Threaded rod is designed for use in constructing things. It is meant to hold things together, to suspend things, or to run through things for internal strength. They are designed to have washers and nuts put on them and for those things to hold their place.
The goal of the design and manufacture of threaded rod is to create a product that you can easily thread a nut onto and get that nut into position and have it stay in position. In short, they are designed to bind when there is any (non-rotational) force applied to a nut (including in-axis force like the weight of a z-stage).
Tension vs Compression
They are also designed to be used in tension . If a printer does use threaded rod for motion, the results will generally be better if the rods are suspended from the top of the printer , supporting weight in tension rather than if the motors are on the bottom holding the rods up and supporting weight in compression. Keeping the rods in tension rather than compression will minimize bowing and bending, though a bent rod can still cause problems. While the deflection from compression may not be very visible, you can bet it’s playing a role in your accuracy when your carriage is near the top. Many printer designers minimize the compression issue by using very large diameter smooth rods that are more resistant to bending, but a tensile arrangement with or without larger rods is a better solution.
Though not related to the linear motion, it’s also worth noting that most RepRap designs that incorporate threaded rods utilize most of them in varying degrees of compression, rather than tension. This is a poor choice of material here, as these members can flex and bend; construction of this type will require much more constraint to make a solid frame, possibly leading to over-constraints that can cause issues. Metal extrusion (such as is used in the Mendel Max) is a much better choice for a framing element. Other newer designs, such as the Prusa i3 also avoid this problem by using a large plate of material instead of the rods.
Because of the usual applications of threaded rod, there are also no guarantees about the straightness of the rod to begin with. Poor handling in transportation and storage, or the manufacturing processes themselves may produce rods that have some natural bend to them. To some extent this is overcome by using solid, stable smooth rods or other precision linear motion mechanics to constrain x/y-axis motion and letting the un-driven end of the threaded rod float so it can only influence the z-axis motion, but the quality of that linear motion system (both components and design) will have a direct impact on how much or little of an impact the curvature of a threaded rod will have.
The quality of threaded rod will also vary wildly, but in general won’t be great as compared to leadscrews. For its standard applications, the tolerances of the rod only need to be good enough to reliably thread a nut of the proper size. There can sometimes be a fair degree of slop between the nuts and the rods. Rods of different sizes, or from different manufacturers will vary, and you can bet they’re not optimizing their processes for consistency beyond what is sufficient for its intended application.
Smoothness and Surface Finish
The surface finish on a threaded rod is a combination of roughness from the treatment against corrosion, and burs that have only been removed so well as to make sure a nut will thread. Again, this comes back to the idea that they are designed to bind. Threaded rod should not be any smoother than is required to thread a nut; the roughness on the surface helps things to bind when non-rotational force is applied.
Post time: Oct-11-2016