This article explains CNC machining tolerances, ISO 2768 standards, and when tight tolerances are truly needed for custom machined parts. It helps buyers and engineers choose practical tolerances to balance part performance, inspection needs, lead time, and machining cost.
When ordering custom CNC machined parts, tolerance determines how much a finished dimension may vary from the nominal size. For example, a 10.00 mm hole with ±0.05 mm tolerance can measure from 9.95 mm to 10.05 mm and still be acceptable.
Tighter tolerances can improve fit, sealing, alignment, and assembly performance. They can also increase machining time, inspection workload, scrap risk, and cost. The goal is not to make every dimension as tight as possible. The goal is to define the right tolerance for each function-critical feature.
This guide explains how to use standard CNC machining tolerances, ISO 2768, tight tolerance CNC machining, and clear RFQ notes to get accurate parts without paying for unnecessary precision.
| Part Feature | Practical Tolerance Decision |
|---|---|
| Non-mating outside profile, cover edge, simple bracket shape | Use ISO 2768-m or the supplier’s standard CNC machining tolerance unless the feature affects assembly. |
| Bearing seat, press-fit hole, shaft, locating pin | Add a specific tolerance or fit requirement. Consider ISO 286 fit classes such as H7 where appropriate. |
| Hole pattern or alignment feature | Use position tolerance or clearly define datum references, especially when multiple holes must match an assembly. |
| Thin wall, deep pocket, long slot, large flat plate | Avoid unnecessary tight tolerance. These features are more affected by clamping, tool deflection, stress relief, and material movement. |
| Anodized, plated, polished, or coated surfaces | State whether tolerance applies before or after finishing. Coating thickness can change final dimensions. |
| Cosmetic surfaces | Prioritize surface finish, coating, and visual acceptance criteria instead of tight size tolerance. |
CNC machining tolerances define the acceptable variation for a machined feature, such as length, hole diameter, slot position, flatness, thread depth, or mating surface location. A good drawing separates critical dimensions from non-critical dimensions. Critical features may need tight tolerance, GD&T, or inspection notes. Non-critical features can often follow a general tolerance standard such as ISO 2768.
TY practical note: In many RFQs, buyers apply ±0.01 mm to nearly every dimension. In most cases, only a few functional features actually require this level of control. Separating critical features from general geometry helps reduce quotation time, machining cost, and inspection disputes.
Standard tolerances are suitable for many brackets, covers, housings, plates, and non-mating surfaces. Tight tolerances should be used only when the dimension directly affects fit, assembly, movement, sealing, alignment, or safety.
| Tolerance Level | Best Used For | Buyer’s Note |
|---|---|---|
| General tolerance | Non-critical shapes and overall profiles | Suitable for many simple covers, brackets, plates, and non-mating surfaces. |
| Standard CNC tolerance | Most functional machined parts | Often enough when there is no press fit, bearing fit, sealing surface, or precision alignment requirement. |
| Tight tolerance | Critical mating or alignment features | Requires clearer drawings, stable setup, suitable process, and defined inspection method. |
| Ultra-tight tolerance | Selected high-precision features | May require boring, reaming, grinding, EDM, CMM inspection, controlled temperature, or special fixturing. |
ISO 2768 is commonly used as a general tolerance standard when individual dimensions do not have their own tolerance callouts. It helps keep drawings cleaner, but it should not replace specific tolerance notes for bearing bores, press fits, datum-related hole patterns, sealing surfaces, or other critical features.
Important note: The simplified ISO 2768-1 linear tolerance chart below is for general engineering reference only and does not replace the official ISO standard. Always confirm the tolerance class, drawing notes, inspection method, and supplier capability before production.
| Nominal Dimension Range | Fine | Medium | Coarse | Very Coarse |
|---|---|---|---|---|
| 0.5-3 mm | ±0.05 mm | ±0.10 mm | ±0.20 mm | - |
| >3-6 mm | ±0.05 mm | ±0.10 mm | ±0.30 mm | ±0.50 mm |
| >6-30 mm | ±0.10 mm | ±0.20 mm | ±0.50 mm | ±1.00 mm |
| >30-120 mm | ±0.15 mm | ±0.30 mm | ±0.80 mm | ±1.50 mm |
| >120-400 mm | ±0.20 mm | ±0.50 mm | ±1.20 mm | ±2.50 mm |
| >400-1000 mm | ±0.30 mm | ±0.80 mm | ±2.00 mm | ±4.00 mm |
| >1000-2000 mm | ±0.50 mm | ±1.20 mm | ±3.00 mm | ±6.00 mm |
| >2000-4000 mm | - | ±2.00 mm | ±4.00 mm | ±8.00 mm |
For linear and angular dimensions, a drawing may state: General tolerances: ISO 2768-m unless otherwise specified. This means dimensions without individual tolerance callouts follow the ISO 2768 medium class. If the drawing also needs general geometrical tolerances such as straightness, flatness, perpendicularity, symmetry, or circular runout, it may use a combined note such as ISO 2768-mK, where m refers to ISO 2768-1 and K refers to ISO 2768-2.
Instead of listing every possible factor separately, use the function of the feature to decide the tolerance. The table below combines material behavior, geometry, process choice, finishing, and inspection into practical drawing decisions.
| Situation on the Drawing | Why It Is Tolerance-Sensitive | Recommended Action |
|---|---|---|
| Bearing bore, press-fit pin hole, or precision shaft | Fit depends on a narrow size window. A drilled hole alone may not be accurate enough for a precision fit. | Use a specific tolerance or ISO fit class where appropriate. Consider boring or reaming, and define pin gauge, micrometer, or CMM inspection if required. |
| Multiple holes must align with another assembly | Even if each hole size is acceptable, accumulated location error can cause assembly failure. | Use datum references and position tolerance, or clearly define the hole-to-hole and hole-to-edge relationship. Machine related features in one setup when possible. |
| Thin walls, deep pockets, long slots, or large flat parts | Clamping force, tool deflection, heat, and stress relief can move the part after machining. | Avoid applying ±0.01 mm broadly. Ask for DFM review and allow realistic tolerances for non-mating areas. Add ribs, increase wall thickness, or adjust geometry if needed. |
| Stainless steel, titanium, copper, or very soft aluminum | Hard materials increase tool wear. Soft or thin materials may deform during clamping or finishing. | Confirm material grade early. Use tight tolerance only on functional features and allow standard tolerance for non-critical areas. |
| Anodizing, plating, passivation, polishing, bead blasting, or powder coating | Surface finishing can change dimensions or edge condition after machining. | State whether dimensions apply before or after finishing. For critical fits, define masking, allowance, or post-finish inspection. |
Measurement note: A tight tolerance is only useful if it can be measured reliably. For very small tolerance windows, inspection equipment resolution, measurement uncertainty, temperature, and part cleanliness can affect pass/fail decisions. If a feature requires ±0.01 mm, define the inspection method and acceptance condition before production.
Tight tolerance CNC machining often costs more because the supplier must control more variables, not simply because the machine is more accurate. The cost increase usually comes from process planning, tooling, setup stability, and inspection.
| Cost Driver | What It Means for the Quote |
|---|---|
| Slower finishing passes | More controlled cutting reduces variation but increases cycle time. |
| Special tooling or processes | Boring, reaming, grinding, EDM, or custom gauges may be needed for selected features. |
| More stable fixturing | Critical features may need to be machined in one setup to reduce accumulated error. |
| More inspection work | CMM reports, first article inspection, micrometers, pin gauges, or optical inspection may be required. |
| Higher scrap risk | A very narrow tolerance window leaves less room for material movement, tool wear, or finishing variation. |
| Tolerance Type | Example | When It Helps |
|---|---|---|
| Bilateral tolerance | 20.00 mm ±0.05 mm | General dimensions that can vary in both directions. |
| Unilateral tolerance | 20.00 mm +0.05 / -0.00 mm | Features that must not go below or above a functional limit. |
| Limit dimension | 19.95-20.05 mm | Clear inspection because minimum and maximum sizes are shown directly. |
| GD&T / position tolerance | Position, flatness, perpendicularity, profile, runout | Complex assemblies where datum relationships matter more than simple plus/minus dimensions. |
For a faster and more accurate CNC machining quote, send both the 3D model and the 2D drawing. The model defines geometry; the drawing defines what must be controlled.
| Item to Send | Why It Matters |
|---|---|
| 3D CAD file | STEP, IGS, X_T, or native CAD helps the supplier evaluate geometry and machining strategy. |
| 2D technical drawing | Shows tolerances, datum references, threads, surface finish, critical dimensions, and inspection notes. |
| Material grade | Aluminum, stainless steel, brass, copper, titanium, and steel behave differently during machining. |
| General tolerance note | ISO 2768-m, ISO 2768-mK, or custom notes reduce ambiguity. |
| Critical dimensions | Mark only the features that truly require tight tolerance. |
| Surface finish requirement | Coating or finishing may affect final size and acceptance criteria. |
| Inspection requirement | State whether you need CMM report, dimensional report, material certificate, or first article inspection. |
| Quantity | Prototype, low-volume, and production runs may need different machining and inspection strategies. |
TY provides precision CNC machining services for custom metal parts, including CNC milling, CNC turning, 5-axis CNC machining, surface finishing, and inspection support.
Not sure whether your drawing is over-toleranced? Send your 3D CAD file and 2D drawing to TY Hardware. Our engineering team can review critical dimensions, ISO 2768 notes, surface finish requirements, and inspection needs before quotation.
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A normal CNC machining tolerance depends on material, geometry, part size, process, setup, and inspection method. For many general metal parts, ISO 2768-m or standard CNC tolerance is enough. Critical mating features should have their own tolerance callouts.
Yes, ±0.01 mm can be possible for selected features, but it is not a general default tolerance. It depends on feature size, material, tool path, fixturing, inspection method, temperature, and production quantity.
Use ISO 2768 for non-critical dimensions that do not directly affect fit, function, sealing, alignment, or inspection. Use specific tolerances for critical features such as bearing seats, press-fit holes, datum-related locations, and sealing surfaces.
No. If a tight tolerance does not support part function, it may only increase cost, inspection time, and scrap risk. Good drawings apply tight tolerance only where it improves fit, assembly, or performance.
Send a 3D CAD file and a 2D technical drawing with material grade, surface finish, quantity, general tolerance note, critical dimensions, threads, datum references, and inspection requirements.
Written by: Tongyong Industries Content Operations Team
Technical review: TY Hardware Engineering Team
Last updated: May 23, 2026
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