Lean Assembly

The Nuts and Bolts of Making Assembly Operations Flow

Michel Baudin

• Publisher: Productivity Press
• Year: 2020
• Print ISBN:
  9781563272639
• eBook ISBN:
  9781000285635
• Topic: Tecnología e ingeniería
• Subtopic: Derecho mercantil

With examples drawn from aerospace, electronics, household appliance, personal products, and automotive industries, Lean Assembly covers the engineering of assembly operations through:

• Characterizing the demand in terms of volume by product and product family, component consumption, seasonal variability and life cycle.
• Matching the physical structure of the shop floor to the demand with the goal of approaching takt-driven production as closely as possible.
• Working out the details of assembly tasks station by station, including station sizing, tooling, fixturing, operator instructions, part presentation, conveyance between stations, and the geometry of assembly lines as a whole.
• Incorporating mistake-proofing, successive inspection, and test operations for quality assurance.

  Lean Assembly differs from most other books on lean manufacturing in that it focuses on technical content as a driver for implementation methods. The emphasis is on exactly what should be done. This book should be the "dog-eared" and "penciled-in" resource on every assembly engineer's desk.

PART C
Detailed design

CHAPTER 9
Assembly station sizing

Torsmallproducts, assemblers work only on one side of the station andpickparts across theproductfrom the otherside. Part replenishmentfrom the back does notinter- fere with assembly. The stations do nothave to be wider than operators and can be arranged into U-shaped cells with the assemblerwork area inside. Theproducts are often lightenough to bepassed between assemblers by hand, without conveyors.

Large products move on conveyors or carts and are usually worked on from both sides of the station, with parts delivered behind orto the side of the assemblers. Presenting the point of assembly to the assembler is a challenge with large products, which is sometimes met through fixtures, as described in chapterlO, and sometimes through cantilevered “raku-raku” seats.

The norm is for assembly work to be performed standing, and moving orrotating between stations as needed. Green corners are provided near the line for assembly teams to sit during breaks and hold meetings. The work height needs to be uniform among stations, and platforms cannot be used, as they restrict assembler mobility. This makes providing the work atan ergonomically appropriate heighta challenge when the workforce includes both short and tall people. The height of kitchen counters in the local area of the plant is a startingpoint, and some limited adjustability can beprovided in thefixtures.

Stations atwhichglue must dry orsealantcure holdproductsformuch longerthan the takt time. They are often flow-through tunnels, which must be laid out perpendicular to the line to allow product units to come out next to theirpoint of entry and avoid separating upstream and downstream stations.

9.1. Issues with assembly station sizing

The dimensions of the product set lower bounds for assembly station sizing, but the line designer then has latitude in deciding how much wider and deeper than the product the station should be and how high the work surface will be. These decisions, in turn, have a great impact on the flow of materials and on ergonomics.

9.2. Assembly stations for small products

Small in this context means small enough for the assembler to pick parts across the station. Products in this category include most manufactured goods for consumers—such as electrical razors or VCRs—as well as subassemblies or spare parts for larger products, ranging from shock absorbers for cars to cockpit switches for airliners.

The small product size creates opportunities in station design that are not available for larger products, such as the following:

• Since work takes place on one side only, such stations can easily be arranged into U-shaped cells with the assembler work area inside.
• Part deliveries do not interfere with the work, and stations can be arranged to face transportation aisles.
• Many small products are light enough to be handled manually.

Most manufacturers fail to exploit these opportunities today. The most common concern is station size. You do not need a 60”-by-30” bench to work on a product whose longest dimension is six inches, and using these unnecessarily large stations causes the following problems:

• Neighboring assemblers are too far apart. Their communications are reduced, they are unable to relieve each other, and one assembler cannot conveniently work at more than one station when volume drops.
• The excessive distance between stations complicates conveyance and hinders the product flow.
• Just because it is there, the extra bench space fills up with unnecessary WIP, tools, fixtures, and other items unrelated to the operation at hand.
• The floor space used by a line of these large stations is twice or three times more than it should be.

Conveyor abuse is also common. Conveyors are commonly used to move parts that are small and light enough to be passed around by hand, for the sole purpose of enforcing pace. The assembly station is then frequently off the conveyor, as in Figure 4-8 on page 54. In short operation cycles, the additional handling this generates can account for 40% of the assemblers’ time.

The example in Figure 9-1 summarizes these issues. Even though the assembler picks parts across the station, a closed wall not only prevents replenishment from behind, but also blocks the view to the next station.

FIGURE 9-1. Large station for small product

Such benches are used today, not because they meet the needs of assembly, but because they are available off the shelf from multiple suppliers.

By contrast, let us go back to the Ford magneto assembly line of (see Figure 9-2). Since it is the first assembly line ever built, it is clearly not made from off-the-shelf modules but custom-designed. Each station is not much wider than the product itself.

FIGURE 9-2. Critique of the 1913 Ford magneto assembly line

SOURCE: From the collections of Henry Ford. Museum & Greenfield. Village.

The magneto assembly line, however, had many questionable features that we would not design into a modern line. In 1913, of course, the designers had the excuse that they were breaking new ground. The most obvious points are as follows:

• The assemblers’ head gear suggests that the shop floor is not properly heated.
• Placing fasteners under the work is ergonomically inadequate, forcing the assembler to pick in an uncomfortable position.
• This layout is also prone to parts contamination, since fasteners can easily drop from assemblers’ hands into the wrong bins.

As shown in Figure 9-3, it didn’t take long before some companies started addressing these issues. The exact date of the picture is unknown, but the workers’ clothing and hair styling points to the 1920s. In this picture, assemblers pick parts across the conveyor, at a comfortable height and without risk of contamination. This approach is still appropriate today, whenever the size of the product makes it feasible.

FIGURE 9-3. Phonograph assembly at Edison in the 1920s

Many lamps hang from the ceiling. This indicates that the stations are well lit, but the windows are behind the assemblers, so that the assemblers cast their own shadows over the work, and artificial light may be needed even in broad daylight. We may also note that the space above the stations is not put to any use other than lighting. In a modern line, powered screwdrivers would be hanging on balancers above the stations.

Figure 9-4 shows a contemporary descendant of the Edison line. This station is used to assemble a component of a laser printer. It has two levels of components to pick across the product, and a screwdriver hanging on a balancer. There is no conveyor, and the assemblers pass the units to each other through single-unit buffers.

FIGURE 9-4. Assembly of laser printer component at Canon (1990s)

SOURCE: Courtesy of Canon, Kajo Kanri, Vol. 45, No. 7, 1999, Nikkan Kogyo Shimbun-sha.

The station itself is built out of plastic-coated metal tubes fastened with metal fittings. Unlike its ancestors in Figure 9-2 and Figure 9-3, it can easily be taken apart and reconfigured.

9.3. Assembly stations for large products

There is a product size beyond which picking across an assembly station is no longer an option. This limit is not expressible as an exact number, but it is easy to see, for example, that VCR assembly is on one side of the limit and car engine assembly on the other. The part presentation challenges associated with large products ...

Table of contents

• Cover
• Title Page
• Copyright Page
• Dedication Page
• Contents
• Figures
• Tables
• A guided tour
• Part A Analysis techniques
• Part B Assembly concepts
• Part C Detailed design
• Part D Assembly quality
• Bibliography
• Index