Error mapping and error budgets

Assuming that all of the sources of error in a precision machine can be identified and quantified, it is helpful to be able to determine the degree to which they impact the quality of the part, i.e. dimension, form, surface roughness, principally. In Chapter 2 we discussed generally error sources and their interactions, for example, Figure 2.4. We will discuss in greater detail the determination of the “sensitive direction” of a machine tool which is, in most cases of machining, normal to the machined surface. The degree to which the elements of the machine tool, structure, spindle, tooling, and fixture interact with the error sources (thermal, mechanical, etc.) to cause displacement or other undesirable movement in a sensitive direction with respect to the workpiece, indicates the inaccuracy that will be imparted to the workpiece. There are several determinations to this — machine components and error sources mapping onto directions affecting the work. This is, in short, error mapping. And, the formulation of a list of these errors and magnitudes, as they affect the workpiece (or desired motion, or any other output of the system) is called an error budget.