Velmex UniSlides are lead-screw driven positioning assemblies available in a huge variety of models and configurations. Over the years I have owned more than a dozen of these in 1.5 inch, 2.5 inch and 4 inch size units using them for focus rails and XY positioning units.
The main advantage in a Unislide over a rack and pinion type focus rail, like something from Novoflex, or a linear stage micrometer slide is that the Unislide is not affected by gravity so the position of the camera will not change when you tilt the camera up or down. A Novoflex focus rail always needs the lock to be firmly in place unless the unit is perfectly level. A linear micrometer driven stage can only be used in one direction. The Uniside can be used in X, Y or Z setups.
Pros and Cons
What I like:
Good supply on the used market
Made in USA
Customer support from Velmex
Very good cost vs performance ratio
Very easy to convert to a Arca Swiss QR system without any cutting or drilling
What I don’t like:
Nothing so far