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  1. #1
    Is aluminium/steel box section straight enough to mount a supported linear rail on opposite sides and expect it to run smoothly without binding.

    Here's an example to clarify. I'm thinking of putting a piece of 100x50 box section between those two horizontal rails.
    Click image for larger version. 

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  2. Depends on the spec... and how it was manufactured.. Extruded may well be, welded steel box section maybe not, might need a pass with a mill..

  3. #3
    'Pass with a mill'
    Skim it with a fly cutter on both sides? What about the internal stresses in the material, is that going to be an issue?

    [Shame they sold the surface grinder at school :(]

  4. surface grinder would be better still...

    I've not had much experience of large 'beam' of that material... JohnS might be able to give a more informed opinion

  5. #5
    Assuming the rail is straight and if it's only a few tenths here or there you could use a thin layer of epoxy.

  6. #6
    Extrusions are very accurate, either profile Ali or RHS Ali section. Bit more expensive than steel, but alot of steel is ERW (electric resistance welded) with a seam down one side. I remember reading a post where they wished they'd skimmed the steel section before mounting the rails. Factor in the cost of getting two faces ground and the Ali profile is your best bet.

    By the way, in your sketch showing the box section assembly wrapped around the gantry this could probably do with a slight re-arrangement. As drawn the top and bottom plates fit on top of the sides. This means that the sides have to be very accurately cut to get a nice just-right preload on the bearings.
    If you make the sides longer, and have the top and bottom plates inset, then the play in the bolt mounting holes (~1mm in total up and down) which are now in the sides and not the top will allow you to put it together better, and set the system up with just the right load. Otherwise you risk either squashing the bearings or pulling them apart.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  7. #7
    Thanks for that. I reckon I'll go for 80x80x3mm aluminium box section then. There's someone selling it near me for £20 per meter. That does make the centre distance of the Y-rails only 134mm - is that OK?.

    Excellent point about the bearing preload - that's probably saved me a lot of hassle! I'll change the drawing now.

  8. #8
    The machine I got from ebay has, from memory, 90mm Rhs, a 9mm wall and its welded (erw) and that has been milled for the rail mounting surface (I guess thats why they used 9mm to allow for the removed steel) this has only been done on one face so internal stress wouldnt have been a problem.

    Ali is also lighter, dosnt rust (not like steel anyway) and dosnt need painting


    Are you using the rails back to back on the y axis to counter the uneven load distribution? (ie 25% reduction at the open side) if so have you looked at doing the same with the zaxis? if you mounted them on a c channel (or box with a slot cut out, then you would have space for the Drive screw as well)

    Just a thought and is some thing I was looking at a while ago for an Ali box and supported round rail machine. Not enough hours in the day..........
    Last edited by Ross77; 24-08-2010 at 01:33 PM.

  9. #9
    Quote Originally Posted by Ross77 View Post
    Ali is also lighter, dosnt rust (not like steel anyway) and dosnt need painting
    True, but it's not as strong and more easily transmits vibrations since it's less dense...

    Quote Originally Posted by Ross77 View Post
    Are you using the rails back to back on the y axis to counter the uneven load distribution? (ie 25% reduction at the open side)
    Exactly.

    Quote Originally Posted by Ross77 View Post
    if so have you looked at doing the same with the zaxis? if you mounted them on a c channel (or box with a slot cut out, then you would have space for the Drive screw as well)
    I looked at it, but didn't think of using C-channel which would make it quite simple.
    The place near me has some 3"x1"x0.25" aluminium angle. There's also U, 2" x 3" x 2" ... only 3/16" thick though. Time for some more drawing / calculation!

  10. #10
    True, but it's not as strong and more easily transmits vibrations since it's less dense...
    Personally for moving gantry with steppers I would go for Ali. Fixed gantry then heavy duty steel box...You pays your money you takes your chance.....

    You can design for extra strength and reduce the vibrations in the first place and it dosnt become such an issue. (I'm sure someone will disagree with that.:naughty:)

    This is what I was looking at, the idea was that the two horizontal box sections top and bottom with the c channel and vertical box in the middle would provide a pseudo I beam but with improved torsion due the central box, the c channel would house the Ball screw and there fore keep the z axis in tight to the gantry

    The idea for the z axis was similar in that the spindle and hence cutting forces were in close to the gantry and the back to back rails to give an even load distribution. I figured that a wider z carriage was acceptable as the increase in rails needed is marginal compared to the bearings.

    Anyway thats as far as i got and I know there are many improvements but hope it of some use or sparks another idea
    Attached Thumbnails Attached Thumbnails U channel frame 1PDF.pdf   U channel frame 3PDF.pdf   U channel frame 5PDF.pdf   U channel frame 4PDF.pdf   U channel frame 2PDF.pdf  

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