CNC Machining for Medical Parts, Explained Simply

What makes a medical part different, why "looks clean" is not enough, and how to ask for the right thing without a regulatory handbook.

Three things that make medical parts different

A machined part for a medical device often looks like any other machined part. Same aluminium, same plastic, same shape. But it is judged on three extra questions.

1. What is left on it? Residue in a small hole, oil from the machine, or a burr on an edge can all matter.

2. Can you prove where it came from? Which metal batch, which machine, which inspection - the customer may ask months later.

3. What happens if the process changes? Even a small change on the shop floor may need to be reported before the next batch ships.

Dimensions still matter. These three just come on top.

Which parts are machined

Most medical machining falls into five groups:

Instrument parts - handles, jaws, shafts. Usually stainless steel, small features, and edges that must be smooth.

Device housings - covers, frames, brackets. Often aluminium or plastic, with a good-looking outside and a precise inside.

Fluid and air parts - manifolds, valve bodies, connectors. The inside channels and sealing faces matter most.

Lab and diagnostic equipment - stages, mounts, trays. Flat and stiff is more important than pretty.

Fixtures and tools - assembly jigs and test fixtures. Easy to forget, often the item that delays a project.

Metal or plastic?

One rule here: do not swap materials to save money without asking. In medical work, a different grade can mean the approval has to be repeated.

Clean is not the same as shiny

A part can look perfect and still be hard to clean. Four common problems:

Blind holes (holes that do not go all the way through) trap liquid and dirt. A through-hole drains and cleans.

Rough surfaces hold residue, even when they look fine.

Burrs - tiny rough edges - can break off and become contamination.

Blasting or tumbling media can get stuck in small features. This has to be avoided by choosing the right process, not by checking afterwards.

Why stainless steel gets "passivated"

After machining, stainless steel is usually passivated. In simple terms, the process cleans the surface and helps the natural protective layer grow back. It also removes tiny bits of steel left by tools.

Two things worth knowing:

Passivation cleans the surface. It does not make a rough or torn surface smooth.

Very fine features and threads can be affected, so the process should be written down, not just the result.

"Can you prove it?" - traceability in plain words

Traceability means a finished part can be connected to its history: which material batch it came from, which machine made it, who inspected it, and when. If a problem appears later, the shop can find every part made the same way.

For the buyer, this usually means asking for four things: material certificates, process records for special steps (like passivation), inspection data per batch, and a warning whenever a process changes.

Two different questions to ask a supplier

This is the part most buyers mix up.

Question 1: Can you make this part well? Look at the machines, the sample inspection reports, and the surface finish they can achieve.

Question 2: Are you approved for medical work? This is about the quality system, not the machines. Many medical programs ask for ISO 13485, which is the medical-device quality standard. ISO 9001 and IATF 16949 are good systems too, but they are not the same thing.

A shop can be excellent at machining and still not be set up for the paperwork a medical program needs. Ask both questions before the quote, not after the first sample.

Keeping tolerances (and cost) sensible

Put tight tolerances only on the features that touch something else: holes, sealing faces, mating surfaces.

Leave the rest at general tolerance. Tightening everything raises cost and inspection time without making the device better.

For surface finish, give a number (Ra) and a direction. "Smooth" means different things to different people.

Thin or soft parts should be measured gently - a probe can push a thin wall out of shape.

Six design tips that save money later

1. Use through-holes instead of blind holes wherever possible.

2. Round the inside corners; sharp corners are harder to clean and to cut.

3. Keep wall thickness even, especially in PEEK and PC, which warp when thickness changes.

4. Use metal inserts anywhere a screw will be taken out and put back many times.

5. Decide early where laser marking will go - marking changes the surface.

6. Keep the "good-looking" surfaces separate from the "must be exact" surfaces, so finishing can be applied only where needed.

What to send with your request

Send the drawing (with tolerances and surface finish), the 3D model, the material and grade, the quantity plus a forecast, cleaning and passivation requirements, the documents you need, and the quality standard your program requires.

That last item is the one that most often changes the answer - and the one most often mentioned too late.

About the manufacturer

Shenzhen Strongd Model Technology Co., Ltd. produces metal and plastic parts for robotics, automotive, consumer electronics and medical equipment customers from a 40,000 m² factory in Shenzhen, China. The company runs 85 CNC machining centres, 18 of them five-axis, inspects parts on Hexagon CMM systems, and holds ISO 9001:2015 and IATF 16949:2016 certification.

Website: https://www.strongdcnc.com/