ERCF V2 Selector Power Drive 60T pulley Mod NEMA17

Only modified to use a NEMA17 stepper motor

## BOM:

* cnc 60T pulley (2GT, always 2GT type)
* one more 20T pulley ( the same type of the ERCF original design for selector motor)
* 154mm belt, (2GT close loop)
* nylong washer M5 0.5mm thick, OD 7mm preferred. 3pcs. or you can print
* nema14 stepper motor
* M5 D shaft about 46mm to 50mm long, slightly longer or shorter shouldn't matter. if no D shape shaft is available, we can easily grind the place for pulley set screw.
* bearing MR115 ( OD 11mm, ID 5mm, height 4mm )   2pcs
* M3 screws , 8mm 2pcs,  10mm 4pcs, 12mm 2pcs
* M3 hex nuts X2,
* M3 heat insert X 2 ( and other heat inserts required by the original design)
* may need a little bit longer open belt (the long open belt for selector) than the orignal ERCF_V2 design, since our 20T pulley is a bit further from the original position, since the 60T pulley need space to stay.
### optional 
only if plan use M4 screw to mount pulley_mount_default and EndBlock_Bypass
* M4 30mm X 2 
* M4 lock nuts X 2

## Config file change

* since we now use 60:20 gear ratio, so a new line is needed in the hw config file manual_stepper selector_stepper section -- gear_ratio: 60:20 .
> manual_stepper selector_stepper section
> 
> rotation_distance:40
> 
> microsteps: 16
> 
>  full_steps_per_rotation: 200 # 200 for 1.8 degree, 400 for 0.9 degree
> 
> gear_ratio: 60:20   <=== add this

More details in ntchris's original model…

https://www.printables.com/model/970372-ercf-v2-selector-power-drive-60t-pulley-mod/files

Only modified to use a NEMA17 stepper motor.