# Why using two motors?

Posted by Fizpok
 Re: Why using two motors? March 17, 2016 05:37PM Registered: 7 years ago Posts: 5,018
That looks pretty solid!

Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
 Re: Why using two motors? March 18, 2016 05:14PM Registered: 4 years ago Posts: 33
Hi DD, I've seen you state several places that belt-coupled screws help maintain orthogonality of the axes, but I'm not getting it.

Isn't the direction of the z-axis defined by the linear support (rail/rods)? If you rotate one screw alone, that will raise/lower that side, tilting the platform and potentially binding the screws.
But even tilted, turning both screws in sync will still move the platform along the same vector - and the tilt can be compensated through bed leveling.
What am I missing here?
 Re: Why using two motors? March 18, 2016 07:01PM Registered: 7 years ago Posts: 5,018
The Z axis is defined by the Z axis guide rails. The X axis is defined by its guide rails. If one of the Z axis screws turns independently of the other for any reason, the X axis will tilt relative to the Z axis. If you try to use auto leveling to correct that problem, you may get the first layer to stick to the bed, but the print will be skewed because auto leveling assumes that the axes are orthogonal (it has to).

When you level the bed you are putting it parallel to the XY plane defined by the X and Y guide rails. That plane should be perpendicular to the Z axis or your prints will never match your CAD designs- they will be distorted. Think about what happens if the X and Y axes are not orthogonal - circles become ellipses and squares become rhombi. The same thing happens in the XZ and YZ planes if the Z axis isn't orthogonal to X and Y.

There are two problems with having two screws driven by two motors- first, the potential for the screws to get out of sync due to a mechanical misalignment that causes binding of the mechanism, or an electrical problem, and second, someone who doesn't understand the importance of axis orthogonality manually turning one of the screws in an ill conceived effort to "level the bed". It doesn't really matter much if you're just printing tugboats and Yoda heads, but when you try to print parts that have to fit together or fit other objects you quickly find out how important it is to have orthogonal axes.

Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
 Anonymous User Re: Why using two motors? March 19, 2016 03:40AM
Quote
rmlrn
Hi DD, I've seen you state several places that belt-coupled screws help maintain orthogonality of the axes, but I'm not getting it.

Isn't the direction of the z-axis defined by the linear support (rail/rods)? If you rotate one screw alone, that will raise/lower that side, tilting the platform and potentially binding the screws.
But even tilted, turning both screws in sync will still move the platform along the same vector - and the tilt can be compensated through bed leveling.
What am I missing here?

ABL will not compensate if the tilt happens during printing.
People have to stop compensating design/construction errors instead of fixing them. It is what auto bed leveling is doing for the current cartesian machine ! It may even introduce other issues and always more weight on the printhead.
Of course on a Delta, similar machine or to gain the ulimate micron if not nanometer on a machine built out of granite and cast iron, ABL is quite a must.

Here is how I modified my geetech Two leaders, one motor !

Edited 3 time(s). Last edit at 03/19/2016 03:46AM by MKSA.
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