Lean implementers: don’t forget engineering!

Just about everybody says that the involvement and personal engagement of top management is the main challenge in Lean implementation. “Key to the success of lean manufacturing,” said the keynote speaker at an industry association meeting in Santa Clara, CA, “is that the leadership team needs to fully buy into the method and remove workplace obstacles so that employees can achieve results.” While he didn’t say it, I am sure his audience heard that management doing as he says is all it takes to implement Lean successfully.

In an informal survey taken recently by blogger Vivek Naik among his readers, no one mentioned insufficient mastery of the engineering and management tools of Lean as a cause of failure. The existence of tools is generally recognized, but most consultants and implementers take them for granted. They are assumed to be simple, widely known and not new. Some, like Bill Kluck, even call the tools “trick shots.”

When you read that the details of Lean tools are widely known, you wonder where and by whom. How many manufacturing managers or engineers do you know who understand heijunka, cell design, work-combination charts, SMED,  the proper use of andons, mistake-proofing, or jidoka? Considering that these tools are the results of 75 years of development at Toyota, and that most of the Japanese literature on Lean is about technical content, I find this dismissal cavalier, to say the least.

It does not happen in other human endeavors, like building a world class soccer team.  Top management commitment is obviously required, but no one would claim that it is sufficient. You don’t hear of dribbling, passing, shooting, receiving or throw-ins as low-level skills that everybody has anyway and that you don’t need to focus on. Soccer teams actually train relentlessly to develop and maintain these skills, and everybody involved, even the fans, fully realize their importance and admire the star players for their mastery.

To understand the issues, and remedy this situation, I would like to dive deeper into the following topics:

The engineering dimension of Lean, and the other dimensions

So why is it different in the competitive game of manufacturing? For one, it is more complex than soccer, and few people have a holistic view of it. One who does is my colleague Crispin Vincenti-Brown, and he has identified four dimensions to this game, and you must pay attention to all if you want to win. They are as follows:

  1. The engineering of production lines.
  2. Logistics and production control.
  3. Organization and people.
  4. Metrics and accountability.

4 dimensions of manufacturing

In the US, the Lean movement has ignored the engineering dimension. Logistics receives some attention, but Lean programs are overwhelmingly focused on the last two: organization and people issues, and metrics. It is out of balance, and I believe this is a key reason for Lean programs to fail.

Lean implementers, whether employees or consultants, come from a variety of backgrounds. In the US, few are engineers. You see MBAs, psychologists, marketing people, and the occasional cognitive sociologist and defrocked priest. There is nothing wrong with having all these different perspectives, as long as they don’t blind you to the whole picture. The psychologist takes engineering for granted while the engineer does the same for people issues and the production control manager thinks that everything revolves around planning and scheduling….

On the one hand, you cannot have a successful implementation unless you address all these dimensions in the right sequence. On the other hand, you cannot expect any individual to master all of them, but you need a team that does, in which every member understands that his or her perspective is not the whole picture, and leadership that can pull all the strings into a coherent approach.

What is special about engineering on a factory floor?

This still does not explain why it is Engineering of production lines that is given short shrift, even in a country like the US, that has contributed so much to this field, and that still has a vibrant engineering community in other technical specialties. What is it about the engineering of production that sets it apart?

The heat of the forge, the sparks from welding, or the din of the assembly line, and interactions with the people who work in these environments,… are not for everybody. Most universities do not know how to teach this kind of engineering. Its subject matter straddles what they call Industrial Engineering and Manufacturing Engineering.

Industrial Engineering (IE), as taught in American universities, is generically about how people work, and gives you no process-specific knowledge. Manufacturing Engineering (ME), on the other hand, is heavily focused on metal working operations, as if these were the only processes worthy of the name “manufacturing.” In principle, you should be able to become a Manufacturing Engineer specialized in all sorts of other fabrication or assembly processes, but the label is in fact used only in metal working.

We could expect, however, those who pursue degrees in IE or ME  to be comfortable on a production shop floor, but most aren’t. Some years ago, my colleague Hormoz Mogarei and I gave a seminar to PhD candidates in Industrial Engineering at Stanford University. We wanted to tell them what we did to get them interested in working with us. Their response, however, was that it was beneath them, and that they had not gone this far in school to do such low-level work.

For factory work, Shigeo Shingo had identified two types of engineers to avoid: the catalog engineer, whose solution to every problem is buying new equipment, and the “no” engineer, who always has a reason why it can’t be done, has been tried before, or won’t work. One more category that did not exist in Shingo’s day but does today is the PowerPoint engineer, whose focus is animating slides.

Historically, with the exception of Lillian Gilbreth who had a PhD, the key innovators in this field, from Frederick Taylor, Frank Gilbreth and Alexei Gastev to Taiichi Ohno and Shigeo Shingo were all self-taught and had no advanced degrees. To this day, the engineers who are most comfortable and effective on a production shop floor started working there as operators in their youth and later went back to school or  learned  through continuing education or apprenticeship programs that alternate extended internships with classroom training. These engineers combine the requisite technical knowledge with an understanding of the operator experience and the ability to work with operators on improvements.

Engineers in Lean implementation

And having practical, shop-floor minded engineers in your plant is still not sufficient. You also need to use them effectively. In manufacturing, if you provide an “engineering sandbox,” organize for people to tinker in it, and provide some form of recognition, you will get results. The engineering sandbox is a space set aside and outfitted with the resources needed for tinkering, experimentation, and prototyping. It is used both by individuals and teams.

Engineering Sandbox

Engineering Sandbox

In Wikipedia, the space you can use to draft an article or an edit before publishing it is called your “sandbox,” and it is similar in concept to the engineering sandboxes you find in factories, that are often called “Kaizen areas” even though the experimentation that takes place can exceed the scope of what is commonly designated as Kaizen. This space is best located in a secluded area, away from heavy traffic and prying eyes and, as it is shared by multiple individuals and teams, access to it must be managed accordingly and often takes place outside of regular working hours. Chihiro Nakao calls this activity “moonshine.”

The implementation of Lean involves engineering projects at multiple scales, from continuous improvement to new plant design. Which no engineering group can be large enough to execute on its own. While many companies set up a “Lean Engineering” group and task it with transforming the entire plant, it cannot work. The engineering group does not have the bandwidth to do no matter how hard its members apply themselves and, even if they did, the production organization would not own their output and would reject it.

Lean engineering group forging ahead

Lean engineering group forging ahead

The only way it can practically be done is by the production organization, under the leadership of its management at the appropriate level, with the engineers in a supporting role. The concepts emanate from the production organization. The engineers help with calculations, research available resources, generate technical drawings, and coordinate the use of contractors if needed. And they apply the lessons learned through improvement to new plant and new line designs where they play a central role.

NIX Octopus

Retrofit to a multicavity injection molding machine

At the start of setup time reduction projects, the focus is on organizing to prepare better before stopping the machine, which achieves initial results, mostly through the work of production operators. But to reduce setup times from 1 hour to less than 10 minutes, you need to go further and modify the machine, which requires engineering. And it’s not all about hardware. More and more machines are computer-controlled, and also require changes to their process programs. The picture to the left shows a device conceived and built by plant engineers, and retrofitted to an injection molding machine to separate the parts by cavity and better trace quality problems.

Correcting the imbalance in the US Lean movement

I am not the only one who has been working to lack of attention to engineering in the US Lean movement. J.T. Black, a professor of IE at Auburn University in Alabama, now in his seventies, was possibly the first American academic to recognize the significance of Lean and make it central to his teachings. Art Smalley, a consultant who is a Toyota alumnus, has also been vocal. But it is an uphill battle. Two of my books, Lean Assembly and Working with Machines, are on this subject, and both are outsold by Lean Logistics, which isn’t.

The baton-touch approach

The following question came this morning from Diogo Cardoso:

What is baton-touch in terms of product oriented manufacturing systems? I have made a deep research about this on Science Direct and other resources but I can find nothing more than an inconclusive paragraph.

Your researched the wrong sources. You could have found your answer in Working with Machines, pp. 140-142. Baton-touch is one of three approaches used to design operator jobs in cells, the other two being the caravan/rabbit-chase and bucket-brigades. The key differences are as follows:

  • In the baton-touch method, each operator performs a fixed subset of the cell’s operations, organized in a fixed sequence. It is commonly used in cells requiring three or more operators making a narrow range of products with similar work content.
    The baton-touch

    The baton-touch

  • In the caravan or rabbit-chase method, the operators follow each other through the entire sequence of operations in the cell. It requires each operator to be skilled in all the operations of the cell, and works well with up to two operators but breaks down with three or more operators, as they queue behind the slowest member of the team.
    The caravan/rabbit chase

    The caravan/rabbit chase

  • In the bucket-brigade method, the operators are in sequence, but the scope of each operator’s tasks varies. When the last operator finishes a unit, he or she takes over the next unit from the preceding operator, who in turn takes over from his or her predecessor, and so on, until the first operator, who starts the next unit. Bucket-brigades are used with a broad mix of custom or configurable products, and work when the faster operators are always downstream from the slower ones. For details, see John Bartholdi’s article on bucket brigades.
    Bucket brigades

    Bucket brigades

Wanna Sabotage Your Lean Implementation Effort? Try This | Lonnie Wilson | IndustryWeek

See on Scoop.itlean manufacturing

Most facilities that fail in a lean implementation have failed to create stable process flow. And by stable I mean statistically stable — a process that is predictable. (Wanna Sabotage Your #Lean Implementation Effort?

Michel Baudin‘s insight:

The way I read Lonnie’s article, he is saying that neglect of the engineering dimension of Lean manufacturing is the primary cause of implementation failure. I agree. It is a long article, but worth reading.

See on www.industryweek.com

From Ybry charts to work-combination charts

Ybry chart used on French railroads in 2013

Ybry chart used on French railroads in 2013

This is a screen shot from yesterday’s evening news on the France 2 channel, part of a story about TGV high-speed trains used on regular tracks to bring vacationers to ski areas. The TGVs, of course run at regular speeds on these single line tracks and must stop at sidings to let regular trains through in the opposite direction. In an earlier post, I discussed the charts invented by Charles Ybry in 1846 for railroad scheduling, and this newscast shows that they are still used in railroads today. Besides railroad scheduling, they are also used in the management of multiple, concurrent projects, and  I believe they were the basis for Toyota’s work combination charts.

The x-axis is time; the y-axis, position along the line. On the chart, the downward lines represent trains going down the line; the upward lines, trains coming up the line. When and where the lines cross, trains cross, and there must be a siding available. The news story had the TGV pilot call in his position on a siding to a control center in Chambéry where the chart was displayed. On the high-speed TGV lines, the signalling is all electronic, and the system automatically knows where the trains are; when you run a TGV train at reduced speed on a regular line, however, it seems that the driver has to report what happens the old-fashioned way.

I learned about these charts in Edward Tufte’s Envisioning Information, where he describes them as a special case of a “narrative of space and time.” Among the examples he gave were a similar railroad scheduling application from Switzerland 80 years ago and the development of Wagner’s operas over almost 50 years in the 19th century:

Trains running up and down between Neuchatel and Chaux de Fonds in Switerland in 1932

Trains running up and down between Neuchatel and Chaux de Fonds in Switerland in 1932

Development timeline of Wagners operas from 1835 to 1892

Development timeline of Wagner’s operas from 1835 to 1892

Work combination charts are a tool to design and communicate about production jobs that require operators to perform a sequence of operations on multiple machines that operate automatically between visits. This is a Japanese example of such a chart:

A Japanese work-combination chart example

A Japanese work-combination chart example

The concept looks similar, doesn’t it? I found this chart particularly useful when you need to plan the activities of more than one operator, as in the following example:

Work combination chart for machining operations

Work combination chart for machining operations

In the Legend, “Manual In” refers to time spent by the operator on the machine with it stopped; “Manual Out,” time spent on the machine while it runs.

To this date, in the US, this powerful technique is far from enjoying the popularity it deserves. It is generally perceived as “too complicated” and I still don’t know of any software tools that fully support it. In designing jobs that involve interactions between human and machines, however, the consequence of not using it is leaving about 50% of the potential productivity improvement on the table. It may take a project team an extra day to do it, but the result is achieving a 40% productivity increase instead of 20%. Details are discussed in Chapter 7 of  Working with Machines.

Lean in administration at St. Luke’s Internal Medicine | David C. Pate

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TEAMwork is St. Luke’s application of lean principles. It’s our management operating system. TEAMwork stands for timely, effective, accountable, measureable work. And it’s making its way through St. Luke’s Health System as we gain on our Triple Aim of better health, better care, and lower costs.

Starting last summer, SLIM embarked on a top-to-bottom examination of how it conducted its work. They wanted to eliminate waste by tapping into the potential and knowledge of every member of the clinic team and build a culture of continuous improvement.

 

 

Michel Baudin‘s insight:

The improvements described are all about supplies and the handling of patients by nurses and administrative staff.

There is not a word about any changes to the work of doctors themselves or involvement by doctors in the improvement process. What form might that take? I don’t know, but, the last industrial engineers to work on health care before Lean were Frank and Lillian Gilbreth 100 years ago, and their focus was the work of surgeons inside operating rooms, not patient handling before and after they see a doctor.

The result of their work was the now standard mode of operation in which the surgeon calls for tools that are handed to him by nurses. It seems hard to believe today but, earlier, surgeons would actually leave patients to fetch tools.

Following in the Gilbreths’ footsteps today would mean for Lean Health Care to get involved with the core of the activity: what doctors do with patients.

In manufacturing, successful Lean implementations start with the work of production on the shop floor, not with the logistics upstream and downstream from production. First you worry about line layout, work station design, and the jobs of production operators. Then you move on to keeping them supplied and shipping their output.

See on drpate.stlukesblogs.org

New assembly methods at Toyota

Toyota’s latest plants in Ohira, in Japan’s Miyagi prefecture and in Tupelo, Mississippi, feature new approaches to assembly. According to press reports, the Miyagi plant is small, with 900 employees making 250 cars/day for export to the US, with a plan to double output and employment. It was designed to require a minimal investment and be easy to change. The plant started operations shortly before the Fukushima earthquake and, even though it is the Northern part of Japan that was most affected, it resisted well and was able to resume operations about six weeks later.

This is how Barry Render described it:

“The Miyagi factory is designed for advanced low-volume, hyperefficient production, with 1/2 the workers and 1/2 the square footage of Toyota’s 16 other plants. Inside, half-built Corollas and Yaris sit side-by-side, rather than bumper-to-bumper, shrinking the assembly line by 35% and requiring fewer steps by workers. Instead of car chassis dangling from overhead conveyor belts, they are perched on raised platforms. This is 50% cheaper, and also reduces cooling costs by 40% because of lower ceilings. Finally, the assembly line uses quiet friction rollers to move the cars along. The rollers use fewer moving parts than typical chain-pulled conveyor belts.”

Toyota is not providing details, but I have been able to glean some information about it from the press and Barry Render’s blog, on the following features:

This is followed by a few conclusions.

Side-by-side assembly

Side-by-side assembly at Toyota Miyagi

Side-by-side assembly at Toyota Miyagi

I have seen side-by-side assembly at the Volvo Bus factory in Turku, Finland. In the picture of the building below, bus bodies are assembled in the hall on the left, side-by-side under they are mounted on a chassis and move forward on their wheels, laid out front to back in the hall you see in the background.

Volvo Bus assembly building in Turku, Finland

Volvo Bus assembly building in Turku, Finland

Volvo bus main assembly flows

Volvo bus main assembly flows

The ratio of width to length  is more favorable to this arrangement for buses than for cars. A straight assembly line with a front-to-back arrangement throughout would require a long and narrow building and a snaking line would have problematic turnarounds. With cars, the side-by-side arrangement seems suitable for work done at the front or the back of the car, such as installing headlights or bumpers. but less for work that requires access from the middle, such as installing instrument panels or upholstery. The following press picture (AP), however, shows an assembly operation done inside the car body in what appears to be a side-by-side layout. It implies that space for the part cart must be provided between cars, which forces them apart.

Assembly operation at Miyagi

Assembly operation at Miyagi

None of the available pictures from the Miyagi plant shows the raku-raku seat that was a prominent feature of the early 1990s designs and made it easier for operators to work inside the car bodies. Not only is a raku-raku seat an added investment, but it is also easier to use in a front-to-back than in a side-by-side layout.

Raku-Raku seat

Raku-raku seat in a 1990s plant

Modular paint booths

I could not find pictures or sketches of the Miyagi painting system. Following is how CNN Money described it on 2/18/2011:

“…Toyota developed a modular paint spray line. The modules can be built somewhere else and are assembled at the plant in a much shorter time. Advantage: Cost savings. However, you don’t build a modular paint spray line factory somewhere unless you intend to build a lot of paint spray lines. Usually, cars get three coats of paint, usually water-based, and usually each coat is dried with heat. Not in Ohira. Here, the third coat is applied onto the still wet second coat and both are dried together. Advantage: Huge energy savings, faster paint time. Lower expenses…”

Friction roller conveyors

Toyota assembly line new concepts 2011 Miyagi plant Conveyance

Following is how CNN Money described the Miyagi conveyor systems on 2/18/2011:

“Where the car moves along the floor, factories usually have below ground pits that house the motors, chains and gears that keep the line moving. Not in Ohira. Here, the cars move on maybe a foot high conveyor system that is simply bolted into the concrete flooring. Advantage: Cheaper to build, cheaper to tear down and rebuild somewhere else. The line can be lengthened or shortened at will. The assembly line doesn’t ‘grow roots’ as they say in Toyota-speak.”

Note that the sketch shows car bodies without wheels. In this system, the bar supporting the cars forms

A photographs of final assembly at Ohira shows operations done further downstream, with the wheels on:

toyota--ohira-plant-in-japan-front-to-back assembly line 2011

Assembly operations after wheels are put on

In this picture, the floor the operators stand on is flush with the assembly line,  meaning that it is either a classical line with the drive mechanism in a pit under the floor, or the operators are in a raised platform spanning the length of this assembly line segment.

Elevated platform versus suspension conveyor

Toyota assembly line new concepts 2011 Miyagi plant Suspension

From suspension conveyor to elevated platform

The following photographs contrast the suspension conveyor approach as previously used at Toyota with the elevated platform at Tupelo, Mississippi:

From these pictures, it is clear that the elevated platform is a cheaper system to build, but I can see two issues with it:

  1. Flexibility in vehicle widths. The Yaris and the Corolla differ in width by less than half an inch, and therefore the same elevated platform can accommodate both. A Land Cruiser, on the other hand, is 11 inches wider, which makes you wonder whether it could share an elevated platform with the Yaris. The jaws of the suspension conveyor, on the other hand, look adjustable to a broad range of widths.
  2. Ergonomics. Working standing with your head cocked back and your arms overhead is just as ergonomically inadequate in both cases. By contrast, the VW plant in Dresden, Germany, uses suspended conveyors that can tilt the body, which is both ergonomically better and much more expensive:
VW Dresden suspended adjustable conveyor

VW Dresden suspended tilting conveyor

Conclusions

The journalists take on the Ohira plant is that it is intended to prove a design for low-volume, low-cost, high-labor content plants that can be deployed easily in emerging economies with small markets. The designs of the early 1990s instead used more automation to make the work easier for an aging work force, with tools like the raku-raku seat. This is a different direction, addressing different needs. But why build it in Northern Japan rather than, say, the Philippines? It shows Toyota’s commitment to domestic manufacturing in Japan, and it is easier to test and refine the concept locally than overseas.

Does US manufacturing need more universities?

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Needed: ‘manufacturing universities’ to keep U.S. competitive
SmartPlanet.com (blog)
The erosion in manufacturing capability weakened the U.S. economy over the past two decades, Atkinson and Ezell add.

Michel Baudin‘s insight:

Universities have essentially contributed nothing to the art of manufacturing in the past, and I have a hard time understanding how they could be essential to US manufacturing competitiveness in the future.

See on www.smartplanet.com

Finding local roots for Lean – Everywhere

Lean is from Japan, and even more specifically from one Japanese company. Outside of Japan, however,  the foreign origin of the concepts impedes their acceptance. In every country where I’ve been active, I have found the ability to link Lean to local founders a critical advantage. The people whose support you need would like to think that Lean was essentially “invented here,” and that foreigners at best added minor details. Identifying local ancestors in a country’s intellectual tradition takes some research, and then you may need to err on the side of giving more credit than is due.

Feeder line at Ford

In the US, using the word “Lean” rather than TPS is already a means of making it less foreign, and it is not difficult to paint Lean as a continuation of US developments from the 19th and 20th century, ranging from interchangeable parts technology to TWI. Ford’s system is a direct ancestor to Lean, as acknowledged by Toyota. On this basis, the American literature on Lean has gradually been drifting towards attributing Lean to Henry Ford. Fact checkers disagree, but it makes many Americans feel better.

Elsewhere, it is not as obvious to find a filiation. Following are a few examples of what I found:

  • Russia has Alexei Gastev, who started an industrial engineering institute in Moscow in 1920, was shot by Stalin in 1939 and largely forgotten afterwards, but our OrgProm colleagues have now named a prize after him, that is given to Russian companies for excellence in manufacturing. It was awarded for the first time in 2011. Here are, from 1924, Gastev’s 9 steps to automate a riveting operation:

Gastev’s 9 steps to automated riveting

  • Poland has Karol Adamiecki, whose “harmonogram” is the same as a Gantt chart, and was invented independently and a few years earlier. If you google “harmonogram,” you get pictures of Gantt charts. I am sure there must be some differences between the two, however minor, but I can’t tell what they are.
  • Italians can connect Lean to the shipyard in which Venetians assembled galleys in the Renaissance. Jim Womack identified it as a early flow line. As he wrote in Walking Through Lean History:

“…  Dan Jones visited the Arsenal in Venice, established in 1104 to build war ships for the Venetian Navy. Over time the Venetians adopted a standardized design for the hundreds of galleys built each year to campaign in the Mediterranean and also pioneered the use of interchangeable parts. This made it possible to assemble galleys along a narrow channel running through the Arsenal. The hull was completed first and then flowed past the assembly point for each item needed to complete the ship. By 1574 the Arsenal’s practices were so advanced that King Henry III of France was invited to watch the construction of a complete galley in continuous flow, going from start to finish in less than an hour.”

Galley assembly hall in Venice

Britain, as the Olympic opening ceremonies reminded us, was home to the industrial revolution. In terms of worldwide share of market for manufactured goods, however, Britain peaked about 1870, and the thinkers that come to mind about British manufacturing are economists like Adam Smith or David Ricardo, whose theories were based on observations of early manufacturing practices, but whose contributions were not on the specifics of plant design or operations. They are too remote to be linked in any way to Lean.

For France, I have asked everybody I know there for nominations but have yet to receive any. The French have invented many products and processes, but I have not been able to identify French pioneers in production systems who could provide a link to Lean. And there are many other countries where the search may be fruitless.

Even though people in China and India have been making things for thousands of years,I don’t know any names of local forerunners of Lean in these countries. China has only emerged as a world-class manufacturing power in the last few decades and I have, unfortunately, never been to India. There are many other countries on which I don’t have this kind of information, and nominations are welcome.

A Lean Journey: Meet-up: Michel Baudin

See on Scoop.itlean manufacturing

Interview on Tim McMahon’s A Lean Journey.
See on www.aleanjourney.com

Yet another (wrong) definition of takt time

This is from a blog post published today that claims to clarify what a takt time is:

Takt Time:  This is the rate of time at which a product or service is being purchased.  For example, a Nissan commercial mentioned that every minute, someone in the world buys a new Nissan.  Selling a car every minute is an excellent example of takt time!

Writing a definition for a thing or an idea is tricky. Following Aristotle, I would say that you have done a good job if you have described what kind of a thing it is and how it differs from other things of the same kind, using terms your reader already understands.

In this definition, takt time is described as a “rate of time.” If there were such a thing as a rate of time, in what units would it be expressed? In production, a rate is expressed, for example, in pieces per hour; a time, in minutes or seconds. Takt Time, as its name suggests is a time, not a rate, and certainly not a rate of time, whatever that may be.

This definition then relates takt time exclusively to “a product or service [..] being purchased,” and gives the example of a Nissan being bought every minute in the world, suggesting that 1 minute is the takt time of a Nissan. Incidentally, if this figure were true, Nissan would sell about 500,000 cars/year, versus the 4 million it actually sells.

Takt time, as we use it in manufacturing and industrial engineering, is in fact not a parameter associated with just a product but with a production line making this product. Given the demand that is given to it and the amount of time that it actually works, the takt time of this production line for this product is the time that must elapse between two consecutive unit completions.

If a line is expected to produce 400 units of a product in a 400-minute shift, then, if you stand by the last station of the line, you will see one unit come out every minute, meaning that its takt time is 1 minute. If you switch from working 1 shift/day to 2 to meet the same demand, you double the takt time to 2 minutes.

This is why it is calculated as follows:

Takt\, time(Product, Production\, line)=\frac{Net\, available\, production\, time}{Demand}

It has a numerator and a denominator, and both matter. They are obviously calculated for the same time period.