That's an interesting layout if you're committed to cable drive. I see three main problems with cable drive. Installing the cables, maintaining the mechanism, and dealing with the motors.
We tend to think of pulleys in ideal terms as being lossless, but the friction at each pulley (and force required to wrap the belts/cables around the pulleys) adds up fast. A corexy mechanism is a surprisingly difficult load for the motors - try pushing the carriage around in XY with motor power off. Your layout adds four more pulleys to a minimal pulley count corexy layout.
Cables are a friction drive system and it usually takes multiple wraps of cable around the drive pulleys to ensure sufficient friction that the mechanism won't slip. Look up the capstan equation. As soon as you use multiple wraps, the cable walks along the length (width?) of the drive pulley as it spins. That walking will change the angle that the cable makes with the pulley(s) on the ends of the X axis. That will in turn change cable tension depending on the pen carriage position in XY space. That will also change steps/mm depending on position, so drawings will be distorted. If the cable tension varies enough in the wrong direction, the mechanism may slip because the friction on the drive pulley will change with the tension change.
You can fix the drive pulley problem by using steering pulley(s) to ensure that the cable doesn't walk along the drive pulley, but that may require some modification of your pulley positions. You're using a single cable loop for the whole system which should be OK. You can close the loop at the pen carriage or maybe in one of the motor-to-corner pulley segments, depending on how large the drawing area is compared to the mechanism. Here's the maintenance problem- in order to do things like replace pulleys, bearings, etc., you have to release the tension on the cable(s). As soon as you do, the cable will jump off all the pulleys if there are no mechanical means to prevent it. The biggest problem occurs at the motors with steering pulleys and multiple loops around the drive pulley. Repositioning the cable at all the pulleys can be difficult.
Here's one way to fix the angles of the cable going on and off the drive pulley:
Here's one I made when I made a cable driven corexy sand table:
I used very cheap pulleys for the steerers (and all the other pulleys), and the mechanism was very noisy. Those tiny pulleys had even tinier bearings that would not last. I recommend you use much bigger bearings for quieter and longer life operation.
You can hear the noise it made here: [
vimeo.com]
If you consider these problems from the start you can probably design a mechanism that is quiet and reliable enough not to require a lot of service and is relatively easy to service when necessary- i.e. learn from my mistakes and try not to repeat them...
Edited 1 time(s). Last edit at 03/20/2021 10:48AM by the_digital_dentist.
Ultra MegaMax Dominator 3D printer: [
drmrehorst.blogspot.com]