Page 157 - Engineering drawing from first principles using AutoCAD

Chapter!

Pattern development

By the end of this chapter you will be able to draw patterns for various hollow objects and
make useful models to check dimensions, shape, appearance and the position of joins.
These practical exercises assist in the overall comprehension of three-dimensional forms.
Applications of this type of work are found in thin sheet metal and plastics fabrications.
Development exercises include:

• Parts of rectangular and triangular prisms.
• Cylinders with branches.
• Cones and intersections.
• Hexagonal pyramid developments.
• True length applications.

Parallel line

This section introduces methods of drawing patterns from thin sheet material. If a print is
then taken and the pattern cut out, it can be folded to make the component. One of the
more difficult aspects of projection is trying to visualise a three-dimensional object on a
two-dimensional piece of paper. Models are ideal in this respect for comprehension
purposes, and also to appreciate the problem of true lengths where lines are placed at
angles away from the plane of the drawing sheet. The exercises here are examples from
rectangular, cylindrical, and conical applications.
Fig. 7.1 shows a component to be made from thin sheet material. We are required to draw
a pattern. The left- and right-hand sides in the solution meet along the vertical centre line
at the rear, while the bottom remains open. No allowances are made for the joins. It is only
necessary to draw half of the pattern and then use the MIRROR feature to complete the
drawing. Print a copy of the solution and cut out the pattern. The dotted lines show the
positions of the bend lines.
In Fig. 7.2 the sides of the component are angled. Note that the lengths AB, BC, CD and
DE represent distances along the centre line of the pattern. The widths CD and FG are
taken from the plan view and should be drawn on the pattern to determine the true
lengths of the corners. Print a copy and again make a model. True lengths are sometimes
very difficult to visualise; measure lengths on the model to check.
Patterns are needed to manufacture the component shown on Fig. 7.3, which consists of
parts of a triangular prism and a cylinder. The pattern is required in three separate parts,
namely the top and bottom surfaces and a continuous strip for the cylindrical section
between the two end wings.
Draw the given profile and rotate the component anticlockwise through 30°. Divide the
plan view into 30° segments and transfer the points on the circumference to the front
view. The back and front profiles of the cylinder are similar.
Divide half of the circumferential length of 361t into six strips of 18.85 width using the
RECTANGULAR ARRAY feature. Transfer the heights from the front view as shown and
draw a polyline through each. Use FITon the MODIFYPOLYLINE option to complete the