Profile rail for gantry, laying flat, or on edge?


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Thread: Profile rail for gantry, laying flat, or on edge?

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    Default Profile rail for gantry, laying flat, or on edge?

    For supporting the heavy gantry I see some machines with the profile rail flat, and some vertical.
    Does it really matter? I feel like flat is better.

    My gantry is going to be about 200lbs, and using 20mm profile rail. Now the simple way for me to mount is to have it vertical, But I think making it sit flat would be best?


    On that note if I am going to make it sit flat would a piece of 2.5" square tube welded to the side middle of a 2x4 tube be plenty strong? To make a T shape, and I would put my rack on the bottom of that little part sticking out then.
    My thought for that is that I could use a straight edge to weld that 2.5x2.5 on after the rest of the table is built and get it pretty close to flat, compared to trying to keep my table top edge flat.

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    Default Re: Profile rail for gantry, laying flat, or on edge?

    pretty close to flat isn't good enough when you consider the stiffness of the system and how much force will actually be exerted on the rails during operation; profile rails are of several varieties, some resist moment (torque) more than others. Most of them are of equal stiffness in any direction, because of that it doesn't matter how you mount them (horizontal or vertical) (for the most part). if mounting them horizontal or vertical changes their spacing, then that is often a big deal. (for example if the bottom rail is horizontal, then mounting the top rail on top of the gantry increases the distance between the two rails as much as 10%. This increased distance would make the gantry bearing blocks at least 20% stiffer) (if the bottom rail is mounted on the bottom of the gantry, then you lose the rail height for gantry clearance obviously, so this isn't usually done)

    Some people shim the rails, others use rails large enough such that the gantry conforms to the bearing, not the other way round. Either way usually you try to get one as straight as possible and then bolt the 4 bearing blocks to your Z axis assembly, and as you slide the assembly across the gantry you measure the deflection of the other rail (hopefully you can't measure it) and tighten the bolts it as you move it.


    btw i recently witnessed a diy router that used 60mm wide profile rail for x and Y. the Y axis gantry was a bar of steel, about 6 inches thick by 24 inches wide, spanning about 8 feet of y axis travel with a rail bolted to each side of it. why they did it that way I have no idea, I would have used a bunch of I beams welded into box sections. they probably had a 40 hp spindle on it.

    in this particular example, its relatively easy to calculate.. what is the added bearing load, if the distance between the rails changes .001"? and the answer is probably measured in tons...

    Last edited by Eldon_Joh; 12-06-2016 at 02:46 AM.


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    Default Re: Profile rail for gantry, laying flat, or on edge?

    When I say pretty close, I was more meaning as a starting point. Easier than trying to keep my whole frame as perfect. I mean I'll still try and keep it as perfect as I can, but trying to reduce the amount I need to shim or fluid level.

    Had almost thought about using angle iron and capping the ends and then pouring a thick epoxy, but turns out thick apoxy is a pain.

    I'm hoping to have my 4*8 work area within 1mm. And then the starting point of the T part being 0.1mm max out.



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    Default Re: Profile rail for gantry, laying flat, or on edge?

    I feel that linear bearing rails mounted flat to the top surface of profile (aluminum or steel) seems better. But in practice they can work in any orientation. It boils down to what works best for your design.
    I've done it both ways. Mounted on top was the easiest but caused more work to shield them from swarf and I bolted them directly to the top of aluminum extrusion. On my second machine, I mounted them on the side of aluminum extrusion. I had a base of epoxy to use as a reference edge and I used a spacer block between the epoxy and the rail to set the height. This worked well. The gantry glided the entire length of its travel while pushing lightly on one side with my finger. I credit the flatness of the leveling epoxy for this.

    In your case, you could use epoxy to get a flat mounting surface on your side mounted 2.5"x2.5" square tubing but, since you couldnt connect them with an epoxy "bridge" and pour them at the same time, the epoxy wont self level and therefore they wont be co-planar.



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    Default Re: Profile rail for gantry, laying flat, or on edge?

    Mounting of the rails can be dependent on the the type of profile linear rail you use. For example THK HSR/SRS rails are 4 way equal load type and can be mounted in any direction. Their SR type are radial type and best mounted horizontal. It's best to read the manufacturer datasheet for optimal mounting position for your linear rails. THK, iko, ina, nsk, hiwins etc. all have many different kinds of linear rails designed for specific loads.



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    Quote Originally Posted by jfong View Post
    Mounting of the rails can be dependent on the the type of profile linear rail you use. For example THK HSR/SRS rails are 4 way equal load type and can be mounted in any direction. Their SR type are radial type and best mounted horizontal. It's best to read the manufacturer datasheet for optimal mounting position for your linear rails. THK, iko, ina, nsk, hiwins etc. all have many different kinds of linear rails designed for specific loads.

    They list equal load all directions. But still feels wrong. Lol.



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    Quote Originally Posted by 1Jumper10 View Post
    I feel that linear bearing rails mounted flat to the top surface of profile (aluminum or steel) seems better. But in practice they can work in any orientation. It boils down to what works best for your design.
    I've done it both ways. Mounted on top was the easiest but caused more work to shield them from swarf and I bolted them directly to the top of aluminum extrusion. On my second machine, I mounted them on the side of aluminum extrusion. I had a base of epoxy to use as a reference edge and I used a spacer block between the epoxy and the rail to set the height. This worked well. The gantry glided the entire length of its travel while pushing lightly on one side with my finger. I credit the flatness of the leveling epoxy for this.

    In your case, you could use epoxy to get a flat mounting surface on your side mounted 2.5"x2.5" square tubing but, since you couldnt connect them with an epoxy "bridge" and pour them at the same time, the epoxy wont self level and therefore they wont be co-planar.

    Oh I wasn't even thinking of co planer. I could add a trough of some sort to join the sides together.



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Profile rail for gantry, laying flat, or on edge?

Profile rail for gantry, laying flat, or on edge?