A prototype run is usually the first time a design has to survive contact with real tooling, real tolerances and a real delivery date. Before a drawing leaves your desk for a Chinese equipment workshop, the questions below are the ones an engineering team will ask you, and having the answers already written down is what separates a clean two-week iteration from a two-month negotiation over what the drawing actually means.
1. A controlled drawing package, not a single file
Send one package with a revision letter on every sheet, and keep it unchanged while you quote. The package normally contains 2D dimensioned drawings with tolerances and datums, 3D step or x-t files for fixtures and sheet-metal parts, an assembly drawing with the bill of materials, and a short statement of which features are functional and which are cosmetic. If your CAD history allows it, note which dimensions came from a simulation or a fit test — that tells the manufacturer what not to relax.
Version discipline matters more than file format. If a drawing is re-issued mid-project, say so explicitly in writing; a silent change to a single bore diameter can invalidate fixtures and test jigs that were already cut against the previous revision.
2. Your real target quantity — and the quantity after that
Prototype pricing and production pricing are built on different assumptions, so quote both at the outset. Three units and three hundred units change what the factory will do for you: at very low volumes parts are often made on general-purpose machines and hand-finished, while a committed production volume justifies dedicated tooling, fixtures and an automated station. This is also the single most common cause of surprise on the proforma invoice.
Give an honest volume range rather than a rosy one. If you expect to move from a pilot batch to volume, say when — a factory that knows the timeline can design the prototype fixturing so it carries over into production instead of being scrapped.
3. Critical dimensions, tolerances and the inspection plan
Mark the features that actually matter. A drawing where every dimension carries the same tight tolerance is more expensive and slower to build than one that says “this bore is to +/-0.01 mm because a bearing presses into it, everything else is general tolerance.” State the fit for each critical interface, the surface finish you need and where, and the datum the tolerance chain runs from.
Then specify how conformance will be shown. For functional parts this usually means a dimensional report per unit, a first-article inspection, or an agreed go/no-go gauge. If you need reports, ask for them in the purchase order, not after shipment; adding inspection after the parts are made often means rework or nothing at all.
4. Materials, substitutes and what is available locally
Specify the material by standard and grade, and say whether substitution is allowed. If a fastener grade, a gasket compound or a coating is safety-related, mark it as non-negotiable. If a grade is scarce or has a long lead time locally, ask the workshop to propose an equivalent and hold it back for your written approval before they cut metal.
For a prototype this is also the time to decide what to buy and what to make. Off-the-shelf frames, enclosures, guides and actuators are usually faster and cheaper than machining them from a drawing, and they let the build budget go into the parts that are genuinely yours.
5. Function, environment and standards
Describe what the machine or assembly has to do and where it will live: duty cycle, ambient temperature, dust or washdown exposure, load, speed, chemicals it will touch, and whether it runs continuously or in shifts. Workshops that see only geometry will optimise for manufacturability; workshops that know the duty will tell you when a wall is too thin, a fastener will creep loose, or a surface will trap residue.
List the standards and certifications the finished equipment must satisfy — electrical directives, machine-safety requirements, food-contact or cleanroom rules, explosion protection. Compliance requirements change the design, the choice of components and the documentation trail, so they need to be on the table before the first cut, not at commissioning.
6. Interfaces, controls and integration points
A prototype rarely stands alone. Specify what it connects to: power supply and voltage, compressed-air quality and pressure, fieldbus or I/O signalling, the upstream and downstream equipment, and the footprint or mounting pattern it must match. If the prototype has to sit in an existing line, send the layout drawing and the tolerances of the existing structure — as-built dimensions, not design intent.
For anything with motion or sensing, say who writes the control logic and who supplies the software, and what interface you will hand over. This is the boundary where schedule slips most often, because both sides assume the other owns it.
7. Acceptance testing and what “done” means
Write the acceptance criteria before the order, not at delivery. For a mechanism that usually means a cycle or throughput figure to hold for a stated duration, a positional accuracy to be demonstrated, and any cleanliness or sealing requirement. Then agree where the test happens: a witnessed run at the builder's works before shipment, or a site commissioning test after installation.
Ship-before-acceptance terms are a commercial risk on prototypes; a factory-run test report you can review in your own time costs little and prevents an argument about a machine that has already crossed an ocean.
8. Timeline, milestones and change control
Give the date the prototype has to be in your hands and work backwards: design freeze, long-lead component purchase, fabrication, assembly, test, packing and shipping. Long-lead items — a driven unit, a custom heater, a certification test slot — usually set the critical path, so list them and place them early.
Agree a written change-control route at the start: who raises a change, how its cost and lead-time impact is quoted, and what happens to work already in progress. Prototypes change; the cost is uncontrolled change, not change itself.
9. Budget, tooling ownership and commercial terms
State the budget as a range and ask for an itemised quotation: material, machining, standard components, surface treatment, assembly, testing, and tooling or fixtures shown separately from unit price. Separate tooling lines let you see what you are paying for and what can be amortised into later production.
Say who owns the fixtures, jigs, test programs and the resulting know-how after the job. If you expect to move the work to volume production, ownership of the tooling is what makes that transfer possible.
10. IP protection and confidentiality on a practical level
Send an NDA before the drawings, and register a design or utility-model filing in the manufacturing jurisdiction if the geometry is the value. In practice a filing gives you something enforceable; an NDA alone is hard to act on across borders. Keep commercial leverage simple: quote with more than one supplier, and do not let a single workshop hold both the only tooling and the only build knowledge.
11. Your contacts, language and time zone
Name one technical contact and one purchasing contact on each side, and note the working hours and language of the exchange. Drawings with tolerances stated in one language and e-mails in another are a recurring source of error, so keep every decision in the same written thread as the drawing revision it applies to. Photographs and short videos of an assembly at an intermediate stage are often worth more than a page of correspondence.
12. After the prototype: scale-up and support
Decide now what the prototype is for beyond proving the concept. If the same partner will build the production units, tell them, and ask what they would change for volume — a design that machines well at three units is often wrong at three hundred. If you want a partner who can also take the design from prototype to a repeatable line, bring the non-standard automation equipment and integrated engineering team services into the conversation early, so the prototype is built with the production method already in view rather than being rebuilt from scratch later.
For equipment that will need service, spares and documentation after handover, list what you expect to receive: spare-parts list with the standard components called out by model, wiring diagrams, an operator manual and the maintenance interval. These are cheap to specify at the start and expensive to reconstruct afterwards.
A short readiness checklist
- Revision-controlled 2D and 3D files, with the change log kept in one thread
- Prototype quantity plus the production volume you expect, and when
- Critical fits, tolerances, finishes and the inspection evidence you require
- Materials named to standard, and whether substitution is permitted
- Duty, environment and the standards or certifications that apply
- Interfaces: power, air, signals, mounting, and who owns the control software
- Acceptance test, pass criteria, and where the test is witnessed
- Milestones, long-lead items and a written change-control route
- Itemised quotation with tooling shown separately, and tooling ownership agreed
- NDA plus any design filing in the manufacturing jurisdiction
- Named contacts on both sides, and the after-sale documentation you expect
None of this is paperwork for its own sake. Every item above removes a question the factory would otherwise ask in week three, when an answer costs a re-made part instead of an e-mail. Put the package together once, and the same file set carries you from the first prototype through to production.
