Changes to probe height

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Changes to probe height

Post by dfajardo » Mon Jun 16, 2014 2:25 am

Autoleveller is working well on my 2 foot by 4 foot home made CNC Mill. It is doing a great job compensating for
the current table, which has warped a bit over time <grin> (I'm using it to help build parts for the next version of the machine). The 'probe-persistance' feature is going to be a godsend - thanks for adding it!

I have two changes to suggest in controlling the probe height during moves. If they sound interesting, I'll email the patches to you - I have them working on a private copy of the repository.

(1) At the begining of the probe sequence, move the probe to 'safe-z-height' before moving to x0,y0. Then move the probe to the Probe-Clearance-Height, and begin probing. (Currently there is no separate Z move before moving to x0y0)
The argument for this is in the case of restarting the gcode probe sequence from somewhere other than x0y0, you really want the probe to be clear of the work area. I ran afoul of this after a probe sequence failed part way down the piece, and I restarted after making appropriate corrections...

(2) Similar to (1)... move the probe to safe-z-height before moving to the next probe point. Then drop the probe to the probe-clearance-height and begin probing. (Currently the height between moves is controlled by the probe-clearance variable, which is ALSO the height where the probing starts (at its SLOW feedrate)).
The result would allow the probe to be raised above any fixtures holding the workpiece during moves, without paying for it with long probe times.

It seems to me that this use of 'safe-z-height' is consistent with its current meaning.
This would change the meaning of the "probe-clearance-height" to be the height where probing starts.
I believe the resulting changes would be cleaner and easier to understand (I found the original usage to be a bit muddled, until I looked at the generated code).

Doug Fajardo

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Re: Changes to probe height

Post by daedelus » Thu Jun 19, 2014 10:21 am

Thanks Doug, I just got a chance to reply to this.

1. If you look at the prerequisites in the GCode, it says something like "touch-off to zero, when the the tool is slightly less than 10mm / 3/8" off your surface". This ensures that the move to X0Y0Z0 will be away from the work area when the move is made.

However, more than a few people have mentioned this to me, so I conclude that this is both poorly explained by me and not a good, logical approach in the first place.

I have changed it so that you now zero the tool on or near the board, then it rapids to a safe height before moving to the start. This change is introduced in .8.2 to be released today.

2. I can see what you mean but the idea behind doing it this way i.e. keeping to tool low, is to reduce probe travel time. If someone (like me) had particularly slow lead screws then a rapid G0 move might not be much faster than a slow probe. Therefore its best to keep it as low as possible to reduce the overall time taken.

I guess there are pros and cons of both approaches and it is good to see you have modified the code to suit you. Thats what open-source software is about. Software to probe and adjust a GCode file for PCB's or any probe-able surface. A short guide to setting up the probe.


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