Chapter 17 - McGraw-Hill

wistfultitleElectronics - Devices

Nov 24, 2013 (3 years and 9 months ago)

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-
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rt.







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!
!
!
!

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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


(C)Metric


17.2.4

Conventions in program commands

17.3
CNC Machine Motions


17.3.1

Axis moves



(A) Rapid travel

(B) Linear Interpolation

(C) Circular

17.3.2 Axis
combinations
:


-

and
3
-
D motion

17.4
Polar C
oordinates


17.4.1


Absolute and incremental polar values


17.4.2

Positive and negative direction


*
*
There are 14 standard axes defined by the
Electronics Industries Association (EIA) used for
motion and position.

*
In this text we’ll study
nine
of them.


3 Primary
Linear Axes X, Y and Z


3
Primary Rotary Axes A, B and C


3
Secondary Linear Axes U,V
& W

Unless it’s a multiplexed machine with several
auxiliary rotary and linear axes, these nine are
adequate to define most of the equipment in
industry today.

However, for tomorrow’s manufacturing world,
that’s another
question.
Machines continue to
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 90
º
).

*
*
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.

*
That relationship stays the same no matter
how the axis set is rotated to suit the
machine.

*
Toward stronger or more efficient machines
manufacturers arrange the set any way
convenient, but
they don’t change the
inter
-
relationship between axes.

The set (my fingers)
remain in
the
same
orientation to each
other no matter their
world
orientation


*
*
The X axis on many turning centers, is not
parallel to the floor, it slants forward.

*
That provides easy access to the turret for
setup work, since the machine isn’t as wide as
level X axis machines.

*
Plus chips and coolants slide right off to the
catch basin below.

Z

X Slanted

This lathe’s
world axis orientation

is not
level but it’s still an orthogonal set.

90º

*
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.

*
It’s based on the line about which the rotary
axis pivots, X, Y or
Z.



Point the thumb of
your right hand in
the
+ direction
of
the
axis of the
rotation
, X, Y or Z
positive direction.


Positive C
direction

Z+

X+

Positive A direction

Y+

What
motion is this?


Positive
B

*
*
CNC machines move
and locate with
reference to
the axis
origin.

*
They move to locations identified with
co
ordinates
.

*
Co

meaning working together.

*
Ordinate

meaning a single line of position.

For example, on this flat screen:






This is an ordinate


the
point to be identified lies
somewhere along it.

Lets say this line is parallel to
the X axis, and
1.750 above
the X
-
Y
origin

This line is parallel to

the Y
axis and
lies
2.50 to right of
the
X
-
Y
origin.

X2.50, Y1.750

This point
is identified with
a set of coordinates
on this
plane
.

*
*
In CNC work, the
place where the coordinates
are X0
, Y0, Z0, is known as the
Program
Reference Zero (PRZ)

*
It’s the starting point for coordinates

*
Most coordinates in the program refer their
distance from the PRZ.

*
For example.

The tip of the drill is
at

X2.250,
Y1.00, Z
-
1.00

Relative to the PRZ
which is
on the top
corner
on this
part.

Where it All Begins


You may hear different terms for the
PRZ: “program zero,” “program data
point,” “the origin” (a math term) or
others, depending on the region in
which you live. PRZ is the most
commonly used, but they all

mean the place where

X=0.0

Y=0.0

Z=You know


When we want coordinates to refer to
the PRZ we put a code in the program
G90

telling the control “this and all
others until changed are absolute
values.” CAM programs are almost all
absolute values.

*
*
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?


*
*
Occasionally we encounter the need for a different kind
of coordinate.

*
They do not refer to the
PRZ,
but
rather
to their last
position.

*
Incremental coordinates are jumps from where you are to
where you wish to go next.

Incremental coordinates
are useful for hand
-
compiling small setup or tooling programs
written by the machinist at the machine. They
are also used in a limited number of
commands that we’ll study later. To use
incremental coordinates in a program, the
control must read a
G91

code or be told in
some other way that they are not absolute.

It’s as though the current position is a
mini
-
PRZ. So if I want a mill spindle to
go to the left, I write
.


G91 X
-
1.00


So when I push cycle start, it will then
move from where it is 1.0 inch to the
left
.

They are also called “relative” coordinates
since they relate to the current location for
their reference.

*
*
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
ways
:


Rapid
Travel


Linear

Single
or
multi
-
axis straight
-
line
motion


Circular

motion within a single
plane


Circular/Linear
, also called

-
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
.

Due to computer evolution with
ever
-
higher computation rates,
m
ost
new CNC controls today
can perform true 3
-
D
motion.





*


Rapid

as
fast as
the machine can
move but with
the ability to
reduce speed
through
operator
override
control.


Trade Tip


Caution!

Depending
on the power of the
CPU, your machine
will rapid in one of
two
ways

be
sure to
discover why in the
text. Older
controllers take an
unexpected route!

*
*
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.

*
N
either
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.

*
A

B

375.87 IPM, X Axis Motion

137.81 IPM
Y Axis

*


*
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
the form of rectangular dimensions, but rather as
the radius and angle from a starting point.

*
Those points are more easily defined using polar
coordinates

a
bolt circle for example.

*
Polar
coordinates
aren’t used inside
CAM
-
generated
programs, but they are very useful for drawing the
part geometry or when doing hand program writing
of polar entities.

Trade Tip


Using polar
coordinates
often
saves a trigonometry step during
drawing or hand program writing!

If the needed significant point is
defined in radius
-
angle, rather than
X
-
Y, why do an unnecessary
calculation? Define it with
an
R
-
A
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 journey
-
level 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.