Swiss-type sliding-headstock CNC turning can hold ± 0.005 mm dimensional tolerance, ± 0.003 mm concentricity, and Ra 0.2–0.4 µm surface finish on brass production runs of any volume. Grinding-finish after Swiss CNC pushes this to ± 0.002 mm and Ra 0.05 µm. Specifying tighter than these limits is over-spec — and typically costs 2–4× standard CNC. This guide quantifies what is achievable, what each tolerance class costs, and how to know when you genuinely need Swiss instead of conventional CNC.
Swiss-type CNC turning was developed in the early 20th century in the Swiss watchmaking industry, where parts had to be cut from long slender bars with the bar guide closely supporting the workpiece against deflection. The defining feature — the bar slides through a guide bushing right up to the cutting tool, so deflection is minimal — is what enables Swiss CNC to hold tolerances an order of magnitude tighter than conventional turning on long, slender brass parts.
For brass procurement and design engineers, the relevant question is: how tight can a Swiss CNC machine actually hold a feature on a typical CW617N brass production part? This article answers that with real production numbers.
Dimensional tolerance — what is achievable
| Feature type | Conventional CNC (turning centre) | Swiss CNC (sliding headstock) | Swiss + grinding finish |
|---|---|---|---|
| Diameter tolerance | ± 0.02 mm (h7) | ± 0.005 mm (h5/IT5) | ± 0.002 mm (h4/IT4) |
| Length tolerance | ± 0.05 mm | ± 0.01 mm | ± 0.005 mm |
| Concentricity (TIR) | 0.02 mm | 0.003 mm | 0.002 mm |
| Perpendicularity | 0.02 mm | 0.005 mm | 0.003 mm |
| Parallelism | 0.02 mm | 0.005 mm | 0.003 mm |
| Surface finish Ra | 0.4–1.6 µm | 0.2–0.8 µm | 0.05–0.2 µm |
| Surface finish Rz | 2.5–8 µm | 1.0–4 µm | 0.3–1.0 µm |
The "Swiss CNC" column is what Brassland holds in continuous production on a Tsugami B0206 / B0326 or Star SB-20R2 Swiss-type machine with brass. Numbers assume:
- Workpiece diameter 2 mm to 32 mm (the typical Swiss CNC bar capacity)
- Workpiece length up to 5× bar diameter (Swiss CNC excels at slender ratios)
- CW617N free-machining brass as the material reference
- In-process tool measurement and compensation enabled
- Production volume above 500 pieces (where the machine warms up and stabilises)
Why Swiss CNC achieves these numbers
Five engineering features of the Swiss CNC platform combine to enable the tolerance band:
- Guide bushing — the bar guide is positioned within millimetres of the cutting tool, supporting the workpiece against radial deflection. On a long, slender brass shaft, conventional turning would deflect 0.02–0.05 mm under the cutting load; Swiss holds it under 0.005 mm.
- Linear motion of the headstock — the workpiece moves longitudinally through the tool instead of the tool reaching out along the workpiece. The cutting tool stays at the most rigid point of the gang slide.
- Multiple tools simultaneously — front-machining and back-machining tools work in parallel, so the cycle time of a 12-feature brass part is set by the slowest single feature, not the sum of all 12.
- In-process tool measurement — every tool has a touch probe that measures itself before each cut. Thermal drift is automatically compensated. This is what holds the ± 0.005 mm band over long production runs.
- Through-coolant tools — coolant flooding through the tool maintains cutting-edge temperature and clears chips, both improving finish.
When Swiss CNC is the only choice
- Workpiece L/D ratio > 4 (slender brass parts)
- Multiple closely-toleranced features on one part
- Concentricity requirement < 0.01 mm
- Surface finish requirement Ra < 0.4 µm without secondary polishing
- Diameter tolerance class IT5 or tighter
- Position tolerance ⌖ < 0.01 mm on a hole-pattern
When NOT to specify Swiss CNC
- Short stubby parts (L/D < 2) — conventional CNC turning is just as good and faster
- Single-feature parts (one diameter, one length) — multi-spindle machine wins on cost
- Tolerance ≥ ± 0.05 mm — conventional CNC is fine
- Low-volume prototypes < 50 pieces — programming and setup cost on Swiss isn't recovered
- Parts requiring secondary off-Swiss operations anyway (drilling, milling, grinding) — Swiss advantage diminished
Cost ratios — Swiss vs conventional CNC vs grinding
| Process | Setup time | Cycle time (typical brass part) | Per-piece cost relative to multi-spindle |
|---|---|---|---|
| Multi-spindle screw machine | 2–4 h | 4–10 s | 1.0× (reference) |
| Single-spindle CNC turning | 30–60 min | 15–60 s | 1.6–2.4× |
| Swiss-type CNC (sliding headstock) | 1–3 h | 15–90 s | 2.4–4.0× |
| Swiss + centreless grinding finish | +1 h | +5–10 s | 3.5–5.5× |
Surface finish — what you can achieve without grinding
Surface finish is where Swiss CNC most often surprises a procurement team. Standard production CNC on brass typically achieves Ra 0.8 µm; Swiss CNC routinely holds Ra 0.4 µm and can reach Ra 0.2 µm with the right insert geometry, feeds and speeds, and coolant strategy. For brass parts up to about Ø 12 mm requiring an Ra 0.4 µm finish on a single critical diameter, Swiss CNC is typically the most cost-effective path — better than turning to Ra 0.8 µm then polishing.
For finishes tighter than Ra 0.2 µm, secondary operations are required: centreless grinding (achieves Ra 0.1 µm typical), vibratory polishing (Ra 0.05 µm cosmetic), or lapping (Ra 0.025 µm precision).
Materials & cutting parameters used at Brassland
| Brass alloy | Spindle speed (rpm) | Cutting speed (m/min) | Feed rate (mm/rev) | Depth of cut (mm) | Achievable Ra (µm) |
|---|---|---|---|---|---|
| CW617N (CuZn40Pb2) | 5,000–10,000 | 250–400 | 0.05–0.15 | 0.5–2.0 | 0.4–0.8 |
| CW614N (CuZn39Pb3) | 5,500–11,000 | 270–420 | 0.05–0.15 | 0.5–2.0 | 0.4–0.8 |
| CW602N (CuZn36Pb2As DZR) | 4,000–8,000 | 200–350 | 0.04–0.12 | 0.4–1.5 | 0.4–1.0 |
| CW724R (CuZn21Si3P lead-free) | 3,500–6,500 | 180–280 | 0.04–0.10 | 0.3–1.2 | 0.6–1.2 |
| C11000 (Cu-ETP copper) | 2,000–4,000 | 100–200 | 0.03–0.08 | 0.2–1.0 | 0.8–2.0 |
Inspection that matches Swiss CNC tolerances
- ± 0.005 mm diameter — air gauge, blade micrometer (Mahr / Mitutoyo digital with 0.001 mm resolution)
- ± 0.003 mm concentricity — Mitutoyo Roundtest (rotary stage with stylus)
- ± 0.005 mm position — Mitutoyo Crysta-Apex CMM
- Ra 0.2 µm surface — Mitutoyo Surftest SJ-410 contact profilometer
- Profile / GD&T verification — Mitutoyo Quick Scope vision measuring system
Brassland holds Mitutoyo inspection equipment for every tolerance class above and supplies inspection reports against any drawing-specified tolerance — see the about page for factory floor and inspection lab photos.
Sources & references
- ISO 286 — Limits and fits (IT4 to IT18 tolerance grades)
- ISO 1101 — GD&T (geometric tolerancing symbols)
- ISO 4287 — Surface texture parameters (Ra, Rz)
- Brassland — Brass Tolerances Guide
- Brassland — Swiss Turning Guide
- CW617N alloy datasheet (machinability reference)
Frequently asked questions
What tolerance is achievable with Swiss CNC machining of brass?
Why does Swiss turning achieve tighter tolerances?
What surface finish does Swiss machining give on brass?
Sources & references
References:
Last reviewed: June 2026. Standards and regulatory references are checked at each review.