Every RFQ for a turned part reaches the same question: what tolerance can you hold? On Swiss-type work the honest answer is not a number, it is a dependency. It depends on bar diameter, on the guide bushing clearance as a fraction of that diameter, on the unsupported length, and on which side of the bushing the feature sits.
This post walks through that dependency chain, because most tolerance disputes I have seen start with a drawing that specifies a number nobody checked against the process physics.
Why Swiss-type exists at all
A conventional fixed-head lathe holds the bar at one end and cuts a cantilever. Deflection under cutting force scales with the cube of the overhang length divided by the fourth power of diameter:
δ ∝ (F · L³) / (E · d⁴)
The practical consequence: as L grows, deflection grows fast, and as d shrinks, it grows catastrophically. A Ø3 mm pin sticking 45 mm out of a collet is not a tolerance problem, it is a geometry problem.
A sliding-head machine fixes this by supporting the bar in a guide bushing a few millimetres behind the cutting tool. The unsupported length stays roughly constant no matter how long the part is, so rigidity does not collapse as the part gets longer. That is the entire reason Swiss-type exists.
Bar capacity is not the same as part diameter range
Spec sheets quote bar capacity, for example Ø2.0–20 mm. Buyers routinely read that as "any part from 2 to 20 mm diameter". It is not.
Two separate constraints sit behind the number:
- The bar must pass the guide bushing. A 20 mm capacity machine runs a 20 mm bar, and a part turned from it can be 20 mm at its largest diameter. Nothing larger is possible.
- Material removal is expensive. Turning a Ø6 mm part from a Ø20 mm bar wastes roughly 90% of the material and most of the cycle time. Shops quote this, but they quote it high, and rightly.
The practical rule: choose the smallest standard bar that leaves a sensible machining allowance. For most parts that means finished diameter plus 1–3 mm, not finished diameter plus ten.
Guide bushing clearance is a tolerance term, and it is usually invisible
This is the part of Swiss-type work that does not appear on any quotation but shows up in every Cpk study.
The bar rotates inside a guide bushing with a small clearance so it can slide forward. That clearance is necessary. It is also a source of radial float: the bar can move within the clearance while being cut. On a Ø20 mm bar the effect is negligible. On a Ø2 mm bar with 0.01 mm clearance, the float is 0.5% of the diameter, and it appears directly as roundness and diameter variation.
Practical consequences:
- Below roughly Ø3 mm, guide bushing clearance becomes one of the dominant error terms. Only a machine and bushing set designed for that range holds a tight tolerance repeatably.
- Ground or peeled bar is not a luxury at small diameters. Cold-drawn bar has diameter and straightness variation that exceeds the bushing clearance, so the bar either seizes or rattles.
- Bar ends matter. The last 200–300 mm of a bar is where straightness degrades; some shops scrap it, some run it and accept the drift. Ask which.
Features on the wrong side of the bushing
A Swiss-type part is machined in two zones: in front of the guide bushing (main spindle) and behind it (sub-spindle, after pick-off). Positional accuracy is not the same in both.
Features machined in the main spindle, close to the bushing, are the most accurate on the part. Features machined after pick-off depend on the sub-spindle's pick-off repeatability, which is a separate mechanism with its own error. A drawing that puts a tight bore-to-bore position between a front feature and a back feature is asking the machine to do the hardest thing it does.
The design fix is usually free: move the tight relationship to one side of the bushing, or define one feature as the datum and relax the other.
Materials and what each one does to the tolerance
Swiss-type work concentrates on a narrow material set because the process rewards machinability:
| Material | Behaviour on a sliding head | Watch out for |
|---|---|---|
| 303 stainless | Free machining, consistent chips | Not for medical or corrosive service (sulphur) |
| 416 stainless | Excellent machinability, hardenable | Corrosion resistance below 316 |
| 316L stainless | Gummy, work-hardens | Tool wear drives dimension drift over a run |
| Brass C360 | Fastest of the set | Lead content rules it out of potable/medical |
| Copper / tellurium copper | Good chip control | Built-up edge on sharp edges |
| TC4 titanium | Slow, heat at the cutting edge | Springback on thin walls, tool life short |
| Kovar / Alloy 42 | Work-hardens under a dull tool | CTE-driven, needs sharp tool and low speed |
The dimension that drifts most across a long run is not caused by the machine, it is caused by tool wear. On 316L, a boring bar that has cut 800 parts is not cutting the same hole it cut at part 10.
Secondary operations change the tolerance budget
Threads, cross-holes and slots done in the same setup inherit the setup's accuracy. Anything moved to a second operation inherits a new stack:
- Thread depth beyond 1.5× diameter needs a specific tap or thread mill approach, and on small threads the tool is the weak link, not the machine.
- Plating changes the thread. Electroless nickel at 5 µm per surface takes a M3×0.5 pitch diameter out of a 6g tolerance band. Specify plating allowance, or specify that threads are masked.
- Deburring removes material. A vibratory or brush deburr that runs 20 minutes removes measurably more edge material than one that runs 5. On a part with a sharp-edged functional face, this matters.
What to actually measure on a turned part
Most incoming inspection on Swiss-type parts over-measures diameter and under-measures the things that cause assembly failures. The list that matters:
- Roundness at the bearing or sealing diameter — diameter alone does not catch a three-lobe error from guide bushing float.
- Cylindricity on any long journal.
- Runout between the functional diameter and the datum the assembly uses.
- Position of cross-holes relative to the datum, measured on a CMM, not with a pin gauge.
- Surface roughness at any sealing or sliding surface, measured, not eyeballed.
A coordinate measuring machine report covering those five is worth more than a full-dimensional report on every chamfer.
The short version
- Tolerance on a Swiss-type part is a function of bar diameter, bushing clearance, unsupported length and which side of the bushing the feature sits on.
- Bar capacity is a maximum, not a range; pick the smallest bar that leaves allowance.
- Below about Ø3 mm, bushing clearance is a first-order error term.
- Put tight relationships on one side of the bushing.
- Tool wear, not machine accuracy, is what drifts across a long run.
If you are specifying turned parts, the process detail behind this is on our swiss type CNC lathe parts supplier page. We run 70 Tsugami Swiss-type lathes covering Ø2.0–20 mm inside a 220-machine plant in Shenzhen, to ±0.005 mm, with prototypes in 3–7 days and production in 7–15 days from a MOQ of 1 piece.
If you have a part where the tolerance keeps drifting across the run, post the drawing callout and the material in the comments — the cause is usually identifiable from those two things.
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