Purchasing a z-axis?

[Tonyel]

It's all in an attempt to get rigidity in the head. I wanted to have two Y rails/carriages, separated by a distance in the Z direction, to provide rigidity against the Z rail flexing. I wanted to have just one lower carriage, back to back with one of the Z carriages, but then there's no way to attach them both to the plate, because you can't get to the bolt holes of one when the other is in place.

So I ended up with the slight extravagance of two lower Y carriages, and the lower of the two Z carriages, all in a line as close together as possible at the lower end of the bracket. Since I had a spare carriage, I thought I might as well use it. Then there is the top carriage, providing a brace and incidentally holding the other end of the driving plate.

I'm open to suggestions though. My main intention is to make it as rigid as possible, which hopefully would make a good foundation for any further improvements in future.

Cheers,

Tony

[Jeremy]

I'm also planning on having a fair bit of Z travel, as I want to be able to machine moulds from foam. I've opted to raise the height of the X rails well above the table, so the gantry doesn't have to be so tall, and then fit the Y rails with a wide horizontal spacing. At the moment I'm hoping to be able to run the Z axis down between the two Y rails (see other thread on my small diameter DIY spindle to see how this might be possible).

The idea for this configuration came from looking at this machine: http://www.mechmate.com/, although I'm not building something this big (mine's about 800mm x 600mm x 250mm working volume).

Jeremy

[Tonyel]

Sounds like your machine is to be similar size to mine - slightly shorter. I was thinking that for most jobs, if I didn't need the full Z travel, I would pack the job up a bit so the cutter isn't operating at full travel all the time.

[Jeremy]

I'd had the same idea of packing the base up for shallow stuff. I may make up a second tall base from MDF using a torsion box design.

The challenge is to reduce flexure to the minimum, both by designing in stiffness and by restricting the offset distance of the cutter from the centre of each beam.

My Y axis beams are 2" square, 10g, alloy box sections, with 16mm fully supported SBR type rails fitted to the top of each. These beams are on 6" centres horizontally, so I have a gap of 4" between them. The Z axis will (I hope!) fit into this space, so that the cutter can retract back between the rails for maximum usable operating depth. Including the height of the rails, my Y axis beams are about 4.8" tall, so I'm hoping that they should remain stiff enough in the vertical axis to not noticeably deflect under the weight of the carriage and spindle.

I'm mounting the rails on top of 8" long 4" x 2" channel sections, mounted with the short faces at top and bottom, bolted directly to the carriages of the X axis. The X axis rails are also 16mm fully supported SBR ones, bolted to the top of another bit of 4" x 2" channel mounted on edge, with the bed bolted beneath. This gives me a nominal Z axis working height of about 248mm, although I can gain a bit more, if needed, by lowering the table a bit more.

Jeremy

[Tonyel]

The challenge is to reduce flexure to the minimum, both by designing in stiffness and by restricting the offset distance of the cutter from the centre of each beam.

Absolutely! Do you have any pics of your design?