CNC machining tolerances define how much a finished part can vary from its specified dimensions while still meeting the design requirements. For engineers and sourcing managers, choosing the right tolerance is critical because it affects part function, assembly, machining process, inspection requirements, lead time, and cost.
For general CNC machining, a tolerance around ±0.13 mm (±0.005 in) is commonly used as a practical reference for non-critical dimensions. Tighter requirements such as ±0.025 mm, ±0.013 mm, or ±0.005 mm may be achievable depending on the material, geometry, machining process, machine capability, and inspection method.
The key principle is simple: apply tight tolerances only where the part actually needs them.
Kintec Machining provides CNC milling, CNC turning, 5-axis machining, Swiss machining, and precision CNC machining for prototype and production parts.
What Are CNC Machining Tolerances?
CNC machining tolerance is the permitted variation between the actual dimension of a manufactured part and its nominal dimension shown on the engineering drawing.
For example, if a shaft diameter is specified as:
Ø20.00 ±0.02 mm
the acceptable finished diameter is:
19.98–20.02 mm
Tolerance is therefore not the same as accuracy. Accuracy describes how close the machining result is to the intended dimension, while tolerance defines the acceptable dimensional range specified by the designer.
For CNC parts, tolerances can apply to:
- Linear dimensions
- Hole diameters
- Shaft diameters
- Hole locations
- Wall thickness
- Angles
- Flatness
- Parallelism
- Perpendicularity
- Position
- Concentricity
- Surface profile
For more information about general machining capability, see Kintec’s CNC Machining Service.
What Are the Standard CNC Machining Tolerances?
There is no single tolerance that applies to every CNC machined part. Actual capability depends on the machine, material, geometry, feature size, tooling, workholding, temperature, and inspection method.
As a practical engineering reference, the following ranges can help during early-stage design:
| Tolerance Level | Typical Tolerance | Typical Application |
|---|---|---|
| General | ±0.13 mm / ±0.005 in | Non-critical dimensions |
| Tight | ±0.025 mm / ±0.001 in | Mating features and fits |
| Precision | ±0.013 mm / ±0.0005 in | Bearings, seals, alignment |
| High Precision | ±0.005 mm / ±0.0002 in | Critical aerospace, medical and optical features |
These values should be treated as manufacturing guidelines rather than universal limits.
Kintec’s current CNC machining information lists approximately ±0.020 mm as a tight general capability, while tighter requirements can be reviewed case by case from the engineering drawing; selected critical features may reach approximately ±0.0002 in (±0.005 mm) under appropriate conditions.
For comparison, Protolabs states a standard machining tolerance of ±0.13 mm and describes tighter options around ±0.051 mm and ±0.0127 mm for specific features.
ISO 2768 — General Tolerances provides the internationally recognized framework for general linear and angular tolerances without individual tolerance indications.
How Does Material Affect CNC Machining Tolerances?
Material selection can significantly influence dimensional stability during CNC machining.
Different materials respond differently to:
- Cutting forces
- Heat generation
- Thermal expansion
- Tool pressure
- Vibration
- Clamping forces
- Internal stresses
- Material deformation
For example, softer plastics may deflect during machining, while thin aluminum walls can distort because of clamping forces or residual stress. Harder materials may require different tooling and cutting parameters.
A practical reference is:
| Material | Typical CNC Tolerance Consideration | Common Challenge |
|---|---|---|
| Aluminum | ±0.013–0.025 mm possible on suitable features | Thin walls and thermal expansion |
| Stainless Steel | ±0.013 mm possible on controlled features | Work hardening and cutting forces |
| Titanium | Tight tolerances possible with process control | Heat generation and tool wear |
| Brass | Generally good dimensional stability | Burrs and surface requirements |
| Copper | Requires careful machining control | Heat and material softness |
| POM / PEEK | Often tighter control requires compensation | Thermal expansion and deformation |
| PTFE | More difficult to hold tight dimensions | High thermal expansion and flexibility |
Material behavior is one reason tolerance should be considered together with part geometry rather than specified independently.
Kintec’s material selection guide also explains how hardness, thermal expansion, stiffness, and heat generation can affect CNC dimensional accuracy.
Internal link opportunity:
CNC Materials Guide
Why Do Tight CNC Tolerances Increase Machining Cost?
Tighter tolerances require greater process control.
When a drawing changes from ±0.13 mm to ±0.013 mm, the manufacturer may need additional operations such as:
- Multiple finishing passes
- Slower cutting parameters
- Specialized tooling
- Additional workholding
- Temperature control
- Tool wear monitoring
- In-process measurement
- CMM inspection
- Secondary grinding, boring, or reaming
This means a tighter tolerance can increase both machining time and inspection cost.
Xometry similarly notes that tighter tolerances can increase costs because of additional fixturing, measurement equipment, longer cycle times, and potentially higher scrap rates.
How Can You Reduce CNC Machining Costs With Better Tolerance Design?
The best approach is not to specify the tightest possible tolerance everywhere.
Instead:
Tight tolerance → functional features
General tolerance → non-critical features
For example, consider an aluminum housing with 30 dimensions.
If only two bearing bores determine assembly alignment, those two bores may need a tight tolerance while the remaining external dimensions can use a general tolerance.
This approach can reduce unnecessary machining and inspection requirements without changing the function of the part.
Kintec discusses the same principle in its guide to Tight Tolerance Machining.
How Should You Specify CNC Tolerances on Engineering Drawings?
A good CNC drawing should clearly identify which dimensions are critical.
For example:
General dimensions
50 ±0.13 mm
Critical shaft
Ø20.00 ±0.01 mm
Critical hole location
Use GD&T rather than simply tightening every linear dimension.
Common GD&T controls include:
| GD&T Feature | What It Controls | Typical Application |
|---|---|---|
| Position | Feature location | Bolt holes |
| Flatness | Surface flatness | Sealing surfaces |
| Perpendicularity | 90° relationship | Mounting faces |
| Parallelism | Parallel relationship | Sliding components |
| Cylindricity | Cylindrical form | Precision shafts |
| Profile | Complex surface geometry | Aerospace components |
GD&T can define the relationship between features more effectively than applying very tight ± tolerances to individual dimensions. Protolabs specifically discusses position, flatness, cylindricity, concentricity and perpendicularity as common GD&T controls for CNC machined parts.
When Do CNC Parts Need Tight Tolerances?
Not every CNC part requires precision machining.
Tight tolerances are generally justified when a feature:
Requires a Precision Fit
Examples include:
- Bearing seats
- Shafts
- Bushings
- Press-fit components
- Sliding components
Requires Accurate Alignment
Examples include:
- Robotic joints
- Optical mounts
- Gearboxes
- Medical instruments
- Aerospace assemblies
Requires a Reliable Seal
Examples include:
- Hydraulic components
- Pneumatic components
- Valve bodies
- Fluid manifolds
Requires Repeatable Assembly
For production parts, consistent dimensional control helps ensure components from different production batches can be assembled without excessive adjustment or rework.
How Do CNC Machining Processes Affect Tolerance?
The selected CNC process can directly affect achievable tolerance.
| CNC Process | Best For | Tolerance Considerations |
|---|---|---|
| 3-Axis Milling | General components | Good for standard features |
| 4-Axis Milling | Indexed complex features | Reduces multiple setups |
| 5-Axis Milling | Complex geometries | Better access and fewer setups |
| CNC Turning | Shafts and cylindrical parts | Excellent diameter control |
| Swiss Machining | Small precision parts | Excellent for long/slender components |
| Reaming | Precision holes | Better hole size control |
| Boring | Accurate larger holes | Useful for precision bores |
| Grinding | Very tight requirements | Secondary precision operation |
Choosing the right process is important because an extremely tight tolerance may require a different manufacturing process.
For example, a hole that can be drilled during normal milling may require reaming or boring when a tighter diameter tolerance is required. In extreme cases, grinding or other secondary operations may be appropriate. Both Xometry and Protolabs highlight this relationship between tighter tolerances and manufacturing-process selection.
How Are CNC Machining Tolerances Inspected?
A tolerance is only useful if the manufacturer can verify it.
Depending on the feature and tolerance requirement, CNC parts may be inspected using:
- Vernier calipers
- Micrometers
- Bore gauges
- Height gauges
- Thread gauges
- Pin gauges
- Optical measurement systems
- CMM — Coordinate Measuring Machine
For high-precision components, CMM inspection provides dimensional measurement against the engineering drawing and can generate an inspection report.
For critical production parts, buyers may also request:
- First Article Inspection (FAI)
- Certificate of Conformance (CoC)
- Material certificates
- Inspection reports
- GD&T inspection results
- Batch traceability
Kintec provides CMM inspection and documentation options for precision CNC machining projects.
What Tolerance Can Kintec CNC Machining Achieve?
Kintec Machining provides standard and precision CNC machining for aluminum, stainless steel, titanium, brass, copper, steel, and engineering plastics.
Depending on the part design and manufacturing process, Kintec can review requirements ranging from general machining tolerances to approximately ±0.005 mm (±0.0002 in) on selected critical features.
Actual tolerance capability depends on:
- Part dimensions
- Feature size
- Material
- Geometry
- Wall thickness
- Machining process
- Workholding
- Tooling
- Production quantity
- Inspection requirements
Therefore, the most accurate way to confirm tolerance capability is to submit the 3D CAD model and 2D engineering drawing for technical review.
Kintec’s precision machining guide recommends applying tight tolerances selectively to functional features and matching the machining process and inspection method to the requirement.
Recommended internal links:
What Is the Difference Between CNC Accuracy, Precision, and Tolerance?
These three terms are often confused.
| Term | Meaning |
|---|---|
| Accuracy | How close the measured dimension is to the target |
| Precision | How consistently the process produces the same result |
| Tolerance | The acceptable dimensional variation specified by the designer |
For example, a CNC machine can repeatedly produce parts within a very narrow range, but if the process is offset from the nominal dimension, the parts may be precise without being highly accurate.
For production machining, both accuracy and repeatability are important.
CNC Machining Tolerance Design Checklist
Before sending a CNC drawing to a manufacturer, check the following:
- Define critical dimensions clearly.
- Use general tolerances for non-critical features.
- Avoid unnecessarily tight tolerances.
- Identify bearing, shaft, hole, and sealing features.
- Consider material thermal expansion.
- Consider thin-wall deformation.
- Use GD&T where feature relationships matter.
- Specify surface finish separately from dimensional tolerance.
- Confirm the required inspection method.
- Ask whether CMM inspection is available.
- Provide both 3D CAD and 2D drawings for critical parts.
- Discuss tight-tolerance features with the manufacturer before production.
This approach helps balance function, manufacturability, quality, lead time, and cost.
CNC Machining Tolerances FAQ
What is the standard tolerance for CNC machining?
A practical general reference is around ±0.13 mm (±0.005 in), although the applicable tolerance depends on the drawing standard, material, geometry, and manufacturing supplier.
How tight can CNC machining tolerances be?
Precision CNC machining can achieve tolerances around ±0.013 mm, while selected critical features may reach approximately ±0.005 mm with suitable equipment, process control, material, geometry, and inspection.
Are tighter CNC tolerances more expensive?
Usually, yes. Tighter tolerances can require slower machining, additional setups, specialized tooling, more inspection, and sometimes secondary operations.
Does material affect CNC tolerances?
Yes. Material hardness, stiffness, thermal expansion, internal stress, and cutting behavior can all affect dimensional stability.
Should every CNC dimension have a tight tolerance?
No. Tight tolerances should generally be reserved for dimensions that affect fit, function, sealing, alignment, or performance.
What is ISO 2768?
ISO 2768 provides general tolerance classes for dimensions without individually specified tolerances. The current published ISO 2768-1:1989 remains applicable while a revised ISO 2768 edition is under development.
What is GD&T in CNC machining?
GD&T, or Geometric Dimensioning and Tolerancing, is a system used to control the form, orientation, location, and relationship of part features rather than relying only on conventional ± dimensional tolerances.
How can I confirm whether Kintec can hold my required tolerance?
Send Kintec your 2D engineering drawing and 3D CAD file. The engineering team can review the material, geometry, tolerance requirements, machining process, and inspection requirements before quoting.
How Can You Get CNC Parts to the Required Tolerance?
Kintec machines aluminum, titanium, stainless, brass, and copper using CNC milling, turning, 5-axis, and Swiss capability under one roof.
Tolerances to ±0.005 mm on critical features- CMM inspection reports and material certs on request
- No minimum order — prototype from 1 piece
- ISO 9001:2015 certified facility
- 24-hour quote turnaround
Send your drawing now and get a free precision metal manufacturing quote in 24 hours.
