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This is about practicality. I'm hearing that people are using their BL Touch 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 BL Touch (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).

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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:

  • simple and cheap mechanical switch
  • no firmware changes necessary
  • no or few soldering
  • already present on most bought printers

Cons for a limit lever switch are:

  • needs fine adjustments counter part to work optimally
  • something can get in between the lever and counter part
  • it doesn't look cool

Pros for a more sophisticated sensor:

  • it looks cool
  • everybody is using it so it must be good

Cons for a more sophisticated sensor:

  • expensive and complicated sensor
  • requires firmware changes (e.g. sensor offset value)
  • requires soldering, or connecting more cables
  • inductive and capacitive sensor work usually better at a higher voltage
  • higher chance of malfunctioning (more parts and electronics)
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  • $\begingroup$ "everybody is using it so it must be good" is not a valid argument lol. Nice try :D $\endgroup$ – user77232 Oct 16 at 23:28
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    $\begingroup$ @user77232 For many people that is actually true, but I was being sarcastic. $\endgroup$ – 0scar Oct 17 at 4:05
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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.

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  • $\begingroup$ Isn't the touch sensor's trigger point under the stow limit, that means there is little more space left. This is also valid for the inductive or capacitive sensors, they do not touch the bed to trigger, so there is enough space for overshooting, if there is a substantial overshoot that is. $\endgroup$ – 0scar Oct 15 at 21:28
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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?

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