G92 problem

NCPlot questions or comments

G92 problem

Postby SD » Tue Oct 07, 2008 7:24 pm

Scott

I think there may be an issue with V2.05 and the way G92 is handled for some machines.
For my Wire EDM-s G92 is used to tell the control the location of the program work 0, much like on other machines.
The difference however is that the definition of the G92 DOES NOT SHIFT!!! the work coordinate with respect to the machine.
Instead it tells the machine that the current location is the location within the work coord. system. It does not shift anything other than define the location of the work 0 with respect to the current location.
At the end of the program you do not need to ( in fact you really do not want to ) cancel or re-define G92, or else you'll loose the work position.
I know this isn't too clear, so let me explain it this way.
G92 is independent of ANY coordinate system AND!!! NOT ACCUMULATIVE.
meaning you can successively enter a whole bunch of G92 blocks, only the last block is recognized.
More specifically G92 does not move the machine's coord system, ite defines the work 0 location with respect to the current position.
As an example, let's say your're somewhere in the middle of the table and call the following program:

G92 X1. Y1.
G92 X7. Y.05
G92 X2 .Y6.
G92 X-1. Y-1.
M02

At the end of this program your coordinate display should read:
X-1. Y-1.

With the latest (V2.05) each time the plot is refreshed, the geometry shifts by the accumulative amount of each G92 instance.
V2.03 was working fine.

Perhaps you can make an addition to the machine configuration where the new method can be unchecked.
SD
 
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Postby scottmartinez » Wed Oct 08, 2008 12:49 pm

Hi,

I'm going to have to check into this. The way you describe it I think is different than the way a mill would handle G92.

Thanks,
Scott
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scottmartinez
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Postby SD » Wed Oct 08, 2008 6:37 pm

Scott, thanks for looking into it.

Actually, now thinking about it and trying a few things, I came to a conclusion that as far as the actual machine's operation it doesn't work all that different!!!
The issue is that while on a typical VMC you have to re-define the G92 either by canceling it at the end or restarting it from a known position in the program, on the EDM all you have to do is to move back to the starting position.

IOW

G92 X1. Y1.
G01 X1. Y2.
G01 X2. Y2.
G01 X1. Y3.
M02

Will shift the plot with each successive refresh,

While:
G92 X1. Y1.
G01 X1. Y2.
G01 X2. Y2.
G01 X1. Y3.
G00 X1. Y1.
M02

Will plot fine.
The difference is that NcPlot knows that the last block has moved the tool back to the original starting point, so the re-run will not shift the display.

The problem is this: On these machines the following codes will accomplish the same thing
G00 X1. Y1.
or
G28
or
G262 P1 X0 Y0

Each one of those blocks will put the tool back to the starting point (when used in the above code example)
G28 is a "start point return" as opposed to machine reference return
G262 P1 X0 Y0 means P1 is a user setpoint ( can be P1 - P12 ), X0 Y0 means to return to P1 X0 Y0 point.
You can also use G54 from within the program, which then overrides all G92 settings ( unlike on conventional controls where G92 is boss)

For 4 axis work there are even more, uncommon ways to return to the start point.

So my suggestion would be is if there is a way to uncheck the accumulative mode of G92 so each refresh would start from the same point.
SD
 
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