Notes on Allocation Spreadsheets
Weight Factor Allocation
WFAlloc uses the Weight Factor allocation method. Use the mouse or arrow keys to select any input cell. Type data entry from the keyboard. Only the entry cells are unlocked.
1. Select an estimated tolerance for each dimension in the loop and enter it in the column labled "Orig," for Original Tolerances. The resulting Worst Case tolerance sum TsumWC is computed and appears below the Orig column. The statistical sum TsumRSS appears below the Tol^2 column.
2. Enter the required assembly tolerance in the cell labeled Tasm. The difference between Tasm and Tsum is the amount of assembly tolerance you have to allocate (the "Pool"). A positive pool allows the component tolerances to be increased; a negative pool requires that they be decreased.
3. Fill in the column of Fixed flags. A one means the component tolerance is fixed and may not be changed by allocation. A zero permits allocation to that component.
4. Choose Weight Factors to direct the distribution of any excess assembly tolerances to the components which need it most. Enter these in the Wt Fact column. Note, they do not need to add up to 100. It is the ratio Wi/Wtot which drives the allocation.
Minimum Cost Allocation
$Alloc reduces the overall cost of an assembly by loosening the tolerance of those parts which are produced by expensive processes and tightening tolerances on less expensive parts. The governing constraint is the tolerance sum allowed for the assembly, which may be calculated by worst case or statistical analysis.
$Alloc uses a cost vs. tolerance model which assumes that the cost of producing a part to a given tolerance varies as 1/tol k. An empirical cost function of the form: C = A + B/tolk is supplied by the designer by specifying the coefficients A, B and k for each part.
A is the "setup cost" for each part, obtained by dividing the total setup cost by the number of parts produced. It may also include the cost of tooling. B/tolk is the cost of producing a single dimension on the part to its specified tolerance. The cost for the part, then, is the sum of the setup cost, the tooling cost and the dimension cost. The overall cost of an assembly is the sum of the costs for all of the parts in the assembly. A is a fixed cost. It does not affect tolerance allocation. It merely shifts the cost curve up. A can therefore be chosen = 0 for the allocation.
1. Fill in the table of B and k for each dimension. A reference table for B and k is provided. Also enter the max and min tolerance permitted for each process.
2. Enter Tasm, and Orig tolerances as in WFAlloc.
3. The allocated tolerances appear in columns WC Ti and RSS Ti. The first tolerance in each column, T1, must be selected manually; the rest are calculated from T1 by formula. You must iterate on the first tolerance until Tsum at the bottom of the column is equal to Tasm. EXCEL has a function which will do this automatically.
4. Check to see that your allocated tolerances do not exceed the max/min for the process.
5. Repeat the allocation with the shaft tolerance fixed.