Today we discuss interchangeability of parts.  This may sound trivial, but you probably would not even consider replacing your food processor blade with your lawnmower blade.  It would be obvious that these are not interchangeable.  However, there are times when a part needs to be replaced or design is reworked altering the composition, that is the constituent parts of the product or design.

Let’s take an example and walked through it.  Consider the ignition key switch to your car.  It has a specific shape, mounts and attached with specific mechanisms, and performs or initiates many actions of the vehicle.  This is often referred to as form, fit and function.  Let’s consider the old ignition switch is going out of production from your supplier.  We would need to find another solution.  As an engineer we would explore alternative or replacement parts.  If we want to minimize costs, we would look for another ignition switch that forms, fits and functions identically to the previous one as this saves time to sort out the differences and modify either the switch the interfaces or the system.

For a truly dropping replacement we would require no change to the part, the interfacing parts, and the ability of the product to meet the system requirements.  The old part or the new part would meet the requirements of the system.  But there is much more to this than just form, fit and function.

There’s more to this function than the contact being made or broken based upon the key switch position or the next position in dickey place.  Specifically, the new switch may have subtle differences which will impact the performance, and certainly the new switches manufacturing processes are likely different.  When parts are assumed to be interchangeable we will likely also assume them to react identical to stimuli.  This assumption can be costly and even dangerous.

Let’s move on to discuss other parts of the function of the key switch.  Let’s consider that the switch we will call old, will support or hold a certain amount of force we will call N.  This means that the key switch requires a force greater than N to move the switch from one detent or switch position to another.  The new switch is not so stiff, it takes a force less than the old switch to move from one detent to another.  Are these switches equal?  No, they’re not, but that may not matter.  It is up to the engineer under the auspices of configuration manager to understand the impact of these different forces and these two ignition switches.  The switch with the greater force requirement is also able to better handle the key load variation especially the higher mass loads.  Meaning when put in a position the key in the ignition switch will tend to stay in that position because it takes more force to move it from that position.  Knowing this enables an informed decision regarding drop in replacement.  In this situation it would behoove the engineer, configuration management, and the organization at large to understand the implications on the system of an ignition switch what requires less force to transition from one detent to another.  The way to understand this would be to perform testing on this new switch to identify the impact of this reduced force on the entire system.  So, this drop-in replacement, at this point does not appear to in fact be a drop-in replacement. However, it is possible this difference means nothing, and the new part can supersede the old.  To determine the impact of this seemingly minor difference can be determined with simulation, calculations and testing. However, it cannot be accomplished by assumptions.