@Tinchus Are you talking about these tolerances?
What I was told, is that the tolerance of width N and height H has no effect on the straightness of the rail, at least for the MGN series rails. There are some rails that are wide enough that there is a straightness tolerance, but everything I have been told is that there is no tolerance for straightness, and that there won't be any guaranteed straightness.
This is one response I received.
"So typically single bolt hole row linear guides do not have a straightness tolerance. The rails do have a dimensional tolerance which is typically +/-0.0002" or so for width and height. The reason for this is that at any significant length (ie 10x the rail width or so), the straightness of the rail installation is determined by the accuracy of the mounting surface or mounting procedure. It is a bit like asking the straightness of a piece of wet noodle. It can certainly be made straight, but it just depends on how it is aligned or what it is sitting on.
For super high accuracy applications, you would want to follow the procedure in the catalog where you machine a grooved shoulder that the rail sits within. You clamp the rail to the machined edge and then tighten down the bolts. This ensure the rail matches the straightness of the machined edge. In practice, very few installations do this especially with the MG series rails. Instead, you would use a precision ground parallel or some other straight edge and tighten the bolt holes sequentially while clamping the rail to the straight edge. Even then, this is uncommon. Many users just loosely thread the bolts in,push the rail fully against the bolts while tightening without any straight edge at all. This does get you straight enough for many applications. What are you aiming for on straightness and over what span?"
This is the response I received from a different company.
"Linear rails get their straightness from what they are mounted to. Linear rails should ideally be mounted against a reference edge, see here:
Though some 3D printer designers don’t heed that, resulting in the alignment needing to be done manually.
The +/-0.023mm refers to something different than straightness. If the rail is mounting on a perfectly flat surface, the position of the carriage would be expected to deviate only 0.023mm in any direction while being slid along the rail. It is a measurement of the machining accuracy of the height of the rail. Some more reading here:
guide rails tend to conform to the surface on which they’re mounted. In order to realize the full benefits of a higher accuracy linear guide, the mounting surface should be machined at least to the same standard as the guide."
I'm not saying you're wrong, but I have people telling me conflicting things, so I can't make heads or tails of it. I know there is a tolerance somewhere, or these rails would be shaped like a piece of wet spaghetti.
@mrehorstdmd Here's the old machine. Probably has 3k hours on it with no real maintenance to the motion platform besides oiling and cleaning up debris/dust.
Here's the new machine. Probably about 10 hours on it.