Side mounting methods for linear rail?

Hi guys,
I'm in the brainstorming stage of a 4'x4' steel frame router build. I'm going with linear rails (25mm) and ball screws. I like the arrangement used on the Shop Sabre routers where the rails and ball screws are mounted on the side of the frame. My concern is how do you ensure that the rails on each side of the bed are square, straight,and level without having it machined? On a router with top mount rails you can level the mounting surfaces with epoxy but I can't think of a way to do that with side mounting. Is there a trick that I haven't found or is that one of the downsides to side mounting?

Thanks!

Well it isnt easy. This is one reason i prefer the rails on top. It should be noted also that the stiffer you make a machine (a good thing) the more important linear rail alignment is. A wood built gantry for example can flex considerably to compensate for rail misalignment. Even a flimsey steel gantry will flex. If you build a steel structure that really takes advantage of steel it will likely be stiff enough to create issues on misaligned rails.

One fab shop i know of achieves precision in its steel frame constructions by milling the ends of tubing square on a large horizontal. Getting your spacer pieces nice and square and the ends parallel with each other makes for precise spacing of the long rails, reduces stress in welding and makes setup for welding easier. So the first thing is to start out with precision parts where needed.

Now this will not assure that the sides are 100% parallel after weld up but it starts you out very close and maybe good enough. If your frame after weld up is small enough the best way to achieve alignment is to mill mounting pads on a suitable mill that can do it all in one setup.

That is an ideal approach! Many DIY dont want to expend that much cash so the alternative is to do it all by hand. There are again many ways but one that isnt considered often is to pour an epoxy surface plate and lay out level lines on each side of the X in the traditional manner. Basically you end up with two scribed lines that are in the same plane. You could also approach the problem with precision levels and precision parallel bars to locate the rails and the transfer drill positions.

Getting the rails in the same plane is likely easier than making sure your two sides are parallel. Even with precisely machined cross pieces you will most likely be out of parallel some. Steel tubing in most forms simply isnt that accurate. It is also far hardwr to describe a home shop solution that solves the problem. It comes fown to how good do yiu need this too be. If you arent off too mich i could imagine scraping parallel with that poured surface plate and precision levels and straight edges. It might also be possible to epoxy level.

It should be noted that steel tubing is often concave or convex on its sides. This throws another sticking point into properly aligning your rails. This can mean mandatory remmediation with machined surfaces being ideal.

wizard,
Thanks for the ideas. I like the idea of a an epoxy surface plate. If the plate is poured on a concrete pad I could count on it staying level.If I place the frame on its side with one rail mounting face pointing up I could epoxy level that high side and then flip the whole thing 180 degrees and level the other side. Assuming I keep the frame at 90 degrees I should end up with two straight and parallel mounting surfaces. Doing it that way would only require a surface plate as long and wide as my mounting surfaces since all it's really doing is ensuring level.

I could then create a level bottom edge on the mounting surface by pouring a narrow channel of epoxy with the frame upright using a bridge across both sides. I image a meniscus would form where the epoxy for the bottom lip meets the vertical mounting surface so I'd need to handle that somehow. Maybe route out the inside edge with a bearing guided hand router.

Anyone see any flaws in my logic here? I wonder if the table top epoxy used for bar tops would work well.

Thanks.

Some epoxy shrinks noticeably as it cures. There are lots of different epoxies each with different properties. I used steel casting epoxy with black pigment to pour a level surface for my router. I also had a machine shop machine pads for the rails and drill and tap the rail mounting holes. There are ways for the DIY'er to do it, but I chose to spend some of my budget at the machine shop since its so important.

I got some side mounting plates laser cut out of 12mm thick steel that had a series of slotted mounting holes along the top edge. This let me get the mounting holes for the gear rack and 25mm linear rails cut in perfect alignment and I just tapped all of the laser cut holes. At each end, I welded a nut on the table leg and out a 30mm long 8mm set screw that the so I could align the rail to be square. The attached photo shows one ot the screws just above the air regs etc. So it was just a matter of squaring the mounting plate up with the set screws and tightening up all of the mounting bolts.

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Originally Posted by Stinkyz

Hi guys,
.................. My concern is how do you ensure that the rails on each side of the bed are square, straight,and level without having it machined? Thanks!

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The reality is: you don't