Sheet metal drafting

This text on "Sheet Metal Drafting" was prepared especially for correspondence study instruction in the Extension Division of the University of Wisconsin. It is also admirably adapted as a text book for Vocational, Evening, and Part time Schools.

The underlying principles of sheet metal pattern drafting are presented, each chapter discussing a different principle. The sequence of principles has been arranged with due regard to the well-known factors governing the student's progress through such a course of instruction. This arrangement has been successfully tested by several years of practical application in the teaching of the subject.

The problems in "Related Mathematics" point out the applications of mathematical principles to sheet metal work and serve as a guide for the proper correlation of the work in mathematics, drawing, and shop practice.

When the text is used for vocational, evening, and part time schools, the articles considered in the various chapters can be manufactured in the shop school. When these are sold, the cost of instruction is considerably reduced without sacrificing the educational value of the course.

1. Sheet Metal Drafting - Sheet metal drafting is merely the application of the principles of ordinary mechanical drawing to objects which, for the purposes of drawing, lack thickness. By this is meant that the materials dealt with are usually in the form of thin sheets and that their thickness is so slight that it may be represented by a line rather than by four lines and an included surface. The ordinary rules and conventions in use in mechanical drawing apply in general to sheet metal drafting. A knowledge of elementary arithmetic is also essential.


Problem 1 - LAYING OUT A METAL CLEAT

9. The Sheet Metal Cleat. The work of this problem will consist in laying out, to full size, the views and pattern for a galvanized sheet metal cleat. In making the layout for this cleat, the following points must be kept in mind:

1. The proper relation of views in a drawing.
2. How to dimension a drawing.
3. Accuracy in the use of the scale rule.

This cleat is to be formed from a flat piece of No. 16 galvanized iron; all the bends to be made to an angle of 90^. Figure 17 represents the cleat as it would appear on a photograph.

Before starting the layout, it must first be determined how many faces the cleat has. By holding the cleat with the largest surface directly in front of the eyes, three of these faces can be seen. A drawing should be made of what is actually seen. This drawing would appear as shown in Fig. 18. This view is called the front elevation and from it the exact sizes of the three faces or surfaces shown can be determined.

If the cleat is turned so that the eyes see the thin edge of the metal, the view will be as shown in Fig. 19. This view is called the profile because it is the exact shape or outline to which the cleat must be formed in the shop.

In drawing the profile, it can be located directly under the front elevation by using extension lines such as shown. In addition to showing the exact outline of the cleat, the profile also shows the dimensions necessary for laying out the pattern. In order to transfer these dimensions to the line of stretch out on the pattern, the profile should be numbered as indicated.

The front elevation and profile furnish all the information required to lay out the pattern. Consequently, there is no need to draw other views.

The line of stretch out is always drawn at right angles to the side of the view from which the pattern is to be taken. Upon this line of stretch out, all of the distances (called the spacing of the profile) of the profile should be placed and numbered to correspond. This has been done in Fig. 20. Perpendicular lines are then drawn through these numbered points. These are called the measuring lines of the stretch out. Extension lines carried over from the elevation locate the top and bottom lines of the pattern. The side -lines of the pattern are formed by measuring lines No. 1 and No. 6. Small free-hand circles should be placed as shown to indicate to the workman where the bends are to be made. The views and the pattern must be fully dimensioned.

10. Related Mathematics on Sheet Metal Cleat. - If the drawing is correct, the sum of all the lines in the profile will be equal to the length of the line of stretch out.

Problem. lA. - Compute the sum of all the lines in the profile from the dimensions given in Fig. 17. Measure the length of the line of stretch out in Fig. 20 and compare with the sum of the profile lines. K the answers do not agree, either the drawing or the arithmetic is incorrect. They should be made to agree.