*
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*You’ve come a long way!
You
understand manufacturing and
machining, and are now totally
ready to study CNC!
*
!!!!
*
*
* Starting with Chapter 17, we begin the lessons on how to
manage a programmed machine tool in a CNC world.
* It’s a bigger job than just putting parts into the setup and
hitting the green button—although that’s where you’ll
probably begin your career: operating a CNC.
* It’s about managing data, making setups, editing programs
and solving problems when they arise!
* In order to manage this level of responsibility we must
study some underlying techno-facts.
* We’ll first learn about the systems that make it work in
Chapters 17 and 18.
*
While they may look like technical beasts when
taken one aspect at a time, the whole subject is
easy to learn.
That’s the way we’ll proceed in Intro to CNC,
the third part of Machining and CNC Technology.
The subject is broken down into individual units
of learning, easily digested, leading to
competency!
After completing Part III you’ll have the baseline
knowledge and be ready to safely and
confidently learn to setup and run your own CNC
machine.
* Like all other PowerPoint sets for this text, this
presentation is not intended to teach the
subject, but rather to show why the units are
important using a sampling of what you’ll be
learning.
* Details have been omitted but will be
explained in the textbook.
*
*
17.1 World Axis Standards
17.1.1 Primary linear axes – X,Y and Z
17.1.2 Primary rotary axes – A, B and C
17.1.3 Secondary Linear – U, V and W
17.1.4 Rules for determination
17.2 Coordinate Systems and Points
17.2.1 Absolute and incremental values
17.2.2 Four quadrants
17.2.3 Points for geometry and reference
(A) Absolute
(B) Incremental
17.2.4 Conventions in program commands
17.3 CNC Machine Motions
17.3.1
Axis moves
(A) Rapid travel
(B) Linear Interpolation
17.3.2 Axis combinations: 2½- and 3-D motion
17.4 Polar Coordinates
17.4.1 Absolute and incremental polar values
17.4.2 Positive and negative direction
(C)Metric
(C) Circular
* There are 14 standard axes defined by the
Electronics Industries Association (EIA) used for
Unless it’s a multiplexed machine with several
motion and position.
auxiliary rotary and linear axes, these nine are
* In this text we’ll study nine of them.
adequate to define most of the equipment in
3 Primary
industry
today. Linear Axes X, Y and Z
3 Primary Rotary Axes A, B and C
However, for tomorrow’s manufacturing world,
Secondary
Linear
Axes U,V
& W to
that’s3another
question.
Machines
continue
evolve as central processors are able to handle
more and more calculations per nanosecond,
thus more functions simultaneously.
*
* Whenever you are assigned to a new CNC
machine, the axis set must be identified as the
first order of business.
* Here are the sets for three common machines.
*
* It’s easy to identify the Z axis: it’s the spindle
or it faces the spindle—it’s the drilling axis!
* Then apply the Right Hand Rule by pointing
your right middle finger in the positive Z
direction.
* Your fingers and thumb then form the
orthogonal axis frame (mutually at 90º).
*
*
*First identify the Z axis.
It’s parallel to the
spindle axis and brings
the work toward and
away from the spindle.
*Pointing your middle
finger in the positive Z
direction, your index
finger and thumb form
the other positive axes.
*
*All CNC machines use the X-Z or X-Y-Z frame,
with each axis mutually perpendicular to the
others.
The set (my fingers)
*
That relationship stays the same no matter
remain in the same
how the axis set is rotated to suit the
orientation
to each
machine.
other no matter their
*Toward stronger or more efficient machines
world orientation►
manufacturers arrange the set any way
convenient, but they don’t change the interrelationship between axes.
* The X axis on many turning centers, is not
parallel to the floor, itXslants
forward.
Slanted
* That provides easy access to the turret for
setup work, since the machine isn’t as wide as
level X axis machines.
Z
* Plus chips and coolants slide
90º right off to the
catch basin below.
*
This lathe’s world axis orientation is not
level but it’s still an orthogonal set.
Whenever a machine features a rotary axis, we
identify it this way:
* If it rotates around a line parallel to
X it’s an A axis
*
Y it’s
B
Z it’s
C
*
* Rotary axes capable of feed rates can move a
cutter head in an arc during machining.
* Or they can move the workpiece in an arc.
* In this video we see A and B auxiliary axes
moving simultaneously with X, Y and Z to cut
this complex turbine blade ►.
Only intelligent 5 axis CAM software can compile this program.
* To determine the direction of rotary motion,
either plus or minus A,B or C (clockwise or
CCW), we use the Rule of Thumb.
Y+
Z+
X+
Point the thumb of
* It’s based on the line about which the rotary
your
right
hand
axis
pivots,
X, in
Y or Z.
the + direction of
the axis of the
rotation, X, Y or Z
positive direction.
Positive
Cisdirection
Bthis?
WhatPositive
motion
A
*
*
* CNC machines move and locate with reference to the axis
origin.
* TheyFor
move
to locations
identified
with coordinates.
example,
on this
flat screen:
* Co
meaning working together.
* Ordinate
meaning a single line of position.
X2.50, Y1.750
This point is identified with
a set
This
is of
ancoordinates
ordinate – on
thethis plane.
Thissay
linethis
is parallel
to the Yto
Lets
line is parallel
point
to be
identified
lies of
axis
lies
2.50
to right
the
Xand
axis,
and
1.750
above
somewhere
the
X-Y origin.
the
X-Y
originalong it.
*
When we want coordinates to refer to
* In CNC work, the place where
theitcoordinates
Where
All Begins
theare
PRZ
putZ0,a code
in the
X0,we
Y0,
is known
as program
the Program
G90Reference
telling theZero
control
(PRZ)“this and all
Youare
may
hear different terms for the
others
until
changed
absolute
* It’s the starting point for coordinates
PRZ: “program
zero,”
“program data
values.”
CAM
programs
are
almost
all
* Most coordinates in the program refer their
point,”
distance
from the
PRZ. “the origin” (a math term) or
absolute
values.
Theexample.
tip of theothers,
drill is depending on the region in
* For
at
which you live. PRZ is the most
commonly
used,
but
they
all
X2.250, Y1.00, Z-1.00
mean the place where
Relative to theX=0.0
PRZ
which is on the
top
Y=0.0
corner on thisZ=You
part. know
* When moving across the origin line using
absolute coordinates, the +/- values change
depending on the quarter circle quadrant in
which the point lies.
* For example:
*
* Point A has positive X and Y values: X1.54, Y1.13
* Point C is X-1.54, Y-1.13
* What are the coordinates of Points B and D?
*
They are also called “relative” coordinates
since they relate to the current location for
* Occasionally we encounter the need for a different kind
It’s asreference.
though
the current
a
their
Incremental
coordinates
are position
useful forishandof coordinate.
mini-PRZ.
if PRZ,
I want
arather
millprograms
spindle
to
small
setup
or but
tooling
* Theycompiling
do
not refer
toSo
the
to their last
position.
written
machinist
go to by
thetheleft,
I write.at the machine. They
* Incremental
are jumps
fromofwhere you are to
are alsocoordinates
used in a limited
number
where
you wish to
go we’ll
next.study later. To use
commands
that
G91 X 1.00
incremental coordinates in a program, the
So when I push cycle start, it will then
control must read a G91 code or be told in
moveother
fromway
where
it is are
1.0not
inch
to the
some
that they
absolute.
-
left.
* To go to a position either
at rapid rate or at a feed
rate, we use significant
points on the part
geometry to create
program coordinates.
*
Trade Tip
In Chapter 25, you will be
drawing a part image using
Mastercam, readying it for
programming. Each line
and arc on the drawing will
be created by defining
significant points.
*Points occur
at the ends of
lines and arcs.
*They also
occur at the
center of arcs
and at
tangent points
where lines
join arcs.
*
*CNC machines move their axes in five different
Due to computer evolution with
ways:
ever-higher
computation rates,
Rapid
Travel
most
new
CNC controls
today
Linear
Single
or multi-axis
straight-line
motion
canmotion
perform
3-D
motion.
Circular
withintrue
a single
plane
Circular/Linear, also called 2½-D motion. Two axes
move in an arc while the third moves in a straight
line.
3-D motion Few older controls have the ability to move in
an arc using three axes simultaneously. Most approximate
these arcs through the power of the cam software.
Trade Tip
Caution! Depending
on the power of the
Rapid—as
as
CPU, yourfast
machine
the
canof
willmachine
rapid in one
move
but withsure to
two ways—be
the
abilitywhy
to in the
discover
reduce
speed
text. Older
through
controllers take an
operator
unexpected route!
override control.
*
*
* The next four motions all move one or more
axes at the machining rate specified in the
program.
* The differences lies in how many axes are
involved—in a straight line or arc.
* As motions become more complex, the CPU
must handle far more calculations per second
by interpolating each axes’ drive commands.
* To move axes simultaneously, to produce a
constant velocity along the line A-B, say at 400
inches per minute.
* Neither the X or Y axis drive will be moving at
400 IPM.
* They will run at lower speeds that combine to
create the tool motion of 400 IPM.
Interpolating means to find an intermediate
value: in this case a feed rate value for each
axis that combines to create the programmed
rate.
*
*
B
137.81 IPM
Y Axis
A
375.87 IPM, X Axis Motion
*
* The operator can override the resultant tool
motion from 0% (no movement) up to 100% or
150% on some machines.
* When the feed rate is changed by the
machinist, the controller must change each
drive proportionately to achieve the rate.
*
* For arc motion at feed rate, the controller is
also interpolating, as with linear.
* The difference is that it is constantly changing
the ratio between the axes involved, as the
curvature changes slants.
*
*Sometimes engineering
information comes not in
Trade Tip
the form of rectangular dimensions, but rather as
Using polar
coordinates
the radius
and angle
from aoften
starting point.
a trigonometry
step defined
during using polar
*Thosesaves
points
are more easily
drawing or bolt
handcircle
program
coordinates—a
for writing!
example.
*PolarIfcoordinates
aren’t used
inside
the needed significant
point
is CAM-generated
programs,
they are veryrather
useful
for drawing the
definedbut
in radius-angle,
than
part X-Y,
geometry
doing hand program writing
why dooranwhen
unnecessary
of polar
entities.Define it with an R-A
calculation?
coordinate on your geometry
drawing.
*
* While one could operate a machine without
the knowledge of Chapter 17, remember that
you are in training to become a full journeylevel machinist.
* The goal is to be a manager of your CNC
machine—more than an operator.
* That requires complete oversight including CNC
motions and how axis drives work, coming up in
Chapter 18.