Z Axis Design Help

[Angler58]

I'm a newbie to CNC and have been reading various posts on the MYCNC site and have now starting to think about my design. Initial thoughts on size are cutting area of 1000mm x 750, 80% wood, 20% non ferrous (aluminium, brass, copper), with an allowance on the X Axis to add a future 4th axis. I have no real overall space issues for the final build, although I am constrained a little by budget.
I want to start with the Z axis, as pointed out in various posts (and I think) this is the most important part (the part nearest to the cutting device) and its design drives the size of the X & Y axis.
I would like to hear other people's thoughts, inspiration, advice on.....

What design features contribute to the Z axis strength & rigidity and should be incorporated into its design in what order, (biggest effect first).

i.e (I've not put the following in any particular order ).

1. Spindle mount and Back plate thickness, Thicker = Stronger ?
2. Best Linear Rail Bearing placement on X, (both front, top & front, bottom & front, top & bottom)
3. X Axis screw placement (behind X, on top of X, in front of X & behind Z, below X), Is closer to the CofG better?
4. Include side bracing of the Z assembly (assuming Bearing or X axis screw placement is behind front face of X), to increase rigidity?
5. Spacing of Z axis bearings (Spindle mount), Further apart less flex?
6. Spacing of Z axis to X Axis bearings, Is wider more stable?
7. Anything else.
Thanks

[Kitwn]

Part of your decision will depend on the range of tools you have access to. You can achieve a simple construction for your Z axis by putting the rails and the ball screw for the gantry mounted axis (Y in my case, X for many others) on the front face of the gantry. This can allow your Z axis back-plate to be a simple flat plate, but it does give you more overhang than other designs and exposes the ballscrew to shavings. This was my choice largely because it was the easiest to achieve in a limited workshop. Getting the ballscrew out of the way behind the gantry is much better for it but you'll need more than a hand hacksaw and a few files to be able to cut the pieces of the Z axis frame accurately enough. Unless, of course, you're a lot better with a hacksaw and files than me!

One of the comments I have seen on here a few times is not to use the smallest sizes of rails for the Z axis itself. It seems like a good idea for reducing overhang but you need enough room between the fixed and moving plates for the ballscrew anyway and it just makes life more complicated to have too narrow a space.

Kit

[Angler58]

Thanks for the reply Kitwin,
I have Bandsaw, Pillar Drill, Heavy Duty hand held Router, small selection of taps/dies and the usual home mechanic/wood worker range of tools (heavy & light duty) and not afraid to try something I haven't done before (hence here ).
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

[JonnyFive]

Have a look on here:

http://www.cncroutersource.com/homemade-cnc-router.html

Lots of useful information.

[Angler58]

Looks like a useful read JennyFive, thankyou

[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

[BigBrand]

has anybody just used the 'premade' ones that are available?