First Edition Dated September, 2000 45
Drawing / Build Tree
Assembly ABC
Key
A
Key
A
Key
A
Part A
Part B
Part C
Key
Part B
Part C
A
Part A
4.5.4_1
The figure above illustrates the flowdown of requirements, datums and
indices (in this case, part-to-part) and the use of key characteristics to
functionally locate datums. Datum A /index on part A of Assembly ABC in the
top build position flows down functionally to the datum A/index on detail part A
in the lower left build position. But the functional datum of the assembly does
not flow down to detail part B. Part B will have its own functional datum/
index, based on how the part is used in the assembly. The key characteristic
on part A is the connection to functionally relate the two datums and is a
feature that must be controlled to reduce variation in the assembly.
The functional datums concept extends to cover another problem caused by a
common design/manufacturing practice. Typically, in the name of design for
producibility, engineers volunteer or are pressured by the manufacturing
shops to place datums on the surfaces of the part that will be indexed in the
machine. Although this is convenient for the fabrication build position, it may
not match the way the customer will use the product.
