Tolerance and Fits Reference

Compiled 2026-04-04 · 40 chunks, 15 posts · tolerance · fits · clearance · interference · dimensions · precision

Summary

Tolerances and fits define the allowable dimensional variation in machined parts and how those parts interact when assembled. This reference provides standard tolerance grades, fit classifications, and practical application guidance for achieving precise mechanical assemblies. Understanding tolerance stackup and proper fit selection is critical for functional parts that must assemble reliably in production environments.

Standard Tolerance Grades

ISO Tolerance System

  • IT01: ±0.0001" (±0.0025mm) - Ultra-precision gage work
  • IT1: ±0.0002" (±0.005mm) - Precision gages, standards
  • IT5: ±0.0005" (±0.013mm) - High-precision fits
  • IT6: ±0.001" (±0.025mm) - Close running fits
  • IT7: ±0.002" (±0.05mm) - General precision work
  • IT8: ±0.003" (±0.08mm) - Medium fits
  • IT9: ±0.005" (±0.13mm) - General machining
  • IT10: ±0.008" (±0.2mm) - Rough machining

ANSI Standard Limits

RC (Running/Sliding Clearance Fits):

  • RC1: 0.0004-0.0016" clearance - Precision sliding fits
  • RC2: 0.0006-0.0025" clearance - Sliding fits for accurate location
  • RC3: 0.001-0.004" clearance - Precision running fits
  • RC4: 0.0015-0.006" clearance - Close running fits

LC (Locational Clearance Fits):

  • LC1: 0-0.0025" clearance - Snug fits for stationary parts
  • LC11: 0.005-0.025" clearance - Loose running fits

LT (Locational Transition Fits):

  • LT1: -0.0006 to +0.0012" - Compromise between clearance and interference
  • LT6: -0.002 to +0.008" - Medium transition fits

LN (Locational Interference Fits):

  • LN1: 0-0.0025" interference - Light press fits
  • LN3: 0.001-0.008" interference - Heavy press fits

FN (Force/Shrink Fits):

  • FN1: 0.0005-0.0015" interference - Light drive fits
  • FN4: 0.002-0.006" interference - Force fits for heavy duty
  • FN5: 0.003-0.010" interference - Force fits for maximum stress

Practical Fit Applications

Bearing Applications

Rolling Element Bearings:

  • Shaft (rotating inner ring): k5 to m6 fit
  • Housing (stationary outer ring): H6 to H7 fit
  • Temperature compensation: Add 0.0001"/inch for each 100°F rise

Plain Bearings:

  • Oil film thickness typically 0.0005-0.002"
  • Clearance ratio: 0.001-0.003" per inch of diameter
  • Bronze bushings: H7/f7 typical

Shaft and Hub Connections

Keyed Connections:

  • Key to shaft: N9/h9 (snug fit)
  • Key to hub: Js9/h9 (slight clearance)
  • Standard key clearance: 0.0005-0.002"

Press Fits:

  • Light press (hand assembly): 0.0002-0.0008" interference
  • Medium press (arbor press): 0.0008-0.002" interference
  • Heavy press (hydraulic): 0.002-0.005" interference
  • Calculate hoop stress: σ = (P × d)/(2t) where P=pressure, d=diameter, t=wall thickness

Threaded Fasteners

Class 2A/2B (Standard):

  • General purpose, commercial grade
  • Typical tolerance: ±0.003" on pitch diameter

Class 3A/3B (Close):

  • Precision applications
  • Typical tolerance: ±0.0015" on pitch diameter

Reference [[thread-standards]] for complete threading specifications.

Tolerance Stackup Analysis

Statistical Method

  • RSS (Root Sum Square): σ_total = √(σ₁² + σ₂² + ... + σₙ²)
  • Use when tolerances are independent and normally distributed
  • Typically allows 20-30% tighter individual tolerances

Worst Case Method

  • Arithmetic sum: T_total = T₁ + T₂ + ... + Tₙ
  • Conservative approach ensuring 100% acceptance
  • Use for critical safety applications

Practical Guidelines

  • Aim for Cp ≥ 1.33 (4σ process capability)
  • Allow 10-20% margin on critical dimensions
  • Consider temperature effects: steel expands ~6.5 × 10⁻⁶ in/in/°F

Measuring and Verification

Equipment Selection

Micrometers: ±0.0001" accuracy, good for 0.001" tolerances Dial Indicators: ±0.00005" resolution, excellent for runout CMM: ±0.0002" typical accuracy, complex geometries Pin Gages: Class ZZ (±0.00002"), Class X (±0.00004")

Gage R&R Requirements

  • Gage precision should be ≤10% of tolerance
  • For ±0.001" tolerance, need ±0.0001" measurement capability
  • Calibrate critical gages every 6-12 months

Surface Finish Impact

Tolerances and [[surface-finish-grades]] are interdependent:

  • 32 Ra: General machining, ±0.005" tolerances achievable
  • 16 Ra: Good machining, ±0.002" tolerances
  • 8 Ra: Fine machining, ±0.001" tolerances
  • 4 Ra: Precision grinding, ±0.0005" tolerances
  • 2 Ra: Precision grinding/honing, ±0.0002" tolerances

Surface finish affects functional performance:

  • Bearing surfaces: 8-16 Ra typical
  • Seal surfaces: 4-8 Ra required
  • Gage surfaces: 1-2 Ra needed

Material Considerations

Thermal Expansion

Expansion Coefficients (in/in/°F):

  • Steel: 6.5 × 10⁻⁶
  • [[aluminum-6061]]: 13.1 × 10⁻⁶
  • [[304-stainless]]: 9.6 × 10⁻⁶
  • [[brass]]: 10.4 × 10⁻⁶
  • [[cast-iron]]: 6.0 × 10⁻⁶

Calculate dimensional change: ΔL = L × α × ΔT

Hardness Effects

Harder materials maintain tighter tolerances:

  • Soft materials (< 30 HRC): Expect 0.0005-0.001" growth under stress
  • Medium hardness (30-45 HRC): Minimal dimensional change
  • Hard materials (> 50 HRC): Excellent stability

Reference [[hardness-conversion]] for material selection.

Shop Floor Tips

Machining for Tight Tolerances

  • Maintain consistent coolant temperature (±5°F)
  • Allow spindle warm-up: 15-30 minutes before critical operations
  • Use climb milling for better surface finish and dimensional control
  • Take spring cuts: 0.0002-0.0005" on final pass
  • Check parts at machine temperature, adjust for inspection room temperature

Assembly Best Practices

  • Measure both parts before assembly - don't assume drawing tolerances
  • For press fits, chill shaft (-80°F dry ice) or heat hub (200-300°F)
  • Use assembly lubricants for interference fits: light oil or soap
  • Press fit force calculation: F = π × d × L × p × μ (diameter × length × pressure × friction)

Quality Control

  • Monitor trends, not just pass/fail - parts drifting toward limits indicate tool wear
  • Use control charts for critical dimensions
  • First piece inspection after tool changes
  • Statistical sampling: inspect 2-5% of production runs

Economic Tolerancing

  • Each additional decimal place roughly doubles machining cost
  • ±0.005": Standard machining capability
  • ±0.001": Requires careful setup, quality tooling
  • ±0.0005": Grinding or precision turning often required
  • ±0.0002": Surface grinding, honing, or lapping needed

Typical cost multipliers:

  • ±0.005" = 1.0x baseline
  • ±0.001" = 1.5-2.0x baseline
  • ±0.0005" = 3.0-4.0x baseline
  • ±0.0002" = 5.0-8.0x baseline
  • [[surface-finish-grades]] — Surface texture requirements for different tolerance grades
  • [[thread-standards]] — Threaded fastener tolerance classes and specifications
  • [[hardness-conversion]] — Material hardness effects on dimensional stability
  • [[aluminum-6061]] — Thermal expansion considerations for aluminum parts
  • [[304-stainless]] — Stainless steel tolerance and surface finish guidelines