A prototype is not an object that only has to look roughly right. In small-batch hardware the prototype is the datum for the next twenty, the next hundred. This is not a medical or diagnostic device. It is written against cases, brackets, and small mechanisms — objects a floor and an office commonly make.
People often revise a drawing three times trying to get the final product in one pass. Treat the prototype as a process-check step and the number of revisions goes down.
What to fix before the drawing speaks
We write three things before we open CAD. Without those three the drawing is pretty and the machining is expensive.
- The force this object holds — a handle, a case around a board, a bracket on a wall.
- The direction it opens — on a product that must come apart, screw seats and slide direction are half the drawing.
- The face you touch — a chamfer on an edge a hand meets is function, not decoration.
The order we pick a material
In a small batch, material attaches to the process you can run now, not to an ideal property. A 3D print checks shape cheaply. Sheet and cut buy strength and a repeat. Mixing both in one prototype is normal.
- Plastic shape check — PLA or PETG print. Same-day shape.
- A housing a hand touches — ABS, acrylic, PC. Finish, clear, paint possible.
- A structural bracket — aluminum 5052 or 6061. Cut and bend both easy.
- A thin cover — SECC or stainless sheet. Repeatable after a flat and a bend.
Print to see interference. Metal to make the seat that takes force. The moment you try to prove strength with one print, layer direction and the real load drift.
Tolerance — do not start too tight
Spray ±0.05 across a drawing and a small-batch quote jumps at once. Working rules at prototype stage split by the job of the face.
- Exterior, a face no hand meets: ±0.2–0.3 mm
- A screw seat, a face that mates: around ±0.1 mm
- A slide or a press fit: lock the number after material and process are set
Paint and anodize eat thickness. Design a tight fit on pre-paint dimensions and the lid will not close after the color goes on. If finish is planned, mark before and after finish on the drawing.
Process choice: print, CNC, sheet
The same box gets a different drawing by process. A prototype drawing is not the final ideal. It is a drawing for the process you will use this time.
On a 3D print we look first at overhangs, wall thickness, and boss thickness for a heat-set insert. If layer direction runs with the force, it splits.
On CNC we give a radius to an inside corner the tool cannot enter. A sharp inside corner exists only on the drawing. A thin floor waves from vibration, so we add a rib or we raise the thickness.
On sheet we draw the flat first. Bend order makes the tool interfere. Miss the minimum bend radius or the hole-to-bend distance and a hole stretches into an oval.
Inspection before assembly
The first part out is not assembled at once. A large share of prototype rejects is leftover chips and a hasty fastener, not a dimension.
- Measure a few drawing dimensions and write them down. Not all of them — mating faces and hole spacing are enough.
- Take off burrs and sharp edges.
- Tap or insert screw seats by hand first. A power tool strips a prototype screw easily.
- Dry-fit for interference only. Glue and rivets wait.
Leave one photo. A prototype becomes “that version then” in a later meeting. Without a date and a material in the filename you cannot tell them apart a week later.
What a prototype tells you, and what it does not
A prototype is good at telling where a hand lands, the open-close path, whether a cable and a board actually go in, and whether the machine order matches the drawing.
A prototype is poor at telling unit cost at a hundred, shrink once you go to a mold, and vibration and scratch in transit.
That is why small-batch hardware is cheaper if you make the prototype once more. The first is shape. The second is process and finish. Trying to get both in one pass is how the drawing gets revised three times.
The minimum to leave as a work note
The next prototype may not be made by the same person. Five lines are enough for the minimum record.
- CAD filename and date
- Material, thickness, finish
- Where it was actually cut (inside, or which outside shop)
- Where force went in at assembly, where it interfered
- Three lines to change on the next version
With that record, a small batch is a repeatable process, not a pile of hand work. The value of a prototype is not in the look. It is whether the next quantity can be made again in the same order.