ERCF Futaba S-U300 Selector Cart

The MOD for the ERCF SturdyBunny MOD replaces the ever-problematic micro-Servo with a regular Servo.

The design has some similarities to Mr Goodman's Large Servo PushMOD but differs in my design not requiring the Servo to remain energized after reaching the DOWN Endpoint ( AFAIK Servos don't appreciate to be under constant load ) as it is using a CAM & Stylus approach instead of a Rack & Pinion one. The CAM Wheel has two radii increasing by 4mm from one to the other as the Wheel rotates with a resting area on each end allowing for the Servo to be deenergized.

The Mod also took inspiration from the ERCF Springy MOD via the use of a Spring in the split CAM Stylus providing both constant force and tolerance when pushing down onto the TopHat / Filament.

BOM ( additional to what is necessary for building the SturdyBunny ):
1x Futaba S-U300 Digital Servo which should come with 1x Servo Horn Screw
1x MR105ZZ Ball Bearing for the CAM Stylus
1x M5x10mm Set Screw or Ultra Low-Profile Socket Head Cap Screw for the Bearing
4x M3x12mm Self-tapping Screw for mounting the Servo
1x M3x6mm Button Head Cap Screw for securing the Spring Steel Wire
1x 1x40mm Spring Steel Wire for lifting the CAM Stylus
1x 6x12mm Compression Spring like the ones found on Bondtech Extruders

Printing:
Some of the parts require Supports and are thus a bit more complex but still doable even without an already running ERCF - Just slice the Parts as if you had an already running ERCF and insert a PAUSE command whenever a ToolChange for switching from Part & Support to the Support Interface Filament and back - The below Picture showing the Part still on the Build Plate with red ASA for the Part & Supports and white HIPS for the Support Interface:

For those not keen on being at the ready for doing the manual Filament Changes I've also included a Selector Cart ALT version requiring only 2 in short succession instead of 6 spanned over potentially hours ( at the cost of some aesthetic 😢 )

I'd suggest using the usual 4 Perimeters & 40% Print Settings for the Selector Cart but changing the Perimeters to 2 & Infill to 90% Rectilinear for the CAM Wheel, CAM Stylus, and TopHat to deal with Material Creep from the constant load of the Springs.

Assembly:

1. Remove the Supports - Be careful with the Barrel shaped Print in Place Support of the TopHat otherwise you may cause the sides to split off…
   
   …and if necessary, ream out the Bondtech Axle Holes with a 2.9 or 3.0mm Drill:
   
2. Open up the 5x blind Holes ( for printability reasons ) in the Selector Cart and 1x in the CAM Wheel:
   
3. The 4x Heat Set Inserts go in as usual where necessary as do the 2x IM8UU Bearings:
   
4. Use either a M5x10mm Set Screw or a M5x10mm ULPSHCS for installing the MR105ZZ Bearing into the Top Part of the CAM Stylus:
   
   In case you absolutely cannot be bothered sourcing the M5 Screw and Bearing there's also a CAM Stylus Top ALT included requiring neither though its functionality is untested ( maybe add some Plastic compatible Grease? 🤔 ) The Spring ( not pictured ) goes in-between the two black Stylus Parts.
5. Slot the 6x12mm Spring in-between the two CAM Stylus Parts and slide the Assembly into the Selector Cart from the top…
   
   …secure the CAM Stylus in place using the 1x40mm Spring Steel Wire by sliding it through the small Hole at the bottom all the way into the hole located in the side of the CAM Stylus Bottom and Secure the Wire with the M3x6mm Button Head Cap Screw:
   
6. Temporarily hook the Servo up to the System to be operational ( mine required servo_up_angle: 0 and servo_down_angle: 180 in the config ) and set it to UP - Attach the CAM Wheel to the Servo using the Servo Horn Screw that came with the Servo to look like below…
   …and test for the DOWN Position ( the CAM Wheel should rotate 120° CCW )…
   
   …before returning it back to the UP position for an easier installation.
7. Attach the 6mm Selector Cart Timing Belt by feeding it through the square Tunnel, around the Idler Wheel and into the toothed Gap of the Selector Cart:
   
8. Attach the Futaba Servo to the Selector Cart using the 4x M3x12mm Self-tapping Screws ( in a pinch you can also try using M3x12mm Button Head Cap Screws ):
   
9. Continue the assembly according to the ERCF SturdyBunny Instructions but using the custom TopHats we prepared at the beginning.

If everything was assembled correctly then there should be like a 1mm Gap between the CAM Stylus Bottom and the TopHat when the former is in the UP position ( as it is being held up with the Spring Steel Wire ) allowing it to move without issues between the Filament Blocks.

When set to DOWN the CAM Stylus Top will move down 4mm whereas the CAM Stylus Bottom will already stop after like 3mm with the Spring between the two dealing with the compression.