Mounting Issues

The care which must be taken when mounting a precision positioning stage is often underestimated, especially if the full accuracy intrinsic to the stage must be realized.

When investigating claims of stage inaccuracy, we regularly determine that inadequate mounting provisions are the root cause of the errors. To better appreciate the sensitivity of mounting errors, consider the idealized example illustrated below (figure 7). A linear stage is bolted to a perfectly flat surface, using fasteners separated by 200 mm. A human hair, which is 75 microns in diameter (and presumed, for discussion purposes, to be incompressible) happens to lie on the mounting surface. The precision ways will now follow an arc, and the resulting Abbé error will produce an X axis positioning error of 75 microns at a position 100 mm above the leadscrew (or linear encoder), or 38 microns if the customer payload was mounted 50 mm above the leadscrew. When you may have spent some money to obtain a positioning accuracy well below this level, the need to carefully consider the mounting surface takes on new meaning.

Figure 7 – Mounting Error

In general, positioning stages are best mounted to precisely flat surfaces of reasonable cross-section. A useful guide is to make sure that the mounting surface flatness exceeds the desired stage flatness. If the stage can hold pitch to below 10 arc-seconds (50 microradians), then the surface should be flat to better than 5 microns per 100 mm. As previously illustrated, care should be taken to ensure that the surface is clean, and that no foreign particles lie under the stage. Tapped holes should be carefully de-burred, and fasteners should be set to designated (and uniform) torques. Lapped and properly plated aluminum and/or steel surface plates of adequate thickness make ideal mounting surfaces. Granite or ceramic surfaces can be made extremely flat, but it is imperative that the threaded inserts for fastening to the surface be installed prior to final lapping. If these inserts are installed afterwards, they may lie slightly above or below the mounting surface, with the potential to seriously degrade accuracy.

Some customers prefer to avoid issues of surface flatness through the use of spherical washers. While these can, in fact, eliminate concerns over surface flatness, this is only the case if three (not four) spherical washers are employed, and most stock stages do not offer a triangular hole pattern. Stage flexure when supported on only three of the four mounting points will degrade accuracy, and even if three equally spaced holes could be provided, flexure will occur in conventional ball or crossed roller stages, as the center of gravity shifts during motion. When mounted to a properly flat and thick base plate, the stiffness of the base of the positioning stage is increased, an advantage which is lost when spherical washers are employed.

In some cases, it is necessary to mount two single-axis stages together. The Dover Motion product line has been designed with this in mind; existing holes permit any linear single-axis stage to be easily mounted to any other such stage, in either an X-Y or coaxial orientation. The only rule is that the underlying stage must be of equal or larger cross section; for example, an RM stage will mount to another RM, RMS, TM, TMS, LM, FM, XM, HM, HMS, etc., but an HM stage cannot be directly mounted to an RM stage.