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Stack-up Analysis Guide

Guide to performing tolerance stack-up analysis in DatumPilot, including Worst-Case, RSS, and Six Sigma methods.

Last updated: 2025-12-23

Stack-up Analysis Guide

Overview

DatumPilot's stack-up analysis combines dimension tolerances using Worst-Case, RSS, or Six Sigma methods. Results include nominal value, total tolerance, min/max range, pass/fail against acceptance criteria, and contribution breakdowns.

Analysis Methods (as implemented)

Worst-Case (Arithmetic)

  • Calculation: T_wc = Sum(|t_i x S_i|) where t_i is bilateral tolerance and S_i is the sensitivity coefficient.
  • All tolerances add directly; most conservative.

RSS (Root Sum Square)

  • Calculation: T_rss = sqrt(Sum((t_i x S_i)^2)).
  • Uses bilateral tolerance derived from plus/minus values.
  • The implementation notes this approximates 99.73% (3-sigma) assembly success.

Six Sigma

  • Uses process capability per dimension (Cp) with a default of 1.33 when not provided.
  • Calculation: sigma_i = (t_i x S_i) / (3 x Cp_i) and T_6sigma = 3 x sqrt(Sum(sigma_i^2)).

Creating a Stack-up Analysis

  1. Name and describe the analysis.
  2. Define the measurement objective (what the closing dimension represents).
  3. Set acceptance criteria: minimum, maximum, or both. At least one limit is required.
  4. Choose the positive direction: left-to-right, right-to-left, bottom-to-top, or top-to-bottom.
  5. Select the analysis method: Worst-Case, RSS, or Six Sigma.
  6. Choose units: mm or inch (all dimensions use the same unit).

Sign Convention

  • Positive: increasing the dimension increases the closing dimension.
  • Negative: increasing the dimension decreases the closing dimension.

Adding Dimensions

Click Add Dimension to enter each contributor.

Required fields

  • Name
  • Nominal
  • Tolerance entry (choose notation per dimension):
    • Symmetric (\u00B1): enter tolerance once
    • Asymmetric (+/\u2212): enter plus/minus independently
    • Limits: enter upper/lower limits, then DatumPilot derives nominal and tolerances
  • Sign (positive or negative)

Optional fields

  • Description
  • Sensitivity Coefficient (default 1)
  • Process Capability (Cp) (Six Sigma only)
  • Source Drawing
  • Source Revision

Limits

  • Minimum: 2 dimensions
  • Maximum: 50 dimensions

Results

Key metrics

  • Nominal Result: sum of signed nominals
  • Total Tolerance: method-dependent
  • Maximum Value: nominal + tolerance
  • Minimum Value: nominal - tolerance

Pass/Fail

  • Uses acceptance criteria limits. Margins are shown when minimum and/or maximum limits are set.

Contribution Analysis

The contribution chart shows which dimensions drive total variation.

  • Worst-Case: contribution is proportional to absolute tolerance.
  • RSS / Six Sigma: contribution is proportional to variance.

Method Comparison

Use the Compare toggle on the detail view to display results for all three methods side-by-side.

Asymmetric Tolerances

The calculator adjusts the nominal by the mean shift:

  • Mean shift per dimension: (tolerancePlus - toleranceMinus) / 2
  • Bilateral tolerance: (tolerancePlus + toleranceMinus) / 2

Tolerance Notation and Storage

DatumPilot stores stack-up dimensions in canonical form:

  • nominal
  • tolerancePlus
  • toleranceMinus

Notation is an input/display mode. Switching between symmetric, asymmetric, and limits does not change the underlying calculation method; it only changes how values are entered and shown.

Next Steps