1) do a large batch of the first part of the assembly then repeat back at the beginning for the second part or 2) fully assemble the product before moving on to the next?
Eric Ries gives a surprising answer in his book The Lean Startup, by using the example of stuffing envelopes.
“..the one envelope at a time approach is a faster way of getting the job done even though it seems inefficient. This has been confirmed in many studies…
The one envelope at a time approach is called “single-piece flow” in lean manufacturing. It works because of the surprising power of small batches. When we do work that proceeds in stages, the “batch size” refers to how much work moves from one state to the next at a time.
For example, if we were stuffing one hundred envelopes, the intuitive way to do it-folding one hundred letters at a time-would have a batch size of one hundred. Single-piece flow is so named because it has a batch size of one.
Why does stuffing one envelope at a time get the job done faster even though it seems like it would be slower? Because our intuition doesn’t take into account the extra time required to sort, stack, and move around the large piles of half-complete envelopes when it is done the other way.
It seems more efficient to repeat the same task over and over, in part because we expect that we will get better at this simple task the more we do it. Unfortunately, in process-orientated work like this, individual performance is not nearly as important as the overall performance of the system.
Even if the amount of time that each process took was exactly the same, the small batch production approach still would be superior, and for even more counterintuitive reasons.
For example, imagine that the letters didn’t fit in the envelopes. With the large-batch approach, we wouldn’t find that out until nearly the end. With small batches, we’d know almost immediately. What if the envelopes are defective and won’t seal? In the large-batch approach, we’d have to unstuff all the envelops, get new ones, and restuff them.
In the small-batch approach, we’d find this out immediately and have no rework required.
All these issues are visible in a process as simple as stuffing envelopes, but they are of real and much greater consequence in the work of every company, large or small. The small-batch approach produces a finished product every few seconds, whereas the large-batch approach must deliver all the products at once, at the end.”