Basics of Tolerance stackup analysis for Beginners.

1. Tolerance stack-up and analysis
By : Mohan Narasaiah

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What is Tolerance ?
A Tolerance is a specified amount a feature is allowed to vary from nominal. This may be
include the form , size ,orientation or location of the feature as applicable.
Types of Tolerance ?
• General tolerance / Local tolerances
• GD & T

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What is Tolerance analysis ?
A method used to determine the meaning of individual tolerances specifications.
What is Tolerance stack up ?
A method of mathematically predicting the resultant effect of part and subassembly
tolerances along with assembly process and fixturing variation on a build objective of the
product.

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Tolerance Loops
• A systematic method of approaching a tolerance stack, and selecting the contributing
tolerances.
• A tolerance loop allows for the evaluation of not only the stack variation, but also the
stack nominal value.
Why to Perform Tolerance stack Analysis ?
• Analyze and optimize dimensional variability within an assembly system.
• Establish piece part tolerances.
• Reduce product costs by increasing tolerances.
• Identify key tolerance contributors.
• Reduce product cycle time and improve quality.
• Determine if existing design and tooling will meet the build objective requirements
(CTQs).

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Tolerance Loop creation.
• Establish start and finish points on each side of the objective.
• Travel from the start point to the finish point in the shortest route, this direction will be
called +.
• Go around the tolerance circuit in the opposite direction from previous step, taking the
most direct route.
• Add signs to the nominal values according to their direction.

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Worst-case analysis
• Determine absolute maximum variation possible for a selected distance or gap.
• Assumes dimensions vary within the entire range of their tolerance zones and that the
accumulation of tolerances will experience all possible variations
Gmin = L1 + L2 + L3 + L4
= 152.75 + (-60.75) + (-42) + (-50.5)
= -0.5 (Interference)
Methods of Tolerance stack-up Analysis.
• Worst case Tolerance analysis.
• Statistical tolerance analysis or RSS method.

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• Worst-case tolerance analysis is dependent on how parts are actually assembled in real
life
• It is incorrect to think max worst case is when the parts are at their largest and min worst
case is when they are at their smallest

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Statistical tolerance analysis.
• Determine the probable or likely maximum variation possible for selected dimension or
gap.
• Assumes it is highly improbable that all the dimensions are at stackup will be at worst
case lower or upper limit at the same time.
When to use RSS ?
• Number of tolerances in part
• Number parts will be produced.
• Method of manufacturing and its controls.
• Design sensitivity or CTQ.

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