Since in sheet metal work a molding is made by bending the sheet until it fits a given stay, a molding may be defined mechanically as a succession of parallel forms or bends to a given stay, and, so far as the mechanic is concerned, any continuous form or arrangement of parallel continuous forms, made for any purpose whatever, may be considered a molding and treated under the same rules in all the operations of pattern cutting. Keeping this fact in mind all parallel forms will be considered as moldings and that word will be used in the demonstrations, remembering that a difference in name simply means a difference of profile, but not a difference in treatment or principle. A molding may be defined theoretically as a form or surface generated by a profile passed in a straight or curved line from one point to another, this profile being the shape that would be seen when looking at its end if the molding were cut off square. A practical illustration of this may be given as follows: In Fig. 233, let the form shown be the profile of some molding. If this shape be cut out of tin plate or sheet iron, as shown in Fig. 234, it is called a stay. For the purpose of this illustration, as will appear further on, a stay, the reverse of the one shown in Fig. 234, or, in other words, the piece cut from the face or outside of the shape represented in that figure, as shown in Fig. 235, will be required.
Saving made a reverse stay, or "outside stay," as it is sometimes called, Fig. 235, take some plastic material - as potters' clay - and, placing it against any smooth surface, as of a board, place the stay against the board near one end in such a position that its vertical lines are parallel with the ends of the hoard, and move this reverse stay in a straight line along the face of the hoard until a continuous form is obtained in the clay corresponding to the profile of the stay, all as illustrated in Fig, 286. By this operation will he produced a molding in accordance with the second definition above given. The purpose in introducing tins illustration is to show more clearly than is otherwise possible the principles upon which the different parts of a molding are measured in the process of pattern cutting.
Fig. 236. - Generating a Molding in Plastic Material by Means of a Reverse Stay.
Suppose that the form produced as illustrated in Fig. 236 be completed, and that both ends of the molding be cut off square. It is evident, upon inspection, that the length of a piece of sheet metal necessary to form a covering to this molding will be the length of the molding itself, and that the width of the piece will be equal to the distance obtained by measuring around the outline of the stay which was used in giving shape to the molding. Now with a thin-bladed knife, or by means of a piece of fine wire stretched tight, let one end of the clay molding just constructed be cut off at any angle. By inspection of the form when thus cut, as clearly shown in the upper part of Fig. 237, it is evident that the end of a pattern to form a covering of this model must have such a shape as will make it when formed up conform to the oblique end of the molding or model.
To cut such a pattern by means of a straight line drawn from a point corresponding to the end of the longer side of the mold, to a point corresponding to the end of the shorter side of it, would not be right, evidently, because certain parts of the covering, when formed up, fold down into the angles of the molding, and therefore would require to be either longer or shorter, as the case might be, than if cut as above described. It is plain, then, that some plan must be devised by which measurements can be taken in all these angles or bends, and at as many intermediate points as may be necessary, in order to obtain the right length at all points throughout its width. This can be done quite simply as follows:
Divide the curved parts of the stay into any convenient number of equal parts, and at each division cut a notch, or affix a point to it. Replace the stay in the position it occupied in producing the molding shown in Fig. 236 and pass it again over the entire length of the model. The points fastened to the stay will then leave tracks or lines upon the surface of the molding. Now, by means of measurement upon the different lines thus produced, the length of the molding at all of the several points established in the stay may be obtained. All this is clearly illustrated in Fig. 237. In the upper right hand corner of the illustration is shown the stay prepared with points, by moving which as above described lines are left upon the face of the molding, as shown to the left
Now, upon a sheet of paper fastened to a drawing board, draw a vertical line, as shown by A B in Fig. 237, and upon that line set off with the dividers the width of each space or part of the profile or stay - that is, make the space 1 2 in the line A B equal to the space 1 2 in the stay, and 2 3 in the line A B equal to 2 3 of the stay, and so continue until all the spaces are transferred - and from the points thus obtained in A B draw lines at right angles to it indefinitely, as shown to the left. The lines an dspaces upon the paper will then correspond to the lines and spaces upon the clay molding made by the points fastened to the stay. Next, measure with the dividers the length of the molding upon each of the lines drawn upon it, and set off the same lengths upon the corresponding lines drawn upon the paper. This gives a series of points through which a line may be traced which will correspond in shape to the oblique end of the molding. Thus, set off from A B on the line 1 on the paper the length of the molding, measured from its straight end to its oblique end, upon the line produced by point 1 of the stay upon its face: and upon each of the other lines on the paper set off the length of the molding on the corresponding line on its face, measuring from the square end each time, which is represented by the line A B of the drawing. By this means are obtained points through which, if a line be traced, as shown by C D, the pattern of the covering will be described. The line A B, containing measurements from the profile, is called the "stretchout line," and the lines drawn through the points in it and at right angles to it are mathematically known as ordinates, but will in this work be called "measuring lines."