Engineering Metrology and Instruments

► Introduction:
Engineering Metrology:

- Measurement of dimensions such as length, hickness, diameter, taper, angle, flatness, profile and others.

Post-Process Inspection:

- Measurements taken after the part is produced
- Term inspection – checking dimensions and determining whether it complies with specifications.

In-process, On-line or Real-Time Inspection:

- Checking part characteristics while part is being produced.

Important concepts:

- Range of measurement:

The physical variables that are measured between two values.One is the higher calibration value H, and the other is Lower value L, The difference between H, and L, is called range.

- Sensitivity:

Is the smallest difference in dimensions that the instrument can detect.

- Accuracy:

The ability of a measurement to match the actual value of the quantity being measured.

- Precision:

The degree to which the instrument gives repeated measurements of the same standard.
The difference between precision and accuracy can be shown by the spread of shooting of a tar-get (as shown in Diagram below).

- Error:

The deviation between the results of measured value to the actual value.

- Correction:

The numerical value which should be added to the measured value to get the correct result.

Measurements Standards:

Two Standards
→ English Speaking Countries (English System):
Pound-mass, Foot, Second
→ Metric System (SI – System International):
Kilogram, Meter, Second

Measuring Instruments:

- classification of measuring instruments:
1- Length measuring Instruments.
- Direct measuring.
- Indirect measuring.
2- Angle measuring Instruments.
3- Instruments for surface finish.
4- Instruments for deviations.

1) Length measuring Instruments:

1.1) Rules and tapes: (Direct measuring)

Rules and tapes are the simplest way of measuring larger dimensions. Steel rules are generally only available to measure dimensions up to 1 metre. Beyond this, steel tapes (measuring to 30 m) or an ultrasonic rule (measuring to 10m) are used.
The steel rule is undoubtedly the simplest instrument available for measuring length.
Measurement accuracy is only modest using standard rules, which typically have rulings at 0.5mm intervals, but the best rules have rulings at 0.05mm intervals and
a measurement resolution of 0.02 mm. When used by placing the rule against an object, the measurement accuracy is much dependent upon the skill of the human measurer and, at best, the inaccuracy is likely to be at least 0.5%.

1.2) Vernier caliper: (Direct measuring)
The vernier calipers found in the laboratory incorporates a main scale and a sliding vernier scale which allows readings to the nearest 0.02 mm. This instrument may be used to measure outer dimensions of objects (using the main jaws), inside dimensions (using the smaller jaws at the top), and depths (using the stem).

When a length is measured the zero point on the indicating scale is the actual point of measurement, however this is likely to be between two data scale points. The indicator scale measurement which corresponds to the best-aligned pair of indicator and data graduations yields the value of the finer additional precision digit.
- Example:
In figure shown, the first significant figures are taken as the main scale reading to the left of the vernier zero, i.e. 34 mm. The remaining two digits are taken from the vernier scale reading that lines up with any main scale reading, i.e. 60 on the vernier scale.

1.3) The Micrometer: (Direct measuring)

The micrometer screw gauge is used to measure even smaller dimensions than the vernier calipers. The micrometer screw gauge also uses an auxiliary scale (measuring hundredths of a millimeter) which is marked on a rotary thimble. Basically it is a screw with an accurately constant pitch (the amount by which the thimble moves forward or backward for one complete revolution).

Take as an example: the micrometers which have a pitch of 0.50 mm (two full turns are required to close the jaws by 1.00 mm). The rotating thimble is subdivided into 50 equal divisions. The thimble passes through a frame that carries a millimeter scale graduated to 0.5 mm. The jaws can be adjusted by rotating the thimble using the small ratchet knob. This includes a friction clutch which prevents too much tension being applied. The thimble must be rotated through two revolutions to open the jaws by 1 mm.

In order to measure an object, the object is placed between the jaws and the thimble is rotated using the ratchet until the object is secured. Note that the ratchet knob must be used to secure the object firmly between the jaws, otherwise the instrument could be damaged or give an inconsistent reading. The manufacturer recommends 3 clicks of the ratchet before taking the reading. The lock may be used to ensure that the thimble does not rotate while you take the reading.

- Example:
In figure shown the last graduation visible to the left of the thimble is 7 mm and the thimble lines up with the main scale at 38 hundredths of amillimeter (0.38 mm); therefore the reading is 7.38 mm.

1.4) Calipers: (Indirect measuring)

These are used to transfer the measured dimension from the workpiece to a steel rule. This avoids the necessity to align the end of the rule exactly with the edge of the workpiece and reduces the measurement inaccuracy by a factor of two. In the basic caliper, careless use can allow the setting of the caliper to be changed during transfer from the workpiece to the rule. Hence, the spring-loaded type, which prevents this happening, is preferable.

Types of calipers:
1- Inside caliper: are used to measure the internal size of an object.
2- Outside caliper: are used to measure the external size of an object.
3- Divider caliper: are used in the process of marking out suitable workpieces.

2) Angle measuring Instruments:

Measurement of angles is one of the less common measurement requirements that instrumentation technologists are likely to meet. However, angle measurement is required in some circumstances, such as when the angle between adjoining faces on a component must be checked. The main instruments used are protractors and a form of angle measuring spirit level.

In some circumstances, a simple protractor of the sort used in school for geometry exercises can be used. However, the more sophisticated form of angle protractor shown in the figure provides better measurement accuracy. This consists of two straight edges, one of which is able to rotate with respect to the other. Referring to the figure, the graduated circular scale A attached to the straight edge C rotates inside a fixed circular housing attached to the other straight edge B. The relative angle between the two straight edges in contact with the component being measured is determined by the position of the moving scale with respect to a reference mark on the fixed housing B. With this type of instrument, measurement inaccuracy is at least š1%. An alternative form, the bevel protractor, is similar to this form of angle protractor, but it has a vernier scale on the fixed housing. This allows the inaccuracy level to be reduced to š10 minutes of arc.

The spirit level shown in Figure is an alternative angle-measuring instrument. It consists of a standard spirit level attached to a rotatable circular scale that is mounted inside an accurately machined square frame. When placed on the sloping surfaces of components, rotation of the scale to centralize the bubble in the spirit level allows the angle of slope to be measured. Again, measuring inaccuracies down to š10 minutes of arc are possible if a vernier scale is incorporated in the instrument.

3) Gages:

Gauge blocks, also known as slip gauges, consist of rectangular blocks of hardened steel that have flat and parallel end faces. These faces are machined to very high standards of accuracy in terms of their surface finish and flatness. The purpose of gauge blocks is to provide a means of checking whether a particular dimension in a component is within the allowable tolerance rather than actually measuring what the dimension is. To do this, a number of gauge blocks are joined together to make up the required dimension to be checked.

The pictured below accessories provide a set of holders and tools to extend the usefulness of the gauge block set. They provide a means of securely clamping large stacks together along with reference points and scribers.

Slip gauges are made from a select grade of carbide with hardness of 1500 Vickers hardness. Long series slip gauges are made from high quality steel having cross section (35 x 9 mm) with holes for clamping two slips together.

They are available in various grades depending on their intended use:
• reference (AAA): small tolerance (± 0.00005 mm or 0.000002 in) used to establish standards
• calibration (AA): (tolerance +0.00010 mm to -0.00005 mm) used to calibrate inspection blocks and very high precision gauging
• inspection (A): (tolerance +0.00015 mm to -0.00005 mm) used as tool room standards for setting other gauging tools
• workshop (B): large tolerance (tolerance +0.00025 mm to -0.00015 mm) used as shop standards for precision measurement

Plug gages:

(a) Plug gage for checking size or diameter of internally threaded part.
(b) Plug gage for checking taper of internally threaded part.
(c) Ring gage for checking size or diameter of externally threaded part.

Ring gages:

(a) The Metric Trapezoidal Ring Gage.
(b) American Standard Adjustable Ring Gage.

Snap gage:

- Snap gage gages diameters, lengths, and thickness.
- The adjustable limit snap gage is used to check outside diameter.
- There are a various types of snap gages:
→ dial indicator snap gage.

4) Comparative length-measuring instruments:

4.1) Dial indicator:

A dial indicator is used to measure shaft runout, shaft thrust, gear backlash, flywheel face runout, flywheel housing concentricity, and valve seat concentricity. You can mount a dial indicator on a test stand or, with clamps and a magnetic base, directly on the equipment to be measured. Figure shows a typical dial indicator with mounting accessories, Most dial indicators have components such as a bezel, indicator pointer, tool post and clamp, magnetic toolholder, and sensor button that are used in taking measurements.

blade micrometer Blade micrometer is a measuring device that has a thin blade to reach inside narrow recesses.
Blade micrometer is used for measuring where the standard micrometer spindle could not be accommodated.
combination set Combination set is used as a rule, a square, a miter, a depth gage, a height gage, and a level. The combination set consists of: blade, center head, protractor head, combination square head, scribe, and spirit level.
comparator Comparator graphically displays and measures dimensions and shapes that would be difficult to measure with regular tools. Optical measurement is a different and valuable aid to quality control. Capable of magnifying an object hundreds of times.
depth micrometer Depth micrometer measures depth of holes, slots, and other recesses.
These micrometers come as a set with interchangeable rods to accommodate various measurements.

dial indicators Dial indicators check alignment of machine components; show the variation of the object being measured. There are 2 types of dial indicators:

a. Balanced reading (has figures in both directions from the zero)
disc micrometer Disc micrometer measures forming tools, cutting edges, narrow slots.
This micrometer finds its application in measuring flanges and hard to reach areas.
gage block set Gage block set is also known as "Jo Blocks". Precision-round square or rectangular blocks are made of steel, chrome, or tungsten carbide. Gage blocks are used for:
- calibration and inspection of precision instruments
- to set comparators/indicators
- setting of sine bars
- precision layout
- machine setups
micrometer Micrometer is used to measure materials. It is available in metric and inch systems. The most common are the outside and inside micrometers.
Available in different shapes and sizes
outside caliper Outside caliper measures an outside surface. Rules are also used for transfer measurements with calipers. It is used for semi-precision and comparison measurement. They depend on "feel" to measure a part.
plug gage A type of "Go" or "No-Go" gage used to admit or refuse the inside dimension of an object.

There are three types:
- plain cylindrical
- cylindrical taper
- thread plug gage They are mounted at the ends of an aluminum handle. "Go" in one end and "No-Go" on the opposite end.
ring gage A type of "Go" or "No-Go" gage used to admit or refuse the outside dimension being checked.

There are three types:

- plain ring gages
- taper ring gages
screw thread micrometer The 60 degree comparator micrometer is usually called a screw thread comparator micrometer or pitch micrometer.
The pitch diameter, the most important measurement of a screw thread.
sine bar Sine bar is used for checking angles, typically used with gage blocks. The sine bar is a precision bar that has been hardened and then ground and lapped to very precise dimensions. It is also used for machine setups.
snap gage Snap gage gages diameters, lengths,and thickness.

Various types of snap gages: