Page 96 - How To Implement Lean Manufacturing

The Significance of Lead T ime 77

and the bottom-side batch took 1.2 minutes. Even though there were several change-
overs per batch, the changeover time was trivial since the tools were already loaded and
the operator only needed to change the program in the computer’s database.
The Assembly Cell
The completed WIP was placed in a holding area, awaiting time on the assembly line,
which had eight work stations. When all the raw materials were available, the product
would be scheduled on the assembly line.

The Analysis
The Lead Time Chart and Minimizing Lot Sizes
To begin the project, we had completed a lead time chart in the classroom, as shown in
Fig. 5-1.
In this case, a large lot was 100 units and the batch lead time was 1220 minutes, or
over 20 hours of production time. This batch of 100 was also about two months demand
for this particular cabinet. We wanted to reduce the batch size and, of course, we would
like to go to one piece flow, but one piece flow with one piece transfer lots was not
possible at this time. Their use of large multipurpose machines made this impractical.
But we still wanted to reduce the batch size. We selected a batch size of 20 because
there were 20 finished units to a pallet and they would never produce less than a pallet,
they said.
This would increase the number of changeovers for the punch press from two to ten
for the 100 unit batch. Recall that the CNC punch press was used for cutting out both
the topside assembly and the bottom-side assembly units. The CNC punch press was
operated by one machinist who handled loading and unloading, as well as did the
changeovers single-handedly.

Balancing the Assembly Cell
Next, we reviewed the operation of the assembly cell and completed a balancing study.
There were eight operators in the cell and although there were only 26 minutes of work
per unit, there was a bottleneck at station 4 lasting six minutes. Even though we only
had 26 minutes of work per unit, with 8 operators and a bottleneck of 6 minutes, we
needed 48 minutes of paid work per unit; a full 80 percent excess labor cost. Nearly all
operations were manual, so by rearranging some work we were able to balance the
work and reduce the bottleneck to 4.5 minutes. With this new constraint time, we only
needed 26/4.5 or 5.8 people. We decided that although it would be possible to use only
six operators in the assembly cell, we would start with seven; still a reduction of one
person. We modified the work stations and work instructions and were ready in the
assembly cell.

Reducing the Changeover Time at the CNC Punch Press
Next, from our prior work in the classroom, we thought there were opportunities in the
changeover times for the CNC punch press. If we could not reduce the changeover
time, we would add almost 300 minutes to the lead time. Again, in the classroom we
did a SMED (single minute exchange of dies—that is, quick changeover) analysis. Recall
that the changeover time was 36 minutes. In this 36 minutes, we found over 11 minutes
of external work; this left 25 minutes of internal work to analyze. It was just 25 minutes
of changing tools for the most part, so we acquired two additional operators and put