An article or part of an article may have a round base and a square or rectangular top, as seen in Fig. 205. Its pattern can be developed by treating the curved portions of the surface as parts of oblique cones, and the flat parts as triangles.

The setting out of the pattern is explained by Fig. 206, in which a quarter-plan and half-elevation is shown. The quarter-circle is divided up into four equal parts, and the lines b 1 and b 2 set along the base line from point t and joined up to t'. A middle line T 0 of the pattern is laid down and made equal in length to t' 0 from the elevation. A line is now drawn through T square to the line T 0, and T B cut off equal to t b from the plan. Now, using B as centre and radii respectively equal to t' 1' and t' 2', arcs of circles are drawn as shown. Then setting the compasses to the length of one of the arcs in plan, say, 1 to 2, and commencing at 0, the points 1, 2, etc., are marked. The triangle T B 0 is next set out, its construction being simple, and a repetition of the first part of the pattern. An approximate centre in this case can be found, as with the pattern in Fig. 201, by simply producing the lines 0 T and 2 B until they meet, as shown by the dotted lines.

Article With Square Top And Round Base 224

Fig 205.

If the top of the article is rectangular or polygonal in form, its pattern can be struck out as above, but in these cases a greater number of lines would have to be used. Also, if the centre of the top does not come vertically over the centre of the bottom, the pattern can be readily marked out with the same method, the only modification being the same as that applied to Fig. 204.