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  1. #11
    I think you're describing changing it round so that the longest axis is the Y-axis and the gantry only moves a short distance? This would make the X-axis (or whatever you want to call it) a 2000mm long unsupported length, hence the question about '20mm tooling plate and ali extrusion be enough' over this increased distance.
    I guess it does mean you would only need one rotating ballnut mount. The mass of the gantry is also increased substantially.

    Edit, didn't see previous post which explains it when I posted this.
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  2. #12
    Quote Originally Posted by Jonathan View Post
    I think you're describing changing it round so that the longest axis is the Y-axis and the gantry only moves a short distance? This would make the X-axis (or whatever you want to call it) a 2000mm long unsupported length, hence the question about '20mm tooling plate and ali extrusion be enough' over this increased distance.
    I guess it does mean you would only need one rotating ballnut mount. The mass of the gantry is also increased substantially.

    Edit, didn't see previous post which explains it when I posted this.
    Exactamundo! Is it a basic rule of thumb to have the longest axis supported?

  3. #13
    Quote Originally Posted by r0bsk1 View Post
    Ok ill rephrase it......The 2000mm axis will be parallel to a wall and move away from the wall on the 1000mm axis.
    Loading the machine with material will be done in parallel to the wall as this side will be open.
    This will not be a moving table design.
    Ok understand now it was the fixed Y axis ref that thru me.!! Well that's ok so long has the long gantry is beefed up enough to handle the length.
    20mm plate and what size profile.?
    What you planning cutting.?

  4. #14
    20MM plate with 90x45 extrusion to mount the 25mm rails on.

    I'll cutting carbon fibre and ali.

    Oh and my other option is to make use of some 60x60x5 box section if needed.

  5. #15
    I've now got the 60mm box section. Its 5mm thick and bleedin' heavy. Should be idea for the table frame.

    Appreciate any thought of reinforcing a 2000mm unsupported span. The difference in cost of the 2 designs would be minimal due to the cost saving with only one ballnut mount will go in to tooling plate. I've been quoted 160+vat for 2000x150x25.
    Ta

  6. #16
    Ok so here's my latest incarnation. Gantry is 2000mm wide. Rails are 25mm.

    I've stacked 3 60x60x5 box sections on top of each other and will be welded together. The edges have a 10mm radius on them so will need several passes to secure them.....at least thats how they did in on Discovery's Extreme Engineering on a bridge build. Do you think i'll need a backing plate?

    Each 60mm section will have 2 supports to prevent the beam falling forward or back. I've used 20mm ali plate for now but may weld box section cut-offs direct to the beam to form a 'T' on the ends instead.

    A spinning ballnut mount is bolted to the back of the Y axis bearings.


    Click image for larger version. 

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    Ta
    Rob

  7. #17
    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.
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  8. #18
    The rails are bolted to the box section, even in post #15. Earlier designs for a 1000mm gantry had them mounted on 90x45 ali extrusion which was bolted to a back plate.
    I was hoping that tacking the sections together first and then do gradual passes would eliminate any heat distortion 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 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.

  9. 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

  10. #20
    Quote Originally Posted by r0bsk1 View Post
    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.

    Quote Originally Posted by r0bsk1 View Post
    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.
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

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