Straightening Linear Rails

[bassman2914]

I have some linear rail that aren't technically made for precise linear movement but from my measurements and the data sheets they seem to be capable of linear movement with a max deviation from center of .0005" or less(that's the best I can measure with my calipers) so they should work fine if i mount them properly. I say this because the rails themselves aren't perfectly flat but the slides(from what I can measure) don't deviate more than .001" from the mounting surfaces meaning if I mount them on something straight the rail will be straight meaning my machine will move straight(at least within my capabilities of measuring). In one dimension the entire rail(actually all 6) are bowed(I assume they're not all bent but bowed intentionally for some design reason) with a calculated radius of 700ft. This means at their full length of 41in in the center it dips .03". At the length I'm using which is 24" it will dip .01"(still unacceptable for cnc work). If I'm wrong and it's got a bend it could be 0 or .05"(depending on where it is). To straighten this out what should I mount it to? Is there any place I can buy aluminum stock that's been machined on one side so I can mount it to the back to straighten it(this would stay there and if it's thick enough it won't bend a considerable amount). Another option is aluminum that's been stress relieved(if that's necessary) so I can machine one side. That is the best option for price but I've read so much on metal warping that I just don't know if it's a problem or not. I would love to hear any other ideas for mounting options(or straightening) that are better or cheaper. Thank You

[CATCH22]

I had the same thing,when you tighten the rails down with your bolts it will flaten them out but you also need to straighten them by running a indicator down the side and tighten them so they are straight that way too.We have a big mill at work so I just mounted my rail plate to the table, machine it flat and tapped all the holes in one setup then snugged them down and ran an indicater along the side and tightened it accordingly.Really you should machine a shoulder so you can pull the rail up to it and tighten it down that way,alot less chance of it moving after also.

Hope this helps!

Mike

[bassman2914]

I was thinking about a shoulder to press it against but since the rail looks extruded with a precision ground inner raceway I can't be sure that they line up perfectly so I will use a reference edge and mount a dial indicator to the slide and adjust the position and tighten the bolts accordingly. My problem is what can I mount it to that can be milled flat without warping? I don't know if warping of 6061-T6511 is a problem but I've read so many different opinions that i just don't know what to believe. Does anyone know about how much the aluminum will warp if at all? What kinds warp less? Is steel a better choice? If they all warp where can I get some stress relieved aluminum flats for a descent price?

[keebler303]

Take a look at MIC-6, it is stress relieved, precision ground aluminum plate.

Matt

[bassman2914]

I've looked at that stuff but it's about 4 times as expensive and it's not even flat on the sides I care about. It needs to be flat on the skinny side meaning I would still have to machine it meaning i just wasted money for precision that i couldn't even use. At least with that stuff i know it wouldn't warp when i surface one side. Since I couldn't possibly use that for my entire machine I still need a cheaper solution. Is there somewhere i can get stress relieved aluminum that hasn't been machined? I just read somewhere that it's possible to mill off .05" on one side, when it warps in the opposite direction just force it straight or press it straight and then do the same to the opposite side and then the stressed layer should be gone and if necessary you can go over it again to get it perfectly flat(without it being forced straight). Would this be a valid test if i used some aluminum from home depot just to test a theory? It is extruded so it should be the same?

[wildwestpat]

Find the data sheet for the section you thinking of using. The data sheet will give you the necessary figures to work out the deflection mid span. Think of the rail being pulled down if it is bent upwards at the middle and supported on blocks thick enough to allow the rail to become straight and the alloy section to bow up. The force that straightens the rail will be equal to the force that will bend the section out of true. What ever true is as all sections will deflect even under their own weight!

Now it is simple arithmetic to work out the forces and the movement. What you will find is that you have to accept that the alloy section will deflect and the rail will need to rest on unequal packing pieces to get the rail flat. Since the rail's flatness is going to help determine the accuracy in terms of level IMO the fact that the alloy section has been deflected is irrelevant. The alloy section will also deflect under the load of the combined loading from the other axis. This comment applies to all the beams that carry weight in your design.

Start by working out the weight of the tool bearing shaft and work backwards to the trucks calculating the weight of the parts to get the total weight on each linear rail bearing. Acceleration will increase the forces exerted by the static weight but this is a later stage in the calculations.

Another approach is to copy a design known to work and give the accuracy you want. Then adjust the packing slips between the section and the rail as necessary. IMO you will need the rail mounted on the narrow side of the section but do the sums on the amount the section will deflect. It is boring but the calculations are necessary. You also need some mounts for the rail and to think about the screws that will hold the rail and the section together.

Do your rails have holes in them at regular intervals (4" or 100mm would be common)? If so it is probable that they were intended to mount on opposite sides of a common section. If so the forces on the alloy section cancel resulting in flat rails and unbent section. As you have more rail than you need this is probably your best option and avoids the arithmetic! No sorry you still need the sums and weight to work out the static and dynamic deflections and the size of the motors unless you are copying an existing known good design and the flattening of the rails was your only question.

Good luck - Regards - Pat

[jsheerin]

The reason people have been telling you to use the tooling plate is because you're talking about wanting to build a very precise machine. I machine extruded 6061 all the time and never think twice about it warping, but I'm not making anything that needs super precision.

The other factors that it would be nice to know about linear rails is how stiff they are and what their load rating is. Load ratings are pretty common, but stiffness numbers are less so. I had to email THK to get some numbers. Of course if you're just building a router to cut wood, the above is probably overkill unless you're building something really big. Honestly you might get better advice by starting a build thread for your router where you put up some sketches and the specific components you're looking at. All these questions can be answered in a vacuum but when you throw out 'very precise', people will default to giving conservative answers that might be overkill for what you're trying to do. But if you just enjoy the learning and doing the engineering, there's nothing wrong with that.

[yancho]

To check the curvature of the rails must for art upholding them at both ends.The easiest way to bring the beam has a liquid metal.Rail mounted at the ends with bolts and check deflection (curvature of aluminum).Cooking liquid metal rubbing under the rail and ready.If you go to straighten aluminum goes that he saw. Accuracy of production of leaves is less than rail.Hope I helped.Good luck

[wdbox]

Yancho

Say again?

[keebler303]

Yancho

Say again?

Google translate falls a little short...

Matt

[yancho]

Geometric accuracy of the rail is greater than that of rolled aluminum sheet.But it also has precision workmanship.Rail supported at each end.Rail supported at each end.By checking the deflection of the beam with indicator,by tightening or loosening,average deflection adjustable bolts.The liquid metal is sticking to the end position when everything is aligned.This development has been the subject of books centering on manufacturing technologies in Bulgaria.If I have not explained exactly do I schedule Autocad if Solidworks ,I do not know how to upload files.

[wildwestpat]

Hi Yancho

I like the sound of what you are doing with that molten metal. Please can I ask you a couple of questions as it might have other uses.

1. What is the metal that is being melted?

2. What temperature does it melt at and do you need to raise the other parts to the same temperature?

As an observation have you tried two part resin as a replacement for the molten metal as I am a little concerned that unless the heating is done slowly in an oven (very even heating) the parts might distort.

Kind regards - Pat