CNC Machining Tolerances: A Technical Reference for Engineers

Dimensional tolerances define the acceptable variation in manufactured parts. Proper tolerance specification balances functional requirements against manufacturing capabilities and costs. Over-specification wastes resources; under-specification risks assembly failures and performance issues.

Tolerance Classification

Standard Machining Tolerances (±0.005” / ±0.127mm)

Standard tolerances represent the natural capability of well-maintained CNC equipment operating under normal conditions. Most machine shops achieve these tolerances without special processes, premium tooling, or environmental controls. Standard measuring equipment like calipers and micrometers provides sufficient inspection accuracy, and typical lead times apply since no extraordinary measures are required.

These tolerances work well for clearance holes and non-critical bores, general mounting surfaces, non-mating dimensions, and prototype parts where precise fit is not yet critical. The key advantage is economic: there’s no premium pricing for standard tolerance work.

Precision Tolerances (±0.001” / ±0.025mm)

Moving to precision tolerances introduces additional process requirements that translate directly to cost increases of 25-50% compared to standard work. The machining environment needs climate control to prevent thermal dimensional changes. Premium tooling with more frequent replacement becomes necessary. Cutting speeds and feed rates must be reduced to maintain accuracy. Inspection requires CMM or optical comparator equipment rather than hand tools, and multiple setup verification checks ensure the process remains in control.

Precision tolerances are appropriate when components must mate accurately: press-fit interfaces for bearings and pins, sealing surfaces for O-ring grooves and gasket faces, alignment-critical surfaces, and moving components like slides, ways, and rotating parts. The cost premium is justified by functional requirements that standard tolerances cannot meet.

Ultra-Precision Tolerances (±0.0005” / ±0.0127mm and tighter)

Ultra-precision work enters a different realm entirely, with cost increases of 100-300% or more compared to standard work. Temperature-controlled environments maintained within ±1°F become essential. Secondary operations like grinding, lapping, or honing typically replace or follow machining. Air-bearing measuring equipment provides the necessary inspection accuracy. Parts require extended stabilization time for thermal equilibrium before final measurement, and 100% inspection with documented traceability becomes standard practice.

These tolerances serve optical component mounting surfaces, precision instrument mechanisms, high-speed rotating assemblies, and medical device components with micron-level requirements. The extreme cost is justified only when no lesser tolerance can meet functional needs.

Material-Specific Considerations

Different materials exhibit varying dimensional stability, which affects both achievable tolerances and long-term performance of finished parts.

Material Class	Tolerance Capability	Stability Considerations
Aluminum Alloys	Excellent	Stable; minimal thermal movement during machining
Carbon Steels	Good	Heat treatment may cause dimensional changes
Stainless Steels	Good	Work hardening requires adjusted cutting parameters
Tool Steels	Good to Excellent	Post-heat treatment grinding often required
Engineering Plastics	Variable	Hygroscopic expansion, creep under load, thermal sensitivity
Copper Alloys	Good	Soft materials require careful workholding

Special Note on Plastics

Engineering plastics present unique challenges that metalworking experience doesn’t prepare you for. Hygroscopic materials like nylon absorb moisture from the air, causing dimensional growth of 0.002-0.003 inches per inch of length. Most plastics expand five to ten times more than metals per degree of temperature change. Sustained loads cause gradual creep deformation over time. Internal stresses from machining can cause warping after the part is released from fixturing.

For critical plastic components, tolerances should be specified at defined temperature and humidity conditions. Allow adequate stabilization time before final inspection—a part that measures correctly immediately after machining may shift as it reaches equilibrium with ambient conditions.

Geometric Dimensioning and Tolerancing (GD&T)

Beyond dimensional tolerances, geometric tolerances control form, orientation, and location relationships that affect function. Flatness controls surface waviness independent of other features. Perpendicularity controls the angular relationship between a surface and a datum. Concentricity ensures cylindrical features share a common axis. Position controls feature location relative to a datum reference frame. Runout controls circular elements relative to a datum axis.

GD&T provides more precise control of functional requirements than dimensional tolerances alone because it separates different aspects of geometry that affect function differently. When specifying precision components, consider whether geometric controls better capture what you actually need the part to do.

Cost Optimization Strategies

Tolerance Analysis

Before finalizing drawings, perform a tolerance stackup analysis to identify which dimensions critically affect function. Many drawings carry tight tolerances on dimensions that don’t actually matter to assembly or performance. The analysis reveals where tolerance can be relaxed without functional impact and where it must remain tight—allowing cost optimization without compromising the design.

Datum Selection

Choose datums that align with functional requirements and are practical for manufacturing. Effective datum features are accessible for both workholding and inspection, sufficiently large to provide stable reference, and machined early in the process sequence so subsequent features can reference them. Poor datum selection forces awkward setups, increases uncertainty in feature location, and may make inspection difficult or impossible.

Design for Manufacturability

For comprehensive DFM guidance, see our 10 Rules for Cost-Effective CNC Parts. The core principles that affect tolerance costs include avoiding specifications tighter than function requires, using standard hole sizes compatible with available tooling, providing adequate material access for machining operations, considering inspection accessibility when designing features, and specifying appropriate surface finishes that align with tolerance requirements.

Inspection and Quality Assurance

Different tolerance classes require different inspection approaches, both for equipment capability and economic practicality.

Tolerance Class	Typical Inspection Equipment
Standard (±0.005”)	Calipers, micrometers, go/no-go gauges
Precision (±0.001”)	CMM, optical comparator, bore gauges
Ultra-Precision (±0.0005”)	CMM with temperature compensation, air gauges

For critical components, drawings should specify more than just dimensional requirements. Consider including inspection frequency (whether 100% or a defined sampling plan), required equipment accuracy relative to tolerance, environmental conditions for measurement, and traceability and documentation requirements. These specifications ensure that inspection provides meaningful verification of conformance.

Working With NextGen Components

Our engineering team reviews all drawings for manufacturability and cost optimization opportunities. We regularly identify tolerance specifications that can be relaxed without affecting function, providing cost savings to our customers while ensuring parts perform as intended.

Request a quote and include your drawings for a detailed manufacturability review. Our team will provide specific recommendations based on your application requirements and production volumes.