IllustrationUse a CNC machining prototype when a part needs real material properties or tight tolerances: aluminum housings, metal brackets, and POM, PC or ABS parts that must flex, carry load or hold a thread. Machining cuts the part from solid stock, so it proves strength and fit well and hides molding issues such as draft and weld lines.
Most device enclosures start as 3D prints. Some parts outgrow printing early: an aluminum housing, a metal hinge, a plastic clip that has to behave like the real resin, a fixture that holds boards for test. A CNC machining prototype covers those cases. This guide explains when to machine a part, how milling and turning differ, which design rules keep machining simple, and what a machined part does and does not prove about a design that will later be molded.
This is an engineering summary. Design limits differ between shops and machines, so confirm figures with the shop that will cut your part. All sources were read on November 12, 2026.
When does a CNC machining prototype make sense for a device?
CNC machining removes material from a solid block or bar with rotating cutters. The part is made of a real engineering plastic or metal, so its strength, stiffness and heat behavior match that material. Protolabs says machining generally produces superior strength and surface finish to any additive process, and lists some geometry limits as the trade-off (rapid prototyping guide, as of November 2026).
Machine a prototype part when:
- The production part is metal. An aluminum housing, a stainless clasp or a brass contact is best tested in the real alloy.
- A plastic part must carry load, flex or hold a thread the way the production resin will, and printing resins do not come close.
- A dimension is tight. A bearing bore, a sealing face or a camera lens seat often needs better than a print can hold.
- You need fixtures, test jigs or programming clamps that will be used hundreds of times.
Protolabs lists ABS, acetal (Delrin), nylon, PEEK, PEI, PMMA, polycarbonate and polypropylene among its machinable plastics, and aluminum, brass, copper, stainless steel, steel alloys and titanium among its metals (CNC machining service, as of November 2026).
Keep printing when the shape still changes weekly, or when the part has internal features a cutter cannot reach. SLA vs SLS 3D printing covers that side.
CNC milling vs CNC turning: which parts suit each?
Milling holds the part still while a spinning cutter moves around it. It makes housings, brackets, plates and pockets. Protolabs lists a 3-axis envelope up to 22 x 14 x 3.75 in. for aluminum at its automated facility (milling design guidelines, as of November 2026).
CNC turning spins the part while a fixed tool cuts it. It makes round parts: knobs, crowns, shafts, standoffs and cylindrical housings. Protolabs lists a factory turning range from 0.16 in. to 3.95 in. in diameter and up to 9 in. long, with a minimum wall of 0.020 in. (0.51 mm). Live tooling adds flats, slots, grooves and holes parallel or perpendicular to the axis. Off-axis holes are offered only in aluminum and brass (turning design guidelines, as of November 2026).
Fictiv advises keeping long, thin turned parts at a length-to-diameter ratio of 8:1 or less to avoid chatter (CNC design guide, as of November 2026). A wearable crown or a pen-style device body usually sits well inside that.
3-axis vs 5-axis: when does prototype machining need more axes?
A 3-axis mill moves the cutter in X, Y and Z. It reaches each face from above, so a part with features on several sides needs several setups. Xometry says 3-axis machines require multiple setups to produce complex parts, while 5-axis machines can work on all sides of a workpiece without manually rotating it (3-axis vs 5-axis, updated April 1, 2026, as of November 2026).
Xometry also says 3-axis machines are far cheaper to buy, program and operate, so cost per part is often lower, and that they suit 2D and 2.5D parts. Protolabs uses 3-axis milling and 5-axis indexed milling, and its 5-axis envelope for aluminum is smaller, at 10 x 7 x 3.75 in. (as of November 2026).
For prototype machining of a device housing, this has a design consequence. A shell with features on the top, bottom and four walls takes several setups or a 5-axis machine. Grouping features so most are reached from one or two sides keeps the part on a 3-axis mill and cuts cost.
Which design rules keep a machined prototype simple?
Cutters are round and have limited reach. Most machining cost comes from small tools, deep pockets and extra setups. These published rules help.
| Feature | Rule as published | Source |
|---|---|---|
| Internal corners | A milled inside corner takes the radius of the cutter. Fictiv says to keep radii above 0.8 mm where possible and to make them slightly larger than the cutter radius, such as 0.130 in. for a 0.125 in. cutter radius. Xometry recommends adding at least 0.02 to 0.05 in. to internal corner radii. | Fictiv, Xometry |
| Pocket depth | Fictiv gives depth-to-diameter limits of 15x the tool diameter for plastics, 10x for aluminum and 5x for steel. Protolabs lists a maximum milled depth of 2 in. (50.8 mm) from either side of the part. | Fictiv, Protolabs milling |
| Wall thickness | Fictiv: above 0.25 mm for metal and 0.50 mm for plastic. Protolabs: minimum nominal wall of 0.040 in. (1.02 mm), with features thicker than 0.020 in. (0.51 mm). | Fictiv, Protolabs milling |
| Drilled holes | Fictiv: keep drilled depth under 12x the bit diameter, and design blind holes 25% longer than the depth you need. | Fictiv |
| Threads | Fictiv: aim for 1 to 1.5x the thread diameter of engagement; a tap deeper than 3x the diameter counts as deep. Protolabs supports UNC/UNF #2 to 0.5 in. and M2 to M12, and asks you not to model internal threads. | Fictiv, Protolabs milling |
| Tolerances | Protolabs quotes ±0.005 in. (0.13 mm) at its automated facility and ISO 2768-1 medium or fine through its network. Fictiv says tolerances tighter than 0.05 mm may be loosened, and below 0.1 mm is hard to hold in plastics. | Protolabs milling, Fictiv |
| Engraved text | Protolabs: minimum stroke width 0.018 in. (0.457 mm) in plastics and soft metals and 0.033 in. (0.838 mm) in hard metals, both 0.0118 in. (0.3 mm) deep. Recessed text is preferred over raised. | Protolabs milling |
Sources, all as of November 2026: Fictiv CNC Machining Design Guide (June 29, 2021), Xometry, Optimizing Internal Corner Radii (updated August 13, 2024), Protolabs milling design guidelines.
The corner rule matters most for enclosures. A housing that will be molded later often has sharp inside corners in CAD, where the board or battery sits. Machining will round those corners, and the board may no longer fit. Add the radius in CAD, or relieve the corner with a small cut-out, so the machined part and the internal parts agree.
Give the shop a drawing for anything that matters. Without one, it works to its default tolerance. Through its manufacturer network, Protolabs says hole and bore features specified on a drawing may be held to ±0.0005 in. (0.0127 mm) (source, as of November 2026). See tolerance stack-up analysis for deciding which dimensions need that.
Which finishes suit machined device parts?
Machined parts show tool marks unless you specify otherwise. Protolabs leaves plastic parts as machined, with visible tool marks, and offers aluminum and other metals either as machined or lightly bead blasted (milling design guidelines, as of November 2026). Bead blasting gives an even matte surface and hides small marks, which helps for looks-like units.
Anodizing is the usual finish for aluminum housings. Protolabs compares two types (July 17, 2023, as of November 2026):
- Type II: 0.00007 to 0.001 in. (1.8 to 25 µm) thick. Can be dyed, for example black. Wear resistance rated good.
- Type III (hardcoat): 0.0005 to 0.006 in. (13 to 150 µm) thick. Dyeing not recommended. Wear resistance rated very good.
The coating adds thickness to the surface. On a press fit, a sealing bore or a thread, that growth can turn a fit into an interference. Agree with the shop which features get masked, and dimension the part for the coated state where it matters. Anodize is also an electrical insulator, so mask any spot that must make ground contact with the board or an antenna.
What does a machined part prove about a molded design?
It proves fit, stiffness of the chosen material, and the function of features such as threads and bearing seats. It proves less than it seems about a part that will be injection molded.
- No draft. A machined wall can be vertical. A molded wall needs draft, which changes fits and gaps. See injection molding draft angle.
- No weld lines or sink. A machined part has uniform strength. A molded part is weaker at weld lines and can sink behind thick sections.
- Different material state. Plastic stock bar or plate is made by a different process and may be a different grade from the molding resin, so strength and impact behavior differ.
- Different wall design. Machined parts tolerate thick walls. Molded parts need even walls and ribs, so the machined geometry often cannot be molded as is.
The practical rule: design the housing for molding from the start, and machine that molding-ready geometry. Then the machined part tests the real design. Prototype plastic injection molding covers the step after machining, and the PCB prototype gate shows where machined parts meet the first boards.
Questions to ask before ordering a machined prototype
- Which question does this machined part answer: fit, strength, heat, finish or a fixture?
- Which material grade will be cut, and how does it compare with the production resin or alloy?
- Which dimensions are critical? Are they on a drawing with tolerances, or left to the default?
- Are internal corner radii larger than the cutter radius the shop will use?
- Which features need a second setup, and could they be moved to one side?
- Which surfaces get anodize or bead blast, and which threads and bores get masked?
- What will the molded version need that the machined part does not have: draft, ribs, gates?
Questions about CNC machining prototypes
When is a CNC machining prototype better than a 3D print?
When the part needs real material properties or tight tolerances. Protolabs says CNC machining generally produces superior strength and surface finish to any additive process (source, as of November 2026). Prints win when the shape changes every week or the geometry cannot be reached by a cutter.
What is CNC turning used for in a device?
Round parts: knobs, buttons, shafts, standoffs, threaded inserts and cylindrical housings. Protolabs lists a factory turning envelope of 3.95 in. diameter by 9 in. long, with live tooling for flats, slots and cross holes (source, as of November 2026).
Is CNC manufacturing practical for small batches?
For some parts. Protolabs lists CNC machining for 1 to 200+ parts (source, as of November 2026). Aluminum housings and metal parts are often machined in small batches. Plastic housings usually move to casting or a mold as quantities rise, because machining cost stays per part.
Should I anodize a machined aluminum prototype?
If the part will be handled, photographed or tested for wear, yes. Protolabs lists Type II at 1.8 to 25 µm and Type III at 13 to 150 µm (source, as of November 2026). Allow for the coating on tight fits, and ask the shop which surfaces it will mask.
How can Duobay help with machined prototypes?
Duobay develops custom devices with a Shenzhen engineering team that covers mechanical, electrical, firmware, app and test work. Enclosure and part prototypes are part of the gated process, and all source files are assigned to you at every paid gate. Initial calls are free, and we do not publish prices. Tell us about your device.
If you want a prototype plan that says which parts to print, machine or mold, tell us about your device.
Sources. All pages accessed November 12, 2026. Machine envelopes and design limits change, so confirm current values with your shop.
- Protolabs, CNC Machining Service (link)
- Protolabs, CNC Milling Design Guidelines (link)
- Protolabs, CNC Turning Design Guidelines (link)
- Protolabs, Comparing Type II and Type III Anodizing for Machined Parts (July 17, 2023) (link)
- Protolabs, Selecting a Rapid Prototyping Process (link)
- Protolabs UK blog, 3D printing vs injection molding vs CNC machining (June 20, 2024) (link)
- Fictiv, CNC Machining Design Guide (June 29, 2021) (link)
- Xometry, CNC Machining Design Tip: Optimizing Internal Corner Radii (updated August 13, 2024) (link)
- Xometry, 3-Axis vs. 5-Axis CNC (updated April 1, 2026) (link)
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