CHAPTER 21 Reading Technical Drawings OBJECTIVES After completing

CHAPTER 21 Reading Technical Drawings

OBJECTIVES • After completing this chapter, the student should be able to – list the types of drawings that can be found in a set of drawings and what information is contained on each of them. – sketch 10 types of lines, identify each, and explain how they are used on mechanical drawings. – explain the difference between mechanical and pictorial drawings. – name all of the various views that can be shown on drawings. – read a set of drawings and explain each item shown and its dimensioning. – discuss why a drawing may be scaled. – compare the differences between sketches and mechanical drawings. – demonstrate the ability to make a sketched drawing. – illustrate how to use graph paper to make a scaled drawing. – list the advantages of using computer-aided drafting software to make mechanical drawings.

KEY TERMS • • • • alphabet of lines bill of materials break line cavalier drawings centerline cut-a-ways cutting plane line detail views dimension line extension line hidden line isometric drawings leaders and arrows mechanical drawings • • • • object line orthographic projections phantom lines pictorial drawings project routing information scale section line section view set of drawings sketching specifications title box vector lines

INTRODUCTION • Drawings are the tools that let us accurately communicate with each other in a very technical way. • As you look at a basic mechanical drawing, you can see the object’s shape, size, and location of its parts. • This chapter will help you see the basic layout of mechanical drawing and how the various parts of a drawing relate.

MECHANICAL DRAWINGS • Mechanical drawings have been around for centuries. • Mechanical drawings have been called the universal language; they are produced in a similar format worldwide. • A group of drawings, known as a set of drawings, should contain enough information to enable a welder to produce the weldment. – The pages may include the following: title page, pictorial, assembly drawing, detailed drawing, and exploded view, Figure 21 -2.

MECHANICAL DRAWINGS (cont. ) • Of the two methods for locating the views, the third angle projection is the most commonly used for welding drawings.

MECHANICAL DRAWINGS (cont. ) • Examples of some of the pages that can be found as part of a set of drawings.

MECHANICAL DRAWINGS (cont. ) – The title box, which will appear in one corner of the drawing, may contain the name of the part, company name, scale of the drawing, date of the drawing, who made the drawing, drawing number, number of drawings in this set, and tolerances. – The specifications detail the type and grade of material to be used, including base metal, consumables such as filler metal, and hardware such as nuts and bolts. – The project routing information is used in large shops where an assembly line process is used. This information lets you know where the parts are to be sent once you have completed your part of the assembly process. – A bill of materials can also be included in the set of drawings. This is a list of the various items that will be needed to build the weldment, Figure 21 -3.

MECHANICAL DRAWINGS (cont. ) • A bill of materials can also be included in the set of drawings. • This is a list of the various items that will be needed to build the weldment.

LINES • To understand drawings, you need to know what the different types of lines represent. • Different types of lines are used to represent various parts of the object being illustrated. • The various line types are collectively known as the alphabet of lines, Table 21 -1 and Figure 21 -4.

LINES (cont. ) TABLE 21 -1 – – – – – Object lines Hidden lines Centerlines Dimension lines Extension lines Cutting plane lines Section lines Break lines Leaders and arrows Phantom lines

LINES (cont. ) • Drawing showing alphabet of lines, Figure 21 -4.

TYPES OF DRAWINGS • Drawings used for most welding projects can be divided into two categories— pictorial and mechanical drawings. – Pictorial Drawings • Isometric Drawing • Cavalier Drawings – Mechanical Drawings

TYPES OF DRAWINGS (cont. ) • Pictorial drawings present the object in a realistic and more easily understandable form. • These drawings usually appear as one of two types, isometric or cavalier.

TYPES OF DRAWINGS (cont. ) • (A) Viewing an object as if it were inside a hinged glass box. • (B) This is what you would see if you traced the views seen through the glass box and unfolded it. • (C) Arrangement of the six views of the sides of the box as shown by the third angle projection method.

SPECIAL VIEWS • • Section view Cut-a-ways Detail views Rotated views

SPECIAL VIEWS (cont. ) • The section view is drawn as if part of the object were sawn away to reveal internal details.

SPECIAL VIEWS (cont. ) • Some drawings use specific types of section lines to illustrate the type of material that the part was made with. • The location of this imaginary cut is shown using a cutting plane line.

SPECIAL VIEWS (cont. ) • Notice that the round drilled hole looks misshapen or elliptical in the right-side view.

DIMENSIONING • The drawing should have all of the dimensions necessary to fabricate the weldment. • Sometimes it maybe necessary to look at other views to locate all of the dimensions required to build the object. • By knowing how the views are arranged, it becomes easier to locate dimensions.

DIMENSIONING (cont. ) – Length dimensions can be found on the front and top views. – Height dimensions can be found on the front and right side views. – Width dimensions can be found on the top and right side views, Figure 21 -10.

DRAWING SCALE • When we use a scale, we are saying that the part being drawn is drawn smaller or larger than it really is. • To aid in making and reading scaled drawings, you can use a drafting tool called a scale, Figure 21 -11.

DRAWING SCALE (cont. ) • The pot rack was scaled down to fit the drawing page, and the detail of the pot hook was scaled up to show more detail.

READING MECHANICAL DRAWINGS • The pictorial drawing of the block in the glass cube shown in Figure 216 A is color-coded. • The front surface in all the views is colored orange, the top view is green, the right side is blue, the left side is yellow, and the bottom is brown.

PRACTICE 21 -1 • Reading Mechanical Drawings – Using a pencil and lined paper, you are going to identify the color of the surfaces shown in the 3 -view drawing as shown on the pictorial view, Figure 21 -13 and the color of the surface identified in the pictorial drawing as they are shown in the 3 view drawing in Figure 21 -14. Use Table 21 -2 to identify the colors of the surfaces.

PRACTICE 21 -1 (cont. )

SKETCHING • Sketching is a quick and easy way of producing a drawing that can be used in the welding shop. • NOTE: When graph paper is used, sketches can be easily drawn to scale, Figure 21 -15.

SKETCHING (cont. ) • A sketch can be any drawing that is made without the extensive use of drafting instruments or computer-aided design (CAD). • Sketching is the process of making a line on a drawing by making a series of quick, short strokes with the pencil or pen, Figure 2117.

PRACTICE 21 -2 • Sketching Straight Lines – Using a pencil and unlined paper, you are going to sketch a series of 6 -in. long straight lines.

PRACTICE 21 -3 • Sketching Circles and Arcs – Using a pencil and unlined paper, you are going to sketch a series of circles.

PRACTICE 21 -4 • Sketching a Block – Using a pencil and unlined paper, you are going to sketch a mechanical drawing showing three views of a block as shown in Figure 21 -21.

NOTE

PRACTICE 21 -4 (cont. ) • Additional blocks to sketching

PRACTICE 21 -5 • Sketching candlestick holders.

ERASERS AND ERASING • Most pencil erasers have an abrasive action on the paper as they are used to erase pencil marks. • If you are not careful in erasing with a pencil eraser, you can damage the paper’s surface, making it hard to redraw over that area. • Plastic erasers are usually white, and these erasers do not have an abrasive and will not damage the paper’s surface like pencil erasers.

GRAPH PAPER • Making a sketch on graph paper is a way of both making your drawing more accurate and speeding up the sketching process. • Graph paper is available with grid sizes ranging from 1/8 in. to 1/4 in. squares, Figure 21 -27.

PRACTICE 21 -6 • Sketching the parts of a workmanship qualification test

PRACTICE 21 -7 • Sketching irregular shapes

NOTE

PRACTICE 21 -7 (cont. ) • Sketching irregular shapes

COMPUTERS AND DRAWINGS • Computers have made it much easier to draw plans for projects. • The welded birdhouse project shown in Figure 21 -32 was drawn on Auto. CAD LT.

COMPUTERS AND DRAWINGS (cont. ) • Drafting programs use vector lines, which is different from most drawing programs, which use raster art. – Computers see vector lines as lines, and they see raster lines as if they were part of a picture. • Because vector drawings are seen by the computer as lines, you can zoom in and out, measure, resize, reshape, or rotate the drawing and the lines stay crisp and sharp.

COMPUTERS AND DRAWINGS (cont. ) • Bitmap line drawing

COMPUTERS AND DRAWINGS (cont. ) • The drawing precision can be set on Auto. CAD LT using the dimension style manager.

COMPUTERS AND DRAWINGS (cont. ) • The Auto. CAD LT rotate feature can be used to check part alignment and fit, such as the trailer ramp on this drawing.

COMPUTERS AND DRAWINGS (cont. ) • The Auto. CAD LT makes it easy to connect lines with its endpoints connecting feature. • Hovering above an icon will provide a one or two-word hint.

COMPUTERS AND DRAWINGS (cont. ) • On-screen help will guide you through many of the computer program’s features. • Active Assistant details the specific functions available.

SUMMARY • Almost every welding fabrication will have a set of drawings detailing exactly how the assembly is to be made. • Welders who can read and accurately follow these drawings have a skill that is valuable to any welding shop. • Even the slightest error in the measuring, cutting, placing, or welding of the fabrication cause it to be scrapped or necessitate expensive reworking.