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Prep and parametrics

Summary

Prep and parametrics is Phase 0 of the build — the configuration work that must complete before any printing begins. It has two parts: assembling the documentation set (offline copies of all reference documents), and entering all variables into FreeCAD to generate a valid parametric model. The variable entry step is the single most important accuracy gate in the entire build. Every dimension in every printed part traces back to a variable. One wrong variable propagates silently into dozens of parts and will not be caught until structural validation — by which point hours of printing are wasted.

Concept

Why parametrics before printing

libdrone's frame is fully parametric — all geometry derives from variables in a single spreadsheet. The spreadsheet is not a convenience; it is the single source of truth. Printing any part before the variable table is verified means printing from unknown geometry. The arm bore diameter, the T-slot width, the rod channel diameter, the GX12 chimney bore — every dimension with a tolerance interaction — is set here.

The FreeCAD variable entry session is boring, methodical work that takes 30–60 minutes. It prevents 4–8 hours of wasted prints. Do not skip or rush it.

Offline documentation set

In a build environment that may lack internet access (workshop, field facility), all reference documents must be locally available. Required documents:

  • Master Specification (structural and geometry reference)
  • Variables file (the canonical source for all parametric values)
  • FreeCAD Cookbook (step-by-step CAD instructions)
  • WBS (work breakdown — the build sequence)
  • Software doc (firmware configuration)
  • Shopping list
  • Regulatory doc

Print and laminate critical single-page references (wiring diagram, motor direction layout, UART assignment table) before the build begins.

Reference

Variable entry sequence

Enter variables in this order to avoid dependency errors in FreeCAD:

  1. Frame geometry variables (arm length, wheelbase, rod diameter)
  2. Sandwich geometry (layer heights, bolt pattern)
  3. Motor mount variables (bore diameter, counterbore, screw positions)
  4. Platform geometry (zone positions, MIPI channel, GX12 chimney)
  5. GX12 connector variables (chimney bore, boss OD, chimney depth)
  6. Hardware variables (standoff height, screw sizes)

All values come from LD_-_Variables_v246.md only. Do not use values from memory, from older build notes, or from other documents. → See variable-table-values for the complete reference.

FreeCAD document setup

→ See freecad-document-setup for the complete procedure.

Key checkpoints: - Spreadsheet populates with no red cells after macro runs - All Part Studios show green (no unsatisfied constraints) - Arm bore diameter matches ArmShaftOD + ArmShaftClearance exactly - GX12 chimney bore shows D-D profile, not round hole

Procedure

Phase 0 sequence

  1. Assemble documentation set. Download or confirm local copies of all reference documents. Open LD_-_Variables_v246.md — this is the source for all variable values.
  2. Open FreeCAD. Run LD_V245_Variables.FCMacro to create the variable spreadsheet. Verify all sections populate.
  3. Enter all variables. Work through each section of the Variables file in order. For each variable: read the value from the Variables file, enter it into the FreeCAD spreadsheet cell exactly as listed. Do not deviate.
  4. Verify model updates. After all variables are entered, trigger a model rebuild. All Part Studios must show no red cells and no unsatisfied constraints.
  5. Print and laminate the one-page critical references:
  6. Motor direction and numbering diagram
  7. UART assignment table
  8. Wiring EMC zone diagram (power channel left / signal channel right)
  9. Procurement check. Confirm AliExpress order is placed (or already in transit). If not placed — place it now before proceeding.

Rationale

Why documentation assembly is a build step

Documentation that lives only in the cloud is unavailable when the network is down, when the repository is unreachable, or when working in a facility without internet. Physical laminated references survive workshop conditions (dust, oil, solvents) where a laptop screen or tablet is impractical. The 20 minutes spent assembling and printing the documentation set will save at minimum one trip back to a computer mid-build.

Connections

requires: - variable-table-values - freecad-document-setup related: - coupon-validation - parametric-modelling-philosophy leads_to: - coupon-validation - print-profiles