# Mass Reduction - Bowden + Direct Drive

Posted by Bryguy
 Mass Reduction - Bowden + Direct Drive April 05, 2018 02:06AM Registered: 5 years ago Posts: 6
I haven't tried any crazy custom ideas yet, so....

I thought I was pretty clever thinking of using a flex shaft from a drill or Dremel to drive a Bowden + Direct Drive system, then I found a post discussing it. The benefit is the mass of the motor driving the filament is located on the printer frame, but the direct drive has better filament control.

Couldn't this also be applied to the traveling stepper motor on the carriage? I know it's not an issue for a XY, but on a normal setup, the motor could be located on the frame of the printer.

(This would be a moving Z axis heatbed, with the X, Y axis staying at the top of the printer, in a single plane)

The moving axis could be driving by the flex shaft, as it moves back and forth, with the motor located on the frame. As I have drawn up the geometries, one potential problem you could have is the flex shaft and its connections could bind in certain places in the XY plane. For example, if it moves to the position closest to the motor, the flex shaft + connections could have some interference. As I have drawn it, the connections could both be installed facing vertically upwards. That way, if it gets too close, the couplings would be parallel to each other.

The problem that could happen with this is that at the max distance position from the motor the flex cable could be stretched, and create a rotating torque on the couplings. Some of this could be alleviated by orienting one coupling vertically, and one horizontally. This could reduce or eliminate the strain on the flex cable and couplings.

Thoughts? Is this useful? Or is this a solution looking for a problem?

B.
 Re: Mass Reduction - Bowden + Direct Drive April 05, 2018 02:33AM Registered: 4 years ago Posts: 5,232
You are describing a H-Bot gantry with the X-motor driven by cable.
Instead look for CoreXY gantry, where X/Y motors are stationary without any cable ( but longer belts )
 Re: Mass Reduction - Bowden + Direct Drive April 05, 2018 06:58PM Registered: 2 years ago Posts: 488
The H bot has both steppers stationary as well, I think you mean he's describing a more standard cartesian setup like the makerbot replicator and the likes.

I personally wouldn't try to use a flex shaft to drive an axis, it wouldn't be precise because of the backlash in the shaft, and it's much simpler (and more accurate) to use a belting arrangement that keeps the motors stationary ala coreXY, Hbot, ultimaker and so on.
 Re: Mass Reduction - Bowden + Direct Drive April 06, 2018 12:01AM Registered: 5 years ago Posts: 6
You are right. There probably is enough flex in a "flex shaft" to introduce inaccuracy. ( Maybe the name "flex shaft" could have been my first clue!)

Could it be more "spring" than just plain "give"? What I mean is that whatever flex is present in the shaft is simply some lag that catches up.

 Re: Mass Reduction - Bowden + Direct Drive April 06, 2018 02:27AM Registered: 4 years ago Posts: 5,232
Quote

The H bot has both steppers stationary as well,

IIRC, the HBOT has two Y-steppers stationary, but the X-stepper rides on the Y-gantry? I might be wrong...are there any drawings of a HBOT?
 Re: Mass Reduction - Bowden + Direct Drive April 06, 2018 04:20AM Registered: 2 years ago Posts: 488
Quote
Bryguy
You are right. There probably is enough flex in a "flex shaft" to introduce inaccuracy. ( Maybe the name "flex shaft" could have been my first clue!)

Could it be more "spring" than just plain "give"? What I mean is that whatever flex is present in the shaft is simply some lag that catches up.

It's a bit of both. In an ideal world, with no friction in the bowden tube of the flex shaft it would turn and only have 'springiness'. But the tube itself has a fair bit of friction which can stop the output turning quite as much as the input. Take a flex shaft and turn the input a little bit, you'll find the output wont turn until you twist the input enough to overcome any friction in the tube. Anyways, even if it was just an issue of springiness and not just the fact the output wont always turn as much as the input, that's still bad. People do their best to minimize the springiness of belts, and those are far less springy than the output of a flex shaft.

Quote
o_lampe
IIRC, the HBOT has two Y-steppers stationary, but the X-stepper rides on the Y-gantry? I might be wrong...are there any drawings of a HBOT?

That's the standard cartesian (ala makerbot, solidoodle etc.), not Hbot. Hbot and corexy are effectively the same thing, just coreXY cross the belt path to move the moment from acting at either end of the x axis to acting at the effector.
have a look at this diagram:
CoreXY [maxdesign1990.files.wordpress.com]
Hbot [maxdesign1990.files.wordpress.com]
They are basically the same setup, just coreXY crosses the belts over at what is the top of those diagrams.

Edited 1 time(s). Last edit at 04/06/2018 04:23AM by Trakyan.
 Re: Mass Reduction - Bowden + Direct Drive April 06, 2018 03:37PM Registered: 5 years ago Posts: 6
That makes sense. It trades one problem for another.

Btw, great explanation. Well written, and easy for everyone to understand.

Thanks.
 Re: Mass Reduction - Bowden + Direct Drive April 07, 2018 12:47AM Registered: 2 years ago Posts: 488
No worries, glad I could help. If you do want to reduce moving mass and keep motors stationary, there are some belting arrangements out there that achieve this like coreXY, Hbot, Ultimaker and more. I'm personally working on two other arrangements with stationary motors and decoupled axis, but I'll wait till I test them and see if they work before I release anything. One of which I'm particularly excited for because it has a built in gear reduction and so doubles resolution and torque. It will probably only be good for low load applications (like 3D printers!) and small-medium setups though.
 Re: Mass Reduction - Bowden + Direct Drive May 12, 2018 07:24AM Registered: 2 years ago Posts: 1
This already exist as a commercial product.

[zesty.tech]
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