Maharashtra HSC Engineering Drawing (Technical Science) key terms

The ratio of a length drawn on the sheet to the corresponding real length of the object, written as a fraction such as 1:50. It fixes the scale of the whole drawing.

A scale in which the drawing is smaller than the real object, so the representative fraction is less than one, for example 1:100 used for buildings.

A scale in which the drawing is larger than the real object, giving a representative fraction greater than one, as in 5:1 for tiny machine elements.

A scale built on a rectangle divided by diagonals so that distances can be read to a second decimal place, typically to measure hundredths of a unit.

A straight-line scale showing two consecutive units, such as metres and decimetres, with the first main division subdivided to read smaller parts directly.

The lines along which an inclined plane meets the horizontal and vertical reference planes; the horizontal trace meets HP and the vertical trace meets VP.

The actual, undistorted length of a line in space, obtained by rotating the line parallel to a reference plane or by constructing a right triangle from its projections.

The shortened length of an inclined line as it appears in a view; it equals the true length only when the line is parallel to that reference plane.

The angle a line or plane makes with HP or VP, seen true size only in the view where the line appears at true length or the plane appears as an edge.

A projection taken on a plane inclined to a principal plane, used to show the true shape of a slanted face or the true length of an inclined edge.

A projection plane perpendicular to HP but inclined to VP, so the view on it gives true lengths and shapes of features sloping in the front view.

A projection plane perpendicular to VP but inclined to HP, used to obtain the true shape of faces that are inclined in the top view.

An imaginary cutting plane passed through an object to expose internal features; its edge view is drawn as a chain line with arrows showing the direction of sight.

The actual outline of the cut surface obtained by projecting the section on a plane parallel to the cutting plane, undistorted by foreshortening.

Thin equally spaced parallel lines at 45 degrees drawn inside a cut surface to indicate solid material; adjacent parts are hatched in opposite directions.

The flat, unfolded shape of a solid's surface laid out on one plane, so that it can be cut from sheet metal and folded back into the solid.

A development technique for pyramids and cones in which true slant edges are swung about the apex to lay out the surface as sectors and triangles.

A development technique for prisms and cylinders in which the surface is treated as a series of parallel generators stretched out along a base line.

The line along which two intersecting solids meet, found by locating common points on corresponding generators or edges in plan and elevation.

A pictorial drawing in which the three axes are inclined at 120 degrees to each other and all three are drawn to a uniform scale, giving an enlarged but undistorted appearance.

A scale of 0.816 times the true length used to mark dimensions along isometric axes so that the resulting isometric projection matches the true projected size.

A pictorial drawing on isometric axes in which true lengths are measured along the axes directly, without applying the isometric scale, so it is slightly larger than the projection.

A pictorial drawing in which one face of the object is drawn true shape parallel to the picture plane and receding edges are drawn at an angle, usually 45 degrees.

An oblique projection in which receding edges are drawn at full true length, giving a bold but somewhat distorted depth to the object.

An oblique projection in which receding edges are drawn at half their true length, reducing the distortion of depth compared with cavalier projection.

A standard simplified symbol, such as crossed diagonals for a square end or a broken line for a shaft, used instead of a full view to save drawing time.

The abbreviated designation written on a drawing to specify a screw thread, giving form, nominal diameter, pitch and tolerance class, for example M20 x 2.5.

A power-transmission thread with a square cross-section, giving high efficiency and low friction but difficult to cut and unsuitable for a split nut.

A trapezoidal power thread with a 29-degree included angle that transmits motion efficiently and can be engaged by a split nut, common on lathe lead screws.

An asymmetric thread shaped to carry heavy thrust in one direction only, used on screw jacks and presses where the load acts along the axis.

A key is a small steel piece seated in matching slots (keyways) cut in shaft and hub to lock them together and transmit torque without relative rotation.

A key partly recessed into the shaft and partly into the hub, giving a positive drive; common forms are rectangular, square and taper keys.

A device joining two shafts end to end to transmit power; it may be rigid, or flexible to absorb misalignment, shock and vibration.

A rigid coupling formed by two flanged hubs keyed to the shaft ends and bolted together, with a projecting spigot locating the two halves.

A machine element that supports a rotating shaft, reduces friction and carries radial or thrust loads; shown in section with hatching for the bush and housing.

A plain bearing in which a replaceable sleeve of bearing metal, the bush, is fitted into the housing so it can be renewed when worn.

The nominal dimension common to both mating parts, from which upper and lower deviations are measured to obtain the limits of size.

The algebraic differences between the maximum or minimum permitted size of a feature and its basic size, denoted by letters such as ES, es, EI and ei.

The total permissible variation in a dimension, equal to the difference between maximum and minimum limits; it is always a positive quantity.

A fit in which the largest shaft is always smaller than the smallest hole, leaving a gap between the mating parts so they can move or rotate freely.

A fit in which the smallest shaft is always larger than the largest hole, so the parts must be forced or shrunk together and hold by friction.

A fit that may give either a small clearance or a small interference depending on the actual sizes of hole and shaft within their tolerance zones.

The arithmetic mean deviation of the assessed profile from its mean line, written beside the surface finish symbol to state the permitted roughness in micrometres.

The tick-like symbol placed on a drawing to specify where and how finely a surface must be finished, with the Ra value and process noted on it.

An addition to the surface finish symbol, such as a circle for no machining required or a triangle for material removal, indicating the production process.

A horizontal section cut through a building at window-sill level, showing walls, openings, room sizes and layout as seen from above.

The external front or side view of a building showing heights of floors, roofs, windows, doors and other features projected from the plan.

A vertical cut through a building showing internal heights, floor levels, foundations, roof construction and the details of walls and slabs.

A simple outline plan drawn to a small scale showing only the arrangement of rooms and walls, used in the early stages of design.

A fully dimensioned plan drawn to a larger scale that shows wall thicknesses, door and window positions, fixtures, and construction details needed for execution.

A thin continuous line terminated by arrowheads or oblique strokes, carrying the numerical value of a dimension with its figures placed above it.

A thin line drawn perpendicular to a dimension line from the feature being measured, showing the exact extent of the dimension.

A thin line, usually with a short horizontal shoulder, pointing from a note or dimension to the feature it describes.

A projection convention in which the object lies between the observer and the plane, so the view from above is placed below the front view.

A projection convention in which the plane lies between the observer and the object, so the view from above is placed above the front view.

The reference plane on which the top view is obtained; it is imagined horizontal, and views on it are projected downward from the object.

The reference plane on which the front view is obtained; it is imagined vertical and stands at right angles to the horizontal plane.

A sectional view in which the cutting plane passes completely through the object, so the whole interior is exposed in one view.

A sectional view of a symmetrical object in which one quarter is removed, showing half the view in section and half in outside view.