Purpose of expected distance in M585 probe?

Otso

What is the purpose of the expected XYZ distance in the M585 command?

I have an Ooznest Workbee CNC with a touch probe. For the Z probe, the macro it runs sets the expected distance to -15mm. The documentation says:

It is mandatory to measure this distance once before M585 can be used reliably.

So what happens when I probe from, say, 30mm instead of 15mm? I tested with different values, and can't notice any difference.

What is the "correction factor" that is mentioned in the M585 documentation?

Also, if I understand correctly, the CNC should use Z15, and not Z-15 as it does now? From the documentation:

f the trigger position of your endstop is at X=210 and you want to drive your tool from X=190 into the endstop switch, you need to specify -20 as your X parameter because you expect to travel 20mm towards the endstop switch and need to correct this factor. If you drive the tool backwards (e.g. from X=210 to X=190), the correction factor should be 20.

On the CNC we're driving the probe towards Z=0, say from Z30 to Z15, so shouldn't that then be M585 Z15 and not M585 Z-15.

dc42

@otso from examining the code I can tell you that after probing, the tool offset along the specified axis is set to (startingPosition - endingPosition) + expectedDistance. So the expectedDistance parameter affects the tool offset that is set.

I am not CNC user and I don't know the purpose of this command. I believe it was added some time ago for an OEM customer.

Otso

@dc42 Thanks for the reply!

Edit: Sorry, I reread, and think I understand now (and misunderstood first). The tool offset will be effected, but that doesn't then really matter in my case.

It would be great if this was explained in the manual.

This is what the Workbee does for a Z probe:

M563 P49 S"XYZ-Probe" ; Define tool 49
T49                                     ; Select tool 49
M585 Z-15 F500 P0 S1    ; Probe, expected distance -15, probe# 0 (P0), towards minima (S1) 
T-1                                      ; Deselect tool
G10 L20 Z5			    ; Set Z offset for tool at 5mm

(The comment indenting got messed up when pasting here even though I replaced tabs with spaces.)

Here the 5mm in the G10 command is the thickness of the conductive touch plate. So the -15mm has no purpose as the tool that it is set for is removed after the probe.

Otso

This post is deleted!

Otso

@dc42 I now understand the purpose of this. It can be useful for routers when you have a Z endstop in the high position and a fixed Z probe. You drive the router to the Z endstop and measure the distance between the router tool holder and the Z probe. This is the M585 Z "expected distance". You insert your endmill and run the M585 command. The tool offset will now be the distance it is sticking out of the router tool holder.