The Potter's Wheel

 

4000 year-old technology solves injury problem, increases output 20%, and eliminates rejects.

 

Dan MacLeod

www.danmacleod.com

January 10, 2006

 

An informal discussion with employees at this plant revealed the task that they considered the most difficult: deburring the “Slotted Pole.”  Consequently, one of the plant engineers and I focused on this task. 

The Slotted Pole was a cylinder about five inches long mounted on a stubby base.  The inside of the cylinder was somewhat hollow, with three slots trisecting the part most of its length, hence its name.  The job was to take the parts that had been recently machined and remove burrs and sharp edges.  Because the part was to go into a mainframe computer, all slivers of metal had to be removed completely and the rough surfaces polished.  Quality requirements were extremely high, since the slightest fleck of metal could ruin an entire computer.

The task of deburring the Slotted Pole was unpopular.  Consequently a variety of people had been trained to do the work and rotated through the job to spread the burden equally.

Issues: Using the left hand as a fixture; right hand pinch grip

One part of the task consisted of holding the part in the left hand and manipulating it, while the right hand performed various motions such as sanding the part’s edges and surfaces.  As shown in the photo, the sandpaper was held in an awkward “pinch” grip with fingers extended to reach the inside of the part, thus creating high compression forces on the right hand wrist.  However, the employees stated that the worst part of the task was with the left hand — the continuous gripping (“static load”) and constant manipulation of the part.

 

Lesson 1:      It is vital to talk to the employee in order to learn about the job and gain insights into problems and solutions.  Some details can be gained in no other way.

 

Lesson 2:      Using the non-dominant hand as a “fixture” is a common source of musculoskeletal disorders.  This is important because there is a natural tendency to focus on the hand doing the work to the neglect the hand manipulating the product.

 

Lesson 3:      Often, the specific problems of a job can be identified relatively easily, and without need for measurements or elaborate studies.  Typically the difficulty is the next step: finding solutions.

 

Based on this assessment and other observations, the engineer and I brainstormed options for improvement, and from time to time floated ideas past the employee and supervisor of the area.  The first goal was to fixture the part to relieve the left hand from the constant gripping forces.  The fixture needed to rotate, however, to accommodate the need to access the part in different orientations. 

 

Initially, we considered a simple “Lazy Susan” fixture.  However, we became concerned that the need to continuously rotate the fixture manually with the left arm would merely transfer the problem to the elbow and shoulder.  Consequently, we thought of the idea of powering the fixture.  But we eventually discarded this idea too, since the “Slotted Pole” needed both to be rotated continuously and at varying speeds to sand the outside surface and then indexed rather precisely at one-third turn intervals to access the slots correctly.  A power mechanism to accomplish both requirements would cost too much, particularly in development time.

 

Our brainstorming then refocused on the Lazy Susan, with the added idea of running a shaft from the rotating fixture on the workbench down the floor where it could be easily manipulated by the feet.  As the engineer and I began to sketch the needed device, it finally dawned on us that a similar device had been invented long ago and was readily available.  We had just reinvented the potter's wheel.

The Potter’s Wheel ─ 4000-year-old technology used to solve a problem in making mainframe computer parts.

Note the fixture on the plate (where the potters would normally work the clay), the kick wheel, the electric motor, . . . and the inadequate seat.

So, the company purchased and installed a potter’s wheel.  We used technology that was 4,000 year old to solve a problem in a plant that made computer parts.  There were many features of the potter’s wheel that could be applied to this task.  The potter’s wheel had a plate, normally used to support the potter’s clay, onto which a fixture to hold the Slotted Pole was attached.  The shaft that connected the plate (and now fixture) to the wheel beneath could be manipulated easily by the feet, both to rotate evenly at varying speeds and to index precisely. 

This off-the-shelf device worked successfully and served as the basis for further refinements.  Specifically, the fixture eliminated all of the repetitions and exertion of one hand in manipulating the part (thus, in one step, reducing the overall wrist stress by 50%). 

 

Lesson 4:      Anything which solves a problem is ergonomic — the solution does not need to be a device normally thought of as an “ergonomic” product.

 

Lesson 5:      The problem-solving process need not elaborate or highly technical.  Creative thinking is often more important.

 

Lesson 6:      Unconventional, “hare-brained” ideas often lead to good results.  Solutions can be found in many areas, even from equipment not normally associated with the industry in question.

 

Lesson 7:      Fixtures, where feasible, can be quite effective in reducing stress on at least one hand.

Deburrer’s Knife ─ Issues: Inadequate grip for right hand (not being used at all); awkward wrist and arm posture; and once again using the left hand as a fixture.

Another part of the task involved use of a generic “Deburrer’s Knife.”  As shown in the photo, this knife was used to reach deep into the center of the cylinder to scrape out burrs.  The photo shows several other issues.  One is the awkward postures of the wrist and elbow, both contributors to musculoskeletal problems.  Another issue is the grip of this generic device, which was so inadequate for the task that it was not used at all.  Finally, the illustration once again shows once again the problem of using the non-dominant hand as a fixture.

Home-made, two-handed knife

Once the part was attached to a fixture, then the left hand became available to help manipulate tools.  In this case, a special two-handled tool was designed specifically to reach into the slots to scrape out burrs.  The tool could simply be inserted into the slot and pulled, providing three great benefits:  (a) the tool placed the wrists and elbows in much better posture, (b) it divided the force between both hands and (c) took advantage of the larger muscle groups in the upper arms and even shoulder and torso.

 

Lesson 8:      Once a product or part is fixtured, it opens the door to two-handed tools, which usually are both easier to manipulate and distribute force to more (and often larger) muscle groups.

 

Lesson 9:       Dedicated tools — designed to accomplish a single task — are often better than generic tools.

Hand sanding ─ Issues: repetitive right shoulder motions; right hand pinch grip; left hand as fixture.

A further problem in the original task was sanding the outside surface of the part.  Considerable repetitive, rather forceful motions of the right arm were required to perform these actions.  Once again, the left hand was used as a fixture in doing this step of the job.

Hand sanding with electric motor.  Note the pinch grips to hold the sandpaper, but the force was light and of short duration..

Fortunately, the potter’s wheel that the company bought was the high end of potter’s wheel design and came equipped with an electric motor to spin the wheel.  Thus, the employee could turn on the motor to spin the wheel, in addition to moving the part slowly by manipulating the feet.  With the motor on, the employee could simply hold a piece of sandpaper rather loosely to the part and it would be polished quickly and cleanly with little physical effort.

 

Lesson 10:     Reducing repetitive motions does not necessarily mean slowing the job down.  On the contrary, with good design, the task can be completed faster, but with less manual movement.

 

Modified Potter’s Wheel.

As experience was gained with the potter’s wheel, a variety of modifications were made.  The plate (to support the potter’s clay) was removed, since it had no function anymore.  The fixture was angled with the use of a gearbox to orient the Slotted Pole to the employee for improved access.  A storage area that was part of the original equipment, but a bit out of reach, was raised up and moved closer.  A task light was added.  The original potter’s stool, which had no back support and lacked cushioning for the seat, was replaced with a car seat, equipped with an electric motor to provide adjustment for varying length legs.

 

Lesson 11:    Continuous improvement applies to ergonomics as much as to any other aspect of the workplace.  There is no final ergonomic fix — there are always ways of improving equipment.

 

Two additional items of special note also evolved.  The first involved an additional problem with the initial potter’s wheel design.  Once the wheel was spinning, there was no way to stop it quickly.  Potters had no particular need to stop the wheel as abruptly and frequently as did these deburring employees.

The solution involved the plant maintenance man, who in the course of events had become actively engaged in redesigning the potter’s wheel.  He happened to be a Harley-Davidson enthusiast and had a supply of spare motorcycle parts available.  To stop the wheel, he attached a Harley-Davidson brake, complete with foot pedal that stood out conspicuously to one side — large and chrome-plated.

 

Lesson 12:    Everyone can make a contribution.  Maintenance personnel, once provided with the principles of ergonomics and the goals of the program, can be especially gifted in making creative improvements.

 

One previous occasions, the supervisor had encouraged the employees to use various powered deburring tools that were available.  However, the employees resisted, stating that the powered tools were awkward to use.  As it turned out, once the employees began to use the potter’s wheel, they also began to use the power tools successfully.  Apparently, the employees had become so used to doing the task in a certain way, that any change, even ones that were seemingly beneficial, seemed awkward.  However, once the entire task was disrupted and whole new techniques needed to be learned, the employees were able to incorporate the power tools.

 

Lesson 13:     Incorporating change can be difficult for a variety of reasons, one of which is the chore of learning a highly refined work technique all over again.  Sometimes it is easier to make a big change than a little one.

 

Results for Potter's Wheel

  1. Injuries — Ultimately, the number of MSDs related to this job dropped to zero.

  2. Quality — Rejects also dropped to zero, since the Potter's Wheel enabled the employees to do their jobs better.

  3. Productivity — Output increased 20%.

 

Improving other deburring tasks

 

After the initial changes was made on the Slotted Pole, attention shifted to other tasks, again involving employees and brainstorming improvements. 

 

Most other deburring tasks involved working on large, traditional work tables.  Employees often worked hunched over these tables, with poor back and neck postures.  The work benches were fixed-height and could not be adjusted for taller workers.  Moreover, since two or more people typically worked at each table, the tables could not accommodate everyone easily, even if they had been adjustable.  An additional issue was that the employees stood all day on a concrete floor.   Finally, other issues were that there no particular place provided for tools, plus occasional long reaches to obtain needed tools and equipment.

 

 

“After” work bench:  At left, the die cart.  At right, a tall worker has raised the die cart six inches above the old table (at his immediate left).  Note the use of the foot rest.

The first decision was to procure sturdy, adjustable-height work tables.  However, none could be found on the market when this case study occurred in 1987.  Happily, once again with creative thinking, an off-the-shelf product — originally intended for another purpose — was modified for use, rather than constructing a suitable table from scratch.

The item purchased was a standard industrial die-cart — designed originally to lift heavy dies into presses — that had many of the features needed, particularly height-adjustment and a sturdy base.  It was modified in several ways.  A one-quarter inch sheet of plastic was attached to the top surface to provide more of an appearance of a work table and to protect tools and parts from the steel.  Brackets were mounted to the sides of the table to hold tools, fixtures and a task light.

Used as a work station, these carts could readily be raised and lowered to adjust for individual height.  Furthermore, the die cart had several additional features which unintentionally came into play:

  • The base of the cart served well as a foot rest, whether sitting or standing. 

  • The dimensions of the top work surface was small, only about three feet wide and two feet deep — quite sufficient for the task at hand.  The unintended benefit was that once all employees were equipped with these tables and the old, larger tables discarded, considerable floor space was gained.  A previously congested area became rather roomy.

  • The die carts came with wheels, since they needed to be moved about in their original application.  These new work stations were thus mobile and could be used as the basis of a modular, flexible manufacturing system.  The engineers created several work cell areas using pedestals attached to the floor and equipped with electric outlets and pressurized air couplings.  Employees then wheeled their workstations to the appropriate cell, hooked up and worked as a unit on a particular part.  As production required, they shifted quickly from one cell to another.

  • These individual workstations could be personalized for each employee to promote an individual’s own identity.

  • Finally, the work stations were designed for standing height, and tall stools provided to permit employees to sit or stand as they chose.  The stools themselves adjustable and had other ergonomic features.  Anti-fatigue mats provided for when standing.

 

Lesson 14.    Once each employee is provided with an individual workbench, then individual adjustment becomes feasible.

 

Lesson 15.    There were unanticipated other benefits.  Many tasks do not require the space allotted to them.  Workbenches and desks are often designed to be large for no reason other than status, and they merely serve to collect clutter.

 

Overall Results

 

Injuries dropped dramatically for the whole facility and workers' compensation costs were cut by 90%.

 

Overall costs of the project totaled about $20,000 for new and modified equipment.  Savings from workers' comp reductions were about $100,000, a 500% return-on-investment in one year.

 

 

Postscript

After a time, the engineers in this facility began thinking about deburring in a way in which they had not previously.  Subsequently, they found ways to improve machining capabilities to reduce the burrs and thus eliminated much of the need for deburring altogether.

 

Lesson 16.  Often, just getting people to think about a problem, perhaps presented in a new way, can be sufficient to induce them to apply their usual skills in solving problems.  Extensive training in the details of ergonomics or complicated task analysis may not be necessary.

 

Lesson 17.  One should not neglect the root cause of the problem.  Don't fix tasks that shouldn't be done in the first place.