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More or less complete design for the single motor, long belt method.
The idlers are both fixed and made from standard skate bearings spaced in the middle with a washer and edged with two penny washers to provide flanges - these will be mounted on a M8x40 bolt taped into the aluminum plate and stood off with a stack of washers. I can adjust the height of the whole motor assembly if necessary and fine adjustment can be made by raising or lowering the motor on 10mm slotted mounts, this was the max I could achieve with the large circular projection (which I assume houses the bearings) at the center of the motor.
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Both lengths of the long belt (top and bottom) are supported on UHMWPE (blue stips on the image below) which will be bolted down to steel angle which in turn will be welded or bolted to the frame with strips of 90x6mm flat bar. This support framework can then double up as the framework for a belt cover.

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The motor this is all based on is the cnc4you 86HS115-4208 8.7Nm Nema 34 Stepper Motor as highlighted on the data sheet below:

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