Fig. 1 Cell with one worker and six machines of workcells.pdf · • This is some time referred as tackt time • Actual cycle time is the actual production capability of a process

National Institute of Technology Calicut Department of Mechanical Engineering

Design of workcells 114 March 2013

DESIGN OF WORKCELLS • Concept of performing all of the necessary operations to make a part, component,

subassembly, or finished product in a workcell is called cellular manufacturing

Fig. 1 Cell with one worker and six machines

• Task in a workcell might be performed entirely by workers

• Example – manual assembly of components

• Role of workers in such a cell is to perform assembly tasks, inspect items, and transfer them to the next station

• Task in a workcell might also be performed by machines at some or all of the workstations

• Role of the workers in that case is to setup and monitor the machines, turn machines on and off, load and unload parts, inspect parts, and transfer parts between machines

• Some workcells can be run with as few as one worker

• Stations are arranged in a U-shape layout so the worker can move quickly around the cell

• Output rate of a workcell can often be manipulated by changing the number of workers



Fig. 2 Cell with two workers (a) Divided subcells (b) Rabbit chase

Typical Workcell End-Items • An entire product can be produced in a single workcell

o Product is usually simple in terms of the number of components and operations

o One- or a few pieced items like metal casting with series of drilling, boring and finishing operations required or somewhat simple assembly such as PC disk drive

• Workcells can also be used for producing more complex product involving numerous operations and component parts

o Like Stereo CD and cassette players and electronic communication equipment for aircraft and missiles

• Physical size of the cell necessary to encompass all of the operations involved is also a determining factor when considering complex end items

• Number of workers and distance between them must be small enough – not overwhelmed by different tasks and excessive time for walking

• Large cell – team work suffers

• For problem solving, work coordination and cohesiveness, a group size of between five and seven people is optimal

• Workcells with 2 to 6 workstations are more common

• 10 or more workstations are less common

• In a workcell production of complex components and assemblies in their entirety is less common

• Workcell commonly focused on producing one product family

• Multiple product family are also produced when the families require similar operations



Fig. 3 Cell for producing multiple product families

• To help workers and avoid confusion about which machines are used on which families, lights are mounted on machines called andons

Linked Workcells and Subcells Complex product manufacturing – dividing the operations into several workcells – linking workcells such that parts flow from one cell to next is in a coordinated manner

Fig. 4 Facility of linked workcells

• Linking cells o Piece by piece with conveyors or mechanical feeders – material flow

somewhat continually between workcells

o Small batches with hand carts or forklifts – flow is intermittent

• Figure below shows two kind of linked workcell processes

(a) a cluster of four clearly distinguishable workcells connected by containers (b) four subcells that function together as one



Fig. 5 (a) Linked cell and (b) subcell examples

• Fig (a) is a kind of linked-cell system and is an example of classic pull production system

• The rates of all workcells should be balanced

• In Fig (a) each workcell has only one upstream cell feeding it

• In a more complex system, each workcell has multiple upstream workcells feeding it

o Looks like a tree diagram with final assembly at the stem

• Rates of the workcells are adjusted by altering the number of workers or length of the workday

• Fig (b) can be viewed either as four separate but linked workcells or as one big workcell with four subgroups

• Subgroups are located close to each other

• Instead of movable containers in the stock areas between the subgroups are kanban squares or racks (denoted a, b, c, and d)

• Rather than sending replenishment orders, workers simply look at the number of items in each square, and when the quantity gets low enough, the upstream station produces a quantity to replenish it

• The production rates of each subgroup are balanced so that both supply and demand rates between subcells are roughly equal

• Conceptually and analytically, the two can be treated as identical, except the time delays in communicating replenishment orders and transporting materials between workcells in fig (a)

• All the workcells in a linked process must produce at a rate that is about the same as the rate required to satisfy demand for the final end-item

• Pull production features o Material flowing to and between workcells must be synchronized and

coordinated



o Setup time must be short

o Equipment cannot breakdown

o Materials ordered from suppliers must arrive on time

o Method of placing orders and transporting materials, size of containers, and frequency of transport all require special attention

o Reduced inventories and smooth production flow using small batches

o Increase in the frequency and reliability of material transport

• Shortening distance between workcells, eliminating double-handling of materials and low cost chutes and light weight carts are ways to improve the handling efficiency

• Balancing the system is a dynamic procedure

• For every change in the final assembly schedule, corresponding changes are required in the production rates of the workcells

Workcell Design • Two kinds of workcells – (a) assembly cells and (b) machining cells

• Assembly cell – mostly manual operation

• Machining cell – work task is usually simpler, more easily automated, and largely or entirely performed by machines

• A basic concept for workcell facility design is cycle time

• Cycle time is the time between when units are completed in a process

• It gives an idea of piece-by-piece product flow and hence it is important in pull production because it implies repetitiveness and smooth, steady flow of material throughout a process

• It also thought as inverse of production rate

Required cycle time and actual cycle time • Required cycle time is the production target of a process or operation

demand Requiredavailable Time

=rCT

• This is some time referred as tackt time

• Actual cycle time is the actual production capability of a process or operation

• Production capacity of a facility is a function of actual cycle time

• Establishing and standardizing work in a process such that the actual cycle time is as close as possible to the required cycle time is an important concept in a lean production

Assembly Cell • Actual cycle time is a function of cell manual time and this time is required for

workers to perform their tasks and move between workstations



• Actual cycle time is the sum of the operation times at every station and the walk time between them

Fig. 6 Example of assembly cell

∑ ∑+= TimesWalk TimesOperation CT Cell a

aCT Cellavailable Time Capacity Cell =

When subcells exit subcell)each of max(CT CT Cell a = Problem Consider a workcell shown in fig. 7(a). The number near the operations is processing time in seconds and number on the arrows is the walk times. Determine the actual cycle time and cell capacity. Assume 8 hour workday.

Fig. 7 (a) One worker assembly cell (b) Two-worker assembly cell Cell CTa =451 sec/unit Cell capacity = 63.9 units/day Assume that the workcell is divided into two subcells as shown in fig 7 (b). Determine the actual cycle time and cell capacity.

Subcell (1) CTa = 231 sec/unit Subcell (2) CTa = 238 sec/unit Cell CTa = 238sec/unit

(a) (b)



If the cell is subdivided among five workers as shown in fig. 8, determine the actual cycle time and cell capacity.

Fig. 8 Five worker assembly cell

Worker Subcell stations CT (Sec) 1 1 and 2 109 2 2 and 4 91 3 5 and 6 101 4 7 90 5 8 90

Cell CTa = 109 sec/unit Cell capacity = 264 units/day

Note: Due to imbalance of cycle times among workers, all workers except the

slowest have to wait

Slowest worker is with cycle time 109 sec

Total idle time of 4 workers = 18+8+19+19 = 64sec/unit

Suppose every workstation is assigned a worker. Walk times are eliminated, but suppose picking up and placing items at each station takes 4seconds. Determine the actual cycle time, total idle time and identify the slowest station/s.

Cell CTa = 84 sec/unit Stations 7 and 8 are slowest Total idle time is 40+30+60+30+40+40 = 240 sec/unit

Productivity improvement

• Workcell productivity improvement efforts are aimed at meeting required cycle time with fewest workers.

Suppose the required cell cycle time is 123 sec/unit and is adopted a five worker assignment as shown in the fig.8. The subcell cycle time and idle time are as shown below

Worker Subcell stations CT (Sec) Idle time (Sec) 1 1 and 2 109 14 2 2 and 4 91 32



3 5 and 6 101 22 4 7 90 33 5 8 90 33 Total 481 134

• Total idle time is 134 sec/unit which is higher than required cycle time

• Theoretical implication is that one less worker is needed than the five currently assigned

• To reduce the number of workers, a way must be found to reduce the task times and reassign the workers so that most of the idle time falls on just one worker

• Suppose through productivity improvement efforts the task times are reduced at some stations and workers are reassigned such that the following results:

Worker Subcell stations CT (Sec) Idle time (Sec) 1 1 and 8 120 3 2 2 and 6 120 3 3 3 30 93 4 7 94 29 5 4 and 5 101 12 Total 465 140

• This assignment may be worst compared to the previous

• Most of the idle time is with station 3. If we can reduce the time at station 3 or 7 by 1 second and if these stations are combined into a single station, a worker can be reduced.

• Who will be removed?

Overall, the most highly skilled worker

• Why? Two reasons

(i) Easier to reassign most highly skilled worker

(ii) A message that reassignment is a reward for good performance

• This practice identifies the top performing worker without exposing average or below average worker

• An example, Allen-Bradley in Milwaukee, where a group of highly experienced and able workers in a volunteer group called SWAT team accept assignment anywhere in the plant, based on demand

Machining Workcells

• Operations are done by machines, with one or a few machines located at every workstation

• Machines are often automatic and single-cycle –stop after the completion of machining operation

• Stations and machines are connected to one another using decouplers



• Decouplers allow machines in a sequence to operate somewhat independently

Functions of decoupler

• WIP control – preceding station stops when the number of units in the decoupler reaches the maximum

• Transportation – Automatically transfer parts from operation to operation

• Worker freedom for movement – the above points allow free movement for worker

• Automatic inspection – mechanical or electronic sensors on decouplers perform inspection of critical dimensions as parts move from one operation to the next

• Route identification – cells with multiple kinds of parts, sensors check features of a part to determine to which possible downstream machines this part to be routed and to what parameters the next machining operation should be set

• Reorienting – reorient part so that it is ready for insertion into the next operation

• Leap-frog or skip operations – identify the operations to be skipped based on the process plan

• Converging or branching – allows multiples machines to feed into a single machine or a single machine to branch into multiple machines

Actual Cycle time determination

• Assume that each station in a machining cell has a single-cycle automatic machine

• Worker cycle time and machine cycle time are required in this case

• Task of worker – Unload, change over, load, start machines and walk between machines

• Time of machines - time per unit to setup the machine (Unload, change over and load machine) and to perform operation

Worker Cycle time = ∑ Task times + ∑ Walk times

Machine cycle time = Time per unit to setup + Operation time

Cell cycle time, one worker = Max (Worker CT, Longer machine CT)

Cells with two more subcells

Cell cycle time = max (Subcell cycle times)

Problem

Consider the one-worker machining cell in fig. 9(a). The number between adjacent stations is the worker walk time. The worker task time (load, unload and start) is 10 seconds per machine. Assume an 8 hour workday. Determine the cell actual cycle time, cell capacity.

If the cell is divided into two subcell by adding one more worker as shown in fig. 9(b). Determine the cell actual cycle time, cell capacity.



Fig. 9 (a) One-worker machining cell (b) Two-worker machining cell Case (a)

Worker CT = 8 (10) + 51 = 131 sec Machine CT = machine automatic CT + machine setup time Longest Machine CT = 70 + 10 = 80 sec/unit CTm = max(131,80) = 131 sec/unit

Cell capacity = 13160608 ××

=219.8 units/day

Case (b) Subcell 1

Worker CT = 3 (10) + 31 = 61 sec Longest Machine CT = 70 + 10 = 80 sec/unit Subcell CT = max(61, 80) = 80 sec/unit

Subcell 2 Worker CT = 5 (10) + 38 = 88 sec Longest Machine CT = 40 + 10 = 50 sec/unit Subcell CT = max(88, 50) = 88 sec/unit

Cell CT = max(subcell CTs) Max(80, 88) = 88 sec/unit

Cell capacity = 8860608 ××

= 327 units/day

• The cell worker cycle time determines the cell cycle time. That is by adding more workers the cell cycle time can be brought down.

• If the longest machine cycle time is greater than worker cycle time then, machine cycle time determines the cell cycle time.

• To reduce the cell cycle time add a machine at the bottleneck station

• Besides adding workers, cell cycle time cam be reduced by manual task time

Determine the cell cycle time for the cell given in fig. 10

(a) (b)



Fig. 10 Three-worker machining cell WORKCELL CAPACITY

• The desired output capacity should be based on demand

• Demand is a variable

• We have seen that the output capacity can be varied by varying workforce size

• However other design alternatives should also be considered Number of machines in a cell

Procurement of new machines

Cross-training of workers

Preparation of backup cell

Expansion of existing cells

QUESTIONS: 1. What are the building blocks of cellular manufacturing system?

2. Why do you prefer a U-shape layout for cells?

3. Explain pull production concepts for linked-cells.

4. What is a linked-cell system? Explain the linked cell concept. Explain the factors affecting the production rate of linked-cell system.

5. Distinguish between a machining workcell and assembly workcell.

6. Distinguish between cell cycle time and required cycle time (takt time).



7. Consider an assembly cell, which consists of 8 workstations and in the production process all the workstations are used sequentially from first to last. The workstations are arranged in the sequence A-B-C-D-E-F-G-H. The cell is operated with one worker, and the work part is small and hence the worker carries a rack that holds six units of the product. The worker does tasks on six units before carrying them to the next workstation. Assume handling of each unit at each workstation takes 3 seconds. The operation time at each workstation for a unit is given below.

Workstations A B C D E F G H Operation time for each unit

40 50 20 50 40 40 80 80

The walk time between workstation/stock areas are given below.

Between Inbound stock area and A

A and B

B and C

C and D

D and E

E and F

F and G

G and H

H and outbound stock area

Outbound and inbound stock area

Walk time

2 5 5 7 5 7 5 5 2 8

What is the average cycle time per unit in seconds? If the cell operates in a day 8 hours, what is the capacity of the cell? If the required cycle time is 310 seconds, suggest various ways of using the cell for this cycle time.

8. A four-workstation cell with one worker has single-cycle machines to perform all operations. The walk time around the cell is 60 seconds. The times (in seconds) for the machine operating cycles and setup (unload, changeover, load and start machines) are listed below.

Machines A B C D Operating Cycle(Sec) 152 173 175 190 Setup (Sec) 23 31 52 28

The cell produces different kinds of parts continuously, one unit at a time.

a. What are the actual cell cycle time and cell capacity (daily capacity, number of working hours 8 per day)?

b. What is the required cycle time when the demand per day is 140 units? Can a cell cycle time of 215 sec. meet this required cycle time? Discuss where in the cell you would have to make changes to achieve this cycle time. Discuss alternatives or possible actions for making the changes.

9. A workcell with two workers is divided into two subcells; one subcell has an actual CT of 323 seconds, the other has an actual CT of 392 seconds. The workcell must produce a part with a required CT of 410 seconds. What is the required production capacity of the workcell per day? Does the workcell have adequate capacity? If it produces at the required CT, what is the resulting amount of daily idle time of the two workers? If it produces at the current maximum



possible rate, how much will its daily output differ from the required output? Assume 8hr workday.

10. A process operates for 480 minutes a day. This process produces two items A and B. The production part mix fraction for the items A and B is 0.7 and 0.3 respectively. The total production quantity of A and B is 450 units. What is the cycle time for both products together? What is the cycle time of individual products?

11. A process that produces part A operates for 480 minutes a day. The required daily output for part A is 320 units. (a) What is the required cycle time for part A? (b) If the same process must also produce 130 units per day of part B, What is the average required cycle time for both products?

12. Use the eight-station machining cell shown in figure given below to answer the following. Assume all machines are single-cycle, automatic, and require a 10-second setup time.

Divide the cell among four workers so the times among them are as similar as possible. What are the resulting cell Cycle Time (CT) and cell capacity?

Suggest a way to reduce cell CT to at least 45 seconds per unit by subdividing the cell among five workers and adding machines. Indicate the number of workers and their machine assignments; also indicate the number of each kind of machine needed at each workstation. Add 2 seconds of walk time for every machine a worker walks past (i.e., bypass without stopping).

13. A workcell shown in figure given below is operated by one worker who walks from station to station. All the operations in the workcell are manual assembly with the exception of operation at workstations 7 and 8, which are each performed by single-cycle machine that runs for 200 seconds and stop when finished. The figure shows the worker’s route around the cell and the relevant times: the number next to each assembly station is the time required for the worker to perform the operation; the number by each arrow is the time to walk between locations. There is no buffer between stations except between stations 7 and 8. There are sufficient components in this buffer for an uninterrupted operation when worker comes to station 8. The worker simply moves the items being worked from station to station with him up to station 7. At station 7 he unloads job to the buffer and loads the job he brought. The unload, load and start time is 12 seconds. After starting the



machine at station 7, he moves to station 8 where he unload the job and place it in out bound buffer and then load the machine at station 8 from buffer. The time for unload, put in outbound buffer, load and start is 12 seconds. After starting the machine he leaves the station 8. Determine the cycle time of the cell.

Fig: A workcell showing stations and worker movement

Fig. 1 Cell with one worker and six machines of workcells.pdf · • This is some time referred as tackt time • Actual cycle time is the actual production capability of a process

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