<< Click to Display Table of Contents >>

How Infinite Scheduling Works

Infinite Scheduling Program Operation

Infinite scheduling is performed through SH-F  Infinite Scheduling.  Infinite scheduling assigns Start Dates and Finish Dates to work order sequences without regard to work center capacity.  It is called Infinite scheduling because it assumes work order capacity is infinite, or unlimited.   Infinite scheduling does not change the work order Start Date and Finish Date.  These dates are manually entered or changed.  The program assigns sequence dates as follows.

The first thing the program does is calculate the number of days available for production.  The formula is:

(finish date) - (start date or today's date [whichever one is later]) - (any non-workdays as defined in the shop calendar) - (any routing lead times for outside processing sequences) = number of days available for production

 

A percentage of total production time is calculated for each direct labor sequence based on the this formula:

(total production time for all sequences) / (total remaining production time this sequence) = % of total production time

The above two results are then multiplied together to calculate this sequence share of available production days as follows:

(number of days available for production) x (% of total production time) = this sequence share of available production days

The program then assigns a Start Date and Finish Date to each sequence.  If production has not begun yet, the first sequence is assigned a Start Date equal to the work order Start Date as long as the Start Date is today or later.  If the Start Date is already passed, it assigns today's date as the sequence Start Date.

If production has already started, those sequences that are completed or partially completed will receive none or little percentage of total production time (see above) and therefore the first Start Date gets assigned correctly to the currently active sequence.  

If the sequence is a direct labor sequence, its this sequence share of available production 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.

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 Start Date (due to contention for that work center capacity), with infinite scheduling the Finish Date for one sequence is always the Start Date for the next sequence.

How to Use Infinite Scheduling

The term Infinite scheduling is misleading, because work center capacity (which is not infinite) is very much a factor when using this method of scheduling.  What it refers to is that when the sequences are scheduled to the work centers, the program assigns start and finish dates to work order sequences without regard for other work orders that are contending for that work center capacity.  In a sense the program is assuming infinite work order capacity (or, more accurately, is ignoring it), which is where the term "infinite scheduling" comes from.

Work Order Start & Finish Dates Are Manually Assigned

When you assign the initial work order Start Date and Finish Date, you should attempt to assign realistic dates, meaning those that you believe can be met either with existing capacity, or, if necessary, by adding overtime or taking other measures.  These initial dates are based on your feel for what the factory can deliver, or they may be dictated by your customer as a condition in giving you the order.

After you create your work orders, you run SH-F  Infinite Scheduling.  You then study your work centers using SH-I Print Work Center Schedule and 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 finish dates as required.

Sequence Dates are Time Windows

Infinite scheduling and finite scheduling both work within the limits of available work center capacity.  The difference is that finite scheduling calculates and sets work order finish dates, whereas with infinite scheduling these dates are set by you, the scheduler. Finite scheduling is much more precise in that it provides routing sequence start and finish dates that correspond to estimated run time, whereas the sequence dates assigned by the infinite scheduling program do not.  Instead, the start and finish date ranges provided by infinite scheduling function as "time windows" within which you should complete the sequence.

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 Functions as the Due Date

Unlike with finite scheduling, the work order Due Date is not important with infinite scheduling.  Because the work order Finish Date does not get changed by the infinite scheduling program, it essentially functions as the due date.  With infinite scheduling usually the Finish Date and the Due Date are entered with the same date and the Due Date field serves as a reference only.

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 infinite 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 infinite 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 infinite scheduling and MRP together.  You will run the infinite scheduling program, evaluate your work centers, make 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

Infinite 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.

If you want to manually assign this type of work center to specific machines, you can 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.

In summary, with infinite scheduling you manually assign work order start and finish dates, run the scheduling program, then study your work centers to see if you can meet your schedule.  If not, you must expand your capacity or move out due dates until the schedule fits your factory's ability to meet it.

Programs Used with Infinite Scheduling

The following are the programs that are most commonly used in conjunction with infinite scheduling.

 

SM-H  Enter Shop Calendar

Use this program to mark weekends and holidays as non-workdays so that those days will be skipped by the infinite scheduling program when it assigns routing start and finish dates.

 

RO-C  Enter Work Centers

Use this program to initially set up your work centers and to change the Total Hours/Day field whenever the work center capacity changes. You can also change work center parameters at any time through SH-C, Manually Schedule Work Centers.

 

WO-A  Enter Work Orders

You can use this program to change the work order Quantity to Make, scheduled Start Date, scheduled Finish Date, or Due Date.

 

SH-A  Edit WO Start/Finish/Due Dates

Use this program to make changes to work orders' scheduled Start Date, Finish Date, or Due Date.  This is an alternative to using WO-A, Enter Work Orders, for the same purpose.

 

SH-B  Manually Schedule Work Orders

Much like SH-A (see above), you can use this program to make changes to work orders' scheduled Start Date, Finished Date, or Due Date.  This is another alternative to using WO-A, Enter Work Orders, for the same purpose.  This program also serves as an inquiry into the status of work order routing sequences.

 

SH-C  Manually Schedule Work Centers

You can use this program as an alternative to RO-C, Enter Work Centers, for changing work center parameters such as Total Hours/Day.

 

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-F  Infinite Scheduling

This is the infinite scheduling program.  See the Infinite Scheduling Program Operation section above for details.

 

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.

 

SH-H  Print Work Order Status

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.

 

SH-J  Print Machine Schedule

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.

 

 

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.