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rigidity problem ?

Posted by DimitriF 
rigidity problem ?
September 13, 2016 11:50AM
I am building a machine for a project of mine, applying layers of silica gel to a glass plate.
It's not really a 3d printer but as I need X, Y and Z movement as well as controlling the syringe, it use the same principle.

I choose an hbot configuration for it simplicity, see in attach the jpg and the openscad file (the openscad contain lots of other stuff but only the xy will be rendered for clarity).

My problem is that I have "play in movement": the X axis will rotate by a few degrees, see the arrows in the jpeg. It is only when the X movement change direction.
I don't need the same accuracy than a 3d printer and I can start working with the machine as is, but I like to understand what is happening.

My assumption is that it comes from the Y linear bearing.
The 90° angle between the axis is not perfect yet, I am waiting for the plate to put bellow that should do it.
From what I read on the forum, the part supporting the 608zz could also lead to rigidity problem as they are not strong enough.

I am a chemist and didn't study mechanic so advise could be more than helpful.

open | download - Layer_printing.scad (39.8 KB)
open | download - DSC_0001_edited.jpg (311.7 KB)
Re: rigidity problem ?
September 13, 2016 02:23PM
Congratulations! You have rediscovered why people build CoreXY instead of HBot machines. The side that is shifting is the driven side- the bearing at the opposite end of the X axis lags behind the driven side because of play in the bearings, flex in the X axis and flex in the Y axis guide rails. You'd be better off driving both sides of the X axis by using a shaft that runs across the machine, and mount the X axis motor on the X axis.

HBot can be made to work with very rigid construction including linear guides instead of end-supported rails, and a much more rigid frame, but I don't see how you'll get it working better with end supported rails and a frame made of threaded rods.

Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: rigidity problem ?
September 13, 2016 04:23PM
I was afraid of such answer but it's not really a surprise...

If I put it back with my word, hbot only drive on side while corexy will drive both, and paf, no more shifting. And because my construction is far from rigid, hbot could not do it.

So I guess I must start over...

Thanks for your quick answer
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