GD&T Tolerance Stack-up Analysis 2 — Questions and Answers
Question 1: What advantage does GD&T cylindrical position tolerance offer over square coordinate tolerancing in a stack-up budget?
- It eliminates the need for tolerance stack-up analysis
- The cylindrical tolerance zone provides approximately 57% more tolerance area than a square zone of the same width (Correct answer)
- It reduces the number of dimensions required on the drawing
- It is always easier to measure on the shop floor
Correct answer: The cylindrical tolerance zone provides approximately 57% more tolerance area than a square zone of the same width
A cylindrical tolerance zone for position has approximately 57% more area than a square coordinate tolerance zone of the same width, effectively increasing the available tolerance budget.
Question 2: In a tolerance stack-up, what is the 'requirement' being evaluated?
- The manufacturing cost target for each component
- The minimum clearance or maximum interference condition that must be met for proper function (Correct answer)
- The surface finish specification on mating parts
- The material yield strength requirement
Correct answer: The minimum clearance or maximum interference condition that must be met for proper function
The requirement is the functional condition that must be satisfied, such as maintaining minimum clearance to prevent interference or ensuring parts assemble correctly.
Question 3: Which analysis method accepts a small percentage of non-conforming assemblies in exchange for allowing larger individual part tolerances?
- Worst-case arithmetic analysis
- Arithmetic analysis
- Statistical (RSS) analysis (Correct answer)
- Virtual condition analysis
Correct answer: Statistical (RSS) analysis
Statistical RSS analysis accepts a small percentage of assemblies that may fail (typically 0.27% for ±3σ), trading a guaranteed function for larger individual part tolerances.
Question 4: What generally happens to each individual component tolerance as the number of components in a stack-up increases while the assembly requirement stays fixed?
- The required component tolerance decreases proportionally
- The required component tolerance remains constant
- Each component must be made to tighter tolerances to maintain the same assembly requirement (Correct answer)
- The assembly requirement automatically becomes easier to meet
Correct answer: Each component must be made to tighter tolerances to maintain the same assembly requirement
As more components are added to a stack-up, each individual part must be made to tighter tolerances if the overall assembly tolerance requirement remains unchanged.
Question 5: In tolerance stack-up analysis, what is a 'contributing dimension'?
- A dimension that increases manufacturing cost
- Any dimension whose variation affects the critical assembly condition being analyzed (Correct answer)
- A dimension shown with a bilateral plus/minus tolerance only
- A dimension specified with a position GD&T symbol
Correct answer: Any dimension whose variation affects the critical assembly condition being analyzed
A contributing dimension is any dimension whose tolerance directly affects the variation of the critical gap, clearance, or interference being analyzed in the stack-up loop.
Question 6: What is 'assembly shift' (datum shift) in the context of tolerance stack-up analysis?
- A change in the assembly sequence on the shop floor
- Additional variation allowed when a datum feature departs from its maximum material condition (Correct answer)
- Physical movement of the assembly during shipping
- A correction factor applied to stack-up calculations
Correct answer: Additional variation allowed when a datum feature departs from its maximum material condition
Assembly shift (datum shift) refers to the additional movement or variation allowed when a datum feature departs from its maximum material condition, creating extra tolerance in the stack.
Question 7: Which of the following best describes 'allocating tolerances' in a tolerance stack-up study?
- Assigning quality control responsibilities to machine operators
- Distributing the total assembly tolerance budget among individual component dimensions (Correct answer)
- Marking dimensions that require special inspection procedures
- Identifying the datum references for each geometric feature
Correct answer: Distributing the total assembly tolerance budget among individual component dimensions
Tolerance allocation is the process of dividing the total permissible assembly variation among the contributing component dimensions based on manufacturing capability and functional criticality.
GD&T Tolerance Stack-up Analysis 2
What advantage does GD&T cylindrical position tolerance offer over square coordinate tolerancing in a stack-up budget?