Flat Bar X Axis for Gemini 3D Printer

As I mentioned in my previous post, I am experimenting with a revised gantry for the Gemini 3D printer.

Here is a test assembly of the new idler ends assembled with the flat aluminum bar support and the 9mm hiwin rail for the X axis mounted.
This design is proving to be adequately stiff for the light weight x-carriage I have been working with on the prototype. Some simple testing shows that it would probably be stiff enough to carry a pair of stepper motors if the design was to use direct drive extruders, however in such a scenario I would probably want a stiffer assembly than the flat bar.
Since the Gemini uses dual bowden extruders, this is a moot point.

This assembly is actually quite simple – there are two printed plastic components, one on each end.

Each end holds three idlers and each idler is composed of two bearings.
The idlers are mounted on M4 bolts which have enough plastic support at their base to handle the tension of the GT2 belts once the gantry is fully assembled.
The lower idlers on the posts that have two idlers are there to prevent the timing belts from rubbing against the idler tower and carry no direct loading. In the initial prototype I observed some rubbing against the plastic support pieces and it occurred to me that the easiest way to solve this problem was to add an extra idler per side.
The double idlers are separated by a small M4 nut. The machining of the bolt head and nut are the perfect size to engage the inner ring of the bearings and not contact the rotating outer assembly and thus eliminate the need for washers.
It would take much more force than this machine will exert to bend the M4 bolts or crack the plastic end pieces.

Two M3 bolts on each side sandwich the 9mm rail and the flat bar to the plastic end piece, with an additional three M3 bolts securing the rail to the bar directly which are spaced evenly along it’s length.
This construction ensures that the bar and the rail the are aligned correctly and that they cannot flex independently which provides the necessary stiffness.

Four more M3 bolts on each end connect the X axis to the hiwin carriages on the Y axis with the flat bar sandwiched between the plastic and carriage. This gives additional stiffness to the assembly and ensures proper squared alignment on the Y rails.