The adjusted time is usually a
combination of the time required to set up a machine for production,
,
and the time required to actually perform the operation, .
The net effect of these two components depends on the number
of units processed for each setup, the lot size *s*.
For purposes of estimating resource usage it is sufficient to
allocate the setup time to the individual unit of product as
To illustrate consider operation A1 with processing
time 3, setup time 120 and lot size 30. The adjusted operation
time for these parameters is 7. It is shown in the *Adjusted
Time* column (M) of the Excel display. Columns M through
Q are interesting for systems with scrap and/or proportions.
For this example with no scrap and all proportions equal to
1, the columns are shown but do not show any variation. The
*Operation Flow* column (R) holds the flow through the
operation expressed in minutes. Since the flow of finished goods
is 60 per week and the number of minutes per week is 2400 minutes,
the operation flow through each operation is 0.025 per minute.
The *WIP* column shows the work-in-process at each operation.
For processing operations the WIP is:
WIP = (adjusted time)*(flow rate)*(lot size)
For operation A1 this is: WIP(1) = (7)*(0.025)*(30)
= 5.25 units
The total WIP for this process is the sum of the
WIP values for the operations. This sum is computed in cell
K3 as 43.5. The *Throughput Time* is computed from Little's
Law as
*Throughput Time* = (*System WIP*)/(*Flow
Rate per min*) = 43.5/(60/2400)) = 1740 minutes
This value is computed in cell K2. The computation
of WIP and throughput time make the assumption that materials
are processed and transferred in lots.
Similar results are computed for each product. |