Key takeaways
- CNC machining remains a genuine production method whenever tolerances, metals, or low annual volume rule out tooling up a mold.
- Machining has a comparatively flat cost curve: little fixed cost, but a per-part cost that barely drops with volume, the opposite of injection molding’s economics.
- Deep pockets, sharp internal corners, and extra setups are what push a machined part’s price up, more than the material itself.
- Aerospace brackets, low-volume medical hardware, and firearm receivers are real categories where machined parts never move to molding at all.
CNC machining beats molding for production parts whenever your annual volume is too low to amortize a mold, your tolerances are tighter than molding reliably holds, or your part is metal rather than plastic. It is a genuine production method, not a placeholder used until a “real” mold gets cut. This guide covers what machining does that molding cannot, why its cost curve behaves so differently, which design choices make it expensive, and how to decide between the two.
CNC is not only a prototyping process
CNC machining shows up early in most hardware projects as a way to make a handful of prototype parts fast, which leads founders to assume it disappears once “real” production starts. For plastic enclosures at meaningful volume, it usually does, because injection molding’s per-part cost drops low enough to win. For metal parts, low annual volumes, and tolerances or material properties molding cannot deliver, CNC machining is the production method indefinitely, not a stopgap. Our guide to choosing a plastic manufacturing method covers CNC alongside molding, 3D printing, and casting as one of five options filtered by volume and geometry; this article goes deeper on the case where machining is the permanent production method.
What machining does that molding cannot
Tolerances, metals, and structural parts
Injection molding shapes a part inside a cavity; CNC machining removes material from solid stock with a rotating cutting tool. That difference is why machining reaches tighter tolerances and runs materials molding cannot process at all, such as aluminum, stainless steel, and titanium.
General numeric tolerances for machined parts come from ISO 2768-1, a standard for linear and angular dimensions produced by metal removal, which defines four classes from fine to very coarse. At the fine end, a dimension between 6mm and 30mm carries a tolerance of roughly ±0.1mm; at the coarse end, the same size range allows roughly ±0.5mm. This is a different standard from ASME Y14.5, which governs the symbols and rules of geometric dimensioning and tolerancing rather than setting numeric default tolerances itself.
| Property | Injection molding (thermoplastics) | CNC machining (metals and plastics) |
|---|---|---|
| Typical materials | ABS, polycarbonate, nylon, polypropylene, TPU | Aluminum (6061, 7075), stainless steel, titanium, Delrin/POM, PEEK |
| Structural strength in metal | Not applicable; plastic only | Full metal strength, suitable for load-bearing brackets and fixtures |
| Tight tolerance work | Constrained by tool wear and shrinkage across the mold | Holds ISO 2768 fine-class tolerances routinely |
| Internal cavities and undercuts | Requires draft angles and multi-action tooling | Limited by tool reach, not by draft |
The cost curve is flat, and why that matters
Injection molding sits at one extreme: high fixed tooling cost, very low marginal cost per part once that tool exists. CNC machining sits closer to the other extreme: little fixed tooling cost, but a per-part cost dominated by machine time and labor that barely drops as volume increases. An open engineering-economics course text comparing manufacturing methods illustrates the shape of each curve: in one worked example, injection molding a small plastic part ran roughly $20 to $50 per unit at a batch of 100 and fell to $0.50 to $2 per unit at 10,000 units, while a separate example machining a steel part on a CNC mill ran roughly $100 to $300 per unit at 50 units and only dropped to $50 to $100 per unit at 1,000 units, since there is no tool to amortize and each part still needs its own cutting time. Treat both ranges as illustrative of the shape, not as a quote for your specific part.
In our experience at Inventornest, this is also how we frame the 3D-printing-to-molding decision for plastic parts, and the same logic extends to CNC: we do not recommend a fixed volume threshold for every product, since the decision depends on part complexity, material, tolerance, tooling cost, and the per-unit savings molding would offer. As a starting calculation, tooling-only break-even quantity equals tooling investment divided by the per-unit savings molding provides: a USD 5,000 mold saving USD 10 per part over machining breaks even around 500 units. This is an illustration, not a supplier quote, and freight, sampling, and tooling changes still need to be added on top.
Design choices that make machining expensive
Deep pockets, internal corners, and setups
Three design decisions drive machining cost more than the base material does:
- Deep pockets. A pocket that is much deeper than it is wide forces a longer, thinner cutting tool, which flexes more, cuts slower, and wears faster. Shallow, wide pockets machine quickly; deep, narrow ones do not.
- Sharp internal corners. A rotating end mill is round, so it always leaves a radius in an internal corner equal to its own radius or larger. Specifying a true sharp internal corner forces either a smaller (slower, more fragile) tool or a secondary operation like EDM, and either way the cost goes up for a feature that is often cosmetic rather than functional.
- Extra setups. Every time a part has to be unclamped, flipped, and re-fixtured to reach a new face, that is a new setup: new alignment, new time, and a new opportunity for the part to shift out of tolerance between operations. A part designed to be fully machined from as few sides as possible, ideally one or two setups, costs meaningfully less than the same part designed without that constraint.
None of these rules are published in a single standard the way ISO 2768’s tolerance classes are; they are shop-floor practice, worth confirming with whoever is cutting your specific part rather than assuming a fixed number applies universally.
Finishing operations and their cost
A part straight off a CNC machine carries visible tool marks and a machined radius in every internal corner. Two finishing paths are common for parts that need to look better or resist corrosion: anodizing for aluminum, which forms a controlled oxide layer under military specifications ranging from a thinner chromic-acid finish to a thicker “hardcoat” anodize, and passivation for stainless steel, a chemical treatment standardized under ASTM A967 that removes free iron from the surface and is verified with salt-spray and copper-sulfate testing. Bead blasting is a common lower-cost option for either material, removing tool marks and leaving a uniform matte texture, though it is shop practice rather than a published standard.
Product categories where machined parts stay in production
Some products never graduate from CNC machining to molding, because volume, material, or the regulatory environment make machining the permanent answer:
- Aerospace structural brackets and fixtures, typically built to AS9100, the aerospace quality management standard layered on ISO 9001, at volumes far too low to justify tooling.
- Low-volume medical device components, common in instruments and fixtures produced under a device quality management system; as of February 2026, the FDA’s Quality Management System Regulation, harmonized with ISO 13485, governs these processes in place of the prior Quality System Regulation.
- Firearm receivers and similar regulated metal components, where strength and precision at small batch sizes keep machining the standard method.
- Test and lab equipment enclosures, typically produced in the tens to low hundreds of units, well below where a mold pays for itself.
How to decide between machining and tooling up
Run the decision in order: confirm the material first (metal rules out molding outright), check your realistic annual volume against the tooling break-even math above, then check whether your tolerances or geometry are achievable in a mold at all. Any one of those three answers pointing to machining usually settles the question regardless of the other two.
Frequently asked questions
Is CNC machining more expensive than injection molding?
Often yes at higher volumes, because molding’s cost drops sharply once tooling is amortized. At low volumes, machining is frequently cheaper overall since there is no tooling cost to pay for.
What tolerances can CNC machining hold?
ISO 2768-1 defines general tolerance classes for machined parts, with the fine class holding roughly ±0.1mm for a 6mm to 30mm dimension. Tighter tolerances are achievable but should be called out explicitly on the drawing.
Can CNC-machined parts be used in a final shipping product, not just prototypes?
Yes. Aerospace brackets, low-volume medical instruments, and firearm receivers are routinely produced by CNC machining as the permanent production method.
What makes a machined part more expensive than it needs to be?
Deep, narrow pockets, unnecessarily sharp internal corners, and a design requiring many separate machine setups are the most common cost drivers, ahead of the base material itself.
At what volume does injection molding become cheaper than CNC machining?
It varies by part geometry, material, and tooling cost. A simple break-even check, tooling cost divided by the per-unit savings molding offers, gives a starting estimate for your specific part.
Where Inventornest fits
Inventornest designs and fabricates enclosures using 3D printing, sheet metal fabrication, and CNC machining depending on what the product and the stage require, and we retain responsibility for engineering coordination when production is handled by our manufacturing partners. If you are trying to work out whether your part belongs on a CNC machine permanently or is headed for a mold, get a quote and bring your volume target and material requirements.
