Walk through any product assembly and count the parts that are neither round nor flat: the housing with a pocket for a PCB, the bracket with three faces milled square, the heat sink with fins and a mounting pattern, the aluminium chassis whose every user-facing surface was cut by an end mill. Those are CNC milled parts, and they are where most custom metal components live. This guide covers what milling can and cannot hold, how axis count changes the price, which materials behave well, and what to put on a drawing before you send it out for quote.
What makes a part "milled"
A milling operation removes material with a rotating cutter while the workpiece stays put (or moves under program control on multiple axes). Anything that can be reached by that cutter becomes a feature: pockets, slots, shoulders, threads, chamfers, contours and freeform surfaces. The family is huge, because unlike turning — which makes rotational parts — milling has no geometric limit beyond tool reach.
The features that define most milled parts:
- Pockets and cavities — for boards, displays, batteries and mechanisms; corner radii are set by the cutter, which is why internal corners are never sharp in a milled part unless the drawing says so.
- Faces and datums — milled faces are the mating surfaces everything else is measured from, which is why flatness and perpendicularity matter more than size.
- Slots and keyways — width tolerance follows slot width, often the single most expensive dimension on the part.
- Threaded holes — milled then tapped, or thread-milled; thread-milling gives better positional accuracy and no tap-breakage risk in tough alloys.
- Contoured surfaces — from simple 2.5D profiles to genuine 3D surfaces that need simultaneous multi-axis movement, covered in our simultaneous 5-axis versus 3+2 comparison.
Axis count: what you are actually paying for
Every additional axis buys you fewer setups, not just more shapes.
| Configuration | What it adds | Typical use | Relative hourly cost |
|---|---|---|---|
| 3-axis | X, Y, Z — one face at a time | Plates, brackets, enclosures, simple pockets | 1.0× |
| 4-axis | Rotation about X — multiple faces per setup | Parts needing features on several sides | 1.2–1.5× |
| 5-axis | Two rotary axes — most faces in one setup | Complex geometry, contoured surfaces, deep cavities | 1.6–2.4× |
The reason to pay for more axes is rarely "we need a complex shape". It is usually setup count: every time an operator unclamps and re-fixtures a part, positional error is reintroduced, and that error — not the machine — decides whether your perpendicularity callout survives. A part needing five setups on a 3-axis mill often comes off a five-axis machine not just faster but genuinely better. Our CNC milling service page covers how capacity is planned around this.
For parts that are fundamentally rotational, CNC turning is faster and cheaper; the turning versus milling comparison covers the boundary, and plenty of real parts use both.
What tolerances are realistic
Milling is the most accurate of the common processes, which is why buyers over-specify it. Prices climb steeply below these levels:
| Feature | Routine | Achievable | Comments |
|---|---|---|---|
| Linear dimension | ±0.05 mm | ±0.01 mm | shop standard across most aluminium work |
| Hole position (true position) | ±0.05 mm | ±0.01 mm | needs proper datum scheme |
| Flatness | 0.05 mm over 100 mm | 0.01 mm | thin walls move during cutting |
| Perpendicularity | 0.05 mm per 100 mm | 0.01 mm | mostly a fixturing question |
| Slot width | ±0.05 mm | ±0.01 mm | below ±0.02 mm consider wire EDM |
| Surface finish Ra | 1.6 µm | 0.4 µm | below 0.4 µm needs grinding or polishing |
Two clarifications save rework. First, tolerance has to be measurable as drawn — a ±0.01 mm callout needs a documented measurement method, or it means different things to your supplier and your incoming inspector. Second, thin-walled parts will not hold machined tolerances once unclamped: a 1.5 mm aluminium wall can spring by several hundredths after the last pass. Our CNC machining tolerance and cost guides cover why the last tenth of a millimetre always costs disproportionately.
Materials that machine well — and three that do not
Material choice drives cycle time more than any other single variable.
| Material | Machinability | Notes | Guide |
|---|---|---|---|
| Aluminium 6061 / 6063 | Excellent | The default. Fast, stable, anodises well | aluminium CNC machining |
| Aluminium 7075 | Good | Stronger, slightly more tool wear, poor for welding | — |
| Brass | Excellent | Best chip control of any common alloy, ideal for fine detail | brass CNC machining |
| Stainless 303 / 304 | Moderate | 303 is the free-machining grade; 304 work-hardens | stainless steel machining |
| Titanium Ti-6Al-4V | Difficult | Low cutting speeds, high tool cost, heat management critical | titanium machining |
| PEEK | Moderate | Thermal management matters more than cutting force | PEEK machining |
If a drawing says "aluminium", ask which alloy. 6061-T6 and 7075-T6 differ in cost, strength and stability, and "7075 because it is stronger" is a decision that should be traceable to a load case, not a habit.
Milling tolerances by feature: what each band costs you
The tolerance on a drawing is never one number — it is a set of decisions about which features actually matter. Standard milling tolerances on our floor fall into three bands: ±0.05 mm on general dimensions under ISO 2768-m, ±0.01 mm on features we hold with a finish pass and in-process measurement, and ±0.005 mm on the small number of features that justify a dedicated setup plus CMM verification. Moving a dimension from ±0.05 to ±0.01 usually adds one finish pass and one measurement step. Moving it from ±0.01 to ±0.005 can double the cycle time on that feature, because it means slower feeds, more rigid workholding and measurement at a controlled temperature.
Three rules follow from that cost curve. Put the tight number only on the feature that needs it and let everything else sit at the general tolerance — a drawing that calls ±0.01 mm across the board is paying for precision it cannot use. Specify the datum structure together with the number, because a ±0.01 mm position callout without datums is not measurable and will be quoted conservatively. And remember the ranking: flatness is cheaper than perpendicularity, and perpendicularity is cheaper than true position. If a face only has to seal, call out flatness and stop there.
Cost drivers, ranked
For most CNC milled parts the price decomposes roughly as follows, and this ordering holds across shops:
- Cycle time — set by material removal volume, tool changes and number of setups. Usually the largest block.
- Setup count — each fixture is programming, fixture build and machine downtime.
- Material — billet price plus the volume you are buying and throwing away.
- Inspection — driven by how many tolerances you wrote and how hard they are to measure.
- Finishing — CNC parts finishing, anodising, bead blast, laser marking.
Notice what is *not* on the list: the hourly rate. Matching cycle time and setup count is worth far more than a rate negotiation, which is why DFM review — unifying corner radii, standardising hole sizes, loosening non-critical tolerances — usually returns 10–20% before a single chip is cut.
Designing parts that quote well
Six rules cover most of what makes a milled part cheap or expensive to make:
- Use the largest corner radius the design allows. A 6 mm cutter removes material several times faster than a 2 mm one. Specifying R0.5 internal corners on a large pocket can triple its machining time.
- Standardise hole sizes. Every drill diameter is a tool change. Three different hole sizes instead of seven saves real money on every part.
- Keep walls above ~0.8 mm in aluminium, and above ~1.0 mm in steel, unless you accept the fixturing cost of holding thin sections.
- Avoid deep pockets. Depth beyond roughly 4× cutter diameter requires special tooling or a 5-axis approach.
- Machine from standard stock sizes where possible — every millimetre of extra billet is material and machine time.
- Tolerance only what matters. This single habit is worth more than any supplier concession.
If the part may change, start with CNC prototyping or low-volume CNC machining and freeze the design against real parts rather than renders.
What to send for a quote
A milled-part RFQ should carry: 3D model in STEP; 2D drawing with dimensions and tolerances; material and temper; quantity and expected annual volume; finish specification including any masking; critical-to-quality features flagged; and inspection expectations (first article report, CMM report, or simply visual and dimensional sampling). Where both machined and formed features exist, our sheet metal bend allowance guide explains the fabrication side of the same drawing.
At Ruijin Fenghui we machine these parts on 200+ CNC machines across 3-, 4- and 5-axis platforms, hold ±0.01 mm on critical features, and run DFM review before quoting — with a 24-hour turnaround on quotation and IATF 16949 / ISO 9001 / ISO 13485 certified processes behind it. Whether your assembly needs robotics components, audio panels or drone frames, the drawing review is the same conversation.
Send your drawing for a DFM review and quote — typically within 24 hours.
FAQ
What CNC machining services does Ruijin CNC offer?
Ruijin CNC provides CNC turning, CNC milling (3-axis and 4-axis), 5-axis CNC machining, sheet metal fabrication, metal stamping, and surface treatment including anodizing, plating and powder coating.
What tolerances and certifications does Ruijin CNC hold?
We hold tight tolerances down to ±0.01mm across all processes, and are certified to IATF 16949, ISO 9001 and ISO 13485.
What is the minimum order quantity and lead time?
We accept prototypes from 1 piece with no strict MOQ, provide free DFM review, and return a quote within 24 hours of receiving your CAD drawing and requirements.
How do I get a quote from Ruijin CNC?
Send your STEP file and drawing to gongtianbao@xhlmarketing.com or via WhatsApp +86 17620300785. We reply with a DFM review and a firm quote within 24 hours.