Assortment of questions on design

[EddyCurrent]

Why would you want to go cheap on the Z axis ? it's one of the most important parts.

I used 15mm rail and blocks for Y and Z but never again. They work fine but unless they have changed the design, getting grease into them is damn near imposssible.
See this thread; http://www.mycncuk.com/threads/7273-Greasing-Hiwin-HG15

This was my solution if it's any use to you.

I got some 4mm tube, push fittings that I turned down to fit in the gap, and there was just enough 'meat' on the push fittings to reduce the 5mm thread as supplied down to a 4mm thread that fits the Hiwin bearings.

Fitted some brackets with 6mm thread push fittings, 6mm thread nipples, and 6mm hex connectors.
It now works very well.

I also fitted a home made extended brass grease pipe for the Z ball nut grease nipple.

Pic1 shows it with Z front plate removed, pic2 shows it reassembled.