scheduling contribution of key topics to scheduling

SchedulingScheduling

CONTRIBUTION OF KEY TOPICS CONTRIBUTION OF KEY TOPICS TO SCHEDULINGTO SCHEDULING

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Objective of this section is to emphasise the importance of some key topics which may become very critical in real life applications in which a schedular is involved or affected in some way or another.

Contribution of Key Topics to Contribution of Key Topics to Scheduling Scheduling

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Development of “Standard Operation Times”

In this case, what we are talking about is the “In this case, what we are talking about is the “standard operation time standard operation time per pieceper piece” which have to be spent for a shop order of a specified part ” which have to be spent for a shop order of a specified part number when it is processed at a work center.number when it is processed at a work center.

The sum of all “standard operation times per piece” in the routing of a The sum of all “standard operation times per piece” in the routing of a shop order multiplied by the number of pieces on it will be equivalent shop order multiplied by the number of pieces on it will be equivalent to its “to its “standard work contentstandard work content” in terms of man or machine hours. ” in terms of man or machine hours.


Op2+Op3

Station 2

Std work content of F004250 for Station 2 = [(10x12) + (10x6)] = 180 min/ (for this mach)

Shop OrderF004250

Part No: A1025-22Quantity: 10

Std OP time per piece = 12 min Std OP time per piece = 6 min

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A schedular must have information or at least an idea of A schedular must have information or at least an idea of “standard work content”s of shop orders to be loaded to a “standard work content”s of shop orders to be loaded to a work center in order to come up with respective “work center in order to come up with respective “remaining remaining work contentwork content” values associated with each respective shop ” values associated with each respective shop

order.order.


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It is usually difficult to find such “standard times” It is usually difficult to find such “standard times” especially in small and medium scale companies.especially in small and medium scale companies.

In large scale companies, they are usually obtained by In large scale companies, they are usually obtained by treating the “treating the “actual operationactual operation timetime”s with some factors ”s with some factors which are found to be appropriate.which are found to be appropriate.

Sometimes, the operation times which are standardized in Sometimes, the operation times which are standardized in this way, this way, are assigned to the work center itselfare assigned to the work center itself. In such . In such cases, it is assumed that all the part numbers passing cases, it is assumed that all the part numbers passing through that specific work center will have through that specific work center will have exactly the exactly the same “standard operation time”!same “standard operation time”!


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Is it acceptable to assign a standard operation time to a Is it acceptable to assign a standard operation time to a work center?work center?


PART B

A Drilling work center having two different drill machines.

PART A

Two parts having different sizes, differenthole diameters, different number of holes and made of different materials.

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Development of “Standard Operation Times” Therefore, development of reliable “standard operation Therefore, development of reliable “standard operation

time”s is a very important subject for realistic scheduling time”s is a very important subject for realistic scheduling especially in job shops. especially in job shops.

The concept of “Earned Hours”The concept of “Earned Hours” ““Earned hoursEarned hours” and “” and “remaining work contentremaining work content” of a shop ” of a shop

orderorder are complementary values some of which is equal to are complementary values some of which is equal to its “its “standard work contentstandard work content” in terms of man or machine ” in terms of man or machine hours.hours.


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The concept of “Earned Hours”The concept of “Earned Hours” Inorder to determine the “remaining work content”, one has to know Inorder to determine the “remaining work content”, one has to know

the “earned hour” value of a specific job at any point in time.the “earned hour” value of a specific job at any point in time.


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The concept of “Earned Hours”The concept of “Earned Hours” Determination of earned hour values in flow shops (either Determination of earned hour values in flow shops (either

flow assembly work or flow fabrication work) is somewhat flow assembly work or flow fabrication work) is somewhat straight forward.straight forward.

On the contrary it is difficult to make exact determinations On the contrary it is difficult to make exact determinations in job shop environments.in job shop environments.

Such environments may require additional tasks of Such environments may require additional tasks of mapping the routing of each part number in terms of mapping the routing of each part number in terms of manufacturing progress steps. In such a case, shop order manufacturing progress steps. In such a case, shop order should only follow the specified routing steps. should only follow the specified routing steps.


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The concept of “Earned Hours” : An ExampleThe concept of “Earned Hours” : An ExampleContribution of Key Topics to Scheduling Contribution of Key Topics to Scheduling

Op 1

Work/center 1

Op 2 Op 3

W/c 2

Op 4

W/c 3

Op 5

W/c 4

Op 6

W/c 5

Op 7

Due date of theshop order (job)

Start date of the shop order (job) F300300 of qty 6 for part no 456789-10

Work center

Operation

Operation time/pc

Work content

CNC Router

cutting

2 min

12 min

4 min

24 min

Press Brake

bending

Age oven

0,5 min

3 min

heat treat

25 min

25 min

Hand Form

forming

Paint

painting drying

3 min

18 min

1,5 min

9 min

45 min

45 min

SampleRule 1

when s/o is reading in a w/c, and if there is a labor charge, we can assume EH for S/o= (1/3)(wrk cont of respective w/c)+Σwrk contents of previous work centers.

EH=12min EH=37min EH=47min EH=70min EH=100min

when s/o is reading in a w/c, and if there is no labor charge for that w/c, EH for S/o= Σwrk contents of previous work centers. Sample

Rule 2EH=0min EH=36min EH=39min EH=64min EH=82min

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The concept of “Group Technology” (GT)The concept of “Group Technology” (GT) GT is a technique for identifying and bringing together GT is a technique for identifying and bringing together

related or similar parts/components or machines in a related or similar parts/components or machines in a production process, in order to take advantage of their production process, in order to take advantage of their similarities by making use of the benefits of flow similarities by making use of the benefits of flow production methods. production methods.

The question is whether GT and MRP can be integrated in The question is whether GT and MRP can be integrated in shop scheduling.shop scheduling.

Both GT and MRP are usefull in planning an control of Both GT and MRP are usefull in planning an control of multiproduct, small lot-sized job shops. multiproduct, small lot-sized job shops.


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The concept of “Group Technology” (GT)The concept of “Group Technology” (GT) However, they appear to be in conflict with respect to their However, they appear to be in conflict with respect to their

individual operational characteristics;individual operational characteristics;

Group Technology wants similar parts manufactured at Group Technology wants similar parts manufactured at the same time regardless of the requirement dates, the same time regardless of the requirement dates,

MRP wants parts to be manufactured at the scheduled MRP wants parts to be manufactured at the scheduled times regardless of the manufacturing impact.times regardless of the manufacturing impact.

Inspite of the above indicated conflict, the answer for Inspite of the above indicated conflict, the answer for possibility of integration is “yes”.possibility of integration is “yes”.


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The concept of “Group Technology” (GT)The concept of “Group Technology” (GT) Especially at workstations where both the number of Especially at workstations where both the number of

unique part numbers and the volume of production is high, unique part numbers and the volume of production is high, respective parts, reardless of their position in the product respective parts, reardless of their position in the product structure, can be grouped by their manufacturing structure, can be grouped by their manufacturing characteristics and by the requirement quantities for fixed characteristics and by the requirement quantities for fixed intervals of time. intervals of time.

For such a group, most probably, the production schedule For such a group, most probably, the production schedule (qty and timing) will be the production schedule of the part (qty and timing) will be the production schedule of the part which has the earliest due date.which has the earliest due date.


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The concept of “Flow efficiency”The concept of “Flow efficiency” Measuring flow efficiency in job shop manufacturing Measuring flow efficiency in job shop manufacturing

environment will give important clues to the schedular environment will give important clues to the schedular about the shop behaviour in terms of the speed of about the shop behaviour in terms of the speed of production flow. production flow.

““Flow Efficiency is defined as the ratio of the fabrication Flow Efficiency is defined as the ratio of the fabrication time (touch labour time) to the cycle timetime (touch labour time) to the cycle time” ” where ““fabrication time, lot process delay, storage delay, and fabrication time, lot process delay, storage delay, and transportation delaytransportation delay” ” are its major components.are its major components.


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The concept of “Flow efficiency”The concept of “Flow efficiency”


Actual labour hours per part per crew member

Total cycle time the part is in the system – router queing Flow Efficiency = ( )*100

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The concept of “Flow efficiency”The concept of “Flow efficiency” ““Factory Flow Benchmarking Report, Lean Aircraft Factory Flow Benchmarking Report, Lean Aircraft

Initiative, 1996”Initiative, 1996” tells that; TThere would be higher flow efficiencies with “lower lot here would be higher flow efficiencies with “lower lot

sizes” and with “shorter distance travelled”.sizes” and with “shorter distance travelled”. The flow efficiency, at its best, will vary inversely with The flow efficiency, at its best, will vary inversely with

the lot size (Max attainable flow efficiency = 1 / lot the lot size (Max attainable flow efficiency = 1 / lot size).size).

It has been detected that, there were no flow efficiencies It has been detected that, there were no flow efficiencies above 2 % if the parts travelled more than 2000 feet above 2 % if the parts travelled more than 2000 feet (609.6 m).(609.6 m).


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The concept of “Flow efficiency”The concept of “Flow efficiency” It has been found that, the process type layout had a It has been found that, the process type layout had a

great deal to do with the flow efficiency. Job shop great deal to do with the flow efficiency. Job shop layouts did not achieve above 0.1 % flow efficiencies layouts did not achieve above 0.1 % flow efficiencies while flow shops, cells or dedicated lines were able to while flow shops, cells or dedicated lines were able to achieve values as high as 18.7 % flow efficiencies.achieve values as high as 18.7 % flow efficiencies.

The median values for routing queuing (mnfg approval The median values for routing queuing (mnfg approval time) ranged from 11% to 32% of the cycle time. time) ranged from 11% to 32% of the cycle time. Maximum value was 83 %.Maximum value was 83 %.

The distances travelled by the parts found to be The distances travelled by the parts found to be averaging from 2416 feet (736 mt) to 5023 feet (1531 averaging from 2416 feet (736 mt) to 5023 feet (1531 mt).mt).


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The concept of “Flow efficiency”The concept of “Flow efficiency” Process control steps ranged from a low of 9.3 % to a Process control steps ranged from a low of 9.3 % to a

high of 81.4 % and averaged from 44 % to 55 %. The high of 81.4 % and averaged from 44 % to 55 %. The predominant process control method (at about 80 %) predominant process control method (at about 80 %) was process verification consisting mostly of manual was process verification consisting mostly of manual inspection.”inspection.”

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An analysis had been carried out at Turkish Aerospace Industries in 1999, about fabricated flat sheet metal parts. This study was covering a total of 599 shop orders (4867 pieces) for which “flow efficiency” & “cycle times” were calculated.

Obtained results were as follows;

Scheduled Flow Efficiency = 0.36 % Actual Flow Efficiency = 0.60 % Attainable Max Actual Flow Efficiency = 30 % Scheduled Cycle Time = 66 M-days. Actual Cycle Time = 70 M-days.

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However, there is a trap in dealing with flow efficiency values, from which a schedular must avoid. This can be better explained by the following example;

Let’s assume that, shop order F200200 is released for manufacturing of 6 pieces of part number 353535-35.

Again, let’s assume that, Only one operator worked on the shop order at each work station it

passed through. The scheduled cycle time for shop order is 10 M-days. The total standard work content (std touch labour) for shop order is 150

minutes. The standard router queue time is 5 M-days. The actual router queue time is realized as 7 M-days. The actual cycle time for shop order is realized as 15 M-days. The total actual touch labour for shop order is realized as 270 minutes.

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Therefore the calculation for realized flow efficiency will yield following result; FE = {[(270/6)/1]/[[(15-7)*7,5]*60]}*100 = (45/3600)*100

= 1,25% and Max FE = 1/6 = 16,7 % Where the scheduled flow efficiency for the case was as

follows; FE = {[(150/6)/1]/[[(10-5)*7,5]*60]}*100 = (25/2250)*100

= 1,11 % , and Max FE has the same value of 16,7 %.

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Activity Grouping in Large Assembly StationsActivity Grouping in Large Assembly Stations In large assembly lines where the end item moves through In large assembly lines where the end item moves through

a series of work stations, each station corresponds to a a series of work stations, each station corresponds to a specific “work breakdown structure” (as discussed specific “work breakdown structure” (as discussed previously).previously).

In this way, the whole end item is represented by a series of In this way, the whole end item is represented by a series of “work breakdown structure”s each of which is a subject of “work breakdown structure”s each of which is a subject of master scheduling.master scheduling.

That is, there is a separate master schedule for each “work That is, there is a separate master schedule for each “work breakdown structure” where each master schedule is breakdown structure” where each master schedule is sequentially dependent on the previous one in the series. sequentially dependent on the previous one in the series.

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Activity Grouping in Large Assembly StationsActivity Grouping in Large Assembly Stations For this reason they are called “scheduled work breakdown For this reason they are called “scheduled work breakdown

structures” (SWBS). structures” (SWBS).

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Activity Grouping in Large Assembly StationsActivity Grouping in Large Assembly Stations

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Activity Grouping in Large Assembly StationsActivity Grouping in Large Assembly Stations Activity grouping in such assembly lines may prove to be Activity grouping in such assembly lines may prove to be

usefull in the operation of job shops by distributing the usefull in the operation of job shops by distributing the work load as evenly as possible through out the time. This work load as evenly as possible through out the time. This grouping is about the grouping of the operations within an grouping is about the grouping of the operations within an SWBS by considering their interdepencies. SWBS by considering their interdepencies.

Thus, it becomes possible to distribute the requirement Thus, it becomes possible to distribute the requirement dates of detail parts over the make span of the respective dates of detail parts over the make span of the respective SWBS. SWBS.

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Activity Grouping in Large Assembly StationsActivity Grouping in Large Assembly Stations In the current example, 14 operations of SWBSC2 are divided In the current example, 14 operations of SWBSC2 are divided

into three groups for which the respective requirement dates are into three groups for which the respective requirement dates are M-Day 4500 (for 2532 pieces), M-Day 4515 (for 1365 pieces) M-Day 4500 (for 2532 pieces), M-Day 4515 (for 1365 pieces) and M-Day 4530 (for 1470 pieces) respectively. The next step to and M-Day 4530 (for 1470 pieces) respectively. The next step to do is to treat the process times of 14 operations as follows;do is to treat the process times of 14 operations as follows;

First Group (6 operations) ; Process time : 45 M-days (no change).First Group (6 operations) ; Process time : 45 M-days (no change). Second Group (4 operations); Change Process times from 45 to 30.Second Group (4 operations); Change Process times from 45 to 30. Third Group (4 operations); Change Process times from 45 to 15.Third Group (4 operations); Change Process times from 45 to 15.

The rest will be performed by MRP software yielding to a The rest will be performed by MRP software yielding to a smoother shop order release activity and in this way, a smoother smoother shop order release activity and in this way, a smoother loading of shops. loading of shops.

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Takt Time and Cycle Time concepsts : What do Takt Time and Cycle Time concepsts : What do they mean to a Schedular?they mean to a Schedular? Both “takt time” and “cycle time” are the powerfull tools in Both “takt time” and “cycle time” are the powerfull tools in

understanding the shop floor.understanding the shop floor. Variances in the respective values may occur from period Variances in the respective values may occur from period

to period due to some expected or unexpected reasons such to period due to some expected or unexpected reasons such as “over loading”, “machine breakdown”, “unavailability as “over loading”, “machine breakdown”, “unavailability of a tool”, “inexperienced worker”, “seasonal high or low of a tool”, “inexperienced worker”, “seasonal high or low temperatures”, or even “the announcement of raise % in temperatures”, or even “the announcement of raise % in monthly salaries”. monthly salaries”.

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Takt Time and Cycle Time concepsts : What do Takt Time and Cycle Time concepsts : What do they mean to a Schedular?they mean to a Schedular? In manufacturing terms, In manufacturing terms, takt time is a takt time is a calculated valuecalculated value. . ItIt is the is the speed speed

at which parts must be manufacturedat which parts must be manufactured in order to satisfy demand. It is in order to satisfy demand. It is

simple enough to calculate;simple enough to calculate; Determine the daily demand (order) volume. Lets say we have orders for Determine the daily demand (order) volume. Lets say we have orders for

215 arm chairs per day. 215 arm chairs per day. Determine the number of working minutes in a day. Let's say that we have Determine the number of working minutes in a day. Let's say that we have

an eight-hour day, with 30 minutes for lunch and two 10-minute breaks. an eight-hour day, with 30 minutes for lunch and two 10-minute breaks. This means we have: This means we have: (8 x 60)-30-10-10=430(8 x 60)-30-10-10=430 minutes, or 430 minutes in a minutes, or 430 minutes in a working day.working day.

Divide the number of minutes by the number of products needed. In our Divide the number of minutes by the number of products needed. In our current example, the calculation would be 430÷215, which equals 2. This current example, the calculation would be 430÷215, which equals 2. This means that one unit must be manufactured every other minute in order to means that one unit must be manufactured every other minute in order to meet demand. Takt time is 2 minutes. meet demand. Takt time is 2 minutes.

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Takt Time and Cycle Time concepsts : What do Takt Time and Cycle Time concepsts : What do they mean to a Schedular?they mean to a Schedular? We also need to mention that there are certain points which one has to We also need to mention that there are certain points which one has to

keep in mind when talking about “takt time”. These points can be keep in mind when talking about “takt time”. These points can be

clarified as follows;clarified as follows; ““Takt time” does not consider the terms “Takt time” does not consider the terms “capacitycapacity” or “” or “hours per hours per

piecepiece”. ”. Work centers for which takt time will be the measure of performance, Work centers for which takt time will be the measure of performance,

should be arranged in a way to process “should be arranged in a way to process “part familiespart families”.”. In a production environment which is going to be monitored by takt In a production environment which is going to be monitored by takt

time, the work content or product mix and part routings, time, the work content or product mix and part routings, should remain should remain samesame as much as possible. as much as possible.

Again in such environments, all of the shop orders must be Again in such environments, all of the shop orders must be

““workableworkable”.”.

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Takt Time and Cycle Time concepsts : What do Takt Time and Cycle Time concepsts : What do they mean to a Schedular?they mean to a Schedular? The second type of time is "The second type of time is "cycle timecycle time”. This is”. This is aa measured measured

valuevalue, not a calculated value as takt time is. In other words, , not a calculated value as takt time is. In other words, you must go out to the floor, and measure the time it you must go out to the floor, and measure the time it

actually does take to manufacture the product.actually does take to manufacture the product. When making time observations, it is important to measure When making time observations, it is important to measure

both the both the total cycle time for each operatortotal cycle time for each operator (how long the (how long the job takes from beginning to end), and the job takes from beginning to end), and the time of each of time of each of the component tasksthe component tasks that make up the cycle. Often it is that make up the cycle. Often it is possible to reassign component tasks to possible to reassign component tasks to rebalance the rebalance the operation. operation.

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Takt Time and Cycle Time concepsts : What do Takt Time and Cycle Time concepsts : What do they mean to a Schedular?they mean to a Schedular? After making the observations, one can draw out an After making the observations, one can draw out an

Operator Loading Bar Chart to graphically express what is Operator Loading Bar Chart to graphically express what is

going on in previous example.going on in previous example.

We get one arm chair every two minutes, but the six operators are working for a total of 8 minutes to do it.

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Takt Time and Cycle Time concepsts : What do Takt Time and Cycle Time concepsts : What do they mean to a Schedular?they mean to a Schedular? Again assume that, we observe enough waste in the work Again assume that, we observe enough waste in the work

cycle to set a team goal for reducing the total cycle time cycle to set a team goal for reducing the total cycle time

from 8 to 6 minutes.from 8 to 6 minutes.

To determine the staffing of this line, we divide the new total cycle by the takt time: 6 ÷ 2 = 3 operators.

Don’t forget that, “takt time is the goal”.

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Takt Time and Cycle Time concepsts : What do they mean Takt Time and Cycle Time concepsts : What do they mean to a Schedular?to a Schedular?

A similar “Load Chart” can be given from a real life example of what has been achieved in assembly of flight deck panel production.

scheduling contribution of key topics to scheduling

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