What is error budgeting?
Error budgeting is the practice of deciding — before you CAD, before you FEA, before you spec any component — how much positional error each part of your machine is allowed to contribute.
You start at the top: the customer needs the tool tip to hit within ±10 µm of the commanded position. That's your total allowable error, dtot. Now you have a budget. Like any budget, you have to split it.
The standard approach does this in two phases. Phase 1 — Apportionment allocates the budget across N axes and the components within each axis (bearings, structure, actuator, sensor, cables) using weighting factors. Phase 2 — Detailed Budget picks real components, models the structural loop with homogeneous transformation matrices, and propagates every error source to the tool tip.
This tool implements Phase 1 today. Phase 2 is next.
Why do it at all? Because without a budget, you'll over-spec the easy parts (a $50 bearing where a $5 one would do) and under-spec the hard parts (a structural rib that contributes 80% of the error and nobody noticed). Error budgeting is how precision machines actually get designed deterministically instead of by hope.