Job Shop Scheduling using Drum Buffer Rope

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Job Shop Scheduling | Machine Shop Scheduling

DEFINITION:  Job shop scheduling is the processes and system for determining which jobs should run in which order.Job Shop Scheduling Whack A Mole

That’s easy to say but can be very difficult to do in many custom job shops and machine shops.  Job shop production scheduling often has to deal with constraints or bottlenecks that move week to week or even day to day.  This moving bottleneck often occurs due to the high mix low volume nature of job shops.  Low volume high mix can mean that the constraint is moving based on the mix of work at any point in time.  Some weeks we have more turning work, other weeks more mill work and other weeks we’re heavy on grinding.

This roaming constraint makes it difficult to apply “off the shelf” Theory of Constraints drum buffer rope because the constraint does not stay in one place.  It’s difficult to leverage a moving bottleneck. A constraint is any resource who’s capacity is less than the demand placed on it.  The assumption when applying Drum Buffer Rope (DBR) is that the constraint(s) is stable.  And this is often true for the traditional production environment but typically NOT true for high mix job shops and machine shops.

There are similar issues when trying to apply lean scheduling to a custom job shop.  How do you determine a takt time when jobs may or may not repeat and the mix is constantly changing?

Common approaches to the job shop scheduling problem include job shop scheduling software and complex optimization algorithms. However, most software solutions add a lot more work for a schedule that seems to be always out of date due to the fact that they can not take into account all the challenges a high mix low volume job shop needs to deal with.  (See the 9 Challenges report for more information.)  As for optimization algorithms — these are typically run in academic environments but in production environments, they need to be constantly run and rerun.  So most shops struggle with chaotic schedules that constantly need to be updated and micromanaged.

Job shop software and optimization algorithms do NOT take into account variability.  A schedule is created using the due date along with the estimated internal process times and estimated outside process times.  These estimates are just that – estimates.  And most estimates will be off by at least a little in one direction or the other.  This, of course, means that the schedule is off.  To counteract this most job shop software and machine shop software allows you to add buffers or queue times.  These have the net effect of adding more variability.  And this is on top of machines going down, employees not showing up, and rest of the 9 Challenges jobs shops are constantly dealing with.  Thus, scheduling is a huge, never ending challenge consuming massive resources.

Adding buffers and queue times can also get your priorities out of whack.  For example if you have 2 jobs, one due sooner than the other and both are using buffers in an attempt to make the schedule more accurate.  And one of the buffers was too long relative to reality, that means that the job was released BEFORE it should have been.  If the difference between the buffer added and reality was enough to change the release order of this job relative to other jobs, it was not only released before it should have been but out of priority order relative to other jobs.  On the surface this doesn’t sound like a bad thing, but consider this.  If a lower priority job is running, it is consuming your limited capacity.  That means that you may not have enough capacity for the higher priority jobs that are due sooner.  So not only is your schedule out of date, but your due date performance may suffer as well.

Job Shop Scheduling Whack-A-Mole

Chasing the schedule is playing scheduling whack-a-mole!

Velocity Scheduling System (VSS) is a visual, manual job shop scheduling system based on drum buffer rope that will stop the Whack-A-Mole scheduling.  It includes the process and systems to determine which jobs should run in which order.  We take into account the due date, the internal process times, cure times, setup times, and any outside processing that needs to be done.  It was designed for the most complex job shops and machine shops.

To determine if VSS will work in your shop, watch the webinar.  Then sign up for a free private strategy session to discuss your unique shop.  Velocity Scheduling System is ONLY for custom job shops and machine shops struggling with chaos, lead-times and due date performance.  VSS works with any ERP system or no ERP.

By Dr Lisa Lang

This article is copyrighted by Science of Business, Inc.

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