CBSE Class 12 Engineering Graphics
Chapters: 2
1. Isometric Projections of Solids
Isometric scale · Isometric projection of solids · Combination of two solids
- Isometric Projection and Axonometric Drawing – An axonometric drawing shows a solid in one picture with three axes: width, depth and height. In isometric projection the three axes are 120° apart and every axis is shortened equally (scale ≈ 0.82). An isometric drawing uses full lengths instead. Dimetric shortens two axes the same, trimetric all three differently. Oblique (cavalier, cabinet) keeps the front face true size and draws depth at 45°, full length (cavalier) or half (cabinet). Circles on faces become ellipses.
- Isometric Projection of Solids – An isometric projection shows a solid in one picture with three axes 120° apart: one vertical, two at 30° to the horizontal. Every length along these axes is shortened by the same factor, the isometric scale: isometric length = 0.816 × true length (√2/√3). Using full lengths instead gives an isometric drawing (view), about 22.5% bigger. Prisms and pyramids are drawn by boxing them in an isometric box. Circles on faces become ellipses, drawn by the four-centre method. A sphere always appears as a circle: in an isometric projection its radius is the TRUE radius, and its centre lies 0.816 × R above the point of contact.
- Isometric Projection of a Combination of Two Solids – In a combination question, one solid (usually the bigger one) is the base and a second solid sits centrally on top of it, so both share one vertical axis. Draw the lower solid first with isometric lengths. Find the centre of its top face; this is the centre of the upper solid's base. Build the upper solid from there, going up the common axis by its isometric height. Heights add: total iso height = 0.816 × (h1 + h2). The direction of viewing (an arrow in the question) decides which faces come to the front, so it fixes how you place the solids on the isometric axes. Hidden lines are not shown.
2. Machine Drawing
Drawing of machine parts · Bearings · Rod joints · Tie-rod and pipe joint
- Machine Drawing: Making and Reading Detail Drawings – A machine drawing is an exact picture of one part, made so a factory can build it without asking any questions. A detail (working) drawing shows the views of the part, section views for hidden insides, every size (dimension), how much each size may vary (tolerance), the surface finish, the material and a title block.
- Assembly Drawings: How Parts Fit Together – An assembly drawing shows many parts joined together as one product. It tells you which parts there are (balloons and a parts list, also called a bill of materials), where each one goes, and how they are joined: with bolts and screws you can undo, or with rivets and welds that stay for good.
- Rod Joints: Sleeve and Cotter, Gib and Cotter – A cotter joint joins two rods end to end so that they can carry a pull or a push along their length, and can still be taken apart. A cotter is a flat steel wedge with a small slope (taper) of about 1 in 30. In a sleeve and cotter joint, a hollow sleeve covers both rod ends and two cotters lock them. In a gib and cotter joint, a strap wraps around the square end of one rod and a gib plus a cotter lock it. In drawing, every size is written as a multiple of the rod diameter d.
- Tie-Rod and Pipe Joints: Turnbuckle and Flanged Joint – A tie rod is a rod that holds parts together by pulling (tension). A turnbuckle joins two tie rods and lets us tighten or loosen them: its body has a right-hand thread at one end and a left-hand thread at the other, so one turn pulls both rods in, and the length changes by two pitches. Pipes carrying water, steam, oil or gas are joined by a flanged pipe joint: each pipe end has a flat ring (flange), a soft gasket goes between the flanges, and a ring of bolts and nuts squeezes them so the joint does not leak but can still be opened.