This is about practicality. I'm hearing that people are using their BLTouch not to adjust the Z offset, but as the limit switch for the machine! Why is this so? What are the pros and cons of using a BLTouch (or any touch sensor for that matter) in lieu of a physical limit switch?
(NB: I'm looking for objective reasons, where no other solution will do as opposed to personal preference reasons where people just like to use it).
Touch sensors (or inductive or capacitive sensors) are generally used to probe the bed to determine the bed shape. For metallic beds that are not perfectly straight this works excellent. But, if your bed is straight and level (e.g. when you are using a straight slate of glass), you do not need to probe the surface as it is level. Instead you can use the probe to determine the Z level.
Pros for a limit lever switch are:
Cons for a limit lever switch are:
Pros for a more sophisticated sensor:
Cons for a more sophisticated sensor:
There's cost.
Or it might be fragility -- there is a little overshoot, i.e. the time between when the Z-stop sensor(whatever type) indicates zero is reached and when the stepper motor actually shuts down. The standard lever-switch has plenty of flexibility and "give" but the BL Touch may not, and may be damaged if the gantry tries to move a couple hundred microns past "true" zero before stopping. I would be cautious, depending on your budget, about trying it out.
I would use both if I had a choice, but my stock "Melzi" board doesn't have enough inputs as near as I can tell. Most of the (scarce) instructions show to disconnect the Z switch and connect the BL Touch to the digital inputs in place of the switch.
I don't see any other connectors to wire up the EZ Touch to. Maybe the ICSP programmer, but I haven't messed around to see if that would be usable (nothing in the stock CR10 box is connected to it anyway).
So I think it's more of a practical issue that you use one in place of instead of adding.
However, less pedantically, why is a BL Touch better than a limit switch?
Two words: Auto Leveling
If you get everything setup right, auto-leveling should be great. I say "Should be" as I still don't quite have the board setup right. It keeps trying to push my nozzle into the PEI sheet, so I really DO wish I had both limit switches and the BL Touch.
Leveling on a hobbyist level 3D printer isn't that hard, but it's time consuming and requires some care. Having to do it every time becomes a colossal pain. So the idea that you could clean off the build plate, hit a button, and have the printer figure out what level was and just start printing is great. Not having prints peel off the plate because you had the gap set wrong, or didn't get the back corner leveled off, or the PEI sheet getting damaged because the nozzle ran into the sheet ... priceless.
But hey, if you're using a 3D printer where you'd even think about a BL Touch, you're on the DIY/Maker side, not the "stick in an SD card and hit print" side, so you should be used to figuring out how to wire one in. It's part of the adventure, right?
One other way a bed sensor may be preferable over a common limit switch: it automatically adjusts when you change out the build surface.
Most glass build surfaces are anywhere from three to five millimeters thick, while the surface they replace (texture coated or uncoated fiberglass/resin, magnetic sheet, etc.) are around one to 1.5 mm thick -- that's a enough change in build surface height to be near the limits of the bed adjustment on my Ender 3, for instance.
There are ways to work around this, for instance with spacers that clip onto the bottom of the X gantry to trigger the Z-stop at a higher position -- but if you forget to install the spacer on a build surface change, you may end up scratching the coating on your glass bed (probably a bad thing) by dragging the nozzle across it, as well as potentially damaging the nozzle (cheap, but takes time to replace once you figure out why your prints are suddenly failing). If you're using a bed sensor in place of a limit switch, this oversight can't happen, because the Z homing will automatically detect where to stop regardless what you've mounted on the bed.
