🔧 Build · Engineer · Intermediate
Custom Parts & Fabrication
From a dimension you need to a part you made: what’s typically legal, the CAD-first workflow, shop discipline, and the fab log that proves it all at inspection.
🔧 What Counts as a Custom Part
A custom part is anything your team cuts, bends, drills, machines, or forms rather than using straight out of the VEX box — a polycarbonate flap, a modified C-channel, a Delrin slider. Teams fabricate when the geometry they need doesn’t exist as a stock part: a ramp with a specific curve, a guard sized to one gap, a spacer at a non-catalog thickness.
Fabrication is also one of the strongest notebook stories you can tell: it proves the team went from a dimension we needed to a part we made, with reasoning at every step.
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Legality first — always. What materials and processes are competition-legal is defined by the
R-rules in the current game manual, and allowances change from season to season. Before you cut anything, verify the material, its dimensions, and the process against this season’s manual — see the
Game Manual guide. Illegal parts don’t just lose points; they get flagged at inspection and must come off the robot. When in doubt, ask your Head Referee or post an official Q&A.
🧱 Common Materials & Where They Show Up
These are the materials teams most often fabricate with — each one subject to the current manual’s allowances on type, quantity, and thickness:
Polycarbonate
The workhorse. Flaps, guards, ramps, intake hoods. Cut with a bandsaw or score-and-snap; bends cold along a scored line. Drill with a pilot hole first — it cracks if you rush.
Delrin / HDPE
Low-friction plastics for sliders, glide pads, and custom spacers. Machines cleanly; doesn’t crack like polycarb.
Modified VEX metal
Cutting a C-channel short, drilling a non-standard hole, filing a slot — modifying VEX parts is fabrication too, and it belongs in your fab log with the same rigor.
Elastic & retention
Rubber bands, surgical tubing, braided rope, zip ties — check the manual for what’s allowed and in what sizes; log where and why each is load-bearing.
3D-printed parts
Legality has changed over the seasons and differs by program — treat this as a check-the-current-manual item every year before designing around a print.
📐 The Workflow — CAD First, Cut Second
1 · CAD the part
Model it in context — attached to the mechanism it serves — so mounting holes and clearances are real. See Onshape: Mechanism CAD.
2 · Make the 2D drawing
A dimensioned drawing is what you fabricate FROM — every hole located, every edge dimensioned, material and thickness in the title block. See Onshape 2D Drawings.
3 · Pick material & process
Match the material to the job (stiffness, friction, weight) and confirm it’s legal this season. Choose the process you can actually do safely with your tools.
4 · Fabricate
Pilot holes before final size. Safety glasses on. Deburr every cut edge. The Tools Guide covers technique per tool.
5 · Fit-check & iterate
Trial-fit on the robot before final mounting. Off by a sixteenth? That’s an iteration — log the change and the new dimension, don’t silently re-cut.
6 · Log it
Every fabricated part gets a fab-log entry (below) and a reference back to its CAD version. See CAD to Build Handoff for the full loop.
📋 The Fabrication Log — Required Fields
Part
Name + which subsystem it serves. “Intake left guard” not “plastic piece.”
Material
Exact material and thickness. “Polycarb, 1/16″” — this is also your legality evidence at inspection.
Process
How it was made: score-and-snap, bandsaw, drilled 5/32″ pilot then 3/16″, cold-bent along scored line.
Tolerance / fit
How close it needed to be and how close it came. “Slot needed ±1/32″; first cut oversize, refit.”
Iteration notes
Version number and what changed since the last one — ties into your CAD version history.
Written / Witnessed
Same as every notebook entry: both names, every time.
❌ Too vague — unusable at inspection or interview
Nov 12 — Cut some polycarb for the intake. Fits now. Written by: Sam
✅ Inspection-ready fab log entry
Nov 12 · Intake left guard v2 · Polycarbonate 1/16″
Process: score-and-snap to 4.5″ × 2.25″, two 5/32″ pilot holes drilled to 3/16″ on 0.5″ spacing, edges deburred.
Why v2: v1 rubbed the roller at full compression — clearance measured 0.03″, needed ~0.1″. New cut moves the inner edge 0.09″ out.
Fit check: 0.11″ clearance at full compression, zero contact in 20 hand-cycles. CAD ref: intake-guard v2.
Written by: Sam · Witnessed by: Coach T.
🛡
Shop discipline. Pilot holes before size drilling (polycarb cracks), safety glasses whenever a tool spins, deburr every edge that a hand or a wire can touch, and never drive VEX 8-32 hardware with a drill — the torque strips it. Full technique per tool in the
Tools Guide.
Real engineering drawings carry tolerances — the allowed error on every dimension — because no process cuts perfectly. Choosing a design your tools can actually produce is called DFM (Design for Manufacturing): a hole pattern you can drill with a hand drill beats a curve that needs a CNC you don’t have. Logging “needed ±1/32″, achieved it on the second cut” is tolerance thinking, and judges recognize it.
🎤 Interview line: “Every custom part starts as a dimensioned drawing, gets fabricated with a documented process, and is logged with its material, tolerance, and fit result — so at inspection we can show exactly what it is and that it’s legal.”
Which fabrication log entry is actually useful at inspection and in the judge interview?
⬛ “Cut some polycarb for the intake, fits now”
⬛ “Intake guard v2 · polycarb 1/16″ · score-and-snap to 4.5×2.25″ · v1 rubbed roller (0.03″ clearance), v2 adds 0.09″ · fit-checked at 0.11″”
⬛ A photo of the part with no text