single piece flow

[email protected]

Par 100 posts (V.I.P)
1. What is it?

a. Single piece flow is the ideal state where parts are manufactured one at a time, and flow throughout the manufacturing and supply chain as single unit, transferred as customer’s order.

b. Manufacturing large batches of parts simultaneously, or accumulating parts in a bin for shipping or transferring 2 or more parts at the same time is opposite or contrasted to the definition of Single Piece Flow.

c. Single Piece Flow (SPF) supports Just-in-Time, Toyota Production Systems, Lean Manufacturing, Theory of Constraints (Drum, Buffer, Rope), and similar types of philosophies and systems

2. How Does it Work ?

a. Batch sizes are recorded for historical system (baseline).

b. Optimum batch size and transfer sizes are calculated, starting with:

i. The most critical work centres
ii. The largest inventory carrying costs
iii. The highest risk processes
iv. The most unpredictable process
v. Other controlling factors

c. Action is taken for improvement at the work centres, rules, methods that have the greatest impact on the throughput, customer satisfaction, risk, cost, or inventory carrying charges. These actions can include:

i. SMED
ii. Kanban
iii. Process re-design
iv. Production sequence
v. JIT
vi. Etc.

3. Resources Required
a. Process Map
b. Calculator
c. Process comprehension
d. Value mapping
e. Authority to make process improvements
f. Process operating date (ie. flows, batch sizes, inventory, etc.)
g. Lean process understanding

4. Symptoms indicating that it is needed

a. Long delivery lead times (customers will consider paying more to defeat them)
b. Obsolete inventories
c. Large batches of defects & rework, all with the same or similar defect.
d. High product velocity ratios (3 is excellent, 60 is poor)
e. Low inventory turns (100 is excellent, 3 is poor)
f. Slow changeovers
g. Left-overs at beginning & end of production run

5. Example/Case Study

a. Penny Lane game


Penny Lane Game



1 Start
















2 3 4 5 6
One piece flow (continuous flow) - lean manufacturing process to eliminate buildup of inventory and queues, to eliminate waste and make it easier to identify the source of problems quickly.
Takt time is used to synchronize the sub processes needed to produce one completed product (for example, a completed car). Thus allowing pieces to flow from one sub process right into the next without creating backlogs of completed product waiting to be used by other parts of the process. Obviously for this to work the overall manufacturing process much be designed to allow flow to proceed properly: WorkCell Design and Layout.
One piece flow is a concept that really requires systemic change to a traditional batch processing setup. Some management improvement concepts can be tried without too much change to the system required. One piece flow does not offer such opportunities. To try it on a small scale you can find some process that can be completely redesigned from start to finish. This allows learning on a small scale before adopting it for large processes.
EXAMPLE:
1) SPF concept can be misleading at times. If you try to implement it right away, you will fail most likely.

What I would suggest is that you should set single piece flow as an ideal and start working by reducing your batch size by half every time untill all your processes becomes tightly coupled with no inventory/WIP in between. Flow in very small batches without queues is what I have always tried to achieve in regards to single piece flow.

2) My company practices "continuous flow" within a cell. For example, we have two cnc lathes, one VMC and three gear shapers in the same cell. Rather than run the first lathe op, then the second, then the mill, etc., we set up all the machines and run the parts continuously through the machines. That way if there is a problem with the first part, we catch it right away and fix it with maybe only one scrap part. Also, even though the part has 12 min. of machining with four operations, we balance our run times to 3 min. ea. That way the cell pumps out one piece every three minutes, rather than batching at each machine. If we batched, the total manufacturing time for 100 pcs would be 100 pcs x 12 min or 1200 min. plus setups. This way the time for 100 pcs is 12 min + (3min x 99 pcs) or 309 minutes plus setups.

Advantages: catch quality issues faster, faster overall manufacturing time, therefore faster lead times, therefore fewer past dues, smaller lot sizes, more inventory turns.
3) "Does 1 piece flow really mean 1 piece flow? so producing lets say 10 items in every cell and passing it(crate of 10 items) down to the next cell would not be considered 1-piece flow? Or would each cell send down 1 unit as soon its cycle is complete? Does that really make sense? all the time lost in travel? Or is this what conveyor belts are made?"[\color]

To answer your questions:
Yes
Correct
Yes
Yes
To have a correct "lean operation" using one piece flow travel needs to be minimized or it won't work.
Setups also need to be minimized. Setups correlate to batch size. Short setup=short batch.

My company has some one piece flow, some batching. The one piece flow is one person handing the part to the next person, or having one-part of inventory in between. not much travel maybe a step or a short piece of conveyor. Conveyor does not always support one piece flow, because as a wise man once said "If conveyor is there it will be full of parts, so keep it short".

To understand this principal in simple language along with lean in general read "The Goal". I forgot the author's name. It's an easy read.



One Piece Flow - Magic or Myth?
or... Selecting The Transfer Batch

One Piece Flow refers to the concept of moving one workpiece at a time between operations within a workcell. At the opposite extreme, we might process an entire batch or lot at each operation before moving it to the next operation.
This idea has many benefits. It keeps WIP at the lowest possible level. It encourages work balance, better quality and a host of internal improvements.
We often hear One Piece Flow pronounced as an absolute must for any workcell. Like the search for the Holy Grail it is taken as a moral imperative. At Strategos, we find it more helpful to think of this question as the "Transfer Batch" or "Internal Lot size." The issue is: "What Internal Lot Size will help the cell meet its performance goals."
To approach engineering design using slogans, edicts, or other grand pronouncements is a dangerous business. Yet many teachers, consultants and practitioners do it. Here are some of the more common edicts:
• Inventory is evil and must be eliminated regardless of all other considerations.
• The only relevant measure of work-cell performance is throughput time.
• Machine utilization is of no consequence and should be ignored.
• Work-cells must have a straight-through flow for all products.
• Work-cells must have one-piece flow.
• Low-tech, manual machine tools are superior to high-tech NC equipment.
Performance goals may vary from one cell or factory to the next. For some, Quality is the foremost measure of performance. For others, delivery speed and customer response dominates. For others, labor efficiency is foremost. Where equipment is expensive and capital scarce, equipment utilization may play a role, although it is often overrated
The question of Internal Lot Size is just one of many decisions that specify the workcell design. Every engineering design is a series of decisions, made in a logical sequence and often interdependent. For more on this, see our page on Rationalized Workcell Design.
One Piece Flow is an ideal that engineers should strive for. But, it simply does not work when the transfer time begins to approach the work time. Nor does it work with certain processes such as shot blasting.
When a cell must address a wide product variety with varied routes, work times, and setup times, One Piece Flow is also counterproductive. This often occurs in jobbing-type machine shops or sheet metal shops.
In situations such as those mentioned above, small-batch flow can be a good answer. For example, the lot size required by a customer might be 100 units of product. This is the "External Lot Size." The optimum "Internal Lot Size" or "Transfer Batch" might be 10 units.
In operation, the first operation in the cell makes 10 units of the 100 and passes the 10 units to the second operation. The first operation then makes 10 more units and passes them along. This continues until the External Lot of 100 is complete.

The Hidden Hazards of One Piece Flow
Precision Aircraft Engine Castings
An investment foundry manufactures small parts for aircraft and stationary turbine engines. The technology is difficult, and unpredictable. Much of the equipment is large scale. There are about 4000 active part numbers with wide variations in routings and work content.
Several years before, some of their engineers had attended a large-scale Kaizen Blitz. These engineers had accepted the usual dictums about "One Piece Flow" and "Sequential Arrangements". But, the complex product and process mix had baffled all attempts to put these dictums into practice. As a result, nothing had been done towards implementing workcells.
A Strategos consultant implemented the first workcell within a few days. It functioned well with impressive results.
Subsequently, the firm's engineers attempted to implement a workcell in different area with very poor results. What had gone wrong with this second cell?
The primary issue was "One Piece Flow". The engineers had accepted this dictum quite literally. But the product and process did not lend itself to One Piece flow. Operation times were very short, about 15-20 seconds. The parts were very small (about 1-1/2"). One process step, shot-blast, required a large number of parts in order to function properly. Compounding this was the excessive distance between machines.
Operators spent more time walking parts than they did processing the parts. The shot blast machine was eating itself alive because the blast impinged on the machine parts rather than workpieces.
When inquiries were made about why only a single piece was moved, the answer was, essentially, "Because that fellow from Nagoya told us we had to do it that way."
One Piece Flow is an ideal that engineers should strive for. But, it simply does not work when the transfer time begins to approach the work time. Nor does it work with certain processes such as shot blasting.
The people involved had great difficulty in reconciling their deeply held faith in "One Piece Flow" with the realities of the situation. The problem was resolved semantically.
The word "piece" was simply redefined as 20 castings . Small carriers were built to carry the 20 castings or in Newspeak, the single piece. In addition, queues were setup to allow accumulation of a reasonable quantity for shot blasting. In this way, each transfer time was amortized over 20 parts instead of just one and the shot wheel had something worthwhile to blast on. The cell began to function effectively.
This project was the genesis of Mr. Lee's subsequent works that bring order, structure, and rationality to the cell design process.
Batch vs Single Piece Flow...in the mind
BATCH vs. SINGLE PIECE FLOW....IN THE MIND

It was an annoying task. Seemingly simple, but I've avoided it all week. As such, it sat there, as unprocessed "inventory" that could be done and helpful to others. But I just kept avoiding it.

I finally sat down and got it done this morning. And realized in it was a principle I know about but haven't applied to this corner of effectiveness.

In the Lean Community, we rail against the "batch and queue" model of production, promoting instead the virtues of "single piece flow." It is correct to do so. Yet how deeply do we apply this passion??

I viewed this task as a "batch," which in fact it was. The single task actually involved multiple steps to get done. Look up this amount. Speak with that person. Get this form signed off. Update a database. Confirm entry. Check the accuracy. Inform the requestor the task was complete.

Yet, I listed the task (and, more importantly, though of it) as a single event. And this "batching" of the task blocked the flow of task.

David Allen in his most excellent book "Getting Things Done" describes the solution to this blockage this way:

"Now write down the very next physical action required to move the situation forward. If you had nothing else to do in your life but get closure on this, where would you go right now, and what visible action would you take?"

Note the Lean principles here. Write it down. Make it actionable. Do it now. Be visible.

I did this. It got done in 8 minutes. A batch that stuck on my back all week took a mere 8 minutes to complete. Once I started thinking about it.

The Batch turns into flow. And gets done, adding value.




One Piece Flow
August 1, 2005
One-Piece flow is one of the most important principles of lean manufacturing. Yet, many people still do not understand what it truly means to achieve one-piece flow. Let us begin by discussing terminology. There are several basic terms used to describe one-piece flow. The most common are as follows:

One Piece flow

Single Piece Flow

Continuous Flow

Make One - Move One

Flow Manufacturing
Each of the above terms describes the same key element of the Toyota Production System, illustrated in the diagram below. As you can see, "Continuous Flow" is a shown as a key element of the pillar, "Just-in-Time."

Thus, one-piece flow is a tool that will help a manufacturer achieve true just-in-time manufacturing. That is, the right parts can be made in the right quantity at the right time. In the simplest of terms, one-piece flow means that parts are moved through operations from step-to-step with no WIP in between either one piece at a time or a small batch at a time. This system works best in combination with a cellular layout in which all necessary equipment is located within a usually U-shaped cell in the sequence in which it is used. To achieve true one-piece flow, equipment must have basic stability:
 
Top