Question 1

What is the difference between worst-case and RSS tolerance stack-up?

Accepted Answer

Worst-case (arithmetic) stack-up adds every tolerance at its extreme at once, giving the absolute limits the assembly can ever reach. RSS (root-sum-square) is a statistical estimate that combines tolerances as the square root of the sum of their squares, assuming each dimension varies normally and independently. RSS gives a tighter, more realistic spread but is not a guaranteed limit.

Question 2

How do I calculate a worst-case tolerance stack?

Accepted Answer

Take the nominal gap as the signed sum of each nominal (add or subtract by direction). The maximum gap adds the maximum of each added dimension and the minimum of each subtracted dimension; the minimum gap does the reverse. The difference from nominal is the worst-case tolerance band.

Question 3

What does direction (+ / −) mean in a stack-up?

Accepted Answer

Direction is the sign of each dimension along the stack axis. Dimensions that add material or distance in the direction you are measuring get a plus; dimensions that close the gap (such as a mating part length) get a minus. The nominal gap is the signed sum of all of them.

Question 4

When should I use RSS instead of worst-case?

Accepted Answer

RSS suits assemblies with several independent dimensions made in volume, where all tolerances reaching their extreme together is statistically very unlikely. Worst-case suits short stacks, safety-critical fits, or low-volume parts where you must guarantee the assembly always fits.

Question 5

Does RSS account for biased or asymmetric tolerances?

Accepted Answer

Not directly. The basic RSS used here assumes symmetric tolerances centered on the nominal. If a process is biased or the tolerance is one-sided, the mean shifts away from the nominal and you should adjust the nominal or apply a mean-shift / modified RSS method before trusting the result.

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