Computer-Aided Assembly Planning

Computer-AidedAssembly Planning

Richard Farrcapacify.wordpress.com

Contents

Quick recap of assembly.

The assembly planning activity.

Computer-based representation of constraints.

Assembly sequence generation and illustration.

Criteria for assembly sequence evaluation.

Conclusions and questions.

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Define the assembly planning task.

Appreciate the usefulness of Computer-Aided Assembly Planning (CAAP).

Understand the requirements in assembly modelling.

Be able to reason about an assembly with geometric and technological constraints.

Know how to evaluate alternative sequences.

Learning Objectives

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What is Assembly?

The process of combining components into sub-assemblies or finished products.

It can form a significant part of the overall manufacturing process.

It has been estimated that it can account for between 25 to 75 % of production costs and 40 to 60 % of production throughput time.

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What is Assembly Planning?

“The act of preparing the detailed instructions for the assembly of a product.”

Assembly Planning includes...

Evaluating the processes and operations that might be employed.

Selecting tooling and fixtures, etc.

Determining the sequence of operations.

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Why?

Most products are assemblies.

Process planning only addresses the operations that produce individual components.

Very few companies have design control over every component they use.

Assembly planning allows us to anticipate problems and solve them before they happen for real on the factory floor.

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Why Computer-Aided Assembly Planning?

The planning task is time-consuming and prone to errors.

The time cost of planning.

Engineers cannot examine all the possible plans.

Computerised tools can ensure that no good assembly plan has been overlooked.

CAAP allows the flexibility to handle changes in available equipment.

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Activities in Assembly Planning

Assembly modelling

Reasoning about the model

Sequence generation

Sequence evaluation

Completed assembly plan

Assembly Models

A description of the assembly for input to planning.

Part models (geometry and other details of individual components).

The spatial relations between components.

Details of the connections between components.

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Computer-based model

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Assembly modelling

Place the base component.

Add further parts, locating them using various types of constraint. For example:

Mate. (Two surfaces that will touch in the final assembly.)

Align. (Two planes coincident and facing in the same direction.)

Insert. (Used to insert a revolved surface into another on the same axis.)ca

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Useful information available within the CAD model

Extraction of additional data from component and assembly models...

Mass properties (weight and centre of gravity).

Clearance and interference calculations.

Bill of materials.

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Reasoning about the Assembly Model

Aim: to determine information not held within the assembly model itself.

Main task: to identify precedence constraints:

Geometric constraints.

Technological constraints.

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Geometric constraints

Precedence (what must be done before what?)

Assembly direction.

Collision checking.

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Technological constraints

Constraints due to the technology used to assemble components.

Typically associated with standard fastening methods (for example, screws and clips).

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The Problem

Like all manufacturing planning tasks, time is limited and experts are in short supply.

The magnitude of the job: how many assembly sequences must be evaluated?

The number of potential sequences rises exponentially with the number of parts in the assembly.

“The Combinatorial Explosion...”

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The Combinatorial Explosion


9.33262154 × 10157100

6,402,373,705,728,00018

The Combinatorial Explosion


Sequence Generation Problem

Fortunately, not all combinations are possible. Some can be eliminated early, because they are physically impossible.

Let’s use a simple example to see how possible assembly sequences can be found.

A ballpoint pen has five components (we’ll call them body, nib, tube, cap and button).

How many assembly sequences?

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Pen components

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What happens

when you assemble things in

the wrong order?

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BodyTube

Button

Cap

Nib

Component liaisons

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= tube, nib

= nib, body

= cap, body

= button, body

Level 1...


= tube, nib

= nib, body

= cap, body

= button, body

Level 2...


= tube, nib

= nib, body

= cap, body

= button, body

Level 3...


= tube, nib

= nib, body

= cap, body

= button, body

Level 4...


= tube, nib

= nib, body

= cap, body

= button, body

Finalversion


Neat version...ca

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Alternative representation: the AND/OR graph

Each node is a feasible subassembly.

All possible decompositions of the product are shown.

The pair of lines leading away from each box is known as a ‘hyper-arc’

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Assembly Sequence Generation

Four main options:

Forward planning

Backward planning

Plan re-use

Rule-based expert systems

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AA

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CCompleteassembly

1. Forward Planningca

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AA

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CCompleteassembly

2. Backward Planning

BBCDE

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3. Plan re-useThe process of generating complete sequences from scratch can be avoided by reusing previously created plans.

Similar to the hybrid approach in CAPP, this uses an expert system technique known as ‘Case-Based Reasoning’ (CBR).

Aims to solve new problems by using knowledge gained from solving similar problems in the past.

This knowledge is held in the form of problem solutions or ‘cases’ held in a database.

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4. Rule-based systems

These systems are based on a more conventional expert system technique, using rules to represent the planning knowledge, and an inference engine to perform the reasoning process to search for a solution.

The rules are in an “IF ... THEN” format, such that if the predicate is true then the conclusion is drawn.

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Sequence Evaluation

Even after assessing geometric and technological constraints, many feasible sequences might remain.

Additional criteria are used to find an optimum sequences – or at least, a set of alternatives that are “good enough”.

Additional criteria (sometimes called strategic constraints) can be:

Qualitative or quantitative.

Product, technology or company specific.cap

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Risk Reduction

Operational Flexibility

Assembly Efficiency

Compatibility

Ass

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Risk Reduction Strategy...

Fit valuable components as late as possible.

Fit fragile components as late as possible.

Perform irreversible operations as late as possible.

Perform precise adjustments as late as possible, and watch out for dependencies between tolerances.

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Operational Flexibility Strategy...

Aim for maximum flexibility when designing the assembly system.

Assembly operations that introduce ‘special’ or unique components... configure the product as late as possible.

Parallelism of operations: can several jobs be done at one time?

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Assembly Efficiency Strategy...

Aim to improve efficiency by reducing non-value adding operations:

Reduce assembly movements.

Reduce tool changes.

Reduce fixture complexity.

Reduce reorientations.

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Compatibility Strategy...

Aim to meet known or predicted assembly system requirements:

Match the sequence to the assembly line configuration.

Aim to produce ‘required subassemblies’ at the right stages.

Avoid unwanted subassemblies.

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In summary

An important concurrent engineering activity, particularly when viewed in the context of globalisation.

Assembly has a considerable influence over product cost, time and quality.

Computer-Aided Assembly planning is necessary, in order to deal with the huge number of permutations that are possible.

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Recap ofProcedure

Drawings or model

Geometric reasoning

Conceptual plan or plans

Revise, based on experienceand the production system

Evaluate for practicality

Issue assembly plans

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Define Assembly Planning.

Draw a flowchart showing the typical manual assembly planning activity.

What are the problems with manual assembly planning?

Why does CAAP offer a valuable alternative?

Review Questions (1)

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Review Questions (2)

Describe four different techniques for assembly sequence generation.

Explain the two methods used to represent assembly sequences pictorially. Choose a product of four to five components and represent it using each method. (Refer to the ballpoint pen assembly example.)

List the four categories of sequence evaluation criteria, and describe the effect each has on sequence selection.

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Further information from Richard Farr can be found

on Capacify, the Sustainable Supply Chain blog

http://capacify.wordpress.com

On Twitter: @Capacified

http://capacify.wordpress.com

Engineering

Computer-Aided Assembly Planning