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How Lead Time Scheduling Works
Lead Time Scheduling Program Operation
Lead Time Scheduling is performed through SH-P Lead Time Scheduling. You can also use Lead Time Scheduling when creating Work Orders in WO-A Enter Work Orders, SO-N Convert Sales Orders to Work Orders or ES-E Convert Estimates if you have set the default in SD-B Work Orders Defaults to use lead time scheduling when creating Work Orders. Lead Time scheduling assigns Start Dates and Finish Dates to work order sequences sequentially either forward (generate a finish date based on start date plus process time) or backwards (generate a start date based on due date and process time). The program assigns sequence dates as follows.
The first thing the program does is calculate the number of days required for production. The formula is:
(Sequence Quantity to be made) x (Time per part) + Setup Time + Positive Overlap + Work Center Queue Time {for all Labor sequences} + Outside Process Sequence Lead Time {for all Outside Processing sequences} = number of days required for production
The number of production days required for a given labor sequence takes the Work Center shift hours and percent utilization into account.
The program then prompts whether to use forward or backward scheduling. If backward scheduling is chosen, it next prompts whether to schedule based on the current estimated Finish Date or the Due Date.Next, for backward scheduling, the available production days between today and the specified Finish Date is compared to the number of days required for production. If there are not enough days to schedule the remaining sequences of the work order, the earliest possible Finish Date is suggested along with a Start Date of today. Otherwise, a Start Date is calculated based on the Finish Date specified and the number of production days required. If a Start Date is specified, the Finish Date is calculated based on the Start Date and the number of production days required. All the date calculations performed will take the non-production days specified on the Shop Calendar into account. Once the Work Order Start and Finish Dates have been specified, each sequence is assigned Start and Finish Dates based on the quantity to be made and the time per part, plus any setup time, overlap and queue times.
If the sequence is an outside processing sequence, the sequence Lead Time (which is always expressed in days) is added to the sequence Start Date, which determines this sequence's Finish Date (skipping over any non-workdays defined on the shop calendar). The Finish Date of this sequence becomes the Start Date of the next sequence.
Each sequence is scheduled in succession as described above. The Finish Date of the last sequence always equals the work order Finish Date.
Unlike finite scheduling where there are usually gaps between one sequence's Finish Date and the next sequence's Start Date (due to contention for that work center’s capacity), with lead time scheduling the Finish Date for one sequence is always the Start Date for the next sequence. The Queue Time represents the estimated contention but is included in the total days a sequence is scheduled for a given work center. Queue times are entered in RO-C Enter Work Centers for Priority 1, 2 and 3 which relates to the Priority of the Work Order being scheduled.
How to Use Lead Time Scheduling
Lead Time Scheduling is most often used as a tool to establish realistic start and finish dates when creating work orders but it can also be used to reschedule work orders after they have been created, especially when conditions change such as Work Center Queue Times or shift hours. If the Use Lead Time Scheduling is set to F or B (Forward or Backward) in SD-B Work Orders Defaults then the programs that create Work Orders will use this lead time calculation logic rather than the fixed lead time specified in the inventory master when creating work orders.
Backward or Forward Scheduling can be Performed
When you assign the initial work order Start Date or Finish Date, the program will calculate the other, either forward or backward. If the Finish Date entered does not allow for enough time to complete all the sequences, a later Finish Date will be suggested based on a Start Date of today.
After you create your work orders, you can study your work centers (via SH-I Print Work Center Schedule or SH-R Work Center Scheduler to see if you have the capacity to meet the commitments you've made. If you've overloaded all or some of your work centers, then you must respond by adding overtime, personnel, plant capacity, or, if you cannot expand your capacity, you must reschedule work order dates as required.
Sequence Dates are Time Windows
Because sequence start and finish dates are time windows, your production foremen have leeway as to which work orders to run on any given day when following SH-I Print Work Center Schedule. As long as a sequence is completed within its time window, the overall work order is still on schedule.
The Finish Date may not equal the Due Date
In lead time scheduling, the Finish Date and Due Date may not be the same. If forward scheduling, the estimated finish date may be before or after the due date. If backward scheduling, the finish date and due date will be the same unless a new finish date was calculated because there were not enough production days available to meet the due date.
Scheduling Works Hand-in-Hand with MRP
Scheduling works hand-in-hand with the Material Requirements module, especially if you have products with multiple levels in their bills of material. Every time you reschedule your work orders, those changes may affect other purchase orders or work orders that are interdependent upon each other. These changes may trigger the MR-F Generate Material Requirements program to produce various EXPEDITE and DELAY messages to advise you of situations where material won't arrive when needed or arrives too early due to scheduling changes you've made.
Be aware that lead time scheduling only schedules work orders. Purchase orders, which must be synchronized with work orders to see that material arrives when needed, are scheduled solely by MR-F, Generate Material Requirements. So in most situations lead time scheduling and MRP will both be utilized, except for companies with single level products and simple material requirements (such as some machine shops, for example).
Typically you will evolve into a cycle of using lead time scheduling and MRP together. You will use the scheduling program, evaluate your work centers, make and needed changes to work order dates and work center capacity, then run MRP to see the effect of those date changes on other orders.
Parent-Child W/C Scheduling is Not Supported
Lead time scheduling does not offer parent-child work center scheduling like finite scheduling does. Therefore, when you set up a work center that is comprised of several interchangeable machines or workstations, you must set it up as a single work center. The Total Hours/Day (as entered in RO-C Enter Work Centers), should be equal to the combined daily hours/day of all the machines or workstations and the Total Shift Hours should be the number of hours per day the work center is staffed (typically 8, 16 or 24 and never more than 24).
If you want to manually assign this type of work center to specific machines, you can do so through SH-D Manually Schedule Machines. These machines are not work centers, but are machines set up through RO-D Enter Machines. You can use SH-J Print Machine Schedule for a daily dispatch report to the shop.
Programs Used with Lead Time Scheduling
The following are the programs that are most commonly used in conjunction with lead time scheduling.
Use this program to mark weekends and holidays as non-workdays so that those days will be skipped by the scheduling program when it assigns routing start and finish dates. The shop calendar MUST be entered before the Work Order Default setting to use Lead Time Scheduling is set to Y.
Use this program to initially set up your work centers and to change the Total Hours/Day field whenever the work center capacity changes and edit Queue times as load conditions change. Only the Average Queue time is used. You can also change work center parameters other than the Queue time at any time through SH-C Manually Schedule Work Centers.
You can use this program to change the work order Quantity to Make, scheduled Start Date, scheduled Finish Date, or Due Date. If the Quantity to Make is changed, you must specify whether to retain the Finish Date and recalculate a Start Date or retain the Start Date and recalculate Finish Date. If the work order is already started, then the Finish Date will be recalculated. If you do not change the quantity and change either the scheduled Start or Finish Date, the other will be recalculated.
SH-D Manually Schedule Machines
If you have work centers comprised of interchangeable machines and wish to manually assign work order sequences to specific machines, you can so through this program. Machines are entered in RO-D, Enter Machines.
SH-G Print Work Order Schedule
This report provides a listing of all your open work orders sorted either by scheduled Start Date or scheduled Finish Date.
This report can be used to see the status of each work order, broken out by its routing sequences. You can see which sequences are late and how many days late they are.
SH-I Print Work Center Schedule
This report is most typically used as a daily dispatch report by the shop foreman as guide to which work orders are to be worked on next within each work center.
If you've assigned routing sequences to specific machines (see SH-D, Manually Schedule Machines, above), you can use this report as a daily dispatch report for the shop.
This is the lead time scheduling program. See the Lead Time Scheduling Program Operation section above for details.
This is the graphical scheduling tool displaying the operations by work center in a Gantt Chart type layout.
MR-F Generate Material Requirements
This program takes the current work order schedule and produces various EXPEDITE and DELAY messages to advise you of situations where material won't arrive when needed or arrives too early due to scheduling changes you've made. The program also suggests new work orders and purchase orders that may be needed.
MR-H Print Order Action Report
This report can be used to get listings of the EXPEDITE and DELAY recommendations made by MR-F, Generate Material Requirements, that you can use to change work order start and finish dates accordingly.