Sheet metalwork application

The design of components to be manufactured from folded sheet metal is a field in which CAD systems can offer great assistance.

In the case of the bracket shown in Fig. 3.3 it would first be necessary to establish the overall dimensions of the part.

The second step would be to imagine that the bracket is folded back gradually as indicated in Fig. 3.4 into the flat sheet form. This shape would then be stamped from metal strip in a power press.

The dimensions of the flat pattern have to make allowance for the bend radius, the metal thickness and

Bend Allowance Definition

the type of metal used. Metals behave quite differently when bent and the CAD system can be programmed to calculate an appropriate bend allowance. After stamping the bracket can be refolded with suitably radiused bends.

In this particular case the dimensions of the stamping are also needed for the design of the press tool set.

The design can be checked for material accuracy, weight, volume, and so on, before being committed to manufacture.

Computerized programs can be produced to operate lathes, mills, flame cutting machines, etc. and many other items of equipment in the manufacturing process.

Models may be constructed in several different ways, including: geometric modelling, meshed surfaces, sweeps, volumes of revolution and ruled surfaces. Each of these is summarized below.

Geometric modellers build models from geometric solids, which have the attribute that mathematical formulae exactly define any point in 3D space occupied by these solids. Shapes include planes, cylinders, spheres, cones, toroids, etc. These shapes are combined using Boolean operations to produce the component. The Boolean operations produce a 3D model by a combination of the following methods:

(a) resulting from the union of any two 3D objects or shapes;

(b) resulting from the difference between any two 3D objects or shapes;

(c) resulting from the volume that is common to any two 3D objects or shapes.

This approach is very successful for modelling machined components but cannot handle anything that might be described as having a freeform shape.

Meshed surfaces. X, Y and Z co-ordinates are either calculated, transferred from 2D drawing views, or measured to provide basic modelling input. The modeller will then generate a 3D meshed surface joining up all the specified points. In order to build up a well-defined surface, the modeller interpolates between points defined in the user input in order to develop a fine enough mesh to show a smooth change in cross-section. This method can be used to produce the freeform shapes used in, for example, styling household appliances.

Sweeps where a 2D outline is defined graphically and then lofted or swept, by the modeller to give the outline a uniform thickness, as the third dimension. This produces objects of any shape in terms of the x and y dimensions, but a constant value for the z dimension. Sweeps can model all of those components that look like they are extruded, or have been cut from steel plate. For a model of a pipe a circular cross-section is swept or moved along a centreline of any shape.

Volumes of revolution for objects the shape of which is symmetrical about a central axis. The wheel is a simple example of this type of 3D object. The input is a half outline, or a cross-section through the object, which is rotated about the axis by the modeller, to produce a 3D illustration.

Ruled surfaces is a simple form of modelling, where any two sections or profiles can be joined at all points by straight lines. An airfoil, or a turbine blade is a typical example where this method can be applied.

Examples of various methods of CAD modelling are shown in Fig. 3.5.

Engineering Drawing Machining Symbols
Fig. 3.5

Pipework systems

There are many aspects of pipework draughtsmanship where the computer can considerably improve productivity. In many cases, by using 3D modelling software the design can be partly automated.

In a typical application a consulting engineer would be appointed to devise an outline solution to a given problem and prepare a specification, defining in general terms, the scope of the job to be built. The plant system needs to be designed and tenders are invited from organizations with experience in plant construction that would be prepared to erect the project and commission the plant for the client in full working order according to an agreed timetable.

On large projects several competitive quotes may be sought from rival construction groups to be delivered by a given date. The client will then make a choice and all responsible parties sign legal contracts. Having received an order to construct the plant, pipework systems basically require two types of drawings. Flow charts are functional diagrams showing the scheme and will include major items of plant. This diagrammatic arrangement is not to scale but shows the relative positions of main items and the connections between them. The diagram illustrates the feasibility of the system.

Equipment may be fixed at various levels. Assuming that a factory is to be built, then separate areas will be allocated to individual teams of draughtsmen who prepare layouts for structural work, manufacturing areas, heating, ventilation, air conditioning, compressed air and electrical services, etc. Ground site surveys are undertaken and various datum levels established to act as benchmarks for reference measurements. Steelwork can now be designed for the factory and manufactured to suit the site contours.

A 3D scale drawing could be constructed showing separate levels on which the items of plant are mounted. Straight lines representing the centrelines of interconnecting pipework are added. Pipes are sized to ensure adequate flow of liquids or gases and to withstand the pressure exerted by the contents. Realistic pipework can now be added. Suitable bends, elbows and other fittings may be directly 'dragged and dropped' at the various corners where pipes change directions and levels.

Software is available with libraries of ready-made standard fittings. Note carefully, however, which Standards are applicable. ISO and US standards are regularly used and specifications need to be checked. Ready-made welding symbols are also available.

The drawing office will be responsible for preparing lists and schedules of equipment required for fabrication and the following are typical:

• Pipe lists quoting sizes and lengths taking into account bend radii. During erection, pipes are cut to length then welded into the pipelines.

• Lists of similar standard bends and elbows.

• Lists of similar welded joints and processes.

• lists of unions joining pipes together for non-welded constructions.

• Valves of all different types, sizes and connections, i.e screwed, bolted and welded.

• Hangers to support pipework and expansion devices to permit movement.

• Pumps and associated fittings.

• Instrumentation devices; pressure gauges, temperature measuring devices and flow meters and filters.

• Equipment will be ordered from manufacturers using these records and costs calculated.

Another vital task that the computer can determine is to check clearances where pipes cross to ensure that there is adequate space to allow erection and operation.

The above are typical process tasks that can be handled by piping software.

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