How to Use Three Types of Time Buffers in Production Scheduling

Time buffers are one of the quieter levers behind reliable production scheduling. Here's why the standard advice gets them wrong for a job shop, and where they actually work.

A buffer is a safety margin built in to protect your ability to deliver on time when something happens. And on a shop floor, it inevitably does: a machine goes down, a part turns up late, a job runs slower than planned, or work backs up waiting on a busy station.

There are three ways to build in that protection: extra material, capacity or time. Time buffers are the least understood of the three, and the hardest for job shops to implement well.

Why are the old rules of thumb for safety buffers out of date for job shops?

The standard advice for time safety buffers doesn't work for job shops or fabricators. It was designed for manufacturers with long runs, repeat parts and predictable flow.

The Theory of Constraints (1990) originally recommended dividing average job lead time by five, then adding 20% extra time as protection against delays. A later recommendation was even higher: add 50% to manufacturing lead time.

But this blanket approach creates problems. You don't want to just drag out your lead time by 50%.

That creates too much work-in-progress sitting around slowing things down. Or having such a relaxed lead time lulls operators into thinking there's no rush, since the job isn't due for ages yet.

The concept has now evolved thanks to the flexibility of automated scheduling. It's a precise operation to add and cut time in the right places.

How do the three types of safety buffers work together?

There are three ways to protect your schedule from the unexpected:

1. Material buffers are stock, like extra steel or spare consumables, held just in case. They're ready the moment you need them, but they tie up cash sitting on a shelf.
   
   
2. Capacity buffers are spare people or machine time, kept in reserve. They're flexible, useful for whatever job comes in, but accounting treats idle capacity as a straight cost, and someone has to approve the overtime to use it.
   
   
3. Time buffers are additional planned lead time built into the production schedule to minimise the effects of variability and uncertainty. They don't tie up cash or need sign-off, but push them too far and lead times stretch and deadlines start losing their meaning.

None of these buffers work alone. Research into how manufacturers actually use safety buffers found that better protection usually comes from combining two buffer types together. They are mutually dependent, so using one type of buffer will reduce your reliance on another. For example, giving yourself more lead time can reduce the need for additional capacity.

How should manufacturers properly set time safety buffers?

Automated scheduling builds safety buffers into the standard work flow. Every step in a mapped digital production process is its own independent task, with customisable buffer time settings. Every job gets the right amount of flexibility at the right moment. Because you're tracking on-time status and completion times, you know exactly where buffers should be added, reduced, or eliminated.

The problem with manual scheduling is you can't fine-tune shop floor operations, so manufacturers are stuck guessing or using rough percentages of overall lead times as guidelines. 

What types of time buffers should job shops use in their production scheduling and how to implement them?

There are three kinds of time buffers, each with a different purpose and employed at different points in the production cycle: inbound safety, queue and outbound safety lead time buffers. You add these time safety buffers at strategic points in your process: when ordering materials, during critical queue points and between completion and delivery. The goal is to ensure production flows smoothly, so it can recover from any hiccups and orders get to clients on time.

Each buffer type protects a different point in the job: the start of production, the process itself, or the promise to the customer; though ultimately, all work to ensure due date performance.

1. Inbound Safety Lead Time Buffer

Purpose: Covers two situations. Material bought for a specific job, timed to arrive before that job's due to start, so you're not paying someone to stand around waiting on a delivery. And general stock you keep on hand, common materials and consumables, reordered before it runs low, so a job never stalls because the shelf's empty.

How to Use It: For job-specific material, add a bump of time before the job's due to start, enough to cover ordering, delivery and any hold-ups from your supplier. For general stock and consumables, set a reorder point, a stock level that triggers a new order automatically, so you're never caught waiting until the shelf's empty.

In Smart Shop Floor, this isn't guesswork. The system checks your bill of materials against your ERP and flags anything missing or running low, whether that's a part ordered for a specific job or general stock due for reorder. Purchasing gets the warning early enough to act, so the job doesn't stall waiting on a delivery that should've gone out weeks ago.

2. Queue lead time buffer

Purpose: Protects delivery lead time, and cushions against internal disruptions like unplanned downtime, quality defects, or labour shortages.

How to Use It: Add a bit of additional time for a job to wait at a work centre before setup. Think of it akin to showing up to an appointment 10 minutes early. This gives operators a wider time span to manage their work. It's a best practice to add a queue lead time buffer directly before your bottleneck, this way work is always available and your capacity-constrained resource never starves.

In Smart Shop Floor, every product has its own custom process flow. You can build in whatever queue lead time is needed. Perhaps a job needs 10 minutes of move time, some additional pick time or even 24 hours of drying time at specific points.

3. Outbound safety lead time buffer

Purpose: Protects delivery dates if lead times run long or if there is insufficient or variable capacity.

How to Use It: When planning, assign a time buffer before client delivery. This way, if a job takes longer than expected, you'll still make the deadline.

In Smart Shop Floor, you can set up customised outbound lead time buffers. For example, you schedule Job G to be completed 24 hours before its due, to give yourself ample time for Q&A, packing and shipping. The job can also be prioritised, so that despite whatever else may happen, you can rest assured that Job G will get done the day ahead for reliable delivery.