The structural mods I described in the last post do indeed help with rigidity, but there's still a bit of an elephant in the room in the form of the lower frame side rails. In order for the vertical frame to oscillate left to right, the lower frame rails have to twist where the vertical frame joins to it. As supplied, those lower frame rails are not very good at resisting torsion - they're just a horizontal 40 x 20mm extrusion hanging in mid air at the point where the vertical frame joins them. You can see this in the picture, along with my solution. I found a bit of 45x45 extrusion in a skip which turned out to be long enough to span the full width of the printer with a bit of overhang. I used the horizontal beam from the top of the vertical frame to mark out four bolt holes that line up with the central tubes of the vertical beams and some longer bolts (M5x60) to to attach it simultaneously to the lower frame and the vertical beams. This makes a huge difference to the rigidity of the vertical frame. I might have a go at making a video to show just how big an improvement this gives.
For the next improvement, I think I'll print some feet to support the printer underneath this extra beam, and so unload the horizontal frame rails almost completely.
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