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Longest Linear rail possible using a Screw driven rail?

Discussion in 'CNC Mills/Routers' started by Hamdi Hirzallah, Oct 17, 2015.

  1. Hamdi Hirzallah

    Hamdi Hirzallah Well-Known
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    Hey there everybody,

    Decided to try my hand at building CNCs. and Boy does this site have resources for this.
    So basically, I want to make a 1.2 x 2m CNC using Aluminum channels. and I wonder if screw driven linear actuators, given that I find screws long enough, can handle such a long run for both X&Y? I can use belts for now as I only intend on using the CNC for marking/scribing, but I have to know if I can use screws for runs that long without issues as I'll probably attempt to add spindles later on and those will need the extra strength.
    I also have to know how long they can go as I will probably wind up wanting to make it even longer later on (up to 4m is still beneficial for my uses.
    would really appreciate links to any builds that are similar to this one.

    Thanks guys.
     
  2. Joe Santarsiero

    Joe Santarsiero OB addict
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    Yes.
     
  3. Joe Santarsiero

    Joe Santarsiero OB addict
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    Just being a smartarse. :D

    There is a critical speed chart in the resource section. I'll link it in a second.

    Joe

    Nook ACME critical speed chart | OpenBuilds

    Follow the nook link for more data.

    For a 4m length you should probably look at rack and pinion instead.
     
  4. Jonny Norris

    Jonny Norris Veteran
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    What type of screw?

    Tho joe is right r & p for 4m is your only option really. Leadscrews will be too slow and likely bind up with screw sag and long ball screws of 2m+ you should use moving y supports to reduce sag so gets complicated not to mention expensive, critical speed drops off the longer you go so you combat that by going up a pitch to something like a 30-35mm with which you need nema 34's or servos to overcome the drop in holding torque. For those size screws and nuts will likely set you back £800 each and the moving supports are basically an extra 2 nuts per screw on a frame at £130 each.

    So yeah r & p is really the better option.
     
    #4 Jonny Norris, Oct 17, 2015
    Last edited: Oct 17, 2015
    Hamdi Hirzallah likes this.
  5. Hamdi Hirzallah

    Hamdi Hirzallah Well-Known
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    I see, a rack and pinion would probably sky rocket the cost.
    I'll cross that bridge when I get to it.

    What about belts for the light duty build? The initial 2m build will be for maker/scribe use. Would a dual belt drive do the trick?

    Thanks
     
  6. Joe Santarsiero

    Joe Santarsiero OB addict
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    A dual belt setup would be fine for the 2m build. Basic acme screw will also work for this length. Just make sure your diameter is right for the length and rpms.
     
    Hamdi Hirzallah likes this.
  7. Jonny Norris

    Jonny Norris Veteran
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    Not too bad if you go for a plastic derived one
     
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  8. Hamdi Hirzallah

    Hamdi Hirzallah Well-Known
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    I risk sounding noobish here, but can you explain the plastic part or at least link a build that uses the concept.
    Thanks man.
     
  9. Jonny Norris

    Jonny Norris Veteran
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    a nylon rack would be the obvious choice and the principles of actuation are the same as with a steel rack and certainly would be an improvement over belt but for reference I believe the guys behind the Area 51 openbuilds did it. Afraid I can't locate the link but you could try a search.
     
  10. Hamdi Hirzallah

    Hamdi Hirzallah Well-Known
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    Thanks, at least now I have a name. I can build on that.
    Much appreciated.

    EDIT: found those plastic racks
    they're really cheap, but are they any good?
     
    #10 Hamdi Hirzallah, Oct 19, 2015
    Last edited: Oct 19, 2015
  11. Flash22

    Flash22 Veteran
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    Anything over 1-1.2m you need to be looking at 14-16mm lead screws tr8x8 at 1m has a fair bit of whip but it doesn't seem to be loosing any accuracy - time will tell tho
     

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