Rugby School Thailand Design & Technology Senior School

2.7 Accuracy in Design

syllabus

By the end of this unit you should have developed a knowledge and understanding of:

how to use a range of measuring and marking out equipment
the importance of accuracy in manufacturing
how testing can eliminate errors
the value of using measuring aids to ensure consistency of accuracy and the reduction of human error

Introduction

Accuracy is essential in order to design, manufacture and test products within the tolerance specified so that they will function effectively.

This can only be achieved by the correct use of appropriate measuring and marking out tools. Additional equipment is also used to reduce the possibility of measuring and marking out errors, particularly in large-scale production.

Measurement is the process of using tools such as rules, tapes and gauges to check existing dimensions.

Marking out is the transfer of designs onto pieces of material or parts to indicate where features such as edges, holes, slots, recesses and bends have to be made during manufacturing processes.

Tools commonly used for measuring and marking out metal

high carbon steel scriber

Marking metal usually requires the use of high carbon steel scriber, which is harder than the metal it is marking. If the scratched lines are hard to see, engineers blue is applied to the surface before using the scriber.

dividers

If circles or arcs are required, dividers are used and function in a similar way to a compass. Beam compasses, sometimes called trammels, can be used for larger radii. External and internal calipers are also used to check dimensions.

Tools commonly used for measuring and marking out wood

Timber is usually marked out with a marking knife or pencil, and there are carpenter’s pencils specifically designed for the this purpose with a thicker lead.

A range of marking gauges, tri squares, mitre squares and adjustable bevels are also available as well as compasses and other specialist wood marking and measuring tools.

High precision work

Laser devices such as cross line levels are becoming more commonly used for measuring and marking out when large and complicated fabrications are being undertaken, as this method projects a perfectly horizontal and vertical reference line onto objects of any shape. They can be used for a variety of trade and manufacturing purposes.

High precision work

Micrometers, vernier scales, calipers and co-coordinate measuring machines (CMM’s) are examples of tools used for precision measurement involving fine tolerances and are covered in Unit 2.9.

Impact of CNC machining

Computer numerically controlled machining processes do not require marking out as they take their data in the form of G-codes generated by the software used to produce the component drawing. This ensures an incredible level of accuracy in manufacturing as long as the drawings are correct.

The importance of accuracy

Accuracy is defined as the level of conformity of a measurement to the required value. It is an increasingly important feature of modern product design to ensure that parts fit and products function correctly, particularly in the light of more prevalent miniaturisation of many devices.

For each part or component the designer must consider what will be an acceptable range of accuracy. The measure of a level of accuracy appropriate for particular situations is known as the tolerance and can vary considerably depending on factors such as material, size, function and need for interchangeability.

Examples include:

cutting the gears for a watch movement
ensuring threads on nuts and bolts fit correctly
fitting a new glazing unit in a window frame
positioning holes for a KD fitting in self-assembly furniture
selecting the correct washer for a dripping tap
positioning holes in a wall to secure a wall bracket

Tolerance

Tolerance is the specification for the allowable upper and lower accuracy limits of the dimension involved. It is usually expressed as X + or -Y where X is the exact size and Y is how much error can be tolerated.

Try this for an example:

A dowel is approx. 9mm allowing for manufacturing variations in wood.

The dowel needs to fit into a hole so the hole should be 9mm + or – 10%

What is the minimum and maximum size the hole can be to still perform as expected?

Answer: between 8.1mm and 9.9mm

A 10mm washer can be used with a bolt diameter ranging from 9.5mm to 10.5mm.

What it’s the tolerance of the of the washer expressed as a percentage

Answer 10mm + or – 5%

When tolerances are not factored in the results can range from simply annoying to disastrous.

At one end of the scale it could mean that a cheap item of flat pack furniture does not fit together properly or is wobbly in use.

At the other end of the scale, an aircraft could end up crashing if the jet engines were not attached using the correct fittings and components.

For your own design work, it could be the difference stoping the product components fitting together and not being able to fully test or evaluate it!

Before undertaking any measurements or marking out in connection with manufacturing a product or component, it is important to consider the tolerance that has been specified in that particular situation.

This will have been dictated by a number of factors as we can see in the following examples:

a hand forged iron scroll for a gate
a rectangular piece of MDF for mounting a picture
a mass produced glass bottle for carbonated drinks
a mould for a die cast car component

a hand forged scroll for a bespoke gate

In this case there is no need for great accuracy and it can be argued that the lack of accuracy might be a good thing as each gate will be unique and reflective of its hand made quality. As long as the scroll fits the design and the space on the gate, it would be acceptable. Tolerances here might be in whole centimetres!

a piece of mdf for mounting a picture

It would be necessary to measure the picture and decide how close the board needs to be to the edges. Typically the tolerance would be + or – 0.5mm but it would not be critical as it could be trimmed to fit later if necessary without affecting the quality of the product.

mass produced glass bottles for carbonated drinks

This product provides a much greater challenge as far as measurements are concerned. Dimensional accuracy will ensure the screw top fits tightly and that all moulding features ae accurate. The correct volume is paramount for compliance with drinks legislation and variations in size and the sagging of the glass will need to be factored in.

a mould for a die cast car component

This part will require great dimensional accuracy to be able to fit perfectly with other components and perform without failure. The tolerance in a part like this could be as fine as + or – 0.002mm. In a precision object like a smartphone, it could e even finer than this as so many intricate components need to fit into a relatively small space.

Measuring aids

Some of the most important measuring tools were dealt with at the start of this unit, but in some cases, accuracy and speed can be improved and potential human errors eliminated by using jigs, fixtures and templates.

This is particularly appropriate when scales of production are greater than one or two products and especially in batch and mass produced items.

The use of non-contact measuring devices such as laser based technologies are becoming more common and can be beneficial when greater accuracy is needed or it is difficult to handle the material due to size, access or safety reasons.

Jigs

are devices used to control the motion of a tool relative to a workpiece. A common type is a drilling jig, used to ensure That holes are drilled in the same location every time. In the images below, a simple drilling jig has been 3D printed and is screwed to a sacrificial board. The wood is pushed in as far as it will go and the drill is guided through the holes ensuring they are in the same place each time. It increases accuracy and speeds up the process.

Fixtures

fixtures are robust frames with holding points and clamps that are used to hold workpieces firmly in place while machining, welding and other manufacturing operations take place. It is essential that every component is held in exactly the same place each time to ensure consistency.

You can see how this works in the construction of a bamboo bike frame below:

Templates

templates usually consist of a rigid shape or pattern, although they can be made of paper or fabric especially in the fashion industry, but these would be less durable. More often they are made of thin board or sheet metal and it is used, like a stencil (which they are sometimes called) to draw around onto the workpiece. It is normal practice to have some system or fixture for holding the template in the same place each time.

In this respect they can be combined with both jigs and fixtures.

Summary

A range of measuring and marking out equipment is available. Tools need to be chosen carefully to suit the material and level of accuracy required.
It is important that parts and products are made accurately so that they function and fit.
An appropriate tolerance needs to be established for each dimension of a part or component.
Unskilled workers rely on devices such as go/no go gauges.
Skilled workers use calipers, micrometers and other equipment for greater accuracy.
Jigs, fixture and templates and be used to improve accuracy and consistency of production.

Prep 57

Revision notes

Much of what you learn in this unit will be valuable and necessary in the production of your own NEA practical work.

Accuracy begins in the design of the product and is equally important in the drawing out of parts to be machined, 3D modelling for rapid prototyping, or marking out or cutting, drilling and shaping.

When making several parts, it is worth considering how gauges, jigs and templates can help you to reproduce the component as accurately as possible more than once.

You should be familiar with a range of digital and analogue measuring tools such as calipers, Vernier scales, micrometers, feeler gauges etc.

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