Please help me understand mesh bed levelling with the IR Probe

BDubs

So I have my DC42 IR Probe configured and working as both a probe and Z end stop. I followed the instructions in the documentation by setting the Z0 height etc with trigger height averaging 2.55mm and then probed the bed (except changed point spacing to 50mm to save time) and was able to view the height map - which was all over the place with max deviation around .75mm. It's a 500mm square piece of glass from China so I'm not surprised about that, but I'd imagine there's still some tweaking to do and a larger data set would probably help.

Here's what I don't understand:

1)If I probe the bed and the data set is stored in the CSV, is that automatically called up before I run a print file via the config file?

If yes, does it automatically transform the Z (in XZ and YZ) on the fly as it is printing
A) Only on the first layer?
B) All layers?
If a code is required to use the CSV to activate MBL while printing, is it advisable to add that to my post processor or somewhere in the config.g file?
Unrelated to above; What's the max Z offset the bottom of the IR probe board to nozzle can be to still be in the 'sweet spot' range of reliable measurement? I set it about 1.5mm above the nozzle, but with a 1.0mm nozzle in my Super Volcano, it occasionally scrapes on the print (and subsequently grates my nerves!) - so I need to bump it up.

Thanks

jay_s_uk

You use G29 S1 to activate the mesh during printing.
It is advisable to add it as part of your start gcode after you have homed the machine (homing the deactivates the heightmap compensation).
It certainly does it for the first layer bit after that I haven't paid much attention.

engikeneer

@BDubs The mesh compensation is applied to all layers when printing unless you set the mesh taper (M376):
https://duet3d.dozuki.com/Wiki/Gcode#Section_M376_Set_bed_compensation_taper

I'd recommend setting the taper command so that you don't put excessive wear in your Z-axis when doing tall prints. Read the advice in the link above about how big to set it.

You can also use the A & S parameters in your G31 in the config to probe each piont multiple times to get an average to help improve your accuracy if you think that's an issue.

RE the height of the IR sensor, the DC42 one should be 1-2mm above the nozzle (from DC42's website: https://miscsolutions.wordpress.com/mini-height-sensor-board/) so you could go a bit higher. Curious as to why your board is hitting the print though. I thought the nozzle should be travelling around at the current printed layer height, so there shoudln't be any plastic above it? Unless you've got some tilt going on in your carriage?

Phaedrux

https://duet3d.dozuki.com/Wiki/Using_mesh_bed_compensation

BDubs

@engikeneer - thank you that's very helpful. I did read the manual about taper - but didn't quite get it - but do now thanks!

Yes - I'd prefer to only have the bed compensation applied on the first few layers and then let whatever slop is left get melted into subsequent layers. I'm certainly not printing anything for aerospace - so aside from being dimensionally in the ballpark and staying stuck down - I'm not worried.

@Phaedrux - thanks - I read it but didn't understand the parts I mentioned in my first post.

I'm going to go back and re-read now...