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 or equipment.

Characteristics:

• No premium pricing for standard tolerance work
• Standard measuring equipment (calipers, micrometers) sufficient for inspection
• Compatible with most materials and feature geometries
• Typical lead times apply

Appropriate Applications:

• Clearance holes and non-critical bores
• General mounting surfaces
• Non-mating dimensions
• Prototype and development parts where fit is not critical

Precision Tolerances (±0.001” / ±0.025mm)

Precision tolerances require additional process controls and more sophisticated inspection equipment. Expect cost increases of 25-50% compared to standard tolerance work.

Process Requirements:

• Climate-controlled machining environment
• Premium tooling with frequent replacement
• Reduced cutting speeds and feed rates
• CMM or optical comparator inspection
• Multiple setup verification checks

Appropriate Applications:

• Press-fit interfaces (bearing bores, pin holes)
• Sealing surfaces (O-ring grooves, gasket faces)
• Mating surfaces requiring accurate alignment
• Moving components (slides, ways, rotating parts)

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

Ultra-precision work requires specialized equipment, environmental controls, and highly skilled operators. Costs typically increase 100-300% or more compared to standard work.

Process Requirements:

• Temperature-controlled environment (±1°F)
• Grinding, lapping, or honing operations
• Air-bearing measuring equipment
• Extended stabilization time for thermal equilibrium
• 100% inspection with documented traceability

Appropriate Applications:

• Optical component mounting surfaces
• Precision instrument mechanisms
• High-speed rotating assemblies
• Medical device components with micron-level requirements

Material-Specific Considerations

Different materials exhibit varying dimensional stability, affecting achievable tolerances and long-term performance.

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 for precision work:

• Hygroscopic expansion: Nylon absorbs moisture, causing dimensional growth of 0.002-0.003” per inch
• Thermal coefficient: Most plastics expand 5-10x more than metals per degree temperature change
• Creep: Sustained loads cause gradual dimensional change over time
• Stress relief: Internal stresses from machining can cause warping after removal from fixturing

For critical plastic components, specify tolerances at defined temperature and humidity conditions, and allow adequate stabilization time before final inspection.

Geometric Dimensioning and Tolerancing (GD&T)

Beyond dimensional tolerances, geometric tolerances control form, orientation, and location relationships that affect function.

Common Geometric Controls:

• Flatness: Controls surface waviness independent of other features
• Perpendicularity: Controls angular relationship to a datum surface
• Concentricity: Controls alignment of cylindrical features
• Position: Controls location relative to 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. When specifying precision components, consider whether geometric controls better capture your functional needs.

Cost Optimization Strategies

Tolerance Analysis

Before finalizing drawings, perform a tolerance stackup analysis to identify which dimensions critically affect function. Apply tight tolerances only where analysis demonstrates necessity.

Datum Selection

Choose datums that align with functional requirements and are practical for manufacturing. Datum features should be:

• Accessible for workholding and inspection
• Sufficiently large for stable reference
• Machined early in the process sequence

Design for Manufacturability

• Avoid specifying tighter tolerances than functionally required
• Use standard hole sizes compatible with available tooling
• Provide adequate material access for machining operations
• Consider inspection accessibility in feature design

Inspection and Quality Assurance

Inspection Methods by Tolerance Class:

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, specify inspection requirements on drawings including:

• Inspection frequency (100%, sampling plan)
• Required equipment accuracy
• Environmental conditions for measurement
• Traceability and documentation requirements

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.

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.