UBL and probe offsets

Hello folks!
I'm currently running 1.1.8 and was in the process of trying to get UBL working, but I kept having the nozzle passionately burying itself into the bed in the midst of the validation mesh. I figured this was due to an unfilled point in the mesh but I swapped back to bi linear until I had time to work with it more. I am now to that point and reviewing the documentation, I'm trying to make sure I get things right before I try running another mesh.

I have an i3 Rework 1.5 and due to the positioning of the endstops (and my declared bed size), my nozzle is already inset safely on the bed when it is at X 0 and Y 0 and at its max travel limits. So I already have an inset margin kind of built in mechanically to keep away from the edges of the bed.

Do I still have to set MESH_INSET? Or can I just make that zero?

Follow up question: does it just figure out what points it can and cannot reach with the probe on the fly (by looking at the offsets and max travel limits), or do I have to set that somewhere manually?

Follow up follow up question: if I don't have to set an inset, that seems like it would effectively make my mesh cover the entire bed. But since my probe can't reach all those points I know I will have to use G29 P2 to fill in those missing points. Is there something in the visual display of the grid on the LCD that indicates to me which points I need to address?

Thanks!

What sort of sensor are you using? If it's an inductive or capacitive sensor you want to keep away frrom probing the very edges of the bed - these probes tend to have trouble at those locations. Marlin 1.1.9 includes a MIN_PROBE_EDGE setting that ensures that only points a certain distance away from the bed edges are probed. If MESH_INSET is less than MIN_PROBE_EDGE, this means that some of the points of your mesh will not be probed and will need to be initialized using G29 P2, but this allows you to have a mesh that covers the whole bed while still only probing points that are safe.

Thanks for the reply!
It is a microswitch probe on a glass bed.

What you're saying makes sense regarding the mesh, but is there something in the interface or a command I can run that tells me which points specifically need to be probed?

If you're using a printer host (like Repetier Host, Pronterface or even Octoprint) you can print out the current mesh with "G29 T". Any points that were not probed will show as ".". Then the recommended procedure is to run "G29 P3" so UBL can interpolate/extrapolate the missing values. It might take more than one run of "G29 P3" to fill in all the unprobed points. Then print out a mesh validation pattern with "G26" and fine-tune the mesh with "G29 P4" as needed.

Ah crap - I should have thought of that. I use G29 T with bilinear but didn't think of using that with the UBL to see the mesh. Thanks! I'll give it another shot!