5. Introduction to Measuring, Testing & Marking Out

In metalworking you will:

Measure parts that you make and fit together
Test that parts are the correct size and shape
Mark out parts to the dimensions given in a drawing

Measuring Accurately

To be able to measure accurately you must learn to:

Handle and use measuring tools correctly
Select measuring tools that are as accurate as required for the work

In Australia, the legal units of length are based on the metre, however it is common to use both millimetres and metres. Centimetres are not commonly used.

Basic Measuring Tools

There are many forms of measuring tools used in industry. The accuracy of these tools depends on the way they are used and treated. Use them carefully and they will retain their accuracy longer. There are several methods used to take measurements with basic measuring tools.

The size may be obtained by the direct reading of a scale by eye when you sight graduations on a rule.

READING SCALES ACCURATELY

To obtain consistently accurate readings when taking a direct reading of size from a scale, take care how you sight the scale. An optical error called 'parallax' must be avoided. Parallax is the apparent change in the position of an object viewed against a more distant object.

The video below explains how parallax error can effect the accuracy of a measurement.

To avoid parallax:

Hold the scale so that its graduations are as close as possible to the part being measured.
Sight the graduations squarely off the scale.

Depth Gauges

A depth gauge is a measuring tool developed from a steel rule. Depth gauges are used to measure:

Depths of holes
Depths of recesses and slots
Distances in from the edges of work.

The depth gauge consists of a narrow graduated steel rule, fitted with a sliding frame that may be clamped along the rule. To use the depth gauge:

Hold the frame base firmly down on the surface and straddling the recess of the work to be measured.
Hold the gauge square to the work by steadying the rule with the first finger of your left hand.
Use the first finger of your right hand to press the sliding rule down until you feel the lower end locate against the bottom of the recess.
Tighten the locking screw with your right hand.
Lift the gauge carefully out of the recess and away from the work and turn the gauge into a position where you can read the depth of the recess.

Combination Sets

The combination set is a useful measuring and testing tool. It consists of a graduated straight edge or blade which can be clamped to several different heads.

Fit the required head to the blade as follows:

Insert the blade into the slot in the head.
Locate carefully the lug of the clamping screw into the recess of the blade.
Slide the head to the required position and lock it there by tightening the knurled nut.

The protractor allows the blade to be set at an angle to the flat face. Use the protractor and blade for setting up or measuring angles.

The square head has one face that forms a right angle with the blade. The other face forms a 45° angle with the blade.

Use the square head and blade as:

A depth gauge to measure from the square face to the end of the blade
A height gauge by setting the square face flush with the end of the blade.

Also use the square head and blade as:

A square to set work at right angles
A bevel gauge to set work at 45°

The centre head is designed to allow one edge of the blade to pass through the centre of two faces at right angles.

Use the centre head and blade for:

Finding or marking the centre of circular work
Checking 45' angles.
Use the centre head and blade to find the centre of circular work as follows:
Fit the centre head on the blade.
Position the head conveniently along the blade.
Place the head centrally on the blade for small work.
Tighten the knurled clamping nut to a firm tight pressure.
Remove any burrs from the end of work with a smooth file.
Hold the right-angle faces of the head firmly against the work with the blade flat across the end.
Scribe a line along the edge of the blade across the end of the work.
Turn the work one-third of a turn and scribe a second line to intersect the first one.
Turn the work a further third of a turn and scribe a third line.
The intersection of the three lines will be the centre of the work.
Lightly witness-mark the position of the centre with a centre punch.

Protractors and Bevel Gauges

Angular measurement is closely associated with linear (straight line) measurement. Parts that are manufactured may have angles or tapers that require to fit accurately to mating parts. Certain measuring equipment has been designed to measure angles or tapers in degrees or parts of a degree.

Protractors

Protractors have a dial face graduated in degrees, with a straight blade that can be swivelled to an angle, then locked in position.

They are used for:

Setting work to an angle
Testing angles
Marking out the position of holes

Bevel gauges

A bevel gauge consists of a body with an adjustable sliding blade that may be set and clamped at an angle to the body.

Use a bevel gauge as follows:

Set the blade of the gauge to the angle required.
Lock the blade to the body with the clamping screw.
Transfer the gauge on to the work.
Compare the setting of the gauge against the angle on the work.

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