Design Specification

Introduction

A design specification facilitates the production of a physical design both at prototype-test stage, when it is used by a fabricator or an assembler, and at production stage, when it used by a factory.

Land-vehicle Design Specification
How do you describe the design of a wheel-barrow? a bicycle? a motorcycle? a car?

A wheelbarrow design example

A wheelbarrow is easy to describe. Its construction is simple. 

2 wheels, a container. 2 handles to push the wheelbarrow.

Then if you want someone to build you that design, you have to get into details and that is a process of specification. 

The wheels have to roll around an axle. 

The axle has to be attached to the container. 

The handles have to be attached to the container. 

(This is structural specification). 

Then you have to choose the width and diameter of the axle. 

The material to be used (solid steel, heavy gauge iron pipe etc). 

Then you have to specify the wheel geometry and Its hub characteristics. 

(This is still all structural specification). 

Then there is construction specification. 

How the axle will fit through the hub.

How the wheel will be prevented from moving across the axle. 

How friction at the wheel-hub axle interface will be handled.

How will you attach the container frame which is likely made of flat or angled iron strips to the circular cross-section axle. 

Similarly, how you will attach the handles to the container, their choice of material (e.g. wooden holding sections attached to iron angled strips).

This could still be called structural specification but relates more to the construction process and less to the resulting structural form.

Surface Materials

Then you might specify surface materials and how they are to be affixed to the frame. Such as rubber coatings for the wooden handles. The material to be used for the bed of the container and the one for the side walls.

Tolerances

Then you might specify tolerances. If there are threads cut onto the outer surface of the axle, how closely should they be spaced which effects the ease of tightening or loosening the nuts (1mm per turn, 1.5mm, 2mm). 

How well do you ensure the axle is perpendicular to both handles. 

How wide apart are the handles, what is their length -- both factors effect how easy the wheelbarrow is to turn.

Summary

Approximately, the process of specifying a design involves describing the final structure and how it will be built -- structural specification and construction process specification.

Observation

Design and construction/production are related to each other in the following way: the ability of the designer to predict the produce-ability of a design improves over time. Once this ability is good enough, design iterations are efficient. Design iterations mostly involve making adjustments to the tolerances in the design. 

Introduction

A design specification facilitates the production of a physical design both at prototype-test stage, when it is used by a fabricator or an assembler, and at production stage, when it used by a factory.

Land-vehicle Design Specification
How do you describe the design of a wheel-barrow? a bicycle? a motorcycle? a car?

A wheelbarrow design example

A wheelbarrow is easy to describe. Its construction is simple. 

2 wheels, a container. 2 handles to push the wheelbarrow.

Then if you want someone to build you that design, you have to get into details and that is a process of specification. 

The wheels have to roll around an axle. 

The axle has to be attached to the container. 

The handles have to be attached to the container. 

(This is structural specification). 

Then you have to choose the width and diameter of the axle. 

The material to be used (solid steel, heavy gauge iron pipe etc). 

Then you have to specify the wheel geometry and Its hub characteristics. 

(This is still all structural specification). 

Then there is construction specification. 

How the axle will fit through the hub.

How the wheel will be prevented from moving across the axle. 

How friction at the wheel-hub axle interface will be handled.

How will you attach the container frame which is likely made of flat or angled iron strips to the circular cross-section axle. 

Similarly, how you will attach the handles to the container, their choice of material (e.g. wooden holding sections attached to iron angled strips).

This could still be called structural specification but relates more to the construction process and less to the resulting structural form.

Surface Materials

Then you might specify surface materials and how they are to be affixed to the frame. Such as rubber coatings for the wooden handles. The material to be used for the bed of the container and the one for the side walls.

Tolerances

Then you might specify tolerances. If there are threads cut onto the outer surface of the axle, how closely should they be spaced which effects the ease of tightening or loosening the nuts (1mm per turn, 1.5mm, 2mm). 

How well do you ensure the axle is perpendicular to both handles. 

How wide apart are the handles, what is their length -- both factors effect how easy the wheelbarrow is to turn.

Summary

Approximately, the process of specifying a design involves describing the final structure and how it will be built -- structural specification and construction process specification.

Observation

Design and construction/production are related to each other in the following way: the ability of the designer to predict the produce-ability of a design improves over time. Once this ability is good enough, design iterations are efficient. Design iterations mostly involve making adjustments to the tolerances in the design.