Tolerance Analysis & GD&T: What Engineers Need to Know

A CAD model looks perfect. Parts fit on screen. But manufacturing fails: screw holes misalign, enclosure halves don't close flush, springs don't transmit force as designed. The cause is almost always the same: Missing tolerance analysis.

Why Tolerances Are Inevitable

No manufacturing process produces exact CAD dimensions. CNC milling works with tolerances of ±0.01–0.05 mm, injection molding at ±0.05–0.3 mm, 3D printing at ±0.1–0.3 mm. Every part has deviations. The question is whether these deviations affect function.

Tolerance analysis means: Controlling the sum of all dimensional changes in a chain of parts so the overall system works.

Tolerance Stacks: The Chain of Uncertainty

A tolerance stack describes the path of a dimension through multiple parts. Example: The distance between two holes in an enclosure depends on:

• Tool tolerance – How precisely the injection mold was manufactured
• Shrinkage – How much the material shrinks when cooling (PA6: 1.5–2%)
• Secondary operations – drilling, threading, deburring
• Assembly – Clearance between parts, spring force, assembly tolerance

If each step has ±0.1 mm tolerance and 5 parts are in the chain, the total tolerance is ±0.5 mm. For tight fits, that's too much.

Worst-Case vs. Statistical Analysis

Worst-Case Analysis

Adds the maximum deviations of all parts. Result: The safest method, but very conservative. Often leads to oversized tolerances and higher manufacturing costs.

Formula: Total tolerance = Σ(tolerances of all parts)

Statistical Analysis (RSS)

Uses Gaussian normal distribution. The probability that ALL parts simultaneously have maximum deviations in the same direction is extremely low. Result: More realistic tolerances, lower costs.

Formula: Total tolerance = √(Σ(tolerances²))

Practical tip: Use worst-case for critical fits (O-ring grooves, enclosure closures). Statistical analysis is sufficient for everything else.

GD&T per ISO 1101: More Than Dimensions

Geometric Dimensioning and Tolerancing (GD&T) is an internationally standardized system for defining form and position tolerances. In Germany, ISO 1101 applies; internationally, ASME Y14.5.

Why GD&T matters:

• Clarity – No misunderstandings between designer and manufacturer
• Functional reference – Tolerances are tied to function, not arbitrary dimensions
• Inspectability – Measurement protocols can be derived directly from GD&T definitions
• Cost optimization – Strategic relaxation of non-critical tolerances reduces manufacturing costs

The 5 Most Important GD&T Symbols for Enclosures

1. Flatness – For enclosure surfaces that must be sealed. Without flatness tolerance, the part warps and the seal fails.
2. Cylindricity – For holes that accept screws or shafts. Determines whether the hole is truly cylindrical.
3. Position – The most important tolerance. Defines where a hole or slot may be located. Combines dimensional tolerances into a function-oriented specification.
4. Parallelism – For two opposing enclosure halves. Ensures the closing surface is actually parallel.
5. Concentricity – For rotating parts like bearing bushings. Determines how far the center axis may deviate from its nominal position.

Practical Example: Enclosure Closure with O-Ring

An enclosure consists of two halves with an O-ring seal. The critical tolerances:

• O-ring groove depth: ±0.05 mm (worst-case)
• Groove width: ±0.1 mm (statistical)
• Height of both halves: ±0.15 mm (statistical)
• Closure surface parallelism: 0.1 mm (GD&T)

Total tolerance (worst-case): ±0.05 + ±0.1 + ±0.15 = ±0.3 mm. This is sufficient for an O-ring seal at 10–15% compression. But only if the surfaces are actually parallel – hence the GD&T tolerance.

Common Mistakes in Practice

1. Ignoring tolerances– "It will fit" is the most expensive sentence in design. Without analysis, you only discover problems in manufacturing.
2. Too tight tolerances – The tighter the tolerance, the more expensive the manufacturing. ±0.01 mm costs 3–5x more than ±0.1 mm.
3. Wrong reference datums – Tolerances must reference functionally relevant surfaces, not arbitrary dimensions.
4. Non-inspectable tolerances – Defining tolerances that cannot be verified with standard measuring equipment.

When Should Tolerance Analysis Be Performed?

At the latest before manufacturing the first prototype. Ideally during the design phase. This saves:

• 60–80% of rework costs – Corrections in CAD are cheaper than reworking metal
• Tool modifications – Rework on an injection mold costs €500–5,000
• Delays – 2–4 weeks for mold corrections, plus shipping time

Anton Steenken

B.Eng. · Hardware R&D Engineer · Founder of engineer your idea