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  1. #31
    Any thoughts on an under slung gantry? I'm just thinking if i'm using 200 or 250mm box section and mounting to the top of the gantry then the Z axis is effectively 200mm + rail / cartridge height before its even descended at all.

    It might be a little tricky to mount the rail, i'd have to jack it up off the bed I suppose. The gantry could be epoxy leveled upside down.

  2. #32
    Quote Originally Posted by Rufe0 View Post
    Any thoughts on an under slung gantry? I'm just thinking if i'm using 200 or 250mm box section and mounting to the top of the gantry then the Z axis is effectively 200mm + rail / cartridge height before its even descended at all.

    It might be a little tricky to mount the rail, i'd have to jack it up off the bed I suppose. The gantry could be epoxy leveled upside down.
    It crossed my mind too as you are right there is much less Z offset doing it that way. I did sketch something up but the problems I encountered were:

    > Side rails cannot easily be supported back down to the bed as the gantry and rails are in the way. The supports down to the bed would have to be outboard of the rail which puts in a bit of an offset.

    > Rails and ballscrews are exposed to swarf and potentially clamp collisions which is not ideal.


    I looked at the commercial machines and they were on top so I went that way. If it helps the box section for the gantry could probably be much smaller as with 2 of them you significant reduce the twisting. I would expect 2off 50x50 sections to be twice as good as 1off 50x50 for bending in X and Z, but importantly, significantly better than x2 for twisting. This is where the advantage is with 2 beams on the gantry.

    I also looked at rails on the inside faces, which is what Jonathan did on the 'sufficiently stiff machine', which is a bit more out of the way and has less Z offset. Downside is it spaces the beams out a bit further for loss of gantry travel. Not a deal breaker if you can make the machine longer.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  3. #33
    Quote Originally Posted by routercnc View Post
    > Side rails cannot easily be supported back down to the bed as the gantry and rails are in the way. The supports down to the bed would have to be outboard of the rail which puts in a bit of an offset.
    I think I understand you. I would mount it normally and reduce the travel by the frame width on each side.

  4. #34
    Just a little update, been a bit busy at the moment unfortunately however it maybe fortunate I didn't get too far as there are a lot of changes afoot, potentially moving house, as a result I can't guarantee I will have a nice big garage to put my machine in anymore. Possibly have to set it up in a bedroom, so it's going to have to be a lot smaller. I'm thinking 500x1000 working area is about the smallest I would want togo. Needs to disassemble into small parts. No pieces larger than about 1900x70 in order to fit through doors, and ideally all parts would be as small and light as possible so it's easy to carry them upstairs etc.

    Still need decent Z travel, tall items, primarily for steel/aluminium.

    Probably my best bet would be a bridgeport mill but that would be a bit of a challenge to carry upstairs...
    Last edited by Rufe0; 07-01-2016 at 03:22 PM.

  5. #35
    it would also be a challenge for the floor boards.

  6. #36
    Indeed indeed, so just brainstorming what my options are now. Thinking the height adjustment on the bed is abit of a hassle I could do without. But I still want to be able to work on tall items. So that leaves me with a very tall machine, would have to use waste boards or something to space the work up, maybe a hassle maybe not. Something like this.
    Click image for larger version. 

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    I'm wondering if there would be a better way though. I had an idea... it's basically a fixed gantry except the bed is fixed and the gantry moves(not shown but bed would sit on supports at both ends). I think it would look really cool, like some sort of scanner out of a sci-fi movie. Would offer better access aswell and I think I could make it significantly lighter. * not a proper drawing just an artists interpretation * shown with spindle hanging out the front for ease of drawing, obviously it would be a dual gantry with centralized spindle.
    Click image for larger version. 

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    I think both would suffer from strength issues, especially as there is no diagonal support to stop skewing.

    Any thoughts?
    Last edited by Rufe0; 07-01-2016 at 09:52 PM.

  7. #37
    Why don't you put the Z axis between the gantry uprights? It would put the C of G much closer to the center of the gantry bearings and you'd lose much less cutting area at the back end of the bed.
    Neil...

    Build log...here

  8. #38
    Quote Originally Posted by njhussey View Post
    Why don't you put the Z axis between the gantry uprights? It would put the C of G much closer to the center of the gantry bearings and you'd lose much less cutting area at the back end of the bed.
    Thanks which one are you referring to? Can you do a drawing?

  9. #39
    Quote Originally Posted by njhussey View Post
    Why don't you put the Z axis between the gantry uprights? It would put the C of G much closer to the center of the gantry bearings and you'd lose much less cutting area at the back end of the bed.
    Ah I think I get you, in the real design the spindle would be centralized to the gantry, not hanging out the front.

  10. #40
    Yes, that's what I meant
    Neil...

    Build log...here

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