In the history of machine tools, spindles have been very good relative to other bearings and structures on the machine. So Quality professionals have developed a cache of tools – like ball bars, grid encoders and displacement lasers – to help them characterize and understand the geometry of the whole structural loop. This makes sense, as this is where the majority of the errors could be found. But as machine tools have improved in their capability and precision, and the demands of part-geometry and surface finish have become more critical, errors in spindles and rotating work tables have become a larger percentage of the total machine error. This is especially true in the manufacturing of gears and bearings because small errors in the work and wheel spindles have large effects on part profile and surface finish.

Once you have done all you can to improve the stiffness, damping, geometry and thermal stability of a machine, and the machine operator is using the best machining practices, the next frontier is to study the errors in the spindles. The ultimate roundness and surface finish that may be achieved by a precision metal-cutting machine tool is determined by the performance of its spindles. By characterizing and routinely checking spindles, part quality can be predicted and controlled.

The focus of this article is to summarize the groundwork already established for using spindle metrology to deterministically improve manufacturing processes. As a bonus, and something the pioneers of spindle metrology would all be quick to point out, is that spindle testing, which is defined in the standards now as “tool to work,” is also an excellent new diagnostic tool for other error sources in the machine. Most importantly, these standards are the standards that are used to qualify roundness and gear measuring machines and so offer needed insight into manufacturing equipment as we try improve the quality of bearings and gears.

The Standards which most-specifically refer to the quality of precision spindles are: ISO-230-7, “Geometric accuracy of axes of rotation” and ASME B89.3.4-1985, “Axes of Rotation, Methods for Specifying and Testing.” These standards are based on the concept of an “axis of rotation,” which is defined as a line segment about which rotation occurs. Spindle “run out” is an often-used term, but it is not consistent with standards for describing spindle precision. Surfaces have run out; axes have error motions.

There are three basic spatial error motions in spindles: Radial, axial and tilt (Figure 1). We will see later that spindle error motions are also characterized by frequency and by sensitive direction.

Summary

So what are the important take always for gear manufactures. First off, if you are manufacturing gears, take care that the error motions of your spindles are not limiting the quality of the gears you manufacture. The synchronous error motions of your work-holding spindles will determine the roundness, flatness or global tooth profile of the parts you manufacture. Residual face error motion of work spindles can cause flatness and profile errors too. Surface finish is dependent on the asynchronous error motion of both the work spindle and the grinding wheel spindle. Dramatic surface finish improvements can be made by characterizing and improving spindles using Axis of Rotation Metrology.

Spindle Metrology offers another tool to use to evaluate the vendors of gear manufacturing equipment, this is the main reason the standards exist. It is how the manufactures of metrology equipment qualify their machines, in a continuing search for improvements, applying these standards to manufacturing equipment is the next logical step.

Additional uses for Spindle Metrology Techniques

Sometimes the sources of variances in gear teeth contact, surface finish or noise levels can be difficult to identify, Axis of Rotation Metrology presents techniques that can be used to understand the system where a gear is operating. For instance these metrology techniques can be used to separate the error components of the bearing supporting a gear; this would be “Bearing Error Motion” in the standard. The shaft or structure supporting the bearing is also likely being influence by forces transmitted though the gear or another nearby load path (this is “Structural Error Motion” in the standards). Basically even a 2 meter diameter slewing ring is an axis of rotation, so the techniques of Axis of Rotation Metrology designed to evaluate spindles may be use to understand the dynamics of gears mounted on even the largest bearings. The techniques are a powerful tool in identifying where the most cost effective improvements can be made.

If you are using oil hydrostatic or air bearing supported work tables your gears may have lower profile errors then the error motions from the rolling bearings they are mounted on in use. Don’t let your gear be blamed for noise that relates to a frequency associated with the rolling elements.

Air Bearing Spindles as a Metrology Reference

Air bearing spindles are an excellent metrology standard, they are the basis of most roundness measuring machines. The large gear industry may do well to consider metrology tools based on air bearing spindles that may be brought to the part rather than taking the parts to a very large and expensive Cartesian metrology machines (conventional CMMs). The design below contemplates a light weight, mobile measuring system that would index to be co-axial automatically to within a few microns of a machine tool work table via a kinematic mount. The measuring machine would then act just like typical CMMs with regard to software and probe systems. The part could be measured as machined and again after the unclamping from the fixture to understand chucking distortions and avoid trips back and forth between the machine tool and the CMM. The measuring machine could be moved from machine tool to machine tool or to a dedicated measuring station. In some cases it could attached directly to the part to be measured. This when manufacturing large round parts this would be a powerful way to increase measuring flexibility, measuring precision and reduce costs at the same time.

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