GD&T Tolerance Stack-up Analysis 1 — Questions and Answers
Question 1: What is the primary purpose of tolerance stack-up analysis in GD&T?
- To minimize the number of dimensions on a drawing
- To determine the worst-case variation in an assembly due to accumulated tolerances (Correct answer)
- To select the most economical manufacturing process
- To calculate surface finish requirements
Correct answer: To determine the worst-case variation in an assembly due to accumulated tolerances
Tolerance stack-up analysis determines how individual part tolerances accumulate to affect the overall assembly variation in the worst case.
Question 2: In a worst-case tolerance stack-up, how are individual tolerances combined?
- They are multiplied together
- They are statistically averaged using root sum square
- They are arithmetically added, assuming all tolerances at their maximum (Correct answer)
- They are subtracted from the nominal dimension
Correct answer: They are arithmetically added, assuming all tolerances at their maximum
Worst-case analysis adds all tolerances arithmetically, assuming each feature is simultaneously at its maximum tolerance limit.
Question 3: What does RSS (Root Sum Square) analysis assume about the distribution of actual part dimensions?
- All parts are at the maximum tolerance limit
- Parts follow a uniform distribution
- Parts follow a normal (Gaussian) distribution (Correct answer)
- Parts are always at the nominal dimension
Correct answer: Parts follow a normal (Gaussian) distribution
RSS analysis assumes part dimensions follow a normal distribution, which is typical for manufacturing processes under statistical control.
Question 4: Which type of tolerance stack-up analysis is more conservative and results in a larger required total tolerance?
- Root Sum Square (RSS) analysis
- Statistical analysis
- Worst-case (arithmetic) analysis (Correct answer)
- Monte Carlo simulation
Correct answer: Worst-case (arithmetic) analysis
Worst-case analysis is the most conservative because it assumes all tolerances simultaneously reach their extreme values, producing the largest possible variation.
Question 5: In a linear worst-case tolerance stack-up, what is the total gap variation if three dimensions each have a bilateral tolerance of ±0.1 mm?
- ±0.1 mm
- ±0.2 mm
- ±0.3 mm (Correct answer)
- ±0.6 mm
Correct answer: ±0.3 mm
In worst-case analysis, tolerances add arithmetically, so three ±0.1 mm tolerances accumulate to a total variation of ±0.3 mm.
Question 6: What is a 'loop diagram' used for in tolerance stack-up analysis?
- To show the manufacturing process flow
- To identify which dimensions contribute to a critical assembly gap or interference (Correct answer)
- To display GD&T symbols graphically
- To document inspection measurement results
Correct answer: To identify which dimensions contribute to a critical assembly gap or interference
A loop diagram traces a closed path through the assembly dimensions to identify which tolerances contribute to a critical gap, clearance, or interference condition.
Question 7: When should tolerance stack-up analysis be performed during product development?
- Only after manufacturing defects are found during production
- During final inspection before shipment to the customer
- Early in the design phase before drawings are released (Correct answer)
- Only when assembled parts fail to meet functional requirements
Correct answer: Early in the design phase before drawings are released
Tolerance stack-up analysis should be performed early in the design phase to identify potential assembly problems and optimize tolerance allocation before manufacturing begins.
GD&T Tolerance Stack-up Analysis 1
What is the primary purpose of tolerance stack-up analysis in GD&T?