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Engineering Marking Out

and checking for accuracy

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

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.

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

Why the difference in vernier scales?

Notice the extra decimal place in the second caliper? This makes the 0.02mm caliper 5 times more accurate that the 0.1mm caliper.
This can be worked out by taking the larger increment by the smaller one, i.e. take 0.1 and divide it by 0.02 and you get 5.

A standard vernier caliper (see below) is accurate to 0.02mm. If a steel rule is accurate to 0.5mm, using the method in the paragraph above,
how many times more accurate is the vernier caliper than the steel rule?

If you take 0.5 and divide it by 0.02, you get 25.

This means a standard vernier caliper is 25 times more accurate than a steel rule! You can now see how something as simple as a second scale can introduce a whole new level of accuracy to everyday measurements. The number of graduations in the second scale determines the accuracy.

Click to have a go at worksheet reading 0.1mm accuracy vernier calipers.
ANSWERS

Click to have a go at worksheet reading 0.02mm accuracy vernier calipers.
ANSWERS

Digital calipers

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.

Disadvantages of digital calipers are that the battery can run out, they are often built cheaply, using poor quality components and rely on optical sensors to return a reading. They are affected by poor care and maintenance and are often not sent for calibration to check their accuracy against standard quality controls.

Manual calipers don't need batteries, but they are affected by dirt and physical damage.

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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