Ender 3 NG v1.2 bolt-on 80 mm spool holder with integrated load cell

This is a bolt-on spool holder with an integrated load cell for the Ender 3 NG v1.2!  It replaces a couple of the enclosure pads on either side of the printer and offers a spool holder with the same look as the rest of the printer.  Don't have an enclosure?  Don't have an Ender 3 NG v1.2?  No problem!  Just stick it anywhere you'd like on an aluminum extrusion for any build you have.

The load cell is read by a HX711 ADC module, and together, the spool acts as a scale for your filament. Most filament manufacturers offer the spool's weight in their specs, and Klipper (and its forks) can accept this weight as a tare.  Then, using Klipper G-Code commands, you can retrieve the amount of filament is left on the spool.  This can be integrated into your own “start” G-code to halt prints that would run out of filament partway, or issue a warning if the spool is running low, as examples.

Here's a screenshot of the spool holder reading the spool weight!  With the LOAD_CELL_READ G-Code call below, you can observe that the spool weighs 424.6 grams.  Polymaker advertises that their 1 kg spools weigh 140 g, so this means that 284.6 g of filament is left on this spool, which is about 25%!

See the Load Cells documentation for more information on how to integrate the spool holder's load cell and ADC into your configuration:

• Klipper: https://www.klipper3d.org/Load_Cell.html

• Kalico: https://docs.kalico.gg/Load_Cell.html

This spool holder is designed around the same parts in the Ender 3 NG v1.2 BOM.  This means that you probably have all the hardware you need as spare parts after building your printer.

• 2 x F695 2RS ball bearings

• 2 x M5 washers

• 2 x M5 heat set inserts

• 2 x M5x20 SHCS screws (from the BOM; M5x8 to M5x40 screws supported)

• 4 x M4x10 SHCS screws (from the BOM; M4x8 to M4x16 screws supported)

In addition, this model requires these parts:

• ShangHJ 5 kg load cell with HX711 ADC Module: https://www.amazon.com/dp/B09VYSHW16

• 4-pin JST XH connector: https://www.amazon.com/dp/B0B2RB524Y

Fits spools with a maximum of 80 mm width, with a minimum 54 mm spool hole.

To build this model, first decide if you want to mount your spool on the left or the right side of your printer (while looking at the bed).  There is a 4-pin JST XH connector hole in the arm of this model, and ideally, it will exit towards the back for easy access to your electronics panel.  There are left and right side arms in this project.  Print the arm with the base face down, like so:

Also print the spool so that it's standing up.  The spool is hallow, with 7 mm solid ends for the heat set inserts.  There are special shapes inside the spool to convince your slicer to make the expected bridges as the 7 mm end is printed on the tall side of the print.  They are located on either side of the model so that the spool can be printed on either side.

With everything printed, put two M5 heat set inserts into either side of the spool.  Attach a F695 2RS ball bearing (with the stop on the inside) and a M5 washer to one side of the spool, as pictured below:

Then, route the four load cell wires through the hole in the back of the spool arm.  There is a small tunnel to route them through the side, where a 4-pin JST XH connector will live:

The tolerances are fairly tight, but the wires are small.  The wires are attached to the side of the load cell, so they can gracefully be pulled around the side, into the hole, and out of the connector.  Here is a view inside the arm with the load cell in-place:

Then, loosely attach the load cell into the arm using the M4 screws.  Ensure that the load cell wires are not packed at the end of load cell, interrupting the mounting position of the load cell.  Loosely attach the M4 screws to the spool holder, using the shim to separate the cell from the spool holder.  Visually align the spool holder and the arm to line up, and tighten the M4 screws.  The construction should look like the picture below, with the shim highlighted, like so:

Then, insert the other F695 2RS ball bearing into the far side of the spool holder, like so:

And finally, insert the spool (with one bearing and washer attached) into the open side of the spool holder, squeeze a washer between the other bearing and the spool, and add the other M5 screw.  With everything assembled, it should look like this:

If you would prefer to build a spool holder without a roller for some friction on the spool, there is also a spool holder in this model that is designed for the spool to simply sit on top of:

From here, cut and solder the four load cell wires to a 4-pin JST XH connector.  Technically, the pinout does not matter (as long as the correct wires eventually are connected to the HX711 ADC module).  However, the wires are connected to the module in this order: red, black, white, and green.  For consistency, I recommend a connector position like below, with the red, black, white, and green wires attached in this order from top to bottom:

Here is a wiring diagram that describes how to connect a ShangHJ load cell to a HX711 ADC module:

It's a good idea to add a drop of super glue at either side of the back of the connector hole before pushing the JST XT connector in place.  This will prevent the connector from being pulled out when disconnecting a cable.  In addition, one face of this hole will be printed as a bridge, so it may be necessary to clean up this face with a hobby knife so that the connector will fit.

With the spool built and the connector situated, you're ready to mount it to your printer!  If you are using an Ender 3 NG v1.2 with an enclosure, remove the two screws (with pads) on the back edge of the printer on the side of your choosing, then attach your spool holder to this location using the same M3 screws.  When attached, it will look like this:

If you would like to, you can also add a filament guide that's also a bolt-on replacement for the mounting pad above the spool holder.  It is threaded for a PC4-M6 fitting for a secure fit and easy maintenance.  There are left side and right side filament guides in this model for your preferred spool side.  If you want a full PC4-M6 tube setup, you can also use these lid corners with PC4-M6 fittings.

The cable to the spool holder can be fished through one of the holes of the electronics vents.  If you'd like a cleaner look, electronic vents are available on this model that include a 4-pin JST XH connector, just like on the spool.  You can build a little cable to connect these connectors together, then run a cable from the vent's connector to your HX711 module.

If you want a tidy card to mount your HX711 module to a DIN rail, check out this universal DIN card model!  Here's what the HX711 card looks like alongside the other cards:

The HX711 module is configured to operate at 10 Hz as-is.  If you'd like to read samples at 80 Hz for a completely optional performance boost, you can cut the ground lead to pin 15 with a razor blade, and bridge pin 15 to pin 16, like the picture below.  Make sure to use a continuity checker to ensure that the cut really did break the trace!

Just so you're not flying blind, here's a screenshot from the HX711 datasheet.  When the RATE pin (pin 15) is pulled high to 1 (connected to power, not ground), the HX711 operates at 80 Hz.  The original configuration is to pull it low to 0 (connected to ground, not power), which makes it operate at 10 Hz.

If your bodge worked correctly, the “measured samples per second” reading from a LOAD_CELL_DIAGNOSTIC G-Code call will read samples higher than 80 samples per second, like so:

From here, connect and configure the HX711 ADC module with Klipper (or one of its forks) to read your module, and you're all set to read spool weights to determine remaining filament with a tidy little low-friction spool holder!  Here's a link to the HX711 configuration documentation:

• Kalico: https://docs.kalico.gg/Config_Reference.html#hx711

• Klipper: https://www.klipper3d.org/Config_Reference.html#hx711