IE 368: FACILITY DESIGN AND OPERATIONS MANAGEMENT Lecture Notes #2 Production System Design Part #1

Production System Design Abstraction of production systems for system design purposes General concepts/definitions that may be used to represent many different systems High level qualitative analysis for the selection of a general system flow concept

Production System Design (cont.) Part #1 Calculations for estimating resource requirements Equipment fraction calculations Extensions Machine assignment Part #2 Calculations for evaluating system performance Review of relevant probability/statistics concepts Application of queuing models

Production System Design (cont.) Part #3 Generalizations of queuing model Generalizations of utilization formula Multiple linked workstations Automated systems Batched arrivals and departures

Production system Workstation Generalization/Abstraction of Production Systems for IE Design/Analysis Production system A collection of workstations that perform operations such as manufacturing, assembly, inspection, finishing, testing, etc. to create products Workstation A collection of machines/operators that perform the same operation for the same set of products A machine/operator may be: An automated machine A machine operated by a human A human operator performing a manual operation * Terminology in this area is not standardized

Generalization/Abstraction of Production Systems for IE Design/Analysis (cont.) The production systems to be addressed are discrete part production systems Each part produced is a distinct entity Vehicle, computer, hamburger, etc. This is in contrast to continuous goods production such as fluids, powders, etc. Often in the domain of chemical process engineers Thus, the product being produced will be referred to generically as a part or job

Production System Design The general arrangement of workstations, dictating the pattern of flow of the products, and the resource requirements at each workstation WS1 WS5 WS4 WS3 WS2

Production System Performance Characterization At the level of abstraction presented, the performance of the production system is evaluated by determining the following: How fast Throughput (e.g., jobs/hour, parts/minute) How long Time-In-System (TIS) Flow Time Cycle Time (we will not use this term) How much Work-In-Progress (WIP)

Examples of Production Systems Simulation models Truck assembly FMS Distribution center Questions What constitutes a job? What are the workstations? Why are jobs flowing as they are?

Production System Design – Different Perspectives Manufacturing engineer Designs/selects the processes and operations required to produce the product e.g., fixturing, tooling, feed rates, cutting speeds, etc. Human factors engineer Design of the individual human operated workstation e.g., bench heights, lifting angles, placement of tools, presentation of visual information, etc.

Production System Design – Different Perspectives (cont.) Higher level IE analysis/business operations Supply chain design The number, level, and location of suppliers Delivery, ordering, inventory policies The number of distributors and their locations …

General Production System Flow Examine product volume versus variety Typically cannot have both Automation Hard Systems Special Purpose Machines Volume of Production General Purpose Equipment Flexible Systems Skilled labor Variety of Products or Parts

Basic Types of Production System Flow Example Job Shop Batch Processing Production Line Continuous Flow

Job Shop Characteristics Many products; low volume General purpose machinery Operators work at only one department and are highly skilled in that operation Process Layout Equipment of the same type is located in the same department Unorganized material flow High material handling High cycle time High flexibility

Job Shop Characteristics (cont.)

Production Line Characteristics Few products; high volume (mass production) Highly standard parts Somewhat similar to continuous production Special purpose machinery Low skill labor Equipment is arranged into a line almost in the same order of required production operations Low material handling Short cycle time Well organized material flow

Production Line Characteristics (cont.) http://www.wk-industries.com/en/sandwich/dachundwand.html

Group Technology/Batch Processing Characteristics Fewer products than Job Shop; higher production volume Products are produced in batches satisfying a few days up to few months of demand Less general purpose machinery than job shop Process layout & cellular layout; machines to produce family of products are located in the same cell Large batches have organized material flow High to moderate material handling Moderate cycle time High flexibility

Group Technology/Batch Processing Characteristics (cont.) Group Technology Cell

Group Technology/Batch Processing Characteristics (cont.) Batch processing layout

Product Volume versus Variety Product Volume versus Product Variety in Production System Design Volume of Production Variety of Products or Parts

Variety-Volume-Flexibility Product Variety High Moderate Low Vey Low Equipment Flexibility Volume Moderate Volume High Volume Very High Volume

Quiz For the following situations, would you suggest a production line, job shop, or hybrid facility? Why? The assembly of vehicle bodies for a popular sport utility vehicle Fabrication and assembly of custom sheet metal parts Fabrication of computer boxes for a line of desktop PC’s plus custom sheet metal parts Assembly of three distinct families of electronic card assemblies for high end printers Production of high end office furniture

Determining Resource Requirements Assume you are running a manufacturing facility. What information would you need to determine how many machines/people are required at each workstation? * Text reading – Chapter 2 pp. 51-53, 56-63

Example 2.5 A machined part has a standard machining time of 2.8 min per part on a milling machine During an 8-hour shift, 200 units are to be produced Of the 480 min available for production, the machine will be operational 80% of the time During that time the machine is operational, parts are produced at a rate equal to 95% of the standard rate How many milling machines are required?

Example 2.5 – “Common Sense” Solution

Determining Resource Requirements Equipment Fraction Number of “machines” required at a workstation Where F = Number of machines required per “shift” S = Standard time per job Q = # of jobs to produce over a fixed time period E = Actual performance expressed as a percentage of standard time H = Amount of time available per machine R = Reliability of a machine, expressed as a percent of uptime

Determining Resource Requirements (cont.) Other considerations If shifts are used In how many shifts a machine can be used Setup times Takes away available production time

Important Observations The equipment fraction as presented is helpful It is more important to understand the fundamentals behind the formula because, as is, it is not applicable in all situations A specified quantity must be produced in some time period and each machine can produce a certain amount in that time period Efficiency, availability and reliability can be expressed in different ways and all these factors affect machine capability Units must be consistent

Incorporating the Production of Scrap in the Equipment Fraction Material waste that is generated in the manufacturing process Affects the number of times an operation must be performed to get a specific number of good jobs Typically due to geometric or quality considerations

Incorporating the Production of Scrap in the Equipment Fraction (cont Nomenclature

Incorporating the Production of Scrap in the Equipment Fraction (cont WS1 WS2 ……. WSn

Example 2.1 97,000 good parts are required. Three operations are used to produce the part with scrap percentages of d1=0.04, d2=0.01, d3=0.03. What are the required inputs to each operation?

Incorporating the Production of Scrap in the Equipment Fraction (cont Calculations with rework WS2 WS1 WS3 Rework station

In-Class Exercise Part X requires machining on a milling machine Operations A and B are required Assume the company will be operating 5 days/week, 18 hours/day The following information is known: The milling machine requires 30 minutes for tool changes and preventive maintenance after every 400 parts Assume that operation A is first and that both operations A and B are completed before the next part is started Find the number of machines required to produce 2,500 parts per week Operation Standard Time Efficiency Reliability Scrap A 2 min 95% 2% B 4 min 90% 5%

In-Class Exercise – Solution

In-Class Exercise – Solution

Modifications to the Equipment Fraction Equation The equipment fraction equation is applicable to machines which can be human operators, a single operator running a single machine, and also automated machines For automated machines, data is often not expressed as in the equipment fraction equation In some cases a single operator runs >1 machine in which case the machines are not producing at their maximum rates (the opposite can occur also)

Modification for Automated Machines Automated Workstation A workstation where the movement of jobs in and out of the workstation, and the processing of jobs is performed by machines While the workstation is operating, the move times and processing times are predictable

Modification for Automated Machines (cont.) Cycle time Represented as C Total time required to produce a single job on a workstation when it is operating Normally C = Process time + Move time Move time The time to move a job into and/or out of the workstation Does the movement of a job into a workstation occur at the same time as the movement of a job out of the workstation?

Modification for Automated Machines (cont.) Mean (Operating) Time Between Failures (MTBF) The average time between unplanned failures of the machine Excludes scheduled down time or non-operating time Mean Time To Repair (MTTR) The average time to bring the machine back to operating status after a failure occurs

Modification for Automated Machines (cont.) Equipment Fraction S = standard time per job Q = # of jobs to produce over a fixed time period E = actual performance expressed as a percentage of standard time H = amount of time available per machine R = Reliability of a machine, expressed as a percent of uptime

Modification for Automated Machines (cont.) Referred to as Stand Alone Availability

Modification for Automated Machines (cont.) Example An automated machine has a move time = 10 sec/job and a processing time of 1 min/job. The machine will be used for a single 8 hr shift and has a MTBF = 75 min and a MTTR = 5 min. What is the number of machines required to produce 1,000 jobs per shift?

In-Class Exercise Part X requires machining on a CNC milling machine Operations A and B are required Assume the machine will be operating 5 days/week, 18 hours/day The following information is known: The milling machine requires tool changes and preventive maintenance after every 400 parts. This takes 30 minutes. Assume that operation A is first and that both operations A and B are completed before the next part is started Move times in and out of the machine occur at the same time Find the number of machines required to produce 2,500 parts per week Operation Process Time Move Time MTBF MTTR Scrap A 5 min 0.5 min 500 min 20 min 2% B 7 min 700 min 30 min 5%

In-Class Exercise – Solution

Rates and Times in Calculations No general rules Assess the general quantity being calculated, then use common sense Test calculations in extreme cases Example 1 – Calculate Average Speed of Vehicle 10 mph 40 mph 5 miles 5 miles

Rates and Times in Calculations (cont.) Example 2 – Calculate Average Job Interarrival Time Example 3 – Calculate Average Job Processing Rate of M1 10 min* M1 5 min 6 min * Time between job arrivals Jobs …CBACBA M1 ? M1 processing rates: A – 20 JPH B – 30 JPH C – 60 JPH

Rates and Times in Calculations (cont.)

Machine Assignment There are many cases where multiple machines are run by a single operator The number of machines running may be limited by the number of operators or the number of machines may determine how busy the operator is

Model for the Number of Machines to Assign to an Operator Assumptions All machines are identical and perform the same task All times are known and constant Use this model as a starting point or approximation

Model for the Number of Machines to Assign to an Operator (cont.) Figure 2.19 from Tompkins

Model for the Number of Machines to Assign to an Operator (cont.) Notation for machine assignment

Model for the Number of Machines to Assign to an Operator (cont.) L = Load UL = Unload T = Transport I&P = Inspect & Pack 11 11 Figure 2.18 Multiple activity chart

Model for the Number of Machines to Assign to an Operator (cont.) A machine uses (a + t) minutes (use min for clarity) per job An operator devotes (a + b) minutes to each machine per job

Model for the Number of Machines to Assign to an Operator (cont.) Since m must be an integer See Figure 2.18 (slide #53)

Model for the Number of Machines to Assign to an Operator (cont.) See Figure 2.18 Idle time per machine in steady state = 1 minute Im = m(a+b) - (a+t) = 3(2+1) – (2+6) = 1 minute

Model for the Number of Machines to Assign to an Operator (cont.) For the example in Figure 2.18 3 parts are produced every 9 minutes  20 jobs/hour Not 22.5 jobs/hour with no idle time Examine the operation of a single machine For a single machine, 1 part is produced every 6 minutes + 1 minute + 1 minute + 1 minute = 9 minutes (Machining + load + unload + idle )

Machine Assignment Impact on Cost Per Job Machine assignment decisions affect the cost per job produced

Machine Assignment Impact on Cost Per Job (cont.)

Machine Assignment Impact on Cost Per Job (cont.) Let then either TC(n) or TC(n+1) will minimize cost per job It is straightforward to show that

Example Problem 2.38 Suppose 5 identical machines are to be used to produce two different products The operating parameters for the two products are as follows: a1 = 2 min, a2 = 2.5 min, b1 = 1 min, b2 = 1.5 min, t1 = 6 min, and t2 = 8 min The cost parameters are the same for each operator-machine combination: Co = $15/hr and Cm = $50/hr Determine the method of assigning operators to machines that minimizes the cost per unit produced

Example

Example

In-Class Exercise Semiautomatic mixers are used in a paint plant. It takes 6 min for an operator to load the appropriate pigments and paint base into a mixer. Mixers run automatically and then automatically dispense paint into 50-gallon drums. Mixing and unloading take 30 min. Mixers are cleaned automatically between batches; it takes 4 min to clean a mixer. Between batches, an operator places empty drums in a magazine to position them for filling. It takes 6 min to load the drums into the magazine. Filled drums are transported automatically by conveyor to a test area before being stored. Mixers are located close enough for travel between mixers to be negligible. What is the maximum number of mixers that can be assigned to an operator without mixer idle time? If Co =$12/hr and Cm=$25/hr, what assignment of mixers to operators will minimize cost per batch?

In-Class Exercise – Solution