# Measuring Instruments in Mechanical Engineering

The measuring instruments in mechanical engineering are dimensional control instruments used to measure the exact size of object. These are adjustable devices and can measure with an accuracy of 0.00 l mm or better.

The gauges are fixed dimension instruments and are not graduated. In most cases, these have no adjustable member for measuring varying dimensions of lengths and angles.

Some important measuring instruments and gauges are discussed, in brief, as follows:

1. Outside micrometer: It is mainly used to measure the outside diameter of a job or length of a small part, to an accuracy of 0.01 mm.
2. Inside micrometer: It is used to measure large internal diameters (over 50 mm) to an accuracy of 0.01 mm.
3. Screw thread micrometer: It is used to measure the pitch diameter of screw threads to an accuracy of 0.01mm.
4. Depth gauge micrometer:  It is used to measure the depth of holes, slots and recessed areas to an accuracy of 0.01 mm
5. Vernier caliper: It is used to measure external as well as internal diameters of shafts, thickness of parts, depth of slots and holes, to an accuracy of 0.02 mm.
6. Vernier height gauge: It is mainly used to measure heights of parts to an accuracy of 0.02 mm.
7. Vernier depth gauge: It is used to measure the depth of holes, recesses, and distances from plane surface to a projection, to an accuracy of 0.02 mm.
8. Combination set: It is an extremely useful instrument and has all the essential features of try square, bevel protractor, rule and scriber.
9. Universal bevel protractor: It is also called Vernier bevel protractor. It is used for measuring and testing angles, within the limits of 5 minutes (1/12 of a degree).
10. Sine bar: It is used either to measure angles more precisely than a bevel protractor or for locating any work to a given angle within very close limits. It is generally used with slip gauges.
11. Ring gauge: It is used to check the diameter of shafts or studs.
12. Plug gauge: It is used to test the accuracy of holes. The standard plug gauge is used in general engineering workshop, tool rooms etc. The limit plug gauge is used where large quantities are to be produced. The single-ended limit plug gauge has separate 'Go' and 'Not go' members. The progressive limit plug gauge has 'Go' and 'Not go' members on the same side of a handle.
13. Snap gauge: It is used to check the external dimension.
14. Slip gauge: It is used to check the accuracy of micrometers, calipers, snap gauges, dial indicators etc.
15. Feeler gauge:  It is used to check the clearances between two mating surfaces.