Monster fixed gantry router for aluminium & wood

[jonboycnc]

I'm trying to organise getting a CNC router at the London Hackspace, and I've been reading the really helpful DIY design advice on this forum.

We want to be able to cut big lumps of aluminium, as well as wood and plastic. We also want XY travels of at least 400mm x 400m, which rules out getting cheap mill as far as I can see - our budget is only around £1500-2000.

What we do have going for us is a very large workshop, on a 5 year lease, with lift access. So little need to worry about space taken up and difficulty moving the machine around.

This gets me thinking that we need a fixed gantry router. As I understand it, most router designs fare poorly cutting deep inside blocks of aluminium, and taking deep cuts, because of lack of rigidity. A fixed gantry machine could be made very rigid, especially if we didn't worry about space consumed and portability.

I've been looking at prestressed concrete lintels. They're concrete and huge so have fantastic vibration dampening, they're designed to take tensile load which concrete is usually poor at, they're precast so have very little shrink/creep left to do, and they're cheap. A lintel 215x140x900mm costs £30 and has a "safe working load" of 83 kN/m (whatever that means).

This leads me to the attached beginnings of a design for a router frame. It would have travels of 400x600x100mm and mount a 2.2Kw Chinese watercooled spindle.



To build I would:
1) build a heavy duty table to carry the whole lot
2) bolt & cement in place the padstones that are the support for either end of the gantry.
3) level the table top with self-leveling epoxy
4) Fastened together the 2 base lintels by steel straps at their ends, and epoxy level on side.
6) place the base lintels on the levelled tabletop, level side of the lintels down (level touching level)
7) epoxy level the front face of the gantry, cement the gantry in place using a digital spirit level to ensure the face is true vertical; then epoxy level the top face of the gantry if we need that.
7) epoxy level the other (now top) side of the base lintel pair.
8) Put a bolt though one corner of the base lintels, and hold the other corners down with bolted straps.
9) mount the gantry & Z carriages, wire up everything
10) mount the moving t-slot table
11) machine the moving tslot tabletop flat
12) adjust the base lintels (swinging around the bolted corner) to square up the machine, using test cut pieces and a micrometer to test for squareness.

QUESTIONS
(many not new, sorry ...)

1) Can someone point me to a design for heavy-duty X & Z carriages for the gantry? There's so much out there but little of it is fixed gantry and even most of that is still thinking much more lightweight. Given that I've got this monster frame, I might as well exploit its potential!

2) For this short 10mm Z travel, is supported rails better or worse than unsupported?

3) If I can afford to upgrade one axis from supported round rails to profile rails, which should it be? Would 15mm profile rails be beefy enough?

4) Which face/faces is best for the X axis rails on the gantry? I'd rather not have any on the bottom as that either reduces travel or increases gantry span.

5) Can anyone help me figure out how wide the X axis travel can afford to be before the gantry starts deflecting too much? I can always add a second lintel, bolted and epoxy cemented to the top of the first one, if that helps.

6) Am I barking up the wrong tree with this concrete lintel idea?

Any other advice?

thanks
Jonathan