Machine Drawing: Machine Parts

Main Idea

Machine drawing represents machine parts and assemblies through standard conventions that communicate shape, dimensions, manufacture and assembly clearly and economically. Threaded fasteners, including bolts, nuts, washers, studs and screws, are selected according to joint requirements, accessibility, loading and whether the joint must be removable, while rivets are used for permanent joints.

Key Concepts and Definitions

• Thread: A continuous helical ridge or groove formed on a cylindrical or tapered surface.
• External thread: A thread formed on the outside surface of a rod or bolt.
• Internal thread: A thread formed inside a hole, as in a nut or tapped hole.
• Major diameter: The largest diameter of a thread, measured over the crests of an external thread or the roots of an internal thread.
• Minor diameter: The smallest diameter of a thread, measured at the roots of an external thread or the crests of an internal thread.
• Pitch: The axial distance between corresponding points on adjacent threads, given by $p=$ distance between corresponding points on adjacent threads.
• Lead: The axial distance moved by a screw in one complete revolution; for a single-start thread, lead equals pitch, while for a multiple-start thread, lead $=$ number of starts $\times$ pitch.
• Crest: The top or highest part of a thread.
• Root: The bottom part of the groove between two adjacent threads.
• Flank: The inclined surface joining the crest and root of a thread.
• Bolt: A headed threaded fastener used with a nut to join two or more parts temporarily.
• Nut: A block with an internal thread that fits onto the thread of a bolt or stud.
• Washer: A thin plate, usually with a central hole, placed under a nut or bolt head to distribute load and protect the surface.
• Stud: A threaded rod, usually threaded at both ends or along part of its length, used where a bolt cannot be conveniently inserted.
• Screw: A threaded fastener that engages directly with a tapped hole or material and may not require a separate nut.
• Rivet: A cylindrical permanent fastener with a head at one end; its other end is formed into a second head after insertion.
• Through bolt: A bolt passing completely through the joined parts and secured with a nut.
• Tap: A tool used to cut internal threads in a drilled hole.
• Clearance hole: A hole slightly larger than the bolt diameter so that the bolt can pass freely through it.
• Blind hole: A hole that does not pass completely through a component.
• Hexagonal bolt and nut: A common fastener combination having six-sided heads or bodies for tightening with a spanner.
• Countersunk screw: A screw with a conical head designed to lie flush with or below the surface of the component.
• Riveted lap joint: A permanent joint in which two plates overlap and are joined by one or more rows of rivets.
• Riveted butt joint: A permanent joint in which two plates meet edge to edge and are connected using one or two cover plates.
• Conventional thread representation: A simplified drawing method using standard thick and thin lines instead of drawing the actual helical thread profile.

Supporting Arguments and Evidence

• Engineering drawings require standard conventions because simplified, consistent representation communicates manufacturing and assembly information more effectively than a pictorially exact drawing. Dimension lines, extension lines, centre lines and object lines must be distinguished by correct line types and thicknesses.

• In an external thread, the major-diameter line is generally shown by thick continuous lines and the minor-diameter line by thin continuous lines in longitudinal views. In an end view, the major diameter is represented by a complete thick circle and the minor diameter by a thin circular arc, commonly about three-quarters of a circle.

• Internal-thread representation is reversed relative to external-thread representation: the visible thread boundaries are shown according to the major and minor diameters of the hole. Sectional views are particularly useful for showing internal threads, holes, contact surfaces and the arrangement of fasteners.

• Thread geometry must be interpreted using pitch and lead. Pitch is the distance between corresponding points on adjacent threads. For a single-start thread, lead equals pitch; for a multiple-start thread, lead equals the number of starts multiplied by the pitch.

• A bolt assembly normally includes a bolt, the parts being joined, a washer when required, and a nut. The usual drawing sequence is to draw the centre line, bolt shank and head, nut, washer if specified, and then apply standard line conventions and dimensions.

• Bolts, nuts and screws generally form temporary joints. A through bolt passes completely through the joined parts and is secured with a nut, whereas a screw commonly engages directly with a tapped hole and is tightened from its head side. A tap is used to cut the internal thread in a drilled hole, and a clearance hole permits the bolt to pass freely through the joined parts. A blind hole does not pass completely through a component.

• A stud is useful when one part is thick, when repeated assembly is required, or when a bolt head cannot be accommodated. The choice between a bolt, screw, stud or rivet depends on whether the joint must be removable, how accessible it is and the type of load it must withstand.

• Washers improve bolted joints by distributing the clamping force, preventing damage to the assembled surface and potentially reducing loosening. Hexagonal heads and nuts are drawn using true geometric construction in end views and simplified conventional forms in many assembly drawings. A chamfer is often provided on bolt heads and nuts to remove sharp edges and help the tool engage smoothly.

• Rivets are permanent fasteners; removal normally requires drilling, cutting or otherwise destroying the rivet. Common rivet-head forms include snap, pan, conical, countersunk and flat heads.

• In a riveted lap joint, two plates overlap and are joined by one or more rows of rivets. In a riveted butt joint, two plates meet edge to edge and are connected using one or two cover plates. The pitch of rivets is the centre-to-centre distance between adjacent rivets in the same row, while edge distance is the distance from the centre of a rivet hole to the nearest edge of the plate.

• Assembly drawings should show the relationship between parts, not merely the shape of each individual component. Parts that touch each other should be shown clearly, and hidden lines may be omitted when they reduce readability.

• Standard dimensions and proportions should be used wherever specified by the applicable engineering drawing standards, and unnecessary repetition of dimensions should be avoided. Accurate centre lines, correct conventional thread representation, proper sectional views and complete dimensioning are essential for interpreting and manufacturing machine parts.

What to Remember

Machine-part drawing depends on standard conventions for threads, fasteners, sectional views, line types and dimensions. Bolts, screws and studs generally create temporary joints, with washers distributing pressure and protecting surfaces, whereas rivets form permanent joints. For examinations, retain the specific thread representations, the pitch–lead relationships, the functions of each fastener, and the defining features of riveted lap and butt joints.