Hello all.

I'm going to scratch build a CNC lathe.
I've converted a 7x16. It was crap out of the box, with significant quality problems. I've improved some things, but would prefer to put in effort building a better machine rather than polishing a turd.

Objective
Build a CNC lathe - designed for CNC from start

Working area - flexible, but I'd like to be able to turn 300mm long and 200mm diameter.
Auto tool turret at some point, possibly gang tooling first. Probably just start by using my QCTP.
- probably this tool turret: CNC Lathe Turret - Other, STEP / IGES - 3D CAD model - GrabCAD

For hobby use. I don't ever foresee using it for production.

I don't require particularly heavy cuts. I'd be happy with 1-2mm depth of cut for 'heavy work' in aluminium. Would like to be able to handle steel, including the occasional bit of stainless.

What I have
HSR30 linear rails 2x 1metre long, with 6 bearings
- got these cheap, if other rails suit better, I'm happy to buy them. For instance, HSR30 are quite tall (45mm from bottom of rail to top of bearing), which means higher cross slide - the spindle axis has to be higher which brings potential issues.
Current design uses these cut staggered, such that on the Z axis there is a 650mm length with two bearings, and a 520mm length with 1 bearing - this allows 400mm travel.

Steppers + contoller

750W Servo for spindle motor
G320X

Current design:
Based on large section steel RHS. E.g. 200x200x8mm tube.
- possibly with epoxy granite cast in for support and mass.

400mm travel in Z
190mm travel in X

Planning on:
- linear rails
- double nut ballscrews

Spindle:
Looking at cartridge spindles such as: Spindle CNC Lathe Machine,Belt Driven Spindle-in Machine Tool Spindle from Industry & Business on Aliexpress.com | Alibaba Group

Budget
Very flexible. I'd prefer to pay more to do this right the first time. I've done a lot of projects that I've not been satisfied with, often because I've tried to cheap out.
However, I don't really want to spend $10,000

Wants, but not essential
Some live tooling.
Basic drilling (e.g. being able to drill a bolt circle but rotating spindle in increments) - possibly full C Axis

Tools
I've got a CNC converted G0704 clone
General tools

I'm building a metal casting furnace
I plan to purchase a decent size surface plate

Just bought a inverter stick welder, but I've not welded before and don't plan to use welding for this project (except for stand / enclosure)

Thoughts that need addressing
I'm really coming around to the idea of a tailstock, most likely a simple tailstock to allow turning between centres / supporting long work.
For this I envisage a wider base, with the Z axis rails offset to the side (meaning they are more directly under where the turret will be) and a set of rails for the tailstock as well.

What is the optimal offset of the Z axis rails?

Orientation of X axis bearings - upright or upside down? (See attached)
- upright means a long carriage plate, but short cross slide plate.
- upside down means rails contribute to stiffness of cross slide plate.

Other designs
jakemestre has an excellent building happening: http://www.cnczone.com/forums/bencht...ilt-lathe.html
dumpstercnc
Guldberg [urlhttp://www.cnczone.com/forums/vertical-mill-lathe-project-log/51688-slant-bed-cnc-lathe-scratch.html[/url]