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Engineering Marking Out

What is marking out?

Marking out is the term used to describe the preparation of materials for cutting, forming, shaping or machining (e.g. drilling).

Here are some of the most common marking out tools and equipment.

Engineers Square

This ‘all-metal” square is similar to the Try Square used on wood. However, the Engineers Square is smaller and more accurate. Its purpose is the same, to draw lines at 90 to an edge and to check if work is a perfect right-angle; as below.

Scriber

These are used to draw/scratch lines onto the surface of a piece of metal. They must not be used for anything else, as the thin, sharp point can cause harm If misused.

Engineers Marking Blue

Special sharp and hard tools are needed to mark-out lines on metal. As these lines become very difficult to see, scratched on a thin layer of “marking-out blue" is brushed on first. A permanent marker-pen can be used in place of the “blue ink-like” liquid.

Odd-leg Calipers (Jenny Calipers)

Once “odd-legs” have been set to a required measurement, they are used to draw a line along a piece of metal. This line will be parallel to the edge it is drawn along: as below. Odd—legs are the ‘metal equivalent” to a marking gauge, used when marking out wood.

Centre Punch

These are used to draw/scratch lines onto the surface of a piece of metal. They must not be used for anything else, as the thin, sharp point can cause harm If misused.

Dot Punch

Sharp lines tend to disappear while filing metal, as Burrs are constantly produced on the edge of a piece of metal. In order that lines can be seen more clearly feint dots, called WITNESS MARKS can be lightly punched around the entire shape (that will be cut-out). A dot punch is used, along with a hammer: to create witness marks. Rest the work on a block of metal to do this, not in a vice. (The point is ground or sharpened to make an angle of 60 degrees).

Dividers

The sharp points on both ends are used to mark out circles and curves onto a piece of metal. Also once the dividers have been set at a measurement they can be used to mark off lots of equal distances along a line.

Another highly useful use for dividers is marking out the PCD (pitch circle diameter) which runs through evenly distributed holes on a circular part. The video below, whilst complicated, shows this fascinating process.

V Block and Saddle Clamp

V Blocks are tools designed to secure and hold objects, usually rods or pipes, for drilling or cutting operations. They are made of steel or cast iron block and consist a V-shape channel at the top. V blocks in many situations are supported by a saddle clamp for extra security and the operator safety.

Surface Plate

A surface plate is a solid, flat plate used as the main horizontal reference plane for precision inspection, marking out (layout), and tooling setup.

Surface Table

A surface table is essentially a surface plate which is raised up to working height and has permanent legs attached to it. Modern surface tables are often made of granite.

Angle Plate

An angle plate is a work holding device used as a fixture in metalworking. Angle plates also may be used to hold the work-piece square to the table during marking-out operations. Adjustable angle plates are also available for work-pieces that need to be inclined, usually towards a milling cutter.

Vernier Calipers (and worked examples how to read them)

A vernier caliper is a precision measuring tool. It can be used for three types of measurements, outside distance (such as the length of an object) , inside distance such as the width of a groove or the diameter of a large hole, and depth, such as the depth of a hole or the height of a step.

A visual example of how to read a vernier scale

Have a go at the example on the left.

Reading from the main scale on the left we can see the zero of the vernier scale lines up with 23 mm on the main scale. Therefore we have 23mm (so far).
The vernier scale has got 10 increments on it. 1/10 is equal to 0.1 This means that this scale is accurate to 0.1mm.
The vernier scale is read after the main scale. We can see that the 7th line is the one which lines up with the main scale the most. Therefore we also have 0.7mm
Adding them up we get: 23 mm + 0.7 mm = 23.7 mm

See, that wasn't so difficult was it?

Let's test your ability to read a 0.1mm accuracy caliper.

Example 1

Example 2

Example 3

Example 4

Were you right?

Example 1 = 10.2mm
Example 2 = 31.5mm
Example 3 = 9.1mm
Example 4 = 26.0mm

What about different vernier scales? How do I tell the difference?

Different vernier calipers have got different levels of accuracy. How do we tell them apart? Lets look at the example below of a 0.02mm accuracy caliper.

We can see that the vernier scale goes from 0 to 10 and has got 5 minor increments for each major increment.

10 x 5 =50. Therefore this is a 1:50 vernier scale.

If we divide 1 by 50 we get 0.02.

Therefore each increment on the vernier scale is equivalent to 0.02mm. This is its accuracy.

This is ADDED to the main scale when taking a reading to give us our total decimal reading.

The example on the left shows that the main scale lines up with 2.8cm or in engineering terms, 28mm.

The vernier scale lines up at 6.0

Therefore this reading is 28.6mm

Whilst it is important to be able to read a manual vernier scale (this is an essential engineering skill), there are alternative tools available to make life easier for us.

For example we can use digital vernier calipers.

It is important to remember to calibrate digital calipers to zero BEFORE using them.

A disadvantage of digital calipers is that the battery can run out. Manual calipers don't need batteries.

Now let's really test your understanding of reading vernier scales...

You can't cheat this time!

Vernier Height Gauge

These are used to draw/scratch lines onto the surface of a piece of metal. They must not be used for anything else, as the thin, sharp point can cause harm If misused.

Vernier height gauges should be used on a surface plate or surface table.

Micrometer

A micrometer is a device which has a calibrated screw inside and is widely used for the accurate measurement of components. It is frequently used alongside other instruments such as dial, vernier, and digital calipers.

Internal and external calipers

External (outside) calipers are used to measure the external size of an object whilst internal (inside) calipers are used to measure the internal size of an object.

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