What CNC prototyping is, when to machine a prototype instead of 3D printing it, how to design and specify parts that machine quickly, and what affects lead time.
By Tom Davies-Kent, Director, Complete Fabrication. October 2026.
CNC prototyping means machining a prototype from a solid block of the real material, such as aluminium, stainless steel, PEEK or an engineering plastic. The part has the strength, stiffness and heat resistance of that material, so you can test how it will really behave. At our workshop near Cambridge we hold ±0.10 mm on most work and turn most parts round in 3 days, with a 24-hour rush when you need it sooner.
A prototype is a pre-production representation of some aspect of a concept or final design (Camburn et al., 2017). A CNC prototype is one made by computer-controlled machining. A program is written from your CAD model, and the machine follows it to cut away material until the part is left.
Two methods cover most prototype work:
| Method | How it cuts | Typical features | Our machines |
|---|---|---|---|
| Milling | A rotating cutter moves across a fixed block | Faces, pockets, slots and holes | Haas VF-2SS 4-axis mills: the rotary axis turns the part during cutting, so several sides can be machined in fewer set-ups |
| Turning | The workpiece spins on a lathe while the tool cuts | Shafts, bores and grooves | Haas SC10 lathe |
Many prototypes need both, plus drilling, tapping and hand finishing. Because all of that happens in one workshop, a part can go from the mill to the lathe to the paint shop without leaving the building.
Choose the process by the question the prototype has to answer. We run CNC machining, 3D printing and vacuum casting side by side, so we can recommend whichever suits the test.
| What the prototype must prove | Usual best route | Why |
|---|---|---|
| Overall shape, size and look | SLA 3D printing | Fastest and cheapest for complex shapes, with a surface that paints well |
| Strength, stiffness or wear in a specific material | CNC machining | The part is the real metal or plastic, not a resin that imitates it |
| Performance under heat or chemicals | CNC machining, for example in PEEK or stainless steel | You test the material you will produce in |
| Close fits between machined parts in an assembly | CNC machining | ±0.10 mm on most work, and tighter on request |
| Tens or hundreds of identical plastic parts | Vacuum casting | Batches of 10 to 500 from one master pattern |
| Internal channels or organic shapes a cutter cannot reach | 3D printing | Additive processes are not limited by tool access |
The headline numbers for each process we run:
| Process | Accuracy | Standard lead time | Quantities | Materials |
|---|---|---|---|---|
| CNC machining | ±0.10 mm on most work | 3 days; 24-hour rush | One-offs to 500+ | Aluminium, steels, stainless steel, brass, PEEK and engineering plastics |
| SLA | ±0.15 mm | 24 hours | One-offs to thousands; typically 20 to 200 | Photopolymer resins, including tough and clear grades |
| FDM | ±0.4 mm | Roughly 1 to 10 working days; confirmed at quote | One-offs and large parts | Thermoplastic filaments, including filled nylons and TPU |
| Vacuum casting | About ±0.2 mm on small parts, up to ±0.7 mm on large ones | Confirmed at quote | 10 to 500 | Polyurethane resins, including clear grades |
A common pattern is to 3D print the first versions to settle the shape, then machine the design in the real material once you need to test how it performs. For the printing side of that loop, see rapid prototyping with 3D printing.
We hold these materials in stock: aluminium, steels, stainless steel (303, 304, 316 and 440), brass, ABS, nylon, POM (acetal), polycarbonate, acrylic and PEEK.
Most of the cost and time in a machined prototype comes from programming, set-ups and slow cuts. A few design choices make a big difference.
Cutters are round, so an internal corner in a pocket is left with a radius equal to the tool. A perfectly sharp internal corner needs extra operations or a different process. If a square part has to sit in the pocket, a small relief cut in the corner is usually simpler.
Deep, narrow pockets need long, thin tools, which bend and must cut slowly. Thin walls bring a similar problem: a review of thin-wall machining in light alloys identified vibration and part deflection as the main challenges (Del Sol et al., 2019). Where a wall can be thicker, or a pocket shallower, without hurting the design, the part will be quicker to make and more accurate.
Each time a part is turned over and clamped again, it adds time and a small risk of misalignment. Grouping features on as few faces as possible helps. Our 4-axis mills can rotate the part during cutting, which cuts the number of set-ups for features around a part.
Standard drill sizes and thread forms can be cut with tools already in the machine. Unusual sizes may need special tooling.
We check every drawing before we quote. If a feature is hard to machine, or a tolerance is tighter than the part needs, we tell you before we cut metal and suggest a cheaper way to get the same result. For a wider view, see our guide to design for manufacture.
We hold ±0.10 mm on most work, and tighter on request. Tighter tolerances take more time and careful inspection, so they cost more. Research on tolerance allocation has long recognised that tolerance choices affect both product quality and manufacturing cost (Hallmann et al., 2020).
For a prototype, the practical approach is:
| Feature | What to specify |
|---|---|
| Bearing bores, press fits, mating faces and sealing surfaces | The real tolerance, marked on a 2D drawing |
| Everything else | The general tolerance: ±0.10 mm on most work |
| Faces that will be painted (high gloss) | Allow about 0.2 mm for the paint, or ask us to mask the face |
| Faces that will be painted (satin or matt) | Allow about 0.1 to 0.15 mm, or ask us to mask the face |
Always send the 2D drawing with the 3D model, so we know which dimensions are critical.
A machined surface shows fine tool marks. That is fine for a functional test, but a looks-like model or a part for a show needs more work. We finish machined parts in our own workshop:
Tell us which faces are seen and handled, so we can plan the finish where it matters. See painting and finishing.
Most CNC prototypes are turned around in 3 days, and a 24-hour rush is available. A very tight deadline that needs overtime or overnight work may carry an extra charge, which we confirm in the quote. We quote within two working days of receiving your drawings.
These things add time:
There is no minimum order. We machine one-off prototypes and short runs of 1 to 500+ parts, finished to look like the final product. For larger numbers of plastic parts, we compare machining with vacuum casting and recommend the cheaper route.
Two projects show how far a machined prototype can go:
It is making a prototype by computer-controlled machining from a solid block of metal or plastic. Because the part is the real material, it can be tested for strength, heat, chemical resistance, wear and fit.
We turn most parts round in 3 days, with a 24-hour rush available. Several set-ups, tight tolerances, materials not in stock and painting add time. We confirm the lead time for your part when we quote.
It depends on the part. 3D printing is usually faster and cheaper for complex shapes and early form checks. CNC machining is the better value when the test needs the real material or close fits. We price both routes and recommend one.
Yes, if it can be machined. We stock aluminium, steels, stainless steel, brass, PEEK and common engineering plastics, and can machine the plastic you plan to mould in. For form and fit checks, a cheaper stand-in material is often enough.
Send a STEP or IGES file, or DWG, with a 2D drawing showing critical dimensions and tolerances. Add the material, quantity, finish and deadline.
Yes. One-off and complex parts in a specific material are our everyday work, and there is no minimum order.
Send your drawings and tell us what the prototype needs to prove. We will recommend a process and confirm the cost and lead time. Call +44 1223 836837, email info@compfab.co.uk, or contact us. See also our CNC prototyping service and rapid prototyping services.
Send your CAD files or drawings and tell us when you need the parts. Call +44 1223 836837 or email info@compfab.co.uk.
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