Some definitions:

CEL: a group of machines that are not technologically homogeneous, but fully capable of working families of similar parts.

MANUFACTURING CELL: group of functionally dissimilar machines that are placed together and dedicated to the manufacture of a specific range of parts / products. 

FAMILY: group of similar parts from the morpho-technological point of view

-> POV: shape or process to produce them.

Solution that improves the production planning process and the saturation rate of the machines.

FLEXIBILITY: ability to change the type of product manufactured in a short time and at a low cost;

ELATICITY: ability to change the production volume without appreciable variations in the prouction unit cost.

Cell Manufacturing allows flexibility and elasticity, characterized by intermediate values.

General Features

-> Differences between Job Shop:

• Less flexible than a JS;
• More productive than a JS.

-> Parts are grouped into  families and machines into cells;

-> Machines are grouped on the basis of the processing requirements of the part families.

Layout and organisation of resources in the plant

-> Production System:  results from grouping machines based on a basic association between a set of parts / products to be produced and a set of machines capable to support their production

💡 Parts/products are assigned to families such that all the parts/products in the families are processed on the same group of machines and similar machines are grouped into cells.

Cells offer:

• Same group of machines;
• Different technological capabilities;
• No inter-cell move;
• Re-arranging group of machines;

-> Each product has its own routine;

✔ Material flows are not intertwined (as JS);

FMS: Flexible Manufacturing System.

FMC: Flexible Manufacturing Cell

📌Prerequisite to build the FMS:

• Each part of part families will be trasferred to the machine tools/workcenters by visiting its processing requirements;
• Grouping parts into families and subsequently associating to groups of machines (according to processing requirements);

May lead to:

• Re-arrange exisent equipement on the facotry floor;
• Operate with new equipement, often incorporating various forms of flexible automation;

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Strength & …

• Rationalization of material flows;
  • No inter-cell move =>
    • Material flows less intertwined;
    • All machines are within the cell => time saving;
  • More movements are possible same cost/# resources
• Setup time reduction: minor adjustment;
  • Lot/match size reduction (less time loss for setup => 🔺 capacity effectively utilized)
• Production management is easier:
  • Reducing size of chell/production system;
  • Reduced sets of parts produced within the cell;
  • Reduced WIP
  • Closeness of machines

Overall (Compared to the job-shop):

• WIP reduction (small batch size + more rational material flows);
• Space requirement reduction (lower space to stock the WIP);
• Less LT (low # of production orders/batches as WIP);
• More rliable estimates of delivery LT (queuing times 🔻 for low WIP => 🔻 variability of queuing times => low variability of manufacturing LTs => 🔺 reliable estimates of delivery LT => possibility to guarantee high delivery!.
• Job enlargement + job enrichment for employees
• Team work within the cell
• Unification of product and process responsibilities
• More control on the quality characteristics of the products

Compared with Job-Shop:

• Labor division in functional departments limitate workers’ responsaiblities and task specialization/skills;
• More unification of product and process responsabilities;
• Potentials quality control/ improvements and delivery time control.

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… & Weaknesses

• Difficulties with work load balancing between cells

-> Introduces rigidity (as consequence of families/cell association, avoiding inter-cell moves; autonomy of each cell.

-> e.g. if for a period the mkt request more than one product (we produce 2 products) one cell will be saturated, blocking the other.

• Problems related to production mix variability
• Difficulties with the application to the whole stages of the production chain (cell configuration is not always applicable in the production chain);
• In some cases, necessity of more machines than in a job shop
• Difficulties to manage technological operations outside the cells
• Problems related to breakdowns

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Group Technology (GT)

-> DEF: approach that helps managing diversity by capitalizing on underlying similarities of parts/products and required activities/operations.

-> Is a manufacturing philosophy that:

• Ralizing together similar activities;
• Standardizing similar tasks;
• Filing and efficiently recovering the information regarding recurrent issue.

-> GT facilitates the rationalization of activities/operations in a wide variety of functional areas, including purchasing, design, and manufacturing.

Steps:

-> Cellular manufacturing is an application of GT;

-> When GT is applied as to form manufacturing cells, the following steps are expected:

1. DATA COLLECTION:
   1. collecting data about the product types, already manufactured and expected in the future;
   2. estimating the required production volumes (yearly demand) for each product type;
   3. defining their technological routings.
2. CODIFICATION/CLASSIFICATION of PRODUCTS: all product/part types should be assigned a part code;

-> They should be classified according to their characteristics,

1. RATIONALIZATION/ STANDARDIZATION of PRODUCTS: in-depth analysis of product/part types aimed at avoiding the product variety which is not necessary.

-> Product variety may induce:

1. the introduction of new products in the product portfolio,
2. the existent product modifications and improvements,
3. the customization requests from clients.

-> If we avoid product features that are not really needed, product (re)design will have a great impact on the shop floor and on the definition of the proper configuration of the production system.

1. STANDARDIZATION of TECHNOLOGICAL ROUTINGS of PRODUCTS: reducing the variety of the technological routings of the product types.

-> If all product types had the same technological routings=>

=> production management could be simplified;

=> material flows intertwining could be avoided

=> We have better workload balancing in respect to the production capacity and better performances.

two levels of standardization:

1. MACRO LEVEL: operations sequences
2. MICRO LEVEL: for each operations, standardization as much as possible of tools and fixtures needed to produce work-pieces.
3. IDENTIFICATION of PART FAMILIES: identifying the part families based on similarities of different parts / products in the product portfolio

-> Parts / products are grouped into families based on similar characteristics (shapes, dimensions, materials, required tolerances);

1. IDENTIFICATION of MACHINE GROUPS FORMING the CELLS: grouping machines according to the part families identified so far at previous step;
2. Fifth + six steps simultaneously: identification of part families / machine grouping forming cells can be also achieved simultaneously.

Step 5 and 6 can be supported by different types of methods, presented in the remainder.

Methods:

-> Classification of products: classification based on the product/part characteristics

Definition:

PFI: Part Family Identification, methods based on the classification of products (Step 2-5);

✔Identify part families;

✔Standardize the productcion components such as fidtures.

📌Similarity for what concern the required worktable volume.

PCA: Part Coding Analysis, methods rely on a conding system.

-> DEF: Identification of product families based on the classification of products

Visual/Informal Methods:

-> DEF: or visula methods, rey on the visual identification of the correspondent part families.

• Geometrical features based;
• Technological features based;

-> Trivial when neumber of parts/products is small;;

✔ Quick method// no investment;

❌ Subjectivity and difficult repeatability.

Geometrical Features of Products:

-> DEF: Part/products families are identified (PFI) based on geometrica features of products (shapes and dimensions)

• Helps to standardize production components such as fixtures;

Technological Features of Products:

-> DEF: Part/products families are identified (PFI) based on technological features of products (materials, processing requirements)

 

PCA, Part Coding Analysis methods:

-> DEF: methods rely on a coding system;

• Geometrical features based (shape and dimension);
• Technological features based;

-> Used

1. to assign (alpha-)numerical weights/ digits to the part/ product characteristics and, afterwards,
2. to identify families by using familiarization scheme based on the numerical weights.

✔More formalized method, which leads to repeatability and objective outcomes.

❌The coding system that are normally available in a company are not necessarily ready for GT

-> PFI will be subsequent haven’t all the part codes available.

Optiz:

-> DEF The Optiz Classification System: Operational Technological Indicator

-> It include a morphological part and technology part:

• MORP PART:

-> Original geometric code (5 digit)

• TECH PART:

-> Supplementary technology codes: 4 more digits are added to the coding scheme, in order to increase the manufacturing information:

• Dimensions (diameter or edge length)
• Material type
• Original shape of raw material
• Accuracy (clearance tolerances or surface quality)

EXAMPLE

PCA can be used:

1.To help with blueprint design reuse
2.To form components families
To form the basis for cellular manufacturing design
To allocate new components to existing cells and easily plan process

PFA, Production Flow Analysis:

-> DEF: Identification of product families/machine groups forming the cells.

Index:

• Cluster Analysis:
  • ROC (Rank Order Clustering);
  • Similarity Coefficients;
• Graph partitioning;
• Mathematical Programming.

Cluster Analysis:

 

Rank Order Clustering (ROC): 

-> DEF: procedure that can be classified as an array-based clustering. It’s objective is to group either entities into clusters such that individual elements within a cluster have a high degree of “natural” association among themselves and very little “natural association” between clusters.

• Step 1: read each row as a binary number;
• Step 2: order rows according to descending binary numbers;
• Step 3: read each column as a binary number;
• Step 4: order columns according to descending binary numbers;
• Step 5: if on steps 2 and 4 no reordering happened go to step 6, otherwise go to step 1;
• Step 6: stop;

-> Trasform the table in sequence of zer and one.

-> We have to ordinate the numbers;

-> Each green group identify potential cells:

• The blocks diagonal define the cells formed;
• Cell dependence: there are some exceptional parts: products that have to be processed partly in a cell and partly in another cell.

=> Options to resolve exceptional parts problem:

• Accept inter-cell moves/ cell dependence;
• Duplicate the machine types intrested within the two respective cells;
• Generate alternative technological routing for the exceptional parts;
• Buy production of exceptional parts (sub-suppliers);

Similarity Coefficients –  Based on PFA:

-> DEF: methods that rely on the computation of similarity coefficients in conjunction with the use of some clustering algorithm/ procedure, in order to finally form the manufacturing cells.

• Another technique of cluster analysis;

-> CHAR:

• Hierarchical clustering methods:
  1. Employed similarity or dissimilarity between machines/ parts (=> prepare the creation of machine cells/ part families);
  2. Machines/ parts are aggregated into a few board cells, by means of a clustering algorithm/ procedure;
• Input: processing requirements of parts on machines (rappresented by machine/ part matrix formulation).

Steps:

1. Compute the similarity coefficients:

2. Join the couple (i*, j*) with the highest similarity coefficient, thus forming the machine group k;
3. Remove rows and columns related to both i* and j* from the original similarity matrix and substitute them with the row and column of the machine group k; then, compute the similarity coefficient:

s_rk =max (s_ri*, s_rj*)

4. Go to step 2 (baed on a criterion: single machines

Example:

1. Given the table:

2. Compute Similarity coefficients:

3. Join the couple:

4. Remove rows and colums related to the original similarity:

📌The dendogram is used to show the hierarchy of similarities among all the couples of machines.

5. Define Machine/Part Matrix:

-> Cells are formed after defining the minimum similarity coefficients amongst the couples of machines

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Rough design of a manufacturing cell

-> Cell design can be based on the same approach used for JS (after identification of families and machines):

1. Calculate the number of machines of type i necessary in the cell;
2. Evaluate the number of shifts/day computing the yearly costs adopting 1,2 or 3 shifts/day.

(Part-Family Identification: PFI; Machine Grouping: MG)

-> We have to verificate respect to the capacity limits.

-> Grouping techniques only group machines, next level of detail it is necessary to look at the flow rates and directions between resources in order to establish the best relative positioning of the machines.

=> We can do it manually or using technques.

Improving the Layouts:

-> U shaped reduce emploee movement and space requirements while enhancing communication reducing the # of workers and facilitating inspection.

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Virtual Cellular Manufacturing (VMC)

-> DEF: in the VMC the grouping of resources is not reflected by a physical structure, but in the PP&C system.

• Alternative to traditional manufacturing cells, with the purpose to be more responsive with production mix variablity;
• Machines that are belong to a cell are not physically located togheter, but rgoup only by Production Planning and Control System.

PP&C: enable to associate part families to group of machines.

• Given the physical structure the PPC system identifies group, creating the virtual cells with subsequent advantage due to cell autonomy and rationalization of material flow.