area
is clean and ordered before leaving.