Most machined medical components go through the same path: a handful of prototypes to check fit and function, a few small builds for verification or clinical use, then repeat batches once the device is launched. The drawing may look the same at each stage, but how the part is made and checked changes a lot. Medical device machining prototype to production is mostly about managing those changes so the parts you validate are the parts you keep getting.
This article covers what changes between stages, when to freeze the process, how cost per part moves with quantity, and how to handle design changes and bridge builds.
Who is responsible for what
The device maker owns design controls, design verification and validation, process validation decisions, biocompatibility, sterilization validation and the regulatory submission. Under the FDA’s Quality Management System Regulation in 21 CFR Part 820, which incorporates ISO 13485:2016 by reference, these sit in the device maker’s quality system. The machine shop makes parts to the buyer’s drawing and quality requirements and records how it made them. Medical Device Machining reviews RFQs and matches each part to a partner shop in southern Vietnam; we do not machine parts.
That split matters here because the stage at which you freeze the process is a decision that feeds your own validation. The shop can tell you when its process is stable. Only you can decide when it is locked.
Stage 1: Prototypes, 1-10 pieces
At the prototype stage, the goal is a correct part fast, not an efficient process. With the partner shops we work with, 1-10 pieces typically take 2 to 3 weeks, including a first article inspection (FAI) report.
What prototype production usually looks like:
- Fixturing: general-purpose vises, soft jaws, and simple plates. Setups are built to hold a few parts, not hundreds.
- Programs: written for this run. The machinist may adjust feeds, tool paths or the order of operations during the run.
- Operations: often more setups than a production process would use, because it is faster to add a setup than to design a dedicated fixture.
- Inspection: every feature on every part, typically on CMM, optical comparator and height gauge. The FAI report ties each drawing characteristic (usually ballooned) to a measured value.
- Material: bought with mill certificates traceable to the heat number, same as production. There is no good reason to prototype a Ti-6Al-4V ELI part in a different grade if you plan to test it.
DFM feedback comes with the quote at no charge. The most useful time to act on it is before the prototype run, when a tolerance change or a corner radius change costs nothing.
Stage 2: Small batches, 10-500 pieces
Small batches cover verification builds, clinical builds, and early commercial volume. Typical lead time with the partner shops is 3 to 5 weeks. Here the process starts to look like production.
- Fixturing: dedicated fixtures appear, often multi-part plates or tombstones on a 4- or 5-axis machine. A good fixture removes setups, which removes stack-up error between datums.
- Process lock: the program, tool list, cutting parameters, fixture and machine get documented and stop changing from run to run.
- Inspection plan: moves from “measure everything” to a written plan that names critical features, the method, the frequency and the reaction if a value drifts.
- Sampling: non-critical features often move to an acceptance sampling plan; critical features may stay at 100%.
Stage 3: Repeat production, 500 pieces and up
At higher volume the priorities change again: cycle time, tool life, and statistical control. Shops may move a part to a different machine type (for example, a small turned part from a mill-turn center to a Swiss-type lathe), add bar feeding, or run lights-out. Each of those is a process change, which is why the timing of process freeze matters.
Stage-by-stage comparison
| Item | Prototype (1-10 pcs) | Small batch (10-500 pcs) | Repeat production (500+ pcs) |
|---|---|---|---|
| Typical lead time | 2 to 3 weeks incl. FAI | 3 to 5 weeks | Set per program and forecast |
| Fixturing | Vises, soft jaws, simple plates | Dedicated or multi-part fixtures | Dedicated fixtures, possibly automation |
| Program status | Can change during the run | Documented and revision controlled | Frozen; changes need approval |
| Inspection | 100% of features, full FAI | Written plan: critical features 100%, others sampled | Plan plus process monitoring on critical features |
| Documentation | FAI report, CoC, mill cert | FAI on production setup, lot records, CoC | Lot records, CoC, change notices, periodic re-FAI |
| Design status | Open to change | Mostly stable | Released and controlled |
| Main cost driver | Programming and setup | Fixtures and inspection time | Cycle time and tooling |
Inspection: 100% versus sampling
At prototype volume, 100% inspection is cheap relative to setup cost. At 500 pieces, measuring 60 characteristics on every part can take longer than machining them.
Acceptance sampling, usually based on ISO 2859-1 or ANSI/ASQ Z1.4 tables, lets you inspect a sample from each lot and accept or reject the lot based on the result. The basic concept of acceptance sampling is that a defined sample size and acceptance number give a known level of protection against accepting a bad lot. For medical components, buyers commonly set AQL 0 (zero defects accepted in the sample) or c=0 plans for anything that affects function or safety.
A typical split:
- Critical features (thread form on a bone screw, a bore that locates a bearing, a mating profile on a trial component): 100% inspection or a tight c=0 plan, sometimes with process monitoring.
- Major features: sampling plan per lot.
- Minor features (chamfers, cosmetic edges): sampling at a looser level or first-and-last-piece checks.
The buyer sets the plan. The shop executes it and records the results against the lot number.
When to freeze the process
The simple rule: freeze the process before the builds whose parts or data go into your validation or regulatory file.
If you run design verification on parts made with an interim fixture and a program the machinist was still adjusting, and production then moves to a new fixture and machine, your verification data describes a process that no longer exists. Depending on your quality system, that can mean repeating tests or writing a justification.
A frozen process is defined by a list, usually attached to or referenced in the purchase order:
- Material specification and approved material sources
- Machine or machine type for each operation
- Fixture identification and revision
- CNC program number and revision
- Tool list and key cutting parameters where they affect the part
- Cutting fluid and cleaning method, especially for implant or patient-contact parts
- Outside process vendors (passivation per ASTM A967, electropolish, anodize, heat treatment)
- Inspection methods and equipment for critical features
Why change control matters to your validation
After freeze, any change to that list should need written notice and your approval before the shop uses it. This is change control, and it protects the buyer’s validation, not the shop’s.
A change that looks small at the shop can matter to the device. A new cutting fluid can change the residue your cleaning validation was based on. A different passivation vendor can change the surface. Moving from a 3-axis to a 5-axis machine can change which features are cut in one setup, which changes positional error between them.
Ask for this in the purchase order or quality agreement: a list of changes that require notification, a minimum notice period, and a first article (full or partial) on changed features before the first lot ships.
Cost per part across the stages
The table below is illustrative and hypothetical. It shows how cost per part typically falls as quantity rises for a small titanium component with around 40 drawing characteristics, using an index where the cost per part at 5 pieces equals 100. Actual numbers depend on the part, material, tolerances and inspection plan.
| Quantity per order | Illustrative cost index per part | Main reason |
|---|---|---|
| 5 | 100 | Programming, setup and full FAI spread over 5 parts |
| 25 | 45 | Setup shared; still near-100% inspection |
| 100 | 25 | Dedicated fixture, fewer setups, sampling on non-critical features |
| 500 | 15 | Fixture amortized, optimized cycle time |
| 2,000 | 11 | Cycle time and tool life dominate |
Two points buyers often miss. First, fixture and first article costs are one-time and are often quoted separately; if you order 25 pieces three times instead of 75 once, you may pay setup three times. Second, tight tolerances that do not matter to function cost more at every stage, which is why DFM review before the prototype run pays off.
Design changes between stages
Almost every project changes the drawing after prototype testing. That is normal. What causes trouble is changes that are not controlled.
- Release each change as a new drawing revision, even a small one.
- Make sure the lot records show which revision each lot was made to.
- Expect a partial FAI covering the changed features, or a full FAI if the change affects datums or fixturing.
- Avoid “verbal” changes by email that never reach the drawing. They get lost when the part moves to production.
Bridge builds
A bridge build is a limited run made while the design or process is not yet final, to cover a clinical study, a trade show, or early launch demand. Bridge builds are useful, but they create parts made under different conditions from the final process.
Treat them as their own lots with their own records. Document the drawing revision, the process used and any known differences from the planned production process. Decide in advance, inside your own quality system, how those parts can be used.
For where this stage plan fits when you move a part to a new country, see our guide to offshore medical device machining.
Sending the RFQ
When you send your drawing through the RFQ form, tell us which stage you are at and what comes next. Useful items to include:
- Drawing with revision, material specification and finish callouts
- Quantities for each stage (for example 5 prototypes, then 100, then a yearly forecast)
- Which features you consider critical
- Whether you need an FAI report, CMM data per lot, or a PPAP-style package (on request, quoted separately)
- Your change notification requirements after process freeze
You can review the partner shops’ capability window on the capabilities page. We reply within 24 business hours with a quote and DFM notes, and a mutual NDA can be signed before any drawing review.