How to implement lean manufacturing at ductwork fabrication shops
There are a number of articles and books written on lean manufacturing techniques and lean construction, including many in the pages of Snips about applying lean in sheet metal shops. This article will highlight basic lean principles and tools specific to the spiral fabrication shop.
Lean’s purpose is to add value and drive out waste. Your shop exists to serve your customers — not to produce lots of metal. All fabrication plans and processes should be aimed at delivering better value to your customers. Value is a useful (aka quality) product with the least amount of cost (price paid, time and hassle). There are seven basic wastes shop owners and managers should want to destroy (See sidebar). Focusing on lean can change the way some people operate in their shops. Contractors seek to deliver products when needed and in the best way possible to enable quick installation.
Instead of turning out a lot of spiral pieces in batches to create greater shop efficiency, many contractors seek to match their fabrication schedule to meet the needs of internal installation crews or external contractor customers. This may mean less efficient operation of ductwork fabrication equipment in the shop. Overproduction is a waste you should want to attack. It causes other wastes, especially inventory, and people and material movement.
Go with the flow
“Flow” is an important lean principle. Once you start fabrication, you want the metal to flow through the fabrication steps and out to customers. While turning straight spiral duct is a good example of one-piece flow, what happens next may result in work-in-progress waiting. You do not want to stack batches of duct to be lined or cut-ins added. You want to produce enough to keep the metal flowing as required by the downstream steps. Finish jobs that are in the queue before starting more.
For spiral fittings, the process usually includes cutting all the fitting pieces at once, and then handling and sorting as they move through the various fabrication steps. These steps include welding, beading, assembly, sealing, etc. There is so much rehandling. You want flow, not batch and wait.
Inventory is another waste, though some waste is unavoidable. You want just enough inventory on hand to keep the work flowing, but not so much inventory that it has to be stored, protected, counted and moved.
The key shop metric is not how many tons of metal is fabricated per hour or day. Use measurements such as cycle time and on-time delivery. Working to improve these two measures will drive very different actions, rather than focusing on how much metal — by piece or weight — you push through the shop.
In the spiral shop, there is a lot of setup time required in preparing to turn the next piece of duct. Setup time is waste because you are waiting to create the duct. It is necessary, but still waste. Managers should study the setup process and work with the shop workers to reduce the time and steps needed to set up the spiral duct machine for each run.
Some shops have addressed this before and already improved their process, but workers change and poor practices can creep back into the process over time. A lean principle is to “go and see.” Go watch the spiral ductwork fabrication process several times. Often, tools used in the setup are poorly located and finding them eats up workers’ time. Adjustments to the machine are often repeated for the same piece of duct many times during a month. The adjustments could be marked on the equipment, so they are easily found.
Some workers have better setup techniques than others. Teach all workers to apply the same best practices. Do not try to catch workers doing something wrong, but observe the process and look for ways to improve. Ask the workers for their ideas — and listen.
The same approach should be used while setting up for special shapes, such as oval duct. Because these shapes may not be the main work of the shop — or the shop was not designed to fabricate them when built — there usually is no set space allocated to this task. Often, the worker must find free space and use a forklift to set the molds up for stretching the spiral to the oval shape. A lot of time is used in setting up that does not add value. Shop managers should especially observe how the setup takes place, looking for quicker ways to do it.
Time is lost in the method used to line duct, especially double lining and adding in cutouts. Usually the metal pieces for fittings are cut in batches and placed on carts or tables. They then wait. When ready, workers sort the pieces to find the ones for a specific fitting and assemble it. They usually assemble all the fittings before moving the batch to the next step. Other workers may then do the next operation, possibly lining the duct, and all fittings are done in batches and stacked on the floor. If there are cut-ins or add-ons, they are done in batches and, finally, the pieces are wrapped in plastic. This is also done in batch mode. Many spiral shops do it this way. There is a lot of stacking and moving of the batches, none of which add any value.
The right approach
A lean approach is to start with one fitting and work through the steps from assembly to lining, final add-on and wrapping. Because the same worker is doing all steps, he or she has greater pride in the final product.
One important side benefit of flowing one fitting at a time all the way through the steps is that any quality problems are usually discovered sooner and fixed, rather than discovering it after all pieces have been cut or assembled.
You never want to be without metal to fabricate, but you don’t want tons of it stored around the shop either. There is a trade-off in managing the inventory. A good technique for minimizing stock-outs and excess material is to use the lean tool known as “kanban.”
Kanban is a Japanese word meaning a “signal to replace.” It keeps needed inventories, especially consumables, in stock and excess to a minimum. It works for coils of metal, too.
There are many variations of how kanban works, but all are similar. All kanban techniques have a way — usually a card — of recording the information needed to reorder the inventory item. This information includes the part name, vendor number, quantity to order, vendor names and contact information. The card is attached in some way to the inventory storage area, so when the reorder point is reached, the card is pulled. Examples include:
- The min-max approach, which uses a marked or painted level to show the maximum level to fill parts. A second lower level or mark is shown marking the reorder point. The kanban card is attached to the container and pulled when the minimum level is reached.
- The dual-bin system is similar to the min-max. In this case, two bins of parts are located together. When the first bin is empty, it is removed and it contains the reorder card. The worker draws from the second bin and does not lose time doing value-added work while the first bin is refilled.
- A red/green color-code system is often used to signal time to reorder. Workers pull product from the green marked shelf or bin until empty, then they draw from the red shelf or bin, and a kanban card is used to reorder stock.
For coils of metal, the reorder level is based on how fast you normally use that metal size and the time for the vendor to respond. The card is usually taped to the coil identified as the reorder level. For example, we might have three coils of a specific size metal on hand. When we use up the first two and start on the last one, we initiate the kanban process.
Another lean tool most useful in any shop, including the spiral duct shop, are known as the “Five Ss,” which stand for sorting, simplifying, sweeping, standardizing and self-discipline. If you are not familiar with the term, read about it in a Snips article from April 2009, “Getting in shape: Five steps for mastering lean manufacturing” here: http://bit.ly/2pTNaJl.
Spiral sheet metal shops can benefit greatly from applying lean techniques. It takes willingness among shop managers to learn and apply the basic concepts and tools, and engage the workers to help.
There are seven basic types of waste commonly encountered in manufacturing.
Defects: When you fabricate and/or install the product incorrectly, you make waste. The punch list highlights the detected defects, but the source of each defect could be at any point in the fabrication and installation process. Defects cause rework, and rework always wastes resources.
Overproduction: Fabricating or ordering material too early and stockpiling material either in the shop, in a warehouse or at the job site is waste. Fabricating material at a rate faster than it can be installed also causes waste. Overproduction is the cause of most other waste, including inventory, transportation and motion.
Inventory: Any material not yet installed and being used by customers is inventory. It is also waste. This includes unfabricated material, work-in-progress and finished fabrication. It also includes spare parts, unused tools and consumables. Employees’ personal stockpiles of tools or ductwork are very wasteful. Overproduction by the shop causes inventory, no matter if fabricated parts are stored at the shop or on the job site.
Inventory is very costly, though much of the costs are hidden. Inventory ties up working capital and space. It must be controlled and continually monitored and audited. Inventory often leads to additional handling, especially around the shop.
Some inventory is usually needed to ensure that the work is performed in a timely manner to avoid the waste of waiting. This is called strategic (or necessary) inventory. While this type of inventory is still waste to be reduced as much as possible, it is necessary. Any additional inventory is excess and should be eliminated as soon as reasonable. The challenge is to differentiate between “excessive” and “strategic” inventory. Seek to get rid of the excess inventory, then seek to reduce the strategic inventory as long as it never impacts the shop’s ability to deliver product to its customers as promised.
Overprocessing: This includes overengineering, requiring additional signatures on a shop requisition and multiple handling of material. Any step in a process that is not value-added is overprocessing and is wasteful.
People movement: “Treasure hunts” happen when shop workers go looking for tools, cut metal sheets, material or information. This happens all the time. No value is added while walking around. Movement is not work. We add value with our fingers, not our feet.
Material movement or transportation: This happens when material is moved around the shop, loaded on the truck or trailer, hauled to the job site and unloaded. Though moving material is necessary, it is not value-added and needs to be minimized.
Waiting: Any time workers wait or work waits for workers, it’s waste. This happens when employees wait for instructions, inspections, change orders or for material to be fabricated.