Choice of linear rail size - what to pick!

I am designing a really big gantry style printer for my business with a 60 x 100 cm bed. The printer will have 2 X carriages hooked together to make 2 identical prints at the same time. The X axis is the 100 cm one.
I am looking at linear rails/bearings and am wondering which size to use. Even 9mm rails support far more load than I need but I am thinking that the larger 15mm and 20mm ones may be smoother and better than the really small ones.
The extra mass of the carriages is pretty trivial. I am considering using HGR20 rails for the Y axis and MGN 12 or MGN15 rails for the X axis with the hot ends on it.
I am also considering using HGR20 for the bed which is the Z axis.
Any suggestions/comment?

Gantry-style implies a fixed bed but other things you're saying imply that the bed is moving. Which is going to be fixed in Z, the bed or the extruder?

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Hi Dentist

The bed will be moving up and down (Z axis). By gantry style I was just trying to state it wont be core XY.

Edited 1 time(s). Last edit at 11/19/2017 09:39AM by SplatHammer.

I'd probably go with HGR15 or 20 for YZ and MGN12 for X. Your printer is large enough that you want to be getting most of the rigidity from the extrusion that the rail is being bolted to, as an aluminium tube is stiffer per gram of weight than a solid bar of steel.

You should reconsider making the X axis the long axis. As a general rule you want the moving axis to be as short as possible.

Technically, for smooth motion your block spacing should be at least half of your rail spacing. If your Y rails are spaced 100cm apart (for 100cm of X travel), the Y blocks will need to be spaced 50cm apart which means you are buying 110cm+ rails for an axis that provides only 60cm of travel.

In a low load application you can probably push the blocks closer together but it will still be very space/weight inefficient.

Edited 1 time(s). Last edit at 11/20/2017 06:34PM by 691175002.

I designed a similar large-footprint gantry machine with a 590mm x 350mm X-Y base. I used 12mm shafts and linear bearings for the longer X axis and 10mm shafts and linear bearings for the shorter Y axis. The workbed is 5/16" (~8mm) glass. I called the machine the "Visible Robot."

This worked out better than I'd planned, in that the glass "droops" at just about the same rates as the shafts. The tip of the end-effector, when the bed is leveled and the "home" position for 'Z' is worked out is well within the .3mm thickness I use for 3D-printing. This is true anywhere on the workbed.

So, there's this added consideration of having the end-effector tip, as it moves around the X-Y plane, be in a parallel plane with the workbed.

Like you, I had ideas of mounting more than one gantry--I mounted three in a design I called the Visible 3Bot. Each gantry is separately controlled. For now, each has a dedicated workspace of about 13.5" x 4", but I envisage a controller between them to allow co-operation.

See more at finixsystems.com

Interesting design, finix.

If I get it right all your axis are lead-screw driven, right ?
Have you recorded what maximum speed - or speeds, probably depend on which axis - you can achieve with that design ?


As a side note you are not really on topic for that thread since you don' t use rails - next time you should create a different topic.

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Most of my technical comments should be correct, but is THIS one ?
Anyway, as a rule of thumb, always double check what people write.

Sorry DeuxVis, I read "rails" as "shafts."

Yes, all the axes are leadscrew driven.

And, no. I haven't recorded max speeds for any axis. For X and Y the leadscrew pitch is 1/4" (6.35mm). I've used both NEMA17-60 and NEMA23 steppers. No missed steps with either. The limiting factor in terms of speed seems to be gantry height (gantry stiffness vis-a-vis height). I've tried three different heights. (Check the 3Bot photo). The shortest is clearly the stiffest. The tallest shows visible "wobble." I'm currently toying with the idea of replacing the gantry risers with a bent-metal solution.

Again, sorry for the O-T.

No problem, we all make mistakes. English not being my primary language I might as well be wrong here too.

Thanks for the answer.

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Most of my technical comments should be correct, but is THIS one ?
Anyway, as a rule of thumb, always double check what people write.

Thanks for the advice. I have decided just to make the first version as a more normally sized 60x50cm bed and forget about having it print 2 copies at the same time.
I looked at the amount of droop I would have and realised I would have to make the x axis at least 40x20, which would be a bit heavy.
Just to be awkward I think I am goint to try making the x axis out of a cabon fiber tube with the rail mounted on top!

Actually, to me all info is always useful, if not for this then for something else! I have decided you halve the size and just go with a single unit printer instead of 2 items at once, at least for the first version. I think I am going to try carbon fiber square tubing for the x axis to keep weight down and rigidity up, with the linear rail mounted on top of the tube.