Chapter Ten Drawing Analysis

1. Chapter Ten Drawing Analysis

2. Purpose This chapter provides an overview of how to: • understand the terminology of engineering elements commonly found on engineering drawings • obtain a pictorial comprehension of a complex component by analysing and combining the information on multiple two-dimensional views • analyse a drawing and ‘take off’ information relevant to its manufacture

3. Drawing analysis • Mechanical drawings are constructed as per AS1100 standards. This allows for accurate interpretation by those required to use the drawing. Therefore, it is important the reader can analyse the information recorded on the sheet. This is facilitated by the use of standard drawing techniques to represent common features. • The following is a description of the detail labelled on Figure 10.1, p.281.

4. Figure 10.1

5. Summary analysis • A counterbored hole is used to house a screw or bolt head so that it does not project from the surface. It also provides a surface, square to the hole axis, for bolt head seating. • A bolt is designated by the material, head shape, ISO metric thread diameter (mm) and the length (mm) of its shank. • A spigot is a piece of material (usually circular) which projects from the face of a member. It is used to locate a member precisely when assembling it with another member. It may also be used to carry any shear load which may be applied to bolts holding the two members together.

6. Summary analysis • Note that as this is a sectional view, the cross-hatch lines pass over the internal thread section. • A recess allows a member to engage right to the bottom of a hole without interference from a rounded corner. A recess can also be used externally, for example when turning a thread up to a shoulder. • A centre line is a light, long-short dash line (type G) which is used to indicate axes of holes and the centres of part and full circles. • A countersunk hole in this case is used as an oil hole, but mostly it would be used to house the countersunk head of a screw.

7. Summary analysis • Note that the hatch lines do not pass over the assembled threads, but where the thread stands alone, item 4 above applies. • A stud is a member, threaded both ends and screwed firmly into the main part. Studs are used to attach coverplates and housings as shown. • A seal is generally a plastic ring seal which, when compressed against the main housing, squeezes against the rotating shaft and prevents entry of dust and grit into the main bearing. It also prevents lubricant from leaking out.

8. Summary analysis • 11. A chamfer is generally 45°, its purpose being to eliminate the sharp edge. • 12. A shaft is a rotating member used to transmit torque. Note the chamfer on the end and the method of showing a break in the shaft, that is, the shaft actually extends beyond the length shown in the drawing. 13–14. A washer (13) is used for assembly with the nut (14) on to the stud. It prevents scoring of the plate when the nut is tightened up. 15. A housing is a general term used to describe the location of items such as seals, bearings, gears, etc. Shown here is a seal housing.

9. Summary analysis • A clearance hole is a hole just a little larger than the diameter of the stud, so that assembly is made easy. Recommended diameters of clearance holes for various sizes of metric thread diameter are given in Table 2.4. • Leaders are used to indicate where dimensions or notes are intended to apply. They are thin full lines which terminate in arrowheads or dots. Arrowheads terminate on a line, dots should fall within the outline of the object, as shown by items 30, 28, 23 and 10. • An external or male thread is the representation of the outside view of a threaded member.

10. Summary analysis • A projection line is a thin full line (type B) extending from the outline, but not touching it. These lines denote the extremities of a dimension and should extend a little beyond the dimension line. • A dimension line is a thin full line (type B) extending between projection lines. It has arrowheads on either end to indicate the length of the dimension, which is placed above the dimension line and approximately in the centre. • A runout is used to indicate the intersection of two surfaces which do not meet at a sharp corner.

11. Summary analysis • A surface texture symbol indicates the finish of the surface to which it is applied. See pages 53–54 for more details. • A spotface is an area around a hole which is machined perpendicular to the hole axis. It provides a flat true seating for the head of a nut or bolt. • Flange is a term used to describe a section of a member that carries holes through which bolts or screws pass to fasten the member.

12. Summary analysis • A boss is a raised or extra portion of metal machined on top to support the screw head. The term ‘boss’ can be applied to extra projections of metal which provide additional support as well as an extension of the function; for example, shaft bosses provide extra bearing length, and screw or bolt bosses provide for adequate thread length. • Pitch circle diameter (PCD) is a long-short dash circle which passes through the centres of a series of holes. The holes are generally pitched evenly around its circumference.

13. Summary analysis • Note in the end view of an internal thread that the full circle is on the inside and the broken circle is on the outside as opposed to the end view of an external thread. See page 33 for more details. • Bush is a term used to describe a plain bearing for a shaft. It is a sleeve, usually made of bronze material, which fits tightly into the housing. • A thin short-dash line (type E) is used to indicate hidden detail such as corners or edges which cannot be seen from the outside.

14. Summary analysis • A web is a strengthening or stiffening member. • All castings have fillets on internal corners to prevent the formation of stress fatigue cracks which originate in sharp corners. • The course of a cutting plane is indicated by a chain line (type H), thick at the ends and where it changes direction, but thin elsewhere. The view in Figure 10.1 (section A-A) reveals detail seen at the level of this plane in the direction of the arrows A-A. • A round is similar to a fillet, but is normally found on external corners of a casting.

15. Summary • When interpreting drawings it is often difficult to visualise the parts and their physical location in context to other parts. • The ability to conceptually picture and manipulate objects and situations is an essential need in many engineering-related jobs. This chapter has highlighted the need to become familiar with the use of standard drawing techniques to represent common features.