Machine Coordinates Systems

1
Machine Coordinates Systems

• Lab 2
• Absolute vs Incremental

2
Coordinate Worksheets

• Provide to each student
• 3 Graph Sheets
• 1 Coordinate Worksheet

3
X 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.

4
Absolute 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

5
Absolute 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
6
Answers 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
7
Incremental 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.

8
Incremental Co-ordinate Graph
9
Incremental 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
10
Answers 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
11
Incremental 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

12
Answers

• 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

13
Playing 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

14
Playing 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

15
Playing 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

16
Answer
17
Including 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

18
Including the Z-axis
19
Efficiency 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