Has anyone tried using THK etc rails as structural elements and the machine datum?

[digits]

Hi guys - I have been wondering how us DIY'ers might go about building an accurate machine with reasonably long travels (say 2000mm/6 feet) using off the shelf linear rails etc.

It would seem that one of the biggest problems is making the rail mounting surfaces accurate along such a length without access to an even bigger machine to simply grind or mill the surfaces flat. I was wondering if anyone had sucessfully tried turning the problem on the head, and used the linear rails as the accurate surface to which to align a machine base.

Feel free to shoot me down, but, the rails are straight during manufacture, and so presumably straight when unstressed and resting on a flat surface like a mill's table or a large suface plate. If you could machine some accurately flat short cross pieces, couldn't you then mount them to the back of a pair of rails, and create a rail + sleeper affair, with the rails parallel and at the same height.

If the 'sleepers' had horizontal slots in them, you could then through bolt them to the verticalish side of some angle iron which you'd milled vertical slots into. The horizontal side of the angle iron would have slots or holes to allow it to be bolted down to some solid chunks of imprecisie iron that would run along the length of the rails. If you then adjusted and tightened all the bolts to get the imprecise bits as solid and square as possible, wouldn't you still have a set of precisely aligned rails, now bolted onto a pretty imprecisie but solid base? If you then added a dozen or so adjustable feet to the I-beams, wouldn't you end up with a large floor-standing machine base with precisely aligned linear rails?

Here's a quick CAD mock-up of what I'm on about:

[Drassk]

The reason linear rails have all those mounting holes is that they don't hold themselves straight, you have to mount them so that they're straight. In other words, they can follow a precision surface but they aren't one themselves.

Check this document out for a description of what I'm talking about, look at pages 12-17:
http://www.automation4less.com/pdfs/hiwinaglg.pdf

[digits]

Hmm, I'm not sure I read that datasheet the same way - you are not trying to distort the rail to straightness with the mounting bolts, but restrain it. The rails are precision ground flat and straight in the factory - surely they don't curl up like bananas as soon as they pop off the grinder...

Looking at page 7 of this Nook pdf http://www.nookindustries.com/pdf/NookProfileRail.pdf it seems to say that the rails are straight to within 170um across the length of a 2500mm rail on the lowest precision (C7) grade. C5 is to within 60um.

And on page 8 they have a paragraph all about rail straightness - basically saying that the rails have to be straight to allow you to mount them to a straight surface.

So, assuming that they actually are straight, I guess you could carefully align a couple of rails face-down on a suface table, and then epoxy a paving slab to the rear sides to keep their relative positions constant...

Or have I just had too much coffee and not enough sleep?

[DIYaholic]

digits,
I would think your idea is possible.....
But is it practicle?
First, how accurate are the 2 "I-beams"? Then, how precisely can you align them? Same goes for the cross ties. How accurate are they? You would probably have to precisely set/shim/align each one seperately. The next step would be to precisely set/shim/align each linear rail. Seems like a lot of work with way too much room for error.
I think the real question is: How flexible are the linear rails and can you use shims at attachment points to maintain their straightness?
Just a non-mechanical engineer's thoughts. Good luck with your design.

Randy,

[digits]

The point of my hair brained scheme is to fix the rails in place at the places they want to be in when they are unloaded. The I-beams and most of the rest of the 'base' are assumed to be warped in all sorts of nasty ways. Therefore the bolting points for the rails are fully adjustable, and can be tightened up to fix the rails in position without pushing or pulling the rails out of their natural, more accurate shape.

When you finally flip the thing over, the base will look like it's been run over by an elephant, but the rails should still be parallel to eachother. I'm guessing that some adjustable feet could then be used to securely support the warped I-beam structure on the floor without altering its shape.

Like I say, it's all a bit hair-brained, but it might just work...

[Jason3]

Hey Digits,

Funny Randy should mention railways - check out the THK JR type rails - they are intended for use as a structural member and will absorb some mounting error. I've used them and found them quite nice to work with.

Best regards,

Jason

[Drassk]

Jason's mentioned a decent solution, but the product is quite different from normal linear rails.

The problem isn't how accurate they are, it's how stiff they are. In the end, a linear rail is still a steel bar and will sag and bend just like one unless it's mounted to a reference surface. The accuracy to which they're machined just means that they can maintain the accuracy of the surface they're mounted to rather than degrading it.

[digits]

Jason, those rails do look interesting!

Drassk - I do get what you're saying - the rails are just steel bars at the end of the day. I guess what I'm wondering is whether you can just support them sleeper style at regular intervals along their length, rather than having to have a continuous, accurate surface below them. They will obviously sag/deform between the mounting points but round rails that are only supported at their ends are accurate enough for some applications - I would guess this would be a lot better.

If I can get the construction of the machine structure down to just drilling, aligning and bolting up, I should be able to build some pretty monsterous machines. Drills have the advantage that you can take them to the workpiece. Are there any magic machines that you can just clamp to a wonky I-beam and have it milled/ground flat along its entire length - if so where can I get one?

[mactec54]

I bet the THK JR rail cost is high

digits
You can not use the normal rails or even expect them to work as a structural member, for mounting the rails, You have to think of them as a bearing , for a low grade rail almost anything goes But they still need to be supported, If you want to mount precsion rails you have to mount them as per the manufacture spec in both directions, The straightness tolerance as well as the flat surface tolerance that is needed for them, & yes some are like a bananas when you get them, they are not straight or flat

[lgalla]

If you pickup a long rail it will wobble like a wet spaghetti noodle.It cannot support its own weight.Just laying it down will not be enough to ensure accuracy.Rails must be mounted to precision structural surfaces.The mounting bolts are slightly smaller than the holes in the rail allowing tweaking into perfect alignment.Sorry I do not know how to explain it any better.
Larry

[digits]

OK, so I need a large precision surface to mount my rails on - how do I measure the flatness of this arbitrary length surface to within 25um/m (~0.001"/40")?

And if you were trying to build the world's largest machine, how would you machine this mating surface precisely flat without access to a machine large enough to do so?

[lgalla]

Here is a big one,240"long.
http://www.cnczone.com/forums/showthread.php?t=19418