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Tolerance Allocation How-To

This walkthrough uses a small model you can build in a minute: two inputs, one output, and a specification band that half the output sits outside of.

Press Copy for Excel, then paste the table into a blank worksheet.

Model Value
Wall thickness 10
Bore 4
Outside width =B2*2+B3

Click cell A1 in a blank worksheet and press Ctrl+V. The two headings land in row 1, the labels in column A, and the model in column B: B2 holds the wall thickness, B3 the bore, and B5 the formula that reads them both. The empty row is deliberate, so the output lands in B5 and not B4, which is what every Monte Carlo walkthrough calls it.

Mark B2 and B3 as inputs with Mark Input, giving each a Normal distribution: B2 mean 10 standard deviation 0.2, B3 mean 4 standard deviation 0.1. Mark B5 as an output with Mark Output and give it a lower limit of 23 and an upper limit of 25.

Steps

  1. QXL Monte Carlo New > Monte Carlo > Contribution Tools > Tolerance Allocation...

  2. Leave the three percentages at 10, 25 and 50.

  3. Run it. The sweep reports its progress as it goes, because it is running many simulations rather than one.

What it does, and how long it takes

Seven simulations per input, not one. Both B2 and B3 are Normal with a spread above zero, so both are swept, and the run is fourteen ordinary simulations. The status line counts levels, and the trials inside the level in progress are counted the way any single run's are.

Each level multiplies that input's standard deviation and holds its mean. At the defaults the seven multipliers are 0.5, 0.75, 0.9, 1.0, 1.1, 1.25 and 1.5, so B2's standard deviation of 0.2 is swept at 0.1, 0.15, 0.18, 0.2, 0.22, 0.25 and 0.3.

What to read

B2's curve should be the steeper of the two. It enters the model with a coefficient of 2 against B3's 1, so tightening B2 buys more than tightening B3 by the same proportion. That comparison is the whole output of the tool.

The middle point of every curve is the model as it stands, because the multiplier there is 1. Read each curve outward from that point: left is tighter than today, right is looser.

Things to try next

  • Change the percentages to 5, 10 and 20. The sweep narrows around the current spread, which is the view to use when you are deciding a small tolerance change rather than exploring.
  • Mark B3 as a Uniform instead of a Normal. It is still swept, because a continuous Uniform has a half-width the multipliers can act on.
  • Mark an input as Triangular and it drops out of the sweep, with the report naming the reason.

See Also