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CNC machining tolerances define how precise a manufactured part must be. This guide explains standard tolerance ranges, what modern CNC machines can achieve, and how tolerances affect cost and quality. Use this article when designing or ordering CNC machined parts that require accuracy and repeatability.
What Are CNC Machining Tolerances
CNC machining tolerance is the allowed variation from a specified dimension. It defines how much a part can deviate and still function correctly. Tighter tolerances require more control, better machines, and more inspection.
Standard CNC Machining Tolerance Ranges
General machining tolerances
For most CNC machined parts, a common tolerance range is ±0.005 inches or ±0.13 mm. This range is suitable for brackets, housings, and non critical components. It offers a balance between accuracy, speed, and cost.
Tight tolerance CNC machining
Tight tolerance machining typically ranges from ±0.001 inches to ±0.002 inches. This level is used for precision assemblies, mating components, and mechanical fits. It requires stable machines and consistent inspection.
Close tolerance CNC machining
Close tolerance machining can reach ±0.0005 inches under controlled conditions. This level is common in aerospace, medical, and high precision industrial applications. Not all part designs can maintain this tolerance reliably.
CNC Machining Tolerances Chart
Typical tolerance ranges used in CNC machining include the following.
Standard machining: ±0.005 inches
Precision machining: ±0.002 inches
Tight tolerance machining: ±0.001 inches
Ultra precision machining: ±0.0005 inches
Actual achievable tolerance depends on material, part size, machine condition, and setup stability.
What Tolerance Can a CNC Machine Achieve
Modern CNC machines can achieve very tight tolerances when conditions are controlled. Most CNC mills and lathes can reliably hold ±0.001 inches. With optimized tooling and inspection, ±0.0005 inches is achievable on specific features. Long parts, thin walls, and complex geometries reduce tolerance consistency.
Factors That Affect CNC Machining Tolerances
Material type
Aluminum is easier to machine accurately than stainless steel or titanium. Harder materials increase heat and tool wear, which affects precision.
Machine capability
Newer CNC machines with thermal compensation maintain tighter tolerances. Older machines may struggle with repeatability.
Tooling and wear
Worn tools reduce dimensional accuracy. Tool deflection increases with smaller diameters and longer tool reach.
Part design and geometry
Thin walls, deep pockets, and sharp internal corners reduce tolerance stability. Rigid designs support tighter tolerances.
How to Specify Tolerances on CNC Drawings
Specify tolerances only where function requires precision. Use general tolerances for non critical dimensions. Apply tight tolerances only to mating or functional features. Over specifying tolerances increases machining cost without adding value.
This approach improves manufacturability, reduces scrap, and shortens overall production timelines.
When Tight Tolerances Increase Cost
In many cases, designers specify tight tolerances without functional need. This leads to unnecessary machining steps, slower production, and higher inspection effort. Review part function carefully before assigning tight tolerance values.
Tight tolerances increase cost due to slower machining speeds, additional inspection steps, and higher scrap risk. The best practice is to use the loosest tolerance that still meets functional requirements.
Frequently Asked Questions About CNC Machining Tolerances
What is a standard CNC machining tolerance
A common standard tolerance is ±0.005 inches for general machining.
What tolerance can CNC milling achieve
Most CNC milling operations can hold ±0.001 to ±0.002 inches reliably.
Does tighter tolerance mean better quality
No. Tighter tolerance means higher precision. Quality depends on part function and fit.
How do tolerances affect lead time
Tighter tolerances usually increase lead time due to setup and inspection requirements.
