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How are work offsets calculated

jcdammeyer

jcdammeyer

John
Premium Member
I'm having trouble getting my head around how the various CNC systems deal with the work offsets.
When I home the mill the knee goes all the way down to the home switch. At this point there is 12.704" between the table the bottom of the quill. The machine Z coordinate value is 0.000"

If I place a 1-2-3 block onto the table and probe it with my T99 touch probe which has a length in the tool table of 4.829" the G54.Z value becomes -4.8571" and the machine Z position is also -4.8571"

Therefore at the point the probe trips the knee has moved the table from 0.000" up to position -4.8571 and the G54.Z space reads 0.000" when the probe tip touches the 1-2-3 block.

Now if I insert Tool #4 which has a length of 2.82042" in the tool table the G54.Z value changes to -9.6861". If I move the knee up until this new tool just scratches the top of the 1-2-3 block the Z reads 0.000 and the machine Z is now -6.8883".

I guess that makes sense. TotalDistance - 123Block - ToolLength should equal the machine position. ie. 12.704-3.0-2.82042 = 6.8836 (I have some backlash I think or tool length is 0.005" off) but close enough.

How does the system calculate the new G54.Z value?
If I change to a 4" tool how does it know what value to put into the G54.Z location?
 
Are you adding the Tool Length Offset (G43 H#) to your tool change? A tool change line should be something like T5 G43 H5 M6 where the tool is #5, and it is going to use tool length offset of tool #5 (G43 H5)
 
Mike R said:
Are you adding the Tool Length Offset (G43 H#) to your tool change? A tool change line should be something like T5 G43 H5 M6 where the tool is #5, and it is going to use tool length offset of tool #5 (G43 H5)
Click to expand...
I don't have any trouble with my tool change when I have a value in the tool table. The M06 replacement code does this. What it doesn't do at the moment is the correct math when the tool table value is zero and it goes and measures the tool using my tool setter.

That's what prompted all this. And the end result is I don't know how a new G54.Z is determined. Or for example say I probed while G56 was active. Then changed a tool.

How does it know what to change the G56.Z to after a number of tool changes where each tool is a different length. The tip of the tool is still supposed be able to essentially scratch the surface of what has been probed as Z==0.000

So I understand how each work space has it's own Z and that's dependant on when the Zero was set for that work.

For example. Say I use the simple AXIS interface in LinuxCNC and with a thin piece of paper between Tool #2 with a Table Tool Length of 3.5" set the top part held in the vise to 0.000. (I'm in G54).

Now change to Tool #3 that has a 1.5" length in the tool table. There's a 2" difference. The G54.Z offset should change by 2". But how does it know? Put in Tool 0 with G49 and then put in Tool #3 with G43 and it's remembered where the top of the work was in the vice.
 
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