Which is best 1 large or 2 small rails

[JohnZ]

So, I am in a bit of a pickle here. I am in the process of rebuilding / upgrading my machine, and I had been using a pair of used THK SR20 rails for my X axis. When I was putting everything back together, I found that my rails were not exactly parallel, and the strain on the rails / trucks resulted in the rails flaking in multiple locations and the trucks having a lot of slop on the rails. The motion is no longer smooth, so I plan on replacing them. Looking around, the best deals I can find are on Automation Overstock.

My question is this, for about the same amount of money, I can purchase one of the following. All are Hiwin rails and LGW trucks, with the same load, preload, and accuracy classifications, and would all give me about the same total travel length (48")

Two 20mm rails with two trucks per rail, same size I already have.
Two 30 (or 35) mm rails with one truck per rail
One 65mm rail with two trucks on the rail (This rail is one classification of accuracy lower than the above two)

My concern is this. My current setup was pretty close, without much noticeable binding except for one end which I never used as the rails had some pitting at that end, so I just avoided that end and lived with the reduced travel (The rails were free from a friend). I kept them greased up well, but apparently my setup still put too much bind on the rails and they self destructed themselves in less than 100 hours of use.

So, as part of my rebuild, I am planning on rebuilding the entire bridge supporting the X axis rails and replacing the rails. However I do not have the facilities, or funds available to hire a shop to grind a perfectly flat set of mounting surfaces for the bridge. Once I get the machine built, I plan on building a grinder attachment so I can use the machine to do surface grinding, but I need to get it built first...

My main question is this, I want the machine to be as stiff and accurate as possible. So, if I replace my 20 mm rails with 20mm rails, I could potentially end up right where I am now, and will the 20mm rails and trucks be stiff enough to mill steel? If I go with the 30 (or 35)mm rails, with my budget, I only get one truck per rail, but those trucks are rated for higher loads, and are stiffer than the 20mm trucks and rails. The last option is the single 65mm rails with two trucks. This rail is way stiffer than the smaller rails, and each truck is rated for over 7,000 ft/lbs of rotational load, but there is no rating for how stiff the trucks are for rotational forces, but if the 25mm rails I bought for my Z axis are any indication, those 65mm rails would be extreamly resistant to rotational forces. So... Which option should I select to upgrade my X axis beam?

Thanks - John Z

[underthetire]

Personally, 2 rails,4 trucks. Will give you the best vibration dampening.

[JohnZ]

Personally, 2 rails,4 trucks. Will give you the best vibration dampening.

Any other input.

One additional thought, I do have two 1" diameter unsupported rails and four 1" diameter SPB linear bearings that I could use as a second double upper "rail" if I went with the third option with the single 65mm rail and two trucks. I have been going back and forth between all three of these options, and have decided which one would be best for my machine (and construction skills). What ever options I go with, the rail(s) will be mounted to 1/2" thick steel plate(s) bolted to concrete filled 2.5" square tubing making up the stationary bridge for the X axis.

Thanks - JohnZ

[harryn]

It is an interesting question, that we all face.

I don't know the answer, but I have seen people use self leveling epoxy to level out an area, then bolt down some 1/4 in steel or Al plate to mount the rails on.

You can even extend a bridge over from one area to another to level them to each other.