Hi Ard - I was thinking of a stout aluminium channel, easy to bolt up and quite stiff. But happy to cast a good bridge if needed. Currently seems no need for the bridge but I think it will help in the vibration area, especially when the gantry is down and those tuning forks are sticking up in the heavens. Every time I go through this process I learn something so its not all in vain. The build is the tough part, reality sucks and time travels about 1000x less fast in the real world vs CAD/FEA world. Like I've had a bag of CSA for 2 weeks now but haven't made the moulds, been painting the workshop floor, so many things to do.... then there's my day job...

The bridge is connected at the back so the columns can be adjusted sideways vs in the "middle". I could attach them at the top, maybe next round the hurdy gurdy ... should do a modal FE I suppose. Peter

post script a 20mm "plate" will probably vibrate so it needs to be stiffer then a "plate" plus damp that's why the interest in composites....