BackTap Probe - Ender 3

Back Tap Probe
Accurate microswitch based nozzle Tap Probe for the Ender 3.

Inspired by an idea from @dalias from the Klipper Community Discord

Uses the free coupling ability of an anti wobble coupler on the Z rod to detect when the lead nut screw travels downwards, while the X gantry stays in a stationary position and is supported by the nozzle touching the bed.

No additional downward pressure is put onto the nozzle other than the weight of the gantry.

Advantages

~1 micro step accuracy
-Allows changing nozzles and tool heads without re-adjusting the Z offset
-Direct under the nozzle probing. No more probing offsets.
-No extra weight on the toolhead (frees up weight by removing existing probes)
-No probe deploying motions, docking and undocking procedures.
-No variable Z offsets adjustments
-Cheap, cost of a microswitch salvaged from an old mouse
-Gets you to build and install and anti-wobble coupler witch solves Z rod motion issues :)

50 samples accuracy test (Kailh Black Microswitch):

Disadvantages 

-Can't probe with a hot nozzle, and the nozzle has to be kept clean, otherwise it affects the Z offset.
-Requires a day of break in period for the mechanical components to settle in their place.
-Accuracy might suffer if your bed springs are too soft, or if you carriage/tool head is to wobbly.
-Requires an initial calibration/check for two additional offsets along the X axis.
-The weight of the gantry might be too much for a hotend supported only by the heatbreak, and I would not recommend it for such setup. I've been using mine with a Phaetus Dragon which has a good support structure.

Updates:

-02 May 2023 - Added Lower Support model for POM Nuts (18mm between holes, 12.7mm tube outer diameter)

Requirements:

Hardware:

-D2F type microswitch (salvage a working one from an old mouse)

Screws

-M3 12mm x1
-M3 15mm x2
-M2 8mm x2 (optional, depending on switch type)

Anti Wobble Coupler or Oldham coupler (not tested)

Software:

Klipper Firmware 

Print settings:

PLA (or better, for stiffness) 

-0.2mm layer height
-4 perimeters
-5 top, 4 bottom layers
-40% cubic infill 
 

Assembly:

• Lower part

-Add the 15mm M3 adjustment screw to the lower part and screw the Platform cylinder onto it. 
-Screw the lower part into the Z Brass Nut that rides on the Z rod.

• Upper Part

-Break though and clean the sacrificial bridges for the M2 screws.
-Insert the Z stop wires into two small upper holes and pull them though the bottom.
-Solder the wires to the pins illustrated below. Orientation doesn't matter. This is the “Normally Open” configuration of the switch.

-Measure the continuity of the switch. The circuit should be closed when the switch is pushed, and open otherwise.
-Press the switch into the mount. Measure the continuity of the switch again.
-Insert the M2 screws that provide extra hold onto the switch.
-Secure the cable using a zip tie attached to the upper rectangular opening.
-Mount the upper part onto the Z gantry and insert a M3 12mm screw into one the same mounting hole reaching the anti-wobble top part. 
-Tighten the platform screw until the switch is constantly depressed by the weight of the X gantry. Try to aim so the distance is just at the point of contact. Adjust it so it releases when lifting the nozzle gently. This can be further tuned later.

Setup:

-The probe requires a break-in period for the mechanical components to settle. Any adjustments made to the switch mount require some settling time.
-Adjust your Z offset. Verify by repeatedly homing the Z axis and doing a paper test. This will probably change slightly during the break-in period. 
- Run the Klipper probe PROBE_ACCURACY command a couple of times during the first day to help with the break-in period. If accuracy suffers even after the break-in period and further adjustments, try a another switch with a different actuating force and distance. (Note: any adjustments to the switch mount require a break-in period)
 

Klipper Setup:

https://github.com/lhndo/BackTap-Probe

Enjoy!