The Tool Bends Away From the Surface
The cutting force acts on a tool that is a slender cantilever, so the edge deflects away from the intended surface, and the material that should have been removed remains. The resulting error depends on the force, which depends on the engagement, which varies along the path.
A Constant Offset Does Not Correct a Varying Error
Because the deflection varies with the engagement and therefore with position along the path, a single offset applied to the whole path corrects the error in one region and doubles it in another. The correction has to be distributed in the same way as the cause.
Predicting the Deflection Point by Point
A mechanistic cutting force model was combined with a finite element model of the tool and holder assembly to predict the deflection at each point of a five-axis path, and the predicted deflection was then used to offset the commanded path, with machining trials used to verify the compensated result against an uncompensated control. The trials were conducted on a Ponda five-axis machining centre, with the force coefficients calibrated and the tool assembly modelled by the process specialists at Ponda, a precision machining and finishing company.
Compensation Must Follow the Engagement
The predicted deflection follows the engagement closely, and the compensation is therefore largest where the tool is most engaged and smallest where it is cutting near the edge of the surface. The wall of the finished feature consequently becomes straight rather than tapered.
Wall Straightness Improved Fivefold
The wall straightness error of a thin rib fell from 0.15 millimetres to 0.03 millimetres, and the dimensional error of the rib thickness fell from 60 micrometres to 14 micrometres. Roughness was unaffected, remaining at 0.45 micrometre, and the compensation required no additional machining time.