Title: Machine Coordinates Systems
1Machine Coordinates Systems
- Lab 2
- Absolute vs Incremental
2Coordinate Worksheets
- Provide to each student
- 3 Graph Sheets
- 1 Coordinate Worksheet
3X and Y Co-ordinate Calculation - Absolute and
Incremental Co-ordinates
- Absolute Co-ordinates
- The G-code G90 is used to select this type of
programming. Before programming commences the
points on the path to be machined are defined
relative to the work piece datum - G-code text used for machining that defines
machine movements - Datum a reference point from which ALL other
points are referenced - Examples of these co-ordinates are shown in the
following graph - The Z axis is the vertical axis and the datum
used is normally the surface of the work, for the
Boxford machines to recognize where the material
first needs to be cut (machine slows feed rate to
cut material).Z is positive when moving away
from the surface and negative when moving towards
or into the surface.
4Absolute Co-ordinate System
- Identify the co-ordinates of the origin in 3
dimensions - Assume the Datum is located at the origin
- Create a table of X, Y, and Z co-ordinates of
each letter labeled point assume z0 for all
lettered positions
5Absolute Co-ordinate Worksheet
- The Datum for X and Y are the origin (0,0)
- The format for expressing X, Y, Z is
- (x, y, z) so Point A (5, 4, 0)
- when the datum is located at the origin
- Instructor Go Back One Page for Students
Point Datum A B C D E F G H
X 0 5
Y 0 4
Z 0 0
6Answers to Absolute Co-ordinates
- The Datum for X, Y, and Z is the origin (0, 0, 0)
- So Point B (10, 5, 0) relative to the Datum,
- Point B is NOT relative to Point A
Point Datum A B C D E F G H
X 0 5 10 -4 -9 -7 -4 7 5
Y 0 4 5 5 7 -3 -6 -5 -2
Z 0 0 0 0 0 0 0 0 0
7Incremental Co-ordinates
- Incremental Co-ordinatesThe G-code G91 is used
to select this type of programming.The points
on the path to be machined are defined relative
to the previous position. Identify the sequence
of point co-ordinates needed to describe the
points in the following table moving
alphabetically, beginning at the Datum (0, 0, 0).
The Z axis is again the vertical axis, and the
points are defined relative to the previous
position positive when moving away from the
surface and negative when moving towards or into
the surface. For the problem, assume all Z
co-ordinates equal zero.
8Incremental Co-ordinate Graph
9Incremental Co-ordinate Worksheet
- The Datum for X, Y, and Z is the origin (0,0,0)
- so Point A (2, 3, 0)
- in an incremental move from the datum
- Instructor Go Back One Page for Students
Point Datum A B C D E F G H
X 0 2
Y 0 3
Z 0 0
10Answers to Incremental Co-ordinates
The sequence specified moving alphabetically,
therefore the following table represents
incremental movements.
Point Datum A B C D E F G H
X 0 2 3 -10 -2 0 3 9 -3
Y 0 3 -1 1 2 -9 2 0 -3
Notice that the co-ordinate numbers associated
with each lettered position would be completely
different had the sequence been Datum, C, A, D,
B, F, H, E, G So if one lettered position is in
error, ALL subsequent positions will be in error
11Incremental Co-ordinate Offset
- If the Datum is moved to (0, 0, 1), and A is the
first position to move to from the Datum, what
happens to the incremental x and y values for
position A? - If the Datum is moved from (0, 0, 0) to (0, 1,
0), what happens to the absolute x and y values
for the actual desired position A? - If the Datum is moved to (1, 0, 0) from (0, 0,
0), what happens to the absolute positions of the
lettered positions of the previous table? - Instructor Stop Here for Students
12Answers
- Only the z-axis of the Datum changed, therefore
neither of the x or y values change. - X values are unchanged, however, subtract 1 from
all y values. - The x, y, and z values are unaffected because the
absolute values do not change. - As you can see, the concepts are simple, but one
must be very careful when changing between
absolute and incremental co-ordinate systems
13Playing with G-codes
- G-codes are used to tell the CNC machines how to
move from point to point - Each line of G-code is numbered
- N001
- N002
-
- The code following each line number is G-code
- N001 G20 G90 X0. Y0. Z0.
- Translation move direct to Absolute coordinates
(0,0,0) - G20 (direct to) and G90 (absolute coordinates)
- A period is required within all G-code numbers
14Playing with G-codes
- IF
- G90 says all following commands are in Absolute
Coordinates AND - G91 says all following commands are in
Incremental Coordinates - THEN
- On a piece of graph paper mark a line between
coordinates that show the toolpath that the
following code describes - use a heavy dot to mark each coordinate
- assume all transitions are in straight lines
between coordinates
15Playing with G-codes
- N001 G20 G90 X0. Y0. Z0.
- N002 G20 G90 X-1. Y-1. Z0.
- N003 G20 G91 X0. Y0. Z0.
- N004 G20 G91 X2. Y2. Z0.
- N005 G20 G91 X0. Y-5. Z0.
- N006 G20 G91 X-5. Y0. Z0.
- N007 G20 G90 X-4. Y-4. Z0.
- N008 G20 G91 X4. Y4. Z0.
- N009 G20 G90 X0. Y0. Z0.
- Instructor Stop Here for Students to make graph
16Answer
17Including the Z-axis
- Adding the Z-axis complicates how we mentally
move things in our minds - However, if you havent noticed by now,
incremental movements are very much related to
addition and subtraction - The following table provides a simple method of
keeping track of coordinates when switching back
and forth between coordinate systems notice the
simple addition and subtraction - Remember The table of the CNC machine is
always in absolute coordinates
18Including the Z-axis
19Efficiency Means Money
- In the G-code example presented, certain G-code
statements had no effect upon the toolpath
movement - Basically, wasted statements
- Where time is money what other manufacturing
conditions can you imagine would cost money
without providing significant useful result? - In corporate America it is estimated that at
least 30 of all revenue is spent in the support
of inefficient processes - For example parts being machined to tolerances
tighter than necessary - Think about this as you proceed through this lab
series