Thanks Clive,
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I figured I could get away with a lot as its a printer rather than trying to mill ali with it :)
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I reckon I'll still go with ballscrews and probably supported rails, as the diameters I think I need are way cheaper than the router I built. The frame will be extrusion and of course I can use my existing machine to do some custom parts (mounts etc.).
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I hear you on the heated bed and I'll also probably encase the whole thing in Perspex so it stays nice and warm in there.
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At the moment I'm trying to draw and visualise a z axis that I can mount maybe 2 or more extruders to (extrusions again), so I can either print different colours or possibly synthetic rubber on one job.
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Cheers
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Chris