In this particular example (Fig. 160) it should be noted that the diameter of the top and the width of the bottom are equal; hence its curved surface is formed of two upper halves of an oblique cylinder, together with two upright triangles for its flat sides.

The development of the surface to form the pattern will be followed by referring to Fig. 161. A half-elevation is drawn, and a quarter-circle described on the base, the radius of this, of course, being equal to the half-width of funnel. The quarter-circle is then divided into three equal parts, and lines through each part run square up to the base line. Through the points on the base lines are now drawn parallel to the slant end line. Make a 0' square to t a, and then set up distances a 3', b' 2', etc., equal to the corresponding lines a 3, b 2, etc., on the quarter-circle. Join 0' to 3' with an even curve. This will be a quarter of an ellipse, and will give the half-girth of rounded ends.

To mark out the pattern draw in a centre line as shown (Fig. 162), and a girth line square to it. For the length of the girth line, set along distances equal to the lengths of the separate parts of the girth-curve in elevation, the points being numbered in the same manner. Draw lines square to the girth line through each point, and then mark off distances 2' 2 equal to b' b, l' 1 equal to c' c, and 0' 0 equal to 0' 0 taken from the elevation. The parallel lines on the pattern will now be cut off at the same length, that is to equal a t from the elevation. The triangle 3' T 3' can be easily constructed, for the line 3' 3' will, of course, be equal to twice the length of line a d. The setting out of the end parts of pattern will be nothing more or less than a repetition of the construction followed for the middle portion. For the pattern to be accurate it should be noticed that its curves run into the straight lines without lump or hollow. On examination it will be seen that all the curves arc exactly the same shape, and in practice the pattern would be marked out by making a small template (like the shaded part of the end), and marking all the curves at top and bottom from this.

Funnel With Central Circular Top And Oblong Semici 175

Fig. 160.

The above method of laying out the pattern has been purposely arranged somewhat different to that shown in Fig. 159, but either method can be applied in both cases, the choices depending upon the size and shape of articles.