What a depreciation model does
A model applies a repeatable rule to original cost or a baseline value over time. It can support planning when assumptions are explicit and comparable evidence is limited.
What the model cannot see
A curve usually does not know the exact control, spindle, options, hours, rebuild history, geometry, documentation, market scarcity, location or cost to remove and reinstall the machine.
Why value does not decline smoothly
Machine markets move in steps. Control obsolescence, parts support, industry cycles, shop closures, lead times, new-machine pricing and demand for a particular work envelope can create plateaus or sudden changes.
Use depreciation as a prior, not a verdict
Start with age-related context, then replace generic assumptions with machine-specific evidence. The final working range should explain where market observations support or contradict the curve.
Accounting treatment is separate
Book depreciation, tax treatment, insurance value, liquidation value and market value answer different questions. Obtain qualified accounting, tax, insurance or appraisal advice for those purposes.