Error Budgets and Error Budgeting Techniques
The specifications demanded by ultra precision machining processes mean it is imperative that errors intrinsic to machine axes are minimised in the design phase. When designing special purpose machinery, Cranfield Precision understands that each machine has unique requirements, and so we regularly use error budgets as one of our precision machine design tools.
What are they used for?
An error budget provides an estimate of potential errors within a machine axis that lead to deviations from the desired motion. The principle is that for any given axis, say X in the diagram, there will be a departure from straightness in both the Y and Z directions as it traverses. Similarly roll, pitch and yaw errors within the X axis will affect positioning in the Y and Z directions.
In addition to errors from linear axes, further errors are observed in rotary axes, namely; axial, radial and tilt errors. Also considered are errors within on-machine measuring systems, their placement relative to the cutting point. This is the Abbe principle where any angular misalignment between an axis and its measuring system is amplified by the offset distance between the measuring system and the axis of motion itís measuring.
Why do we use them?
We use them in advance of comprehensive design effort as a method of evaluating the ability of a proposed machine axis configuration to meet the desired specification. When coupled with our finite element and fluid dynamic analysis software, we can create design solutions that are proven to closely match actual machine performance. Reducing potential errors in the design phase also reduces any software compensation effort subsequently required.
How do we use them?
Firstly we use historical data from over 40 years of experience to estimate likely errors from potential axis components such as: rolling element bearings, hydrostatic bearings and encoders. When combined with tool, workpiece and measuring system offsets we can calculate the potential error at the machining point. This method enables us to quickly focus on the critical aspects of the machine design. Added to this would be possible thermal growth caused by temperature fluctuations, which, in turn, helps to determine the requirements of any temperature control systems.
In summary, Cranfield Precision maintain that error budgeting is a fundamental technique for successful machine tool design.