Rc bug fix - Fix for issue #2931

[AlwaysTinkering]

This change should fix issue #2931

If Z_SAFE_HOMING is set, then Z_RAISE_BEFORE_HOMING doesn't happen until after X and Y have been homed. This is actually unsafe on many printers. Disabling Z_SAFE_HOMING is also undesirable, because of the many benefits when using a head mounted Z probe, such as not probing when the probe isn't over the bed.

This change allows the Z raise before X and Y are homed, even if Z_SAFE_HOMING is set. The risk of raising Z before homing, is that if Z is already near max, and the Z_MAX_POS is too high, then it could be a problem. This is noted on the comment for Z_RAISE_BEFORE_HOMING. The risk of not raising Z before homing X and Y, is that the Z probe and head may be crashed through bed leveling knobs, screws, clips, etc. on the way to X and Y home if the head was already at 0, or the head was out of bounds.

I believe that raising Z before homing X & Y (when that is set) is better than homing X and Y first, then raising Z anyway, which is how it works with Z_SAFE_HOMING set in the current RcBugFix branch.

[thinkyhead]

I believe it's possible that the small change you propose could cause unexpected results. But maybe you can confirm that with your change these do what they are supposed to do:

• If you home X and Y, then home Z after, it should safe-home — lifting Z only once!
• If you try to home Z-only when X and Y are unknown you should get an error.

There's been some effort to simplify these options a bit. The purpose of Z_RAISE_BEFORE_HOMING is only to raise Z before deploying a Z probe. Thus it was only ever considered important to do this just before homing Z. For Z_SAFE_HOMING that means the procedure would generally be:

• Home X and Y
• Move to XY: Z_SAFE_HOMING_X_POINT, Z_SAFE_HOMING_Y_POINT
• Raise Z by Z_RAISE_BEFORE_HOMING
• Use the Z Probe to do homing

Since Z is going to get raised and homed as part of this procedure anyway, I agree it makes the best sense to do the raise first. I just want to make sure that removing the Z_SAFE_HOMING condition at the start of G28 doesn't cause a double-raise or anything weird.

Note that currently this (Z_SAFE_HOMING) only applies if Automatic Bed Leveling is in use. But it has been suggested that we allow Z_SAFE_HOMING even if automatic bed leveling isn't in use, because some printers might still want use a Z probe to home Z even though they're not using automatic bed leveling.

Anyway, the number of options has begun to get confusing, so I proposed that we simplify them and just specify the clearance that a probe requires, then let Marlin figure how much to raise Z in each case…

https://github.com/MarlinFirmware/MarlinDev/pull/366

[AlwaysTinkering]

I would definitely prefer a more comprehensive approach to solving the Z raise issues. This is just what I was using to get around the issue. If this can be addressed as part of a broader solution, simplifying the number of Z related options, that is a much better approach.

[thinkyhead]

Just marking this "Don't Merge" while we get some more testing and feedback on this.