DFA - Design for Assembly
Design for Assembly is one of the approaches in Design for Excellence (DFX). The X here can take many forms like Manufacturing, Safety, Cost, Service, Reliability etc. So how is DFA different from others and when should one go for it
DFA should be the preferred if the product that we are designing needs to be assembled and disassembled often. Because in such situations more than anything else, it is more important that the assembly should be
1. easy
2. efficient
3. effective
My top of the mind items that usually require to be assembled and disassembled are military guns and toys (especially track toys and Lego).
Elements that you need to consider in DFA
1. Number of parts - product with lesser number of parts is easier to assemble. Therefore the number of parts should be kept to a bare minimum. Parameters to check if a part can be removed or not are
a. Is it absolutely necessary to have the part made of a different material?
b. Does the part has a movement relative to the other parts of the product?
c. Is the part used as a fastener or for securing other parts?
2. Time taken to assemble or ease of assembly - there are quiet a few things that are considered here
a. Easy to handle parts - neither too small nor too big
b. Symmetry of the parts - symmetrical parts are easy to handle
c. Remove flexible, slippery, sticky parts along with parts that have sharp edges
d. Easy to insert - unidirectional, self inserting and easy to align
For a given design (after considering the above parameters), one could also compare the options using DFA-index i.e. Design for Assembly Index. It is given by the below formula
DFA = 100 Nm tm / ta
Nm - theoretical minimum number of parts
tm - minimum assembly time per part
ta - estimated total assembly time
Higher the DFA, better is the design for assembly.
Taking a hypothetical example below to explain
Soldiers are frequently required to disassemble and re-assemble their guns.
Soldiers will not be using revolvers, but typically their guns are also without too many screws and fasteners. Most of parts are easily assembled using uni-direction motion and fit into one another.
Considering two revolvers here
Gun 1 - revolver with a rotating chamber for each bullet
Gun 2 - revolver with a magazine holder for bullets
Theoretical minimum number of parts in both the guns are same. Therefore Nm = 4
1. Barrel
2. Firing Pin
3. Ammunition Chamber (rotating or magazine)
4. Holder
tm - minimum assembly time per part remains 4 seconds.
ta - total estimated assembly time varies for each gun. In gun 1, it is 90 seconds because before closing you need to match the chamber with the barrel. In gun 2, it is 60 seconds.
DFA for gun 1 = 100*4*4/90 = 17.78
DFA for gun 2 =100*4*4/60 = 26.67
DFA index for gun 2 is better, therefore as a manufacturer you should go for the design of 2nd gun.
Gun 1 - Rotating Chamber Revolver
Magazine Type Revolver
P.S. - Images only for illustration