The Mrs aint gonna be happy about this little project!

[Jonathan]

but if it's out of spec for the bearing before i even start, well that puts the mockers on that idea. Any idea how 90x45 ali extrusion would cope over 2m?

I should have been clearer, it might be out too much even before welding, but you can get round that. Instead of welding three pieces together couldn't you start with one or two bigger sections, then bolt them to a back plate? If you have a fastener every 100mm or so you can use them to adjust and remove the bend enough to mount the rails. Two pieces of 90x45 aluminum extrusion to mount the rails on would be fine, or you could use aluminium box section.

I would have liked to have the ballscrew mount on the Z axis upright so I would have to remove the middle box section and bolt to a 20mm back plate but then then is no room for the ballscrew mount unless the width of Y is 200mm+ to accomodate the mount as i feel it best to surround the Y bearings. Y ballscrew is currently 100mm off centre from the Y bearings.

Spacing the Y-rails out 200mm+ to accommodate the ballnut mount would be good as in addition to making the axis much more rigid due to the ballnut being in the optimal position, it will also make make it stronger because the bearings are further apart. However you don't want the rails too far apart without increasing the spacing of the bearing blocks since it risks them skewing/racking. The ballnut mount could be made a fair bit shorter by moving the mounting holes and having only one available ratio without changing the belt.

[r0bsk1]

You set up index plates on the top and bottom of the box section which not only would help deal with small distortion of welding But also give you a rebated straight edge with which to create a master index edge for setting up the rails with. This also has the advantage of being able to do dis-assembly and certain types of maintenance much much easier. First you have to decide which edge is you master edge and work from that in the actual building. Though bolting works much better in most cases. This is why King bridges that are bolted together from many plates have been know to last over 80 yrs of active use.

Michael

That would be very tricky to do as the box section is 60mm and the rail base plates are 65mm.......unless i've completely misunderstood. I'd have to mount somthing to the side and offset by 2.5 mm. Tricky but doable.....

I should have been clearer, it might be out too much even before welding, but you can get round that. Instead of welding three pieces together couldn't you start with one or two bigger sections, then bolt them to a back plate? If you have a fastener every 100mm or so you can use them to adjust and remove the bend enough to mount the rails. Two pieces of 90x45 aluminum extrusion to mount the rails on would be fine, or you could use aluminium box section.

There's no way im going to be able to bend the box section. I'd expect the ali back plate to bend before the steel. However they do "look" straight.

Spacing the Y-rails out 200mm+ to accommodate the ballnut mount would be good as in addition to making the axis much more rigid due to the ballnut being in the optimal position, it will also make make it stronger because the bearings are further apart. However you don't want the rails too far apart without increasing the spacing of the bearing blocks since it risks them skewing/racking. The ballnut mount could be made a fair bit shorter by moving the mounting holes and having only one available ratio without changing the belt.

Right so that sounds like the ballnut mount is upright mounted against the Z upright and runs between the Y box section beams. I had i laying on it side poking out the back. The rail base plates would be approx 330mm apart if the mount is 200mm. What could you get the ballnut mount down to?

[r0bsk1]

Here's a quick mock up the ballscrew mount running between the Y rails. All uprights are approx 500mm




Back plate is 2190x330x20mm.......quoted.......wait for it........£300 +vat for ecocast!!!
5038 is £205

Back to the drawing board i guess

[Jonathan]

That would be very tricky to do as the box section is 60mm and the rail base plates are 65mm

The SBR25 rail base is 55mm wide unless I'm missing something?

There's no way im going to be able to bend the box section. I'd expect the ali back plate to bend before the steel.

We're only talking small fractions of a milimeter which doesn't require that much force. Both will bend a little bit...

What could you get the ballnut mount down to?

About 160mm if you're happy being limited to only using 1:1 or 1:2 ratio without changing the belt. Don't quote me on that though as I need to draw it first!

Back plate is 2190x330x20mm.......quoted.......wait for it........£300 +vat for ecocast!!!
5038 is £205

It would be fine with 1/2" thick plate, but that doesn't really change the conclusion as it's still expensive.

[r0bsk1]

The SBR25 rail base is 55mm wide unless I'm missing something?

My Bad, been looking at TBR rails

We're only talking small fractions of a milimeter which doesn't require that much force. Both will bend a little bit...

About 160mm if you're happy being limited to only using 1:1 or 1:2 ratio without changing the belt. Don't quote me on that though as I need to draw it first!

It would be fine with 1/2" thick plate, but that doesn't really change the conclusion as it's still expensive.

To be honest I'm seriously thinking about going back to the shorter gantry as in my orginal designs. I'll need to weigh up the pros and cons.....