Whenever a 3D printer that uses linear rails is announced (case in point: the cetus), the Internet (well... at least that corner of it dealing with 3D printing) gets abuzz with excitement.
I researched a bit the topic myself, and while I understand that linear rails can be produced to a fantastic degree of precision for super-heavy machinery, it escapes my comprehension why they are considered superior "by default", relative to the classic linear bearings on a shaft.
3D printing is a lightweight application, and the motion of at least 2 axis does not happen against a solid surface (where you could bolt a linear rail every few cm) but suspended between the 2 ends of the axis. Furthermore, the internals of the bearings used on linear rails are substantially identical to those used on a shaft.
The cetus site says under the heading "Quality Linear Rails":
Self-lubricated | Maintenance Free | High Precision | Long Lifespan | Quiet
but this - in my experience - can be said of "Quality Linear Bearings on a Shaft" as well, and in some cases even bushing deliver to a high standard on 3D printers.
So, what am I missing?
The following is a compilation of the input from a number of sources.
Linear rails in general are mechanical components that - when designing equipment - offer great flexibility.
The profile of the rail can be designed in nearly infinite ways. This in turn allows for:
- Different levels of stiffness in different directions (for example you may have stresses only on a given plane, or you may actually want the rail to slightly flex in one plane but not in another one).
- Placing the surfaces for the rollers strategically, for example in a location that is unlikely to get contaminated, or where the maximum force will be applied.
- Curved paths, so that the carriage can move along a line that is not straight.
Because the contact surface between the rollers and the bearings is flat, cylinders can be used instead of spheres. This in turns diminishes the mechanical stresses, and the amount of play, increases longevity and allows for more bearing capacity, among others.
Linear rails can be anchored along their full length, rather that at their extremes, thus increasing the accuracy of their positioning, their stiffness and their bearing capacity.
Linear rails can be machined while pre-loaded, thus achieving maximum accuracy when in use, rather than when coming out of the factory.
The bearings on a linear rail only allow for one degree of movement. There need to be two rods with linear bearings/bushes to achieve the same result.
All that said, when it comes to the specific application of consumer-grade FDM 3D printers, it seems that none of the above is very relevant, nor confers any real advantage to the printer in terms of quality of the final print:
- the mechanical stresses involved in 3D printing are very small,
- the movements all happen along straight lines,
- most of the axis cannot be anchored to a large, rigid body,
On the other hand, the design with rods + linear bearings is cheap, effective, simpler and lightweight, all characteristics that are highly desirable in a 3D printer.
All in all, it seems that there is no good reason to prefer linear rails over rods in general.
Still, there may be specific designs that may benefit from their adoption. I postulate that the Cetus printer linked in the question may be such a design: the cantilever arrangement of its axis - for example - is well served by the fact that a single rail locks movement in all but one direction, and the orientation of the X rail offers maximum rigidity against the action of gravity.