Design and management of production system

M ANUFACTURING CELL

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It is a production system in between the job shop and the transfer line. We have seen that with the job shop the

machine spends most of the time waiting and this could be a problem. If the production requires a family of objects,

similar parts, it is possible to standardize a lot: actually we are looking for a system that works “from a product point of

view” and not from “the action (milling, grinding, painting, …) point of view”.

Therefore we have a group of machines (cell) of different types that

completely fabricates a range of “similar” products or parts, which are called

families. Machines are grouped and placed according to the manufactured

families. Inter-cellular flows are not “allowed”.

Manufacturing cells and product families are constituted by applying models

of Group Technology, strongly related to cellular/group manufacturing and

having a wide operation range both in terms of volume and product mix.

A single operator can be trained to run different machines. Human-driven

setups are lower than processing time: a single operator can use

simultaneously more machines – well-balanced manufacturing cell: one part

at a time and multiple machine handling. U-shape is generally chosen in order

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to minimize time loss due to the operator repositioning .

Manufacturing cells are a sort of sub-companies because they are independent one on each other, they are only located

on the same shop floor.

Fewer products than job shop but higher production volume (standardized products = families)

 It is often a re-arrangement of a job shop

 Products are produced in batches (few days – few months of demand)

 Less general purpose machinery than job shop (more specific equipment are required for specific families)

 Process layout and cellular layout

 Moderate materials handling

 Reduced set-up time (due to families of products)

 Moderate lead time

 Lower flexibility than job shop but good

 Pro Cons

Job shop Flexible routings and process; skills Complex flow, high WIP, long lead-time,

grouped complex control

Flow line Low WIP, low lead-time, predictable, Limited routing/process flexibility, limited

simple, visible buffers

Cellular manufacturing Combination of advantages of flow line and job shop. Cells are more customer-oriented

and very easy to control

30 It belongs to the fabrication of discrete parts – there is also the assembly cell that works in a similar way.

31 U-shape line may reduce employee movement and space requirements while enhancing communication, reducing

number of workers and facilitating inspection. 36

Some cells are not completely self-contained: sub-contracting or inter-cell movement are possible, and also sharing of

staff. Exceptions are made in terms of products (exceptional parts) and processes (bottlenecks):

Accommodation of parts with no family

 32

Some production processes cannot be integrated into cells



Duplication of resources (trade-off) is possible to prevent inter-cell movements and to

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avoid imperfect cells .

PRO:

 Simplification of materials flows, shorter distance, great supervision (so better

quality)

 Reduction of setup time and cost (similar parts, same tools: lower change-over)

 Reduction of lot size

 Reduction of WIP (improvement of management)

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 Better space utilization

 Reduction of lead time

 Better estimate and control of delivery times

 Better capacity utilization

 Job enrichment and enlargement, self-managed teams

 Streamlined assignment of responsibilities

 More consistent quality and lower reject rates

 Ease of management

 Lower probability of worker frustration

CONS:

 Unbalanced workloads between different cells and between machines

 Possible need of a larger number of machine

 Presence of off-cell operations difficult to manage (sub-contracting capacity or

technology)

 Greater exposure to failures

 Limited flexibility for new products

 Greater exposure to phenomena arising from turbulence in the mix or changes in

volumes

 Coexistence with departments not organized by cells

 No application to the whole production

 High implementation costs

32 E.g. heat treatment.

33 If all the cells have the same starting activity you must buy a machine per cell, but if it costs too much it is unfeasible.

34 Transportation is practically only for raw materials; less damage related to material handling; lower cost of logistic

(lower number of operators involved); improved and rationalized layout in order to simplify internal transport. 37

 Investment “barrier” in CAD, CAPP and technical data base for the generation of the groups

G T

ROUP ECHNOLOGY 35

Manufacturing philosophy based on recognizing and deploying the existing similarities on products/parts . Group

Technology is the realization that many problems are similar, and that by grouping similar problems, a single solution

can be found to a set of problems, thus saving time and effort. Principles:

The new cells produce families of products

 A family is based on similarity of shapes, structures or production processes

 The sub-set of machines (cell) is grouped and placed in the same location

 Material flows are thus simplified



Objectives:

Product rationalization: identify, classify and group parts characterized by being similar for shapes and/or

 technologies adopted;

Process rationalization: identify and group subsets of machines able to manufacture several families;

 Assign product families to cells.



Relevant data are morphological and technological information about the products, technological cycles, production

resources and volumes. Steps for cell forming:

1. Data gathering (products, processes and volumes);

2. Systematic classification of parts;

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3. Design rationalization and standardization;

4. Technological processes rationalization and standardization;

5. Grouping similar parts into families;

6. Grouping machines into cells and assigning families to cells;

7. Sizing of single cells;

8. Fabrication process rationalization and related organizational measures to be adopted.

Families:

 Design similarity: focus on morphological characteristics (shape)

35 Realizing together similar activities; standardizing similar tasks; filing and efficiently recovering the information

regarding recurrent issues.

36 The needed technologies in each cell are established (type of machines): classification of machines, analysis of part

process plans, grouping machines into cells and parts into families, coupling families and cells. The number of machines

is calculated through a job shop sizing. 38

PRO CONS

Fast to be implemented There are always some parts that don’t look like any

other else and don’t find a natural location

Use of CAD systems Morphological affinity doesn’t mean that

technologies to be used are the same: the threat is

to duplicate machines and/or equipment

 Production similarity: technologies involved in manufacturing process

PRO CONS

It solves the issues of morphological method It hinders the start of standardization projects:

morphological similitudes are not taken into account

Facilitation of cycles generation

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Use of CAPP systems

V ,

ISUAL INSPECTION CLASSIFICATION AND CODING

An expert observes and creates the groups. It is a cheap and fast method that may use also photos or prints, but the

disadvantages are that grouping is dependent on the expertise of the employee (subjective, not perfectly reproducible),

and a limited number of parts can be coded.

Classification: arrangement of parts into classes based on their characteristics. There are a lot of benefits:

Engineering

 - Reduction of number of similar parts

- Elimination of duplication parts

- Identification of expensive parts

- Reduction of drafting efforts

- Easy retrieval of similar functional parts

- Identification of substitute parts

Equipment specification and facility planning

 - Flow line layout of production equipment

- Location of bottlenecks

37 Computer Aided Process Planning. 39

- Location of underutilized machine tools

- Reduction of part transportation times

- Improvement of facility planning

Process planning

 - Reduction of number of machining operations

- Shortening of production time

- Improvement of machine loading operation

- Easier prediction of tool wear and tool changes

Coding: assigning a numerical or alphabetical value to item characteristics in order to facilitate classification. Most

widely used is the Opitz code. Problems: which are the characteristics and features needing to be formally coded? Trade-

off between design, planning and production?

Design:

 - Basic external shape

- Basic internal shape

- Function

Design and fabrication:

 - Main dimensions

- Diameter/length ratio

- Materials

- Dimensional tolerances

- Surface finish

Fabrication and planning:

 - Main phase of the fabrication process

- Other phases

- Production routing

- Machines

- Fixtures/clamping systems

- Tools

- Fabrication time

- Volumes

- Lot size

Methods: Monocode, hierarchical (tree code).

 38 39

The meaning of each symbol depends on the value assigned to the previous ones . Usually this method

is preferred for morphological and permanent information, but it is difficult to construct.

38 The digit.

39 Each digit amplifies the information given in the previous one. With a small number of digits, a large amount of

information can be stored. 40

Polycode (chain code, attribute code).

 All digits are independent, thus present information doesn’t depend on previous ones. It is easier to

understand immediately and remember and to accommodate changes.

Hybrid, mixed.

 It has some digits forming monocodes, but strings them together in the general arrangement of a

polycode.

Examples: 41

MICLASS code

Coding, pro and cons:

 Independent from the person coding parts

 Reproducible

 Accessible for PC keywords

 Almost unlimited number of parts that can be coded

 Time consuming

 Complicated

 Expensive

 Long time for implementation 42

P F A

RODUCTION LOW NALYSIS

1) Factory flow analysis.

a. Broad sub-division of plant into departments;

b. Only really necessary for large systems.

2) Group flow analysis.

a. Machine families based on routing, regardless of sequence (ROC, SLC);

3) Line analysis.

a. Flow among machines to provide sequence;

b. Flow may be obvious;

c. Can use From/To chart.

R O C

ANK RDER LUSTERING ALGHORITHM

Mach\Prod P01 P02 P03 P04 P05 P06 P07 P08

A 1 (A is 0 0 1 0 0 0 0

used) 43

B 0 (B is not 1 1 0 1 1 1 1

used)

C 1 0 1 1