Content Preview
TB033 Rev1 Isolating Position Problems to Control or Mechanics
Overview
This document describes the techniques to be used when it is necessary to determine whether a position
problem lies in the CNC control or whether the problem is caused by machine mechanics
Tests
The first test is to verify extra or missing encoder counts looking at the absolute position in the PID display
The machine should be powered up and properly homed
This example will use the X axis and assumes that the machine homes in the X+ direction It uses the "M92
/X" command to send the table to the "X+" home position If homing in the "X " direction the "M91 /X"
command would be used
Steps
At the main menu press ALT+P
then press F3 MDI
you should see a display similar to the one
below
In the MDI Bock type " M92 /X
" then press CYCLE START
The X axis will move to the plus home
switch
NOTE This is a slow move and you may want to jog the axis closer to the home position before
pressing CYCLE START
When the axis stops moving check the "Abs Pos" for the axis under test it should read zero
If the "Abs Pos" is zero then the mispositioning most likely is the result of fixture or part movemet
while being cut
If the "Abs Pos" is NOT
zero then proceed to the next test
The next test for isolating a position problem is basically to remove the control the motor from the ballscrew
set up a reference position perform positioning tests and then return to the reference point and check for
errors In the steps outlined below we will assume that the X axis has position errors
Steps
Power up the control and perform the homing procedures From the main screen in the CNC7
software press the following keys in order to get into the following parameter screen F1 Setup
F3
Config
Enter the password 137
press Enter
then F2 Machine
Write down the current Lash Comp inches values Change these values to 0 0000 and press F10
Save
Position the X axis in the center of travel and set an X part zero
Engage the E Stop which will remove power from the motors
Remove the X axis motor You may leave the pulley on the shaft Attach the motor cable if you have
removed it Release the E stop
Press F3 MDI
and then at the Block? type G90X0
press ENTER
You will be prompted to press
Cycle Start
to move
Mark the motor shaft and case as illustrated in Figure 1
Create a test program that cycles the X axis through its travel envelope Use F6 Edit
F9 New
to
create the test program
G90G1F150
X2
X 2
M102
 Start the program and let it run for an hour
Press the CANCEL
on the Jog keypad
Press F3 MDI
and Block? X0
press ENTER
If the marks line up the control is positioning properly and it is a mechanical issue
If the marks do not line up refer to technical bulletin TB149 Troubleshooting Motor/Encoder
Problems for further troubleshooting
*Warning Never hit or pound on the motor shaft
Figure 1 The Proper Way to Mark a Motor and Shaft Top View
Â
Â
Document History
Rev1 Created on 1999 09 07