Intriguingly enough, the follow design works with only two cuts needed to get the internal rails to the right length. It gives me (by a scrape) enough travel for a 300mm work area on the Y, plenty (up to about 400mm) on the X and approx 160mm on the Z.

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I think I'll eventually take the plunge and cut some profile to length in order to extend the back section. It feels unbalanced as it is and I'd like that the angle to be slighty shallower.

Someone came up with the rather interesting idea of replacing the side plates and front gantry plates originally planned with a single piece folded around the frame. The idea is that it would form a more rigid structure and could also work with thinner plate - the suggestion was to use 3mm, presumably to assist in forming cleaner sharper bends? If memory serves, this would be similar in concept to a monocoque design as the walls provide structural strength? Let's see how that would look (but with 5mm because I forgot to change the numbers)...

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...and the flattened sheet...

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This piece of plate in 5mm comes in around $42 + VAT, 4mm at £34 + VAT, so material-wise it isn't a significant price difference from the original bracing idea. The issue is cutting it out cleanly, then finding a local fabricator to bend it for a reasonable price!

A key piece of the puzzle remains the gantry. Another interesting idea that came up is mocked up below (again not dimensionally accurate)...

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The ballscrew and motor were to be internal, with a slot in the front of the panel allowing the ballscrew to connect to the sled for the Z axis. Pulleys were to be external for ease of changing (note motor shaft and ballscrew sticking out the side).

I presume the idea could still be achieved by doubling up on the ganty supports, to reach a depth of 80mm - which would accomodate NEMA 23, while a cut-away section would be needed to accomodate NEMA 34.

I'm not particularly planning to try and implement this, but there's something about the idea I liked so I thought I'd share it :)