Z Axis Design Help

[Kitwn]

... and not afraid to try something I haven't done before

You're over the biggest hurdle then. I think one of the main obstacles for many people is getting started and being willing to have a go at something new (I should have bought a welder about 2 years before I finally plucked up the courage to have a go). Though you are right to start by asking questions rather than just buying bits off eBay and hoping for the best.

I had read that larger than 15mm rails are better, but is it better to use larger bearings as well ?
I'm trying at this stage to get to grips with what is better for strength/rigidity and direct my design/expenditure towards the most influential areas.
Roger

And the other biggest hurdle is aquiring all this knowledge before comitting your limited budget in the most cost-effective way. I can't answer this particular question from personal exprience but you will see designs where people have had to cut channels for the ballscrew to fit into which isn't ideal, especially if you don't already have a milling machine to hand. The narrower you can make the Z assembly, the more cutting length you will get from a given set of rails and ballscrews on the Y (X) axis so your choice of bearings should take this into acount.

It's also worth checking the materials you have available locally. Not too much of a problem when you live on such a tiny, crowded island, but if you can design around what you can get hold of nearby it helps.

Kit