Thanks Neil and Boyan.
. (see i learned the dot trick)
Really good spot on the x axis bearing bolt access Neil. I hadnt noticed it.
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I can certainly take into account most of the advice kindly offered. I will certainly double the gantry sides and make the back plate a solid piece as recommended. I'll still have a head of 15mm Alum over, so I'll probably use it to double up wherever it will add extra strength. The bed supports being an obvious one to me. I'm sort of stuck with the supported rail for the mo as I already have some, found them locally at a price too good to ignore. However a bit of redesign, so I can easily replace them with profiled rails at a later stage.
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I also take the point about the flatness of the base. The steel plate it sits on is certainly as flat as a straight edge can tell, however before I lay the rails, I'll give that bit extra care with a stone to remove any possible high spots and then clean it to an inch of its life.
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As for the adjustable height gantry, I think I'll drop that idea. I'll adjust the height in the design until the gantry clears my milling vice with a bit to spare for protruding stock.
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Boyan, I studied your comments regarding z-axis vibration and strength. That sound of the whole machine shuddering when you go down into aluminium is exactly the sound my existing MDF machine makes:) So in short if I widen the spacing between the bearings (vertically) and reduce the overhang as much as possible so that the spindle is as close to center as possible? Maybe again double that main block on the Z axis. Do I understand correctly? As you can see from my images, I already have one of those 80mm chinese spindle mounts from my existing machine, so maybe get a second one so that the spindle is supported top and bottom? I don't need to be concerned about weight as once this is in place it ain't ever moving:) However, I don't want overkill weight to affect the + travel of the z-axis. I don't think an extra aluminium block will make too much difference?
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Clive S? I'm interested in your comment regarding accuracy? Can I ask what you consider to be accurate? I would consider my MDF machine to be reasonable accurate, insofar as it cuts to the right dimension as far as my calipers can tell. However where it loses out is if I do a longish complex run, it won't return to exactly the same origin, so clearly it isn't all that accurate afterall. For what I need I don't need micron accuracly but I would like reliability and repeatability.

Thanks again for all the input it is genuinely appreciated.

Regards,

Dave.