The Mrs aint gonna be happy about this little project!

[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.....

[r0bsk1]

Due to a change in plan i've settled on a 1200mm for both X and Y. Ill be using 2 lengths of 60mm box section on a back plate with sbr25 rails to form the Y axis. Y is mounted on the X bearings near enough at the mid-point of the Y bearings. I've not drawn them in yet but ill put some diagonal pieces in to support the Y uprights against the X bearing plates.
Would taking the Y uprights below X bearings and box section and linking up with the opposite side of the X bearing plates offer significant more support to the X bearings?
Y bearings appear to be a little out of position. Everytime i move 1 bearing, they all move! I must have grouped them somehow. Going on the bearing blocks are 65mm i have a 20mm space between the 2 which make 150mm from one end to the other on each Y rail. Is this spacing sufficient?
Is it best to drive Y on the main Z back plate or can i use the shorter Z back plate? I'd like to protect the Y screw from any debris.


Attachment 7198
Cheers

[r0bsk1]

Just a quickie in prep for building a PSU. I've got 5 MA860 drivers rated at 7.2A each driving 3.1nm steppers.
When I come to use all 5 stepper they wont all be maxed out at once. What rating of transformer should i be looking for?
I'm looking at 1200VA 2x50 at 12A each. Wire the output in parallel and get 24A. Yes this is below 7.2x5 but would i ever exceed that machining wood/composites/Ali?

[Jonathan]

Bit of a hand waving explanation here, but in short...The the motor coils are rated for 2.1A, and you'll put them in parallel so 4.2A per motor. Due to them not drawing anywhere near full power most of the time, you can design for about 2/3rds of that, so 2/3*4.2*5=14A. 50V, so P=50*14=700W. So you need the nearest size transformer, or combination of transformers above that rating - so most likely 800VA. Plus some capacitors and a rectifier. I've currently got a 500VA and 325VA to power 5 or 6 of those motors.

[Ricardoco]

From the drawing it looks like the plan is to mount the rails on the top and bottom of the welded box section? If so you'll have problems with how much the steel will distort due to the welding. The rail spacing needs to be very accurate (guessing probably <0.05mm) for them to run smoothly, which is hard to get even before welding. Perhaps consider bolting them to a backing plate instead, since this allows adjustment to get the rails parallel. Also you will get much less deflection on this Y-axis if the ballscrew is placed between the rails. Currently when a force is applied parallel to Y, the Y-axis bearings can deflect in the same direction since there is a long horizontal distance (parallel to X) between the cutter and ballnut/rails.

If you decide to mount the rails on aluminium plate (as in post #15), then yes it is best to use tooling plate, but realistically the tolerances of the bearings are not that good so you would be fine with normal aluminium plate, but definitely not flat bar. If you were using profile rails then tooling plate would be the best option.

With reference to the section marked in bold, you simply cannot make such a sweeping statement, ive just welded my complete 7x4 and i used my dual dial guages one the gantry was clamped and ready to weld and there is no diference in the distorsion after welding or certainly no more than there was before the welding. If you weld it in the correct fashion and you have used a jig to hold the pieces, then it is fine. I will admit that if you use a 2mil rod on an old stick welder crank up the amps and weld 2mm box together you will have problems but a modern mig used to tack the items together before finishing the welding of it turns out fine, im not saying Its better than Ali, just that if care is taken during the welding of steel box no more surfacing and leveling will be required than before you welded it. Obviosly when mounting the rails onto the gantry you will have to do some remedial leveling work but its just not difficult, I will show you how ive done mine in the next week or so and it certaily ticks the 0.05 accuracy box you mention with ease


Rick

[r0bsk1]

Ah the ruddy motor windings! New i was missing something. Ta

[Jonathan]

Even before welding, it's questionable whether steel box section is accurate enough to mount round rails on opposite faces, so no matter how good the welding is it's not a good plan. It certainly wouldn't work for profile rails without further machining or shimming.

it certaily ticks the 0.05 accuracy box you mention with ease

How do you know?