Rugby School Thailand Design & Technology Senior School

3.8 Measuring and marking out

syllabus

• measure and / or mark out using rule, pencil, marking knife, marker pen, scriber, try square, bevel, mitre square, centre square, dot / centre punch, dividers, inside / outside / odd-leg calipers, template, marking / cutting / mortise gauge

• produce datum lines by surface plate and scribing block or calipers

• measure using a micrometer, vernier gauge and/or digital caliper

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 a rule (ruler), 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

Rule

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.

Bradawl

Used for marking a hole into woods ready to be drilled.

measure and/or mark out using rule, pencil, marking knife, marker pen, scriber, try square, bevel, mitre square, centre square, dot/centre punch, dividers, inside/outside/odd-leg calipers, template, marking/cutting/mortise gauge

Dot Punch / Centre Punch

Centre Square

Rule

A Rule is usually a steel 300mm straight edge ruler that the measurement starts at 0mm at one flat end. This means that the end of the rule can be placed against materials and the measurement starts at that point. A standard plastic ruler usually has approximately 5mm of plastic before the scale starts.

Marking Knife

A marking knife is a sharpened steel blade, cut at an angle, and is used to accurately mark out lines onto timbers. The blade is thin and sharp and so can leave a small, thin cut line into timbers, across the grain of the wood and mainly used for wood joints.

T Square

Used for marking out 90 degree / right angles on material.

Scriber

A Scribe or Scriber is used to mark out lines on metals. It is a steel rod that has a grip and a sharp point. It is commonly used with Bluing paint to accurately mark lines onto metals.

Mitre Square

Spring Dividers

Used for measuring metals and cyclindrical forms.

Spring Callipers

Used to set a measurement and work with cylindrical forms on a lathe - to measure the diameter.

Vernier Callipers

Used for accurately measuring work pieces. Works to 100th of a mm.

Micrometer

Used to accurately measure work pieces. Works to 1000th mm.

Bevel

Try Square

Used for marking out a right angle on materials. One edge rests upon the material and the other edge creates the 90 degree right angle. When both parts are made from mild steel it is usually referred to as an Engineers Square. If one part is wood and the other mild steel then it is more commonly called a Try Square.

Engineers square

Dividers

Used for marking out circles on metals and measuring distances.

Inside Callipers / Outside Callipers / Odd-Leg Callipers

Template

Marking Guage / Cutting Guage / Mortise Guage

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.

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