engineering drawings

Download Engineering Drawings

Post on 08-Oct-2014




3 download

Embed Size (px)


1/18/12 Engineering Drawings1/11 are not the natural language of engineers.Drawings are their prose, mathematics their grammarand differential equations their poetr.GleggDRAWINGS IN ENGINEERING DESIGNInodcionEngineering drawing is not only the province oI the draItsperson. It is the language oI the engineer. It is theirmeans oI developing and recording their ideas, and conveying them to others. Every engineer will be using andreIerring to some Iorm oI drawings almost daily. They will oIten be producing or directing the preparation oIdrawings. Usually, they make the preliminary sketches and design drawings in accordance with principles oIengineering drawing. Because this is the most unambiguous way oI to convey and record inIormation. It is alsolikely that every engineer at sometime will be checking the work oI designer draIters and approving drawingsbeIore they are sent to manuIacturing. When engineers sign oII the Iinal approval oI a drawing, they takeresponsibility Ior it. An overlooked error in the drawing could be costly.Ideally, then, engineers should be good draItspersons.They can constructively criticize the work oIinexperienced draIters. However, with the limited timeavailable at the University it is not possible to get thenecessary proIiciency. At the university you are giventhe Iundamentals, and it is up to you to improve yourknowledge and skill as required.This course will emphasize design procedures. However, the design drawings which you will be making must beproperly executed.Deelopmen and Podcion DaingIn their everyday work mechanical engineers must be Iamiliar with production drawings. The Iunction oI theproduction drawing is to impart descriptions, speciIications, and instructions to the shop so that three-dimensional objects and systems may be manuIactured and assembled in their correct location with respect toother components oI a machine.Where do the ideas Ior the creation oI the object originate, and how are these ideas developed? The Iorm oI adesign is progressively developed graphically. For example, much oI the original thinking is involved in thetechnical sketch made by the engineer or designer. Many calculations are done at this stage. As IurtherconIirmation oI the practicability oI the design an accurately made scaled drawing called a layout is made. Thelayout shows the overall dimensions and will show several critical elements assembled in their Iunctionalrelationships. Detail drawings are then made. Usually one drawing is made Ior each part, showing completedetails and instructions necessary Ior its manuIacture. Finally, subassembly and assembly drawings are made toshow how the detail parts are to be assembled and to show general dimensions.Specificaion and he Popoal DaingLayout representation begins with the interpretation oI design speciIications by making up proposal drawing (Exhibit 1a , Exhibit 1b , Exhibit 1c ).Suppose a Space Agency wants to purchase a new attitude control system. From preliminary studies they have1/18/12 Engineering Drawings2/11 of he mo efl pocede in all age a hee of feehand keche o epeen all alenaie in a3D aangemen.Nei e al, ICED 83Iound the Ilight characteristics oI their vehicle. This gives them the control requirements Ior their system. A set oIspeciIications is drawn up and requests Ior proposals are issued to the companies Irom which they wish toreceive quotations. The design engineers at these companies on receiving copies oI these speciIications will beginrough designs. The designers will roughly design the components that will make up the system, sensors,actuators, computers, programs, etc. so that a proposal drawing can be made. These drawings show the generaldesign that will best IulIil Iunctional requirements. They show general dimensions, areas, weights and other basicdesign and manuIacturing inIormation. From these proposal drawings a preliminary estimate oI engineering,tooling and production costs are made. The estimated cost and the proposal drawing are sent to the salesdepartment who add a Iactor Ior proIit and establish a selling price which is to be quoted. The price, drawings,and much other descriptive inIormation are then submitted as a proposal and tender.The proposal including the proposal drawings become an essential part oI a design contract, and it is the basis oIthe eventual design, the drawings are not used Ior Iabrication. When a complicated product is being considered,proposal drawings with the written text indicate only the method to be employed, in obtaining basic the Iunctionalrequirements. They emphasize engineering principles to be used in design. The bulk oI the minor design work isgenerally suggested but not completed. It is expected, thereIore, that the Iinal product although constructedaccording to the principles set Iorth in the proposal drawing, may diIIer considerably Irom it.The degree oI completeness oI proposal drawings is inversely proportional to the complexity oI the product.Thus, Ior a less complicated product the proposal drawing may also suIIice as the layout, and occasionally evenas a working drawing. The reason behind this is that involved systems require many specialists who must expendmuch time and eIIort to arrive at detailed solutions. The development procedure is expensive and can be justiIiedonly when an organization has received a contract to carry a design to its completion. Proposal drawings supplyonly enough inIormation Ior contract acceptance.The major product design work begins aIter the company has received the order and the proposal drawing hasbeen accepted. Meanwhile many revisions may have been made to the proposal drawing to suit the customer'srequirements beIore it is accepted. The principal component parts or sections oI the product are assigned tospecialized design groups, and each group might be headed by an engineer. A project engineer will be in chargeoI the complete product design. This however will vary greatly with the type oI organization.The proposal may include an outline drawing ( Exhibit 2 ), at the time oI submission or shortly aIter the order isconIirmed. Outline drawings become part oI the contract obligation. Its purpose is to provide the customerssuIIicient inIormation about the product that they can go on with the rest oI their design. ThereIore the outlinedrawing gives all the dimensions oI the Iinished device requited to attach it and to connect it to the equipment itwill work with. And it must also give the overall dimensions oI the system so that the space it will occupy. Outlinedrawings are sometimes required to be certiIied. A responsible oIIicer oI the company, typically the ChieIEngineer, must sign the drawing guaranteeing that the system will be in accordance with it. Technical SkechThe designers interpret the requirements shown on theproposal drawing, study the accompanyingspeciIications, and begin thinking out solutions. Thesolutions are recorded in technical sketches. In thetechnical sketch ( Exhibit 3a, Exhibit 3b, Exhibit 3c ) the designer puts down the important Iactors - general1/18/12 Engineering Drawings3/11, clearances to be checked, structural investigations, Iunctional requirements and basic manuIacturingprocesses that may be used. The designer must exercise ingenuity in making approximations beIore an accuratestress analysis is made to decide actual sizes. Technical sketches are not discarded, they are valuable becausethey record most oI the ideas and the directions that contribute to the Iinal design. As much thinking and planningas possible should be shown in the rough sketches. This expedites a more direct solution and lessens thepossibility oI having to change design principles completely on the careIully drawn layout.The LaoutA layout drawing (Exhibit 4a , Exhibit 4b , Exhibit 4c ) by the designer is an exact graphical representation oI thedesign. It is intended Ior engineering rather than manuIacturing use, although sometimes a layout drawing is usedIor experimental production. The layout is an accurate development oI the conception oI the design, or theplacement oI units. Essential elements are developed, and the geometry oI the machine or structure isdimensionally deIined taking into consideration its Iunction, manuIacture and other requirements. The layout is akey drawing Irom which production drawings are made. Several layouts may be required Ior one machine. Forinstance the steering mechanism in a car would require a layout drawing. The rear end would require another. Inmaking the layout, the basic reIerence lines and center lines are located. Adjacent or existing parts are drawn inphantom lines. This conveniently deIines the space available to work. The general shape oI each componentmember is approximated and calculations are carried out simultaneously which Iinally determines the actual sizes.Sometimes the layout, the design sketches and calculations are made simultaneously because each providesinIormation that is needed Ior the other.Layout drawings are always drawn to scale, Iull scale iI possible. CAD is extremely useIul this way. Layoutdrawing can also be done rapidly on squared paper to give the scale. The prime consideration is accuracy - onlya minimum oI necessary essential graphical inIormation is presented. Layout drawings are similar to assemblydrawings, except that cross hatching is conIined to the boarders and may be done Iree hand. Symbols may beused Ior standard components unless details are required Ior clariIication.For stress calculations Ireehand sketches may be used also. The sketches and calculations are Iiled Ior reIerenceand checking purposes. The coordination oI stress analysis, Iunction, manuIacturing, and clearance Iactors are allembodied in the layout. The Production Detail DrawingDetail drawings ( Exhibit