Types of Measuring Instruments

 - With a need to make quality products which meet design specified tolerances large number of firms use measuring instruments that are having high accuracy and precision.

 -The special branch of science that deals with such instruments is known as metrology.

1) Linear Measurements:

A) Non Precision Instruments

a) Steel Rule

 - It is also known as scale. it is a line measuring device.

 - It is simplest and most common measuring instrument used in inspection.

 - The scale can be 150mm, 300mm, 600mm or 1000mm long.

b) Calipers

 - A caliper is a device used to measure the distance between two opposite faces of an object.

 - The tip of the caliper are adjusted to fit across the points to be measured, the caliper is then removed and the distance read by measuring between the tips with a measuring tool, such as ruler.

 - Different types of calipers are Outside caliper, Inside caliper, Spring caliper, Transfer caliper, Hermaphrodite caliper.

c) Dividers

 - It is used for measuring, transferring or marking off distances, consisting of two straight adjustable legs hinged together.

 - It is used principally in drafting for the accurate transfer of dimensions from a measuring scale and in machine shop for scribing lines on surfaces.

d) Telescopic Gauge

 - Telescopic gauge is a measuring tool with spring loaded plunges used together with a micrometer to measure the inside of holes or bores.

 - The telescopic gauge is definitely preferable for gauging simple round holes because of it's potentially greater level of accuracy.

e) Depth Gauge

 - A depth gauge is used to measure the depth of a slot, hole or any other surface of an object.

 - It can be of scale, dial or digital type.

B) Precision Instruments

a) Micrometers

 - It is also known as Micrometer screw gauge.

 - The spindle of micrometer is very accurately machined screw and the object to be measured is placed between the spindle and the anvil. The spindle is moved by turning the ratchet knob until the object to be measured is lightly touched by both the spindle and anvil.

 - The different types of micrometers are External micrometer, Internal micrometer, Vernier micrometer, Depth micrometer, Pitch micrometer.

b) Vernier Caliper

 - The Vernier caliper is primarily intended for measuring both inside and outside diameters.

 - The measuring ranges of caliper can be 0 to 25, 0 to 150, 0 to 200 mm etc.

 - If there is any error in the measuring instrument then it is called zero error. and correction to be made to eliminate the error is called zero correction.

c) Vernier Height Gauge

 - It is used to take precise vertical measurements of various objects.

 - They often have scribbling tool as the measurement level, allowing operator to repeatedly mark vertical distances on pieces of metals.

 - Vernier height gauge and electronic height gauge these are two main types of tools used to make accurate vertical measurements.

d) Slip Gauge

 - Slip gauges were first developed by Johnson, and sometimes also called as 'Johnson Gauge Blocks'. These are rectangular blocks of steel having a cross-section of about 32 mm x 9 mm.

 - If two slip gauges are twisted together under certain pressure, then due to molecular attraction and atmospheric pressure they will adhere to each other quite firmly. This process is known as wringing.
 
This Process is very useful to produce a required size by assemble several gauge blocks.

e) Comparators

 - A comparators works on relative measurements i.e. it gives only dimensional differences in relation to a basic dimensions.

 - Comparator compares the unknown dimensions of a component with some standard or master setting which represents the basic size and dimensional variations from the master.

 - The advantage of comparators are that not much skills is required on the part of operator in its use.

 - Comparators are classified as Mechanical, Electrical, Optical and Pneumatic.

2) Angular Measurements

A) Non Precision Instruments

a) Protractors

 - Protractor is a measuring instrument, typically made by transparent plastic or glass for measuring angles.

 - Some protractors are simple half-discs like bevel protractor have one or two swinging arms, which can be used to help measure the angle.

b) Adjustable bevel

 - A sliding T bevel, also known as bevel gauge or false square is an adjustable gauge for setting and transferring angles.

 - The bevel can be used to duplicate an existing angle, or set to a desired angle by using it with any number of other measuring tools such as protractor.

c) Try Square

 - An engineer's square is a tool used in metal working to check a workpiece is square or not.

 - It can also be used to help draw or scribe a line at 90 degrees to the edge of a workpiece.

d) Combination Square set

 - It is a tool used for multiple purpose in wood working and in metal working. 

 - It consist of blade and moving head called square head, protractor and a center head. The most common head is the standard or square head which is used to lay out or check right and 45° angles.

B) Precision Instruments

a) Bevel Protractor

 - It is used to establish and test angles to very close tolerances.

 - It reads to 5 minutes or 1/20° and can be used completely through 360°.

b) Sine Bar

 - A sine bar consist of a hardened, precision ground body with two precision ground cylinders fixed at the ends.

  - When sine bar is placed on a level surface the top edge will be parallel to that surface. If one roller is raised by a unknown distance, then the top edge of the bar will be tilted by the same amount forming angle that may be calculated by the application of the sine bar.

c) Angle Gauges

 - These are wedge shaped steel blocks and their working faces are finished in the same manner as slip gauges, enabling them to be wring together in combinations.

 - A full set of comprises twelve pieces divided in three series. 1, 3, 9, 27 and 41 Degrees; 1, 3, 9 and 27 minutes; 0.1, 0.3 and 0.5 minute.

d) Spirit level

 - Spirit level, bubble level is designed to indicate whether a surface is horizontal or not. 

 - Spirit level had very slightly curved glass vials with constant inner diameter at each viewing point. These vials are incompletely filled with a liquid (Ethanol- as alcohol have low viscosity and surface tension which allows the bubble to travel the tube quickly). a colorant such as fluorescein typically yellow or green added to increase visibility.

e) Clinometer

 - Clinometer is an instrument for measuring angles of slop, elevation or depression of an object with respect to gravity. 

 - It is also called as tilt meter, slope alert, gradient meter, declinometer, pitch and roll indicator.

 - Clinometers measured both inclines( Positive slopes) and declines (Negative slopes) using three different units i.e. degrees, percent and topo.

3) Taper Measurements

 - Taper angle is measured by Bevel protractor, Tool room microscope, Autocollimator, sine bar, Rollers, slip gauges and micrometers.

 - Taper micrometer itself has the sine bar principle and gives the actual value of the taper of small angles. Larger tapers can be directly obtained from micrometer reading. This makes possible to check both internal and external tapers ten times faster than old conventional methods and does not require sine bars or more elaborate equipment's.

4) Surface Measurements

a) Straight Edge

 - It is used for testing the straightness and flatness of plane surfaces.

 - The ordinary shape is rectangular but for accurate work one edge is beveled or form into thin knife edge.

 - Narrow edge is working surface and wide edge is serves as base when not in use. flatness is checked by a light test. 

 - The narrow edge is applied to the surface to be tested and kept horizontally at eye level, watching for light between the straightedge and surface.

b) Surface Gauge

 - This is largely used in the lying out of parts that have to be fitted or machined.

 - It is used to test the parallelism of work. If scriber is replaced by dial indicator then it becomes a precision instrument.

 - The tool has a V-shaped groove at the bottom of the base for resting the block on a round bar, so that dimensions may be set off from the bar to some other part of the work.

c) Optical Flat

 - An optical flat is an optical grade piece of glass lapped and polished to be extremely flat on one or both sides usually within few tens of nanometers. They are used with a monochromatic light (Single wave length) to determine the flatness of other surfaces.

 - When optical flat placed on another surface and illuminated, the light waves reflect off both the bottom surface of the flat and the surface it is resting on. The reflected waves interfere, creating a pattern of interference fringes visible as light and dark bands.

 - A flat surface is indicated by a pattern of straight, parallel fringes with equal spacing, while other patterns indicate uneven surfaces.

 - Usually only one of the two surface of an optical flat is made flat to the specified tolerance, and this surface is indicated by an arrow on the edge of the glass.