Manufacturability Mistakes That Become Scrap

A model can look perfect and still be un-makeable. These are the mistakes that turn aluminum into scrap.

Mistake: internal corners smaller than the tool. A 1/4 in router endmill physically cannot cut an internal corner sharper than 1/8 in radius (its tool radius); a drilled-only shop cannot cut closed pockets at all. Symptom: the machinist says 'I can't cut that.' Fix: design fillets equal to or larger than the cutting-tool radius for routed/milled parts; for waterjet you can go tighter but still avoid knife-edge corners. Always design for the machine you actually have.

Mistake: no tolerances on a press-fit. A bearing bore drawn at the bearing's nominal OD will be too loose or too tight depending on the cut. Symptom: the bearing falls out or won't seat. Fix: apply the right fit — a light press fit for a bearing OD typically wants the hole a few thousandths under nominal; call it out on the drawing (e.g., bore Ø1.125 +0.000/-0.001). CAD nominal is not a spec until the drawing states the tolerance.

Mistake: hole/shaft mismatch. Sketching a 0.500 in round hole for a 0.500 in hex shaft means it won't seat correctly. Symptom: parts bind or spin on assembly. Fix: use COTS-matched bores — flanged hex bearings are sized for the exact hex stock, and round bearings have defined bore/OD. Pull the real part from a maintained Onshape parts library — FRCDesignLib (via the FRCDesignApp plugin) is the current community standard, having superseded the older MKCad library — rather than guessing dimensions.

Mistake: tube hole patterns that don't index. If a bracket's holes don't fall on the tube's #10-on-0.5-in grid (REV MAXTube / AndyMark pre-drilled), you'll be hand-drilling at the machine. Symptom: holes don't line up. Fix: place bracket holes on the same 0.5 in pitch and verify in the assembly that they coincide with tube holes.

Mistake: skipping the drawing. Without dimensioned drawings, the machinist guesses. Fix: generate a drawing with critical dims, hole callouts, and tolerances for any custom part before it goes to manufacturing.

Pre-flight checklist before sending to the shop: (1) Internal radii ≥ tool radius. (2) Press-fit/clearance holes toleranced on the drawing. (3) Shaft/hole/bearing dimensions pulled from real COTS. (4) Mounting holes on the structural grid. (5) Material and thickness match available stock (e.g., 0.190 in / 0.090 in plate; 2x1 tube in 1/16 in or 1/8 in wall). (6) Part nests within the machine's bed. Running this list at design review catches scrap before it costs material and a build-season day.