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Request Engineering ReviewSwiss CNC Machining Tolerances
Many Swiss-turned features start review at ±0.01 mm. Selected critical ODs may be assessed near ±0.005 mm under suitable conditions. Near ±0.002 mm usually needs grinding or controlled finishing.
2D drawing + matching 3D CAD.
Engineering Review Bands
Start with the practical baseline, isolate features that need tighter control, then assess whether the finishing route must change. These values guide drawing review; they are not automatic process commitments.
A starting point for suitable turned features with straightforward cutting access and stable support.
Drawing datum, material condition and measurement method.
Selected outside-diameter CTQs may be evaluated at this level when process and inspection conditions are suitable.
Support, tool condition, thermal stability and gauge capability.
Requirements near this level usually need grinding or another controlled finishing process.
Finishing allowance, datum transfer and final-size inspection.
Feature-Level Review Matrix
Use these ranges to screen drawing requirements, not to promise a machine-wide capability. Confirm each feature's datum, process conditions and inspection method before approval.
| Feature | Review Starting Point | Main Variables | Verification |
|---|---|---|---|
| Controlled Turned OD | ±0.01 mm baseline; selected OD CTQs may be reviewed near ±0.005 mm. | Support, alloy, tool condition and thermal stability. | Micrometer, air gauge or validated comparative gauge. |
| Step / Shoulder Length | ±0.02–0.05 mm may be a starting range on suitable features. | Datum, tool path, cutoff and feature sequence. | Optical measurement or CMM. |
| Runout | Around 0.01 mm TIR may be reviewed after datum and setup assessment. | Support, bar straightness, datum and fixture. | Indicator with defined fixture or CMM. |
| Cross-Hole Position | For coordinate dimensions, ±0.02–0.05 mm may be reviewed. True position must follow the drawing's GD&T callout. | Indexing, tool access and datum relationship. | Optical measurement or CMM. |
| Small Bore | No blanket range. Review each bore's geometry and process. | Bore depth, chip evacuation, wall stiffness and tool access. | Plug gauge, air gauge, optical method or CMM as appropriate. |
| Threads | Specify thread class and functional acceptance requirements. | Tool wear, thread start, material and post-machining coating. | Go/No-Go or functional thread gauge. |
| Surface Finish | Ra 0.8–1.6 µm may suit some turned surfaces; Ra 0.4 µm needs dedicated review. | Alloy, feed, tool radius and finishing route. | Profilometer at the defined surface location. |
Process Conditions
Tolerance feasibility depends on the actual feature and its machining conditions, not just the nominal machine specification. Evaluate support, cutting forces, stock condition, material response and temperature together before releasing a drawing.
An excessive unsupported span can increase deflection, chatter or taper on slender features. The effective span and support position relative to the active cut must be considered alongside the feature's length-to-diameter ratio. See L/D ratio and slender-part stability .
Unsupported length, local geometry and support arrangement.
Cutting forces can deflect thin sections, while tool wear can change diameter, surface finish and cutting behavior. Tight CTQs need controlled cutting conditions and a defined approach to monitoring dimensional drift during production.
Tool condition, cutting parameters, wear limits and inspection frequency.
Bar straightness, diameter consistency, roundness and surface condition affect how stock behaves through the machine's support system. Incoming variation should be considered when investigating dimensional variation at the finished feature.
Material specification, incoming condition and lot consistency.
Machinability, chip formation, work hardening, thermal response and residual stress can affect cutting stability and dimensional consistency. A material grade alone does not establish achievable tolerance. See our Swiss CNC machining materials guide .
Exact alloy and condition, chip control and material-related distortion risk.
Machine warm-up, coolant conditions and part temperature can influence measured dimensions. For tight features, distinguish variation during machining from variation introduced by the inspection environment or measurement procedure.
Operating stability, part temperature and defined measurement conditions.
Drawing Control Logic
A Swiss-turned feature can meet its dimensional limits and still fail a geometric requirement. Size defines permitted dimensions, runout relates a surface to a datum axis during rotation, and position controls a feature's location relative to a datum reference frame. Each specified requirement needs its own acceptance criteria.
| Control Type | What It Controls | What It Does Not Establish | Review Before Acceptance |
|---|---|---|---|
| Size Tolerance | The permitted size variation of a specified diameter, bore, length or other dimension. | The feature's runout or its location relative to a datum. | Drawing limits, the controlled dimension, and a suitable measurement method. |
| Runout | Surface variation relative to a datum axis during rotation. Circular runout evaluates individual circular elements; total runout evaluates the surface over its extent. | Compliance with a size tolerance or a separate position tolerance. | Datum axis, controlled surface, circular or total runout callout, and the specified inspection setup. |
| Position | The permitted location of a feature, such as a hole axis or center plane, relative to the specified datum reference frame and basic dimensions. | The feature's size or every geometric relationship that is not separately specified. | Datum references, basic dimensions, the position tolerance zone, and applicable drawing modifiers. |
CTQ Verification
Each critical-to-quality (CTQ) feature needs a measurement method suited to its geometry, datum relationship and acceptance criteria. Select the method according to measurement access, required resolution and the condition of the finished part.
Use a micrometer for accessible, stable diameters. Laser or comparative gauging may suit applications where contact, geometry or inspection throughput affects method selection.
Plug or pin gauges can verify specified functional limits. Air gauging, optical inspection or CMM may be appropriate when additional dimensional or geometric data are required.
Use an indicator with a defined fixture or a suitable CMM setup. The datum axis, support condition and specified circular or total runout requirement must be clear.
Use CMM or optical measurement against the specified datum reference frame. A functional gauge is appropriate only when it reflects the drawing's positional requirement.
Use the specified Go/No-Go or functional thread gauge. Confirm the required thread class and whether acceptance applies before or after any specified coating.
Use a profilometer at the specified location and direction. Evaluate the final surface condition against the drawing's stated roughness requirement.
Process Boundary
Swiss turning may establish the main geometry and datum features, but selected CTQs can require a separate finishing operation. The decision depends on the final dimensional tolerance, surface requirement, material condition and the stability of the complete manufacturing route.
Requirements approaching ±0.002 mm need a feature-specific feasibility review. Grinding may be appropriate for selected cylindrical CTQs when the turning route cannot maintain the required size, form or process margin. Stock allowance, datum transfer and final inspection must be considered before choosing the route.
Final tolerance, material condition, grinding allowance and post-finish measurement method.
Bearing, sealing and sliding surfaces may require grinding, honing, lapping or polishing when the specified final condition cannot be achieved reliably by turning alone. Select the process according to feature geometry, surface function and the actual drawing requirement—not roughness alone.
Specified Ra, form requirements, functional contact area and finishing access.
Heat treatment and surface treatments can affect final dimensions, geometry or surface condition. Where these operations are specified, define whether critical features are accepted before or after treatment and whether machining or finishing allowance is needed.
Process sequence, treatment condition, dimensional allowance and final acceptance state.
Provide the controlled 2D drawing, matching 3D CAD where available, material and condition, and clearly identified CTQs. If heat treatment, plating or coating affects final size, define the condition in which the feature must meet its acceptance requirement.
Engineering FAQ
Understand the practical limits of Swiss turning, when secondary finishing may be needed, and how dimensional and geometric requirements should be verified against the engineering drawing.
No. ±0.005 mm is a selected feature-level review target, not a blanket Swiss machining capability. Feasibility depends on geometry, support, material condition, tool wear, thermal stability and the inspection method. Some outside diameters may justify this target; other features require a different process assessment.
It should not be assumed. Requirements approaching ±0.002 mm need a feature-specific feasibility review and may require grinding or another controlled finishing process. Consider the material, geometry, finishing allowance, datum transfer and final inspection method before selecting the manufacturing route.
No. Machining related features in one setup can reduce re-clamping and datum-transfer errors, but it does not guarantee runout or coaxial alignment. Support, spindle condition, cutting forces, feature sequence and datum interpretation still matter. The drawing must define the geometric requirement and its acceptance criteria.
The drawing should identify the condition in which each functional feature must comply. Heat treatment, plating and coating can affect dimensions or surface condition. Plan machining allowances and any secondary finishing around the specified final acceptance condition, rather than relying only on pre-treatment measurements.
There is no single best method. Choose according to the CTQ, datum relationship, feature access and required measurement uncertainty. Micrometers, air gauges, optical systems, CMMs, functional gauges, profilometers and laser systems suit different requirements.
For first-article inspection and documented dimensional verification, see our FAI for Swiss CNC parts guide . For surface roughness requirements, consult the Swiss CNC surface finish guide .
Drawing-Based Tolerance Review
Share your engineering drawing and part requirements for a feature-level feasibility review. SPI can assess critical dimensions, datum relationships, inspection requirements and whether selected features may need grinding or other secondary finishing before quotation or production planning.
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