Swiss-type sliding-headstock CNC turning can hold down to ± 0.005 mm on small, slender diameters, concentricity of about 0.01 mm and Ra 0.4 µm with finishing passes in brass series production. Tighter than that means grinding or honing — operations Brassland does not perform in-house. Specifying tighter than the function needs is over-specification and raises the part cost. This guide shows what is achievable and how to know when you genuinely need Swiss instead of conventional CNC.
Swiss-type turning was developed in the Swiss watchmaking industry, where parts had to be cut from long slender bars with the bar closely supported against deflection. The defining feature — the bar slides through a guide bushing right up to the cutting tool, so deflection is minimal — is what lets a Swiss-type machine hold considerably tighter tolerances than conventional turning on long, slender brass parts.
For brass procurement and design engineers, the relevant question is: how tight can a Swiss-type machine actually hold a feature on a typical brass production part? This article answers that with the figures from our own capability pages.
Dimensional tolerance — what is achievable
| Feature type | Conventional CNC (fixed headstock) | Swiss CNC (sliding headstock) | Beyond turning |
|---|---|---|---|
| Turned diameter | ± 0.05 mm standard · ± 0.02 mm precision | down to ± 0.005 mm on small, slender diameters | tighter: grinding — not in-house |
| Bored inside diameter | ± 0.05 mm standard · ± 0.02 mm precision | ± 0.008 mm | tighter: honing — not in-house |
| Reamed bore | ± 0.01 mm | ± 0.01 mm | — |
| Concentricity | 0.02 mm | about 0.01 mm (part finished complete in one cycle) | — |
| Surface finish Ra | 0.8 µm standard | 0.4 µm with finishing passes | 0.2 µm polished |
The Swiss column reflects what we state for our Swiss-type machines — Tsugami B0206 / B0326 and Star SB-20R2. The figures assume:
- Workpiece diameter 2 mm to 32 mm (our Swiss bar capacity)
- Slender parts — a length-to-diameter ratio above about 4, where the guide bushing makes the difference
- Free-machining brass such as CW614N or CW617N
- The critical dimensions and the measurement plan agreed at the RFQ stage — the tightest figures apply to those features, not to every dimension on the drawing
Why Swiss CNC achieves these numbers
Five engineering features of the Swiss-type platform combine to enable the tolerance band:
- Guide bushing — the bar guide sits within millimetres of the cutting tool and supports the workpiece against radial deflection. On a long, slender brass shaft, conventional turning lets the part deflect under the cutting load; the guide bushing largely removes that error.
- 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.
- Front and back machining — main and sub-spindle tools work in parallel, and the part is finished complete in one cycle, which is what protects concentricity between features.
- Thermal stability and offsets — the machine runs warmed up, and tool offsets are corrected from in-process measurements of the parts, which keeps a tight band over a long run.
- High-pressure coolant — coolant at the cutting edge controls temperature and clears chips, both improving finish.
When Swiss CNC is the right choice
- Workpiece L/D ratio > 4 (slender brass parts)
- Several closely toleranced features on one part
- Concentricity requirement around 0.01 mm
- Ra 0.4 µm on a functional diameter without a secondary operation
- Diameter tolerance tighter than ± 0.02 mm
When NOT to specify Swiss CNC
- Short stubby parts (L/D < 2) — conventional CNC turning is just as good and faster
- Bar diameters above 32 mm — that is fixed-headstock work (our range runs to Ø 150 mm)
- Tolerance ≥ ± 0.05 mm — conventional CNC is fine
- Very small quantities — programming and set-up cost is spread over few pieces
Cost — Swiss vs conventional CNC vs grinding
There is no universal ratio. A Swiss-type machine costs more per hour and takes longer to set up than a simple turning job, but it finishes a slender multi-feature part complete in one cycle, which often makes it the cheaper route for exactly those parts. Every tolerance step adds cost — slower finishing passes, more frequent measurement, more scrap risk — and a ground finish adds a separate operation at a partner. The reliable way to control cost is to put the tight tolerance only on the features that need it.
Surface finish — what you can achieve without grinding
Standard production turning on brass gives about Ra 0.8 µm. With finishing passes — the right insert geometry, feed and coolant — Ra 0.4 µm is achievable on a functional diameter, and Ra 0.2 µm by polishing. For brass parts that need Ra 0.4 µm on one critical diameter, turning it on the Swiss-type machine is normally more economical than turning to Ra 0.8 µm and polishing afterwards.
Finishes finer than Ra 0.2 µm need secondary processes such as centreless grinding, lapping or superfinishing — operations we do not run in-house.
Alloys — how they behave on the Swiss-type machine
- CW614N (CuZn39Pb3) — the machinability reference (100); short chips, best finish, fastest cycles.
- CW617N (CuZn40Pb2) — close behind; the usual choice when the same alloy is also forged.
- CW602N (DZR) — machinability 75–85; somewhat longer cycles.
- CW724R (lead-free silicon brass) — machinability about 85, but harder and tougher on tools; feeds and tooling are adapted.
- C11000 (Cu-ETP copper) — machinability about 20; gummy chips, so finish and burr control need more care.
Inspection that matches Swiss CNC tolerances
- Diameters to ± 0.005 mm — micrometers and bore gauges with 0.001 mm resolution, checked against calibrated masters
- Form, position and GD&T — coordinate measuring machine (CMM), contour measuring instrument and vision measuring machine
- Surface finish — contact surface roughness tester for Ra / Rz
- Threads — calibrated Go / No-Go gauges
Brassland supplies inspection reports against the drawing-specified tolerances agreed at the RFQ stage — see the about page and the manufacturing & quality FAQ for the equipment list.
Sources & references
- ISO 286 — Limits and fits (IT4 to IT18 tolerance grades)
- ISO 1101 — GD&T (geometric tolerancing symbols)
- ISO 21920-2 — Surface texture parameters (Ra, Rz; formerly ISO 4287)
- 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: September 2026. Standards and regulatory references are checked at each review.