08_Machine Scheduling and Flow Shop

Question 1

Basic Input Data of Detailed Scheduling

Answer 1

1. target due dates
   - determined in lot sizing
2. process plans:
   - list of required processing tasks (contains documentations and sequence of tasks, per unit production times, setup and transition times etc.)

Question 2

Goals of Job Shop Scheduling

Answer 2

Minimize:
- job lateness
- idle times of machines
- throughput times of orders
- sequence dependent set up times

Notice how these objectives are all time related instead of cost related

Question 3

Scheduling Dilemma

in context of Job Shop Scheduling

Answer 3

Minimizing
- idles times of machines and
- throughput times of orders

Question 4

Popular Dispatching Rules

6 items

Answer 4

• shortes processing time
• longest processing time
• most work remaining
• least work remaining
• nearest due date
• first-come-first-served

Question 5

Order dispatching in Job Shops

Which method to use according to SPT rule

Answer 5

• shortes processing time rule
• use Gantt Chart

Question 6

Goals of Flow Shop Scheduling

Answer 6

• smoothing of capacity load at bottleneck station:
• starvation: processing at predecessor station takes too long based on chosen product sequence so that idle times occur
• blocking: processing at successor station takes too long based on chosen product sequence so that buffer storages overflow and station is blocked

Question 7

Simplifying Assumptions of Flow Shop Scheduling

Answer 7

• only one single bottleneck station considered for which product sequence is systematically determined

Question 8

Bottleneck Station in Flow Shop Scheduling

Answer 8

• shows the heaviest average workload based on given production volume of product variants

Question 9

Determinatin of product sequence by use of a priority rule

Flow Shop Scheduling

Answer 9

min d(j) = |b(j) + B(p-1) - (p x c)|

where

• b(j) = per unit production time of product j in the bottleneck station
• pxc = projected average workload at the bottleneck station after p product units (c = cycle time; p = counter for products in the sequence)
• B(p-1) = workload of bottleneck station caused by the preceeding p-1 product units
  = ∑b(k) with B(0) = 0

Question 10

Characteristics of
Common Mixed Model Assembly Lines

Answer 10

• manufacture of variants of a product type on the same production line
• no set up time
• similar task sequence with varying task times
• cycle time usually considered as flexible target
• key planning problem: determination of the product sequence under consideration of the flexible cycle time

Question 11

How to calculate the minimum part set in flow shop scheduling?

Answer 11

• the production cycle = ∑ of demand of each product divided by smallest unit of demand of the products

e.g.
Demand per period
A = 120
B = 180
C = 60
∑ = 360

**Minimum part set 360/60 = 6
**
2A; 3B; 1C
-> part set is multiplied by 60 in this case

Question 12

How to determine resulting cycle time for the bottleneck station

Answer 12

• calculate total workload for each station using minimum part set
• choose station with highest workload
• workload time / (minimum part set)
• e.g. 18 min / (3+2+1) = 3

Question 13

Procedure Flow Shop Scheduling

Answer 13

1. Determine Production cycle (minimum part set) [e.g. 2A, 3B, 1C]
2. Determine bottleneck station with highest workload
3. Determine cycle time [workload bottleneck station divided by minimum part set e.g. c = 18 min / (2+3+1)
4. Determine product sequence using priority rule [d(j) = |b(j) + B(p-1) - (p x c)|

p = counter for products in the sequence