Background to CAD drafting

When drafting by hand, it is usual to take a clean sheet of paper of appropriate size and put it on a drawing board (table) along with your scale ruler. You then think about the design, the views and scales that you will use to convey the design effectively and finally put pen to paper, selecting drafting pens of varying thickness as you work. It is necessary to work with a scale ruler all the time.

 Sheets in Australia and Europe are usually the ISO A series – A0, A1. A2 etc. The B series of sheets – B1, B2, B3 and B4 – are less commonly used and are reserved for design work in the US.

Pens

Most technical drafters use special drafting pens such as those made by the Rotring company to give emphasis to parts of the design. These pens have an accurately defined pen nib width (thickness) viz. 0.25 mm, 0.35 mm, 0.5 mm, 0.7 mm, 1.0 mm etc.

Why this emphasis on line weights?

Most drafting tutors will insist that you use these special pens and construct your drawings with varying line weights because your drawings will have ‘depth’; drawings will have good definition and can be easily read and interpreted.

A 0.5 mm Rotring pen has a red ring around it to help you identify the thickness of the particular pen. In Australia, many drafters use the same system when creating CAD drawings. If a line in the drawing is colored red, this implies  (when it comes to plotting), a 0.5 mm pen will be assigned to it.

 Color can be associated by setting a color property to an individual element like a line or by associating color with the layer that the line is located on.

 If you are operating in an environment where the Imperial system is used (feet and inches), the thickness will of course be a different unit (fractions of an inch), but the principle is the same.

Experienced drafters often lay a drawing out with a fine pen and then ‘go over’ different parts with a thicker pen for emphasis. This is good practice and can be mimicked in the CAD environment.

Drawing aids

Various aids, such as set squares and rulers, are often used to assist when drafting by hand. Some of these devices are ‘built-in’ to the drawing table (sometimes called drawing machines). The settings on these drawing machines can usually be altered, for example, changing the drafting angle.

 CAD software such as gCADPlus contain the equivalent of a drawing machine and you can change settings in the software to suit your purpose; it is easy to change the drafting angle and many other settings.

Rubber stamp shapes – symbols/blocks

If the same shape(s) is used over and over again in your drafting; for instance chairs, desks, trees and shrubs, bolts, flanges etc., then it is likely that you will have made templates of these shapes so that you can place copies of your standard symbols into drawings. The same applies in the CAD environment, where we can easily make and subsequently use standard symbols.

  These symbols are called BLOCKS. If the drafter needs to ‘stamp’ shapes into the drawing, these stamps (called blocks by gCADPlus and by AutoCAD, but cells, symbols or stamps by other CAD programs can be called by instructions (Commands) which enable the complex shapes to be easily inserted.

Drafting using a computer – ‘CADDing’ about?

A drafter using a CAD system works in much the same way as a ‘manual’ drafter. You don’t however pull out a piece of paper first, think about scales and start to draw, but concentrate on building the design immediately (creating a full size model of your design in the computer). This is done by very accurately entering data describing the design into the ‘drawing space’ – a space sometimes called the drawing editor.

A mouse, keyboard or a variety of other input devices can be used – light pens, touch pens, trackballs, photography and digitizers can all used as input devices. The ability of CAD software to incorporate images from phone cameras is especially useful.

After the drafter has finished creating the design (the model), the drawing sheet with its title block information is inserted, often in what is called ‘paper space’ or the ‘presentation’ area.

  Keep in mind that the designers of CAD programs such as gCADPlus have tried to make the program mimic the way in which drafting work is done at a conventional drawing table. The correlation is very strong indeed. If you remember this, then it is easy to avoid initial confusion, which comes mainly from the introduction of new terminology. When learning CAD, simply say to yourself “how would I do this on the board? There must be an equivalent somewhere in the software.”

Viewing your design

Just as you do when drafting by hand, a CAD system allows you to work on different parts of the drawing sheet at various times. The drawing might be viewed at close quarters while constructing detailed design and then viewed from further away while composing the design on the sheet.

The developing CAD drawing is viewed on some sort of display monitor. So the software (the CAD program) and hardware (the computer) record the geometry of the design drawing and save the growing bank of information about the drawing in a database.

CAD software such as gCADPlus running under Windows is capable of simultaneously displaying the design on one screen and various tool palettes and plant databases on two monitors.

The drawing is a database

It is most important to recognize that CAD programs such as gCADPlus use a coordinate system to record the location of entities in the drawing. These coordinates are stored in the database describing the drawing. Indeed it is possible to interrogate the drawing and obtain lists of (say) pieces of equipment that have been specified.

The database (the drawing) may be edited (changed) at any time and the screen refreshed to update the drawing (design). Once the drafter is satisfied that the drawing is complete, the image is sent to a variety of ‘hard copy’ output devices including printer/plotters and plotters. The drawing may be printed (plotted) out at various scales. Once printed, the drawing is usually saved in electronic form on disk.

CAD drawings can contain layers

It is likely that numerous (transparent) overlay sheets will be used in CAD drawings in order to incorporate more information into the drawing. Several drawings can then be made from a single drawing, by turning layers off and on as needed.

CAD drawings are easily edited

If mistakes are made in a hand drawings they are removed using some sort of eraser. CAD software has an ERASE command. Many entities can be removed at the same time. There is also a handy UNDO tool.

 If you make a mistake, there is no need to start over. Don’t erase the drawing, just UNDO back to the correct point, and then start again.

Don’t’ forget that COPY is an editing command – editing is not all about correcting your mistakes – we use the many and varied editing commands to build up our geometry!

Precision. Along with the ability to remove mistakes, you can also move (say) a wall 600 mm in a particular direction, change the layer an entity is located on etc.

Printing

When the drawing is finished, the final edited drawing is plotted (printed) at any scale you want and on any size paper. It is then stored (filed) in a plan cabinet. In summary, the CAD process mimics the hand drawing process.

Learning to use CAD software – some advice

It helps when learning CAD to realize that gCADPlus software mimics the ‘hand drawn’ drawing process. Instead of using a pen or pencil as a drafting tool, the pointing device (a mouse) is used to drive an ‘electronic pencil’ around the computer screen. The screen is analogous to the drawing board. The pencil is, metaphorically speaking, positioned and then lowered onto the electronic drawing board. Special instructions are then issued to produce the entities (lines, arcs, circles etc.) which get placed in the drawing. The major advantage of this process is that it is not necessary to actually draw shapes like circles, ellipses etc., these shapes (and even more complex ones), are simply placed into the drawing. The CAD program creates the shape perfectly every time, locating it precisely.

A CAD drafter when operating at maximum productivity, becomes an assembler of pre-drawn parts.

‘Real world’ units

Most drawing involves the production of plans. Any plan is a collection of lines that represent actual objects. The objects can vary in size. They may be doors or windows on a house plan; flanges or details of welding joints in a mechanical drawing; circles representing trees and shrubs in a landscape plan or sketched lines representing the boundaries of a blood capillary as seen down a microscope.

The objects have actual dimensions. The block of land on which a house sits might be rectangular in shape, perhaps 35,000 mm (35 m) by 28,000 mm (25 m), the door in an architectural plan may be 600 mm wide and 1900 mm high, the window 2700 mm by 1000 mm, the canopy diameter of a tall tree in a survey plan 35 meters or something as small as a blood capillary, 6 micrometers wide and so on. Every one of these items has dimensions that mean something to the designer in the real world!

When you draft by hand, you constantly use a scale ruler (1 mm represents 100 mm – 1:100; 1 inch= 8 ft – 1:96 etc.) When drawing with gCADPlus, you do not draw to scale, but simply construct the drawing in drawing units. In gCADPlus, these drawing units are ‘set’ equal to ‘real world’ units. This is a ‘mental’ setting in the mind of the designer. If the cursor (pointer) on the screen moves one drawing unit, this is equivalent to a movement of one unit in the world of the designer.

In Australia, everything in the building trade is measured in mm, so one drawing unit is equal to 1 mm. The window is exactly 2700 units by 1000 units i.e. 2700 mm long by 1000 mm wide, no more, no less.

You would probably work in meters if you were a surveyor.  A landscape designer would use the SCALE command in gCADPlus to scale the drawing so she can work in mm.

Forget scale rulers

Conventional drafters use scale rulers constantly and need to be aware of the scale they are using. gCADPlus users suffer no such restriction; they simply draw in ‘real world’ units where the drawing unit is equal to the actual unit used to size the object. Experienced manual drafters need to forget the traditional way of doing things and metaphorically speaking ‘throw away’ their scale rulers. Many new CAD operators who have previously trained as manual drafters become confused when first seeing a CAD system, because they are not now required to work to a scale.

If you measure a screw 6″ long with a ruler, then draw it 6″ long.  If you have a piston 2″ in diameter, then draw it 2″ in diameter.  If you fail to draw things the same size as they are in the real world, then your will have endless trouble with dimensions, etc., etc. Never scale a drawing up or down. Scale title blocks up and down all you want, but not the drawing itself.

Scales and plotting

It is only at the final assembly of the drawing on the display sheet (border sheet) that one considers scale. We take might take the drawing of (say) a plot of land (the site boundaries), insert a drawing of plans of a house drawing into it and finally insert and scale up the drawing sheet around the block of land at a suitable scale.

If a landscape designer had a plotter that was large enough, it would be possible to plot a site plan at a scale of 1:1 and use it as a template for laying out foundation and brickwork.

This lack of concern about scale when using gCADPlus is the major difference between CAD drafting and the use of conventional methods

Note:  The approach to preparing CAD drawings outlined above is not the only one that is used, but most CAD drafters do work ‘full size’. Rather than inserting part drawings into a sheet of real world size, some (a very few) drafters ‘scale down’ the parts as they bring them in to the presentation sheet and then plot the sheet 1:1. The danger with this approach is that the parts have been scaled down and cannot be stretched and re-dimensioned later without taking into account this scaling factor. You really must learn to draw full size when using CAD!

CAD drawings are very accurate

CAD programs such as gCADPlus keep a database describing the developing drawing that is accurate to 16 decimal places (floating point arithmetic). The accuracy of your drawing is limited only by the accuracy of the printer or plotter you use to produce the printed plan.

The advantages of Computer Aided Design

At this point, beginners always ask about the benefits of using a CAD program. Some of the major benefits are listed below.

Speed

I am sure that you do want to improve the speed and efficiency of your drafting. Rest assured that after a short time, once you gain the necessary skills, you will be able to complete drawings much more rapidly and neatly using CAD, than by hand.

The main reason for the increase in speed is that much drafting is repetitive – think of the amount of lettering found in most drawings. Lettering is essentially repetitive and much more efficiently done by typing the required characters and letting the CAD program insert them, rather than placing the characters by hand using stencils. This applies even if you are not a very fast typist – ‘hunting and pecking’ is quite OK for a CAD operator.

Many other drawing tasks are also repetitive; the architect designing a multi-story building uses the same base plan at each level of the building and many of the same fixtures and fittings in each room on a floor, simply arranging components differently to produce the desired design; a designer of printed circuit boards uses many of the same silicon chips in a design, while a landscape designer will usually use many instances of the same species of plant in a planting plan.

Thus CAD shines where standard components are inserted into assembly drawings.

At its most sophisticated, CAD software may even be used to build very complex drawings simply by assembling a whole series of smaller drawings. If you learn to use gCADPlus templates much of the heavy work can be done before you put pen to paper or more accurately the cursor to the screen.

CAD drawings are easily edited.

Since CAD drawings are easily changed, ‘going back to the drawing board’ is not a phrase to be feared. In design work, it is often necessary to make minor adjustments to a drawing – move a wall, remove a window, alter a layer on a printed circuit board, change the size of bolts used in a mechanical assembly etc. These changes are difficult to quickly and neatly achieve using conventional methods.

A CAD drawing may be plotted at different scales.

The same drawing may be plotted many times at different scales and different views of the drawing can be shown on the a single one presentation sheet. Different colour or thickness of pens can be used in plotters for emphasis.

Semi-automatic and associative dimensioning is possible.

Since objects are drawn at full size, dimensions are easily inserted into drawings because they are taken from the “model” itself. These dimensions change automatically if parts of the drawing are stretched (when what is known as associative dimensioning is turned on).

Image files While working on a design for a small courtyard, we wanted to change the length of a wall. A linear dimension had already been applied to the wall and we show that the STRETCH command can be used to alter the length of the wall and the dimension itself in one step.

It bears repeating that everything you draw with your gCADPlus software must be drawn full size. This means that it any time you make an enquiry of part of your drawing (using any of the enquiry group of commands – DISTANCE, LIST, ID or AREA) the software ‘spits back’ values that mean something to you as a designer. We emphasized this many times in this set of notes, but you need to keep reminding yourself of it when you first work with gCADPlus.

This concept of ‘drawing full size’, is a major change for many people and can be most disconcerting, if you have long been used to producing paper drawings by hand on a drawing board.

Information can be extracted from drawings.

gCADPlus has tools which enables non-graphical information contained within the drawing (botanical name, common name, code, number of plants used etc. to be written to a file which can subsequently be read by other programs such as Excel to prepare bills of materials

To sum up, CAD software helps you to build a computer model of what is to be constructed in the garden space. There should be no surprises when the construction team goes on site!

gCADPlus software is an example of a ‘command driven’ program. There are various groups of instructions for creating drawings. Many instructions (commands) exist to get a new sheet from a FILE cabinet, similarly there are many commands to DRAW different entities, to DISPLAY the drawing, to EDIT the drawing, to define and use BLOCKS as templates, to create and switch between LAYERS etc. Drawing utilities analogous to the rulers and squares mentioned above may be used. It is possible to PLOT the drawing on paper and to FILE the completed drawing.

It is useful to think of a CAD program as a set of drawers in a file cabinet; each draw containing a specific set (group) of commands such as DRAW, EDIT, DISPLAY and so on. Within the DRAW group you will find a LINE command, an ARC command, a CIRCLE command and so on.

Do I need to learn CAD commands by heart?

As we have said, the instructions given in gCADPlus to place elements such as lines and circles into the design space (the drawing) are called commands. There are many commands. Beginners quite reasonably ask at this point ‘do I need to learn the commands by heart or can I rely on clicking on a toolbar icon on the screen or menu item to call the instruction I want?’ The answer is no, everything is available on a drop menu, toolbar or can be typed into the drawing. the choice of how to do it is yours.

How many commands exist in gCADPlus?

There are more than 200 commands available to you. Certainly, you do not need to learn all of them. It is said that a typical user will use 20% of commands 80% of the time. It’s knowing how to use the 20% properly that’s important.

So, besides attempting to show you how to use enough commands to build a CAD drawing in this course, we will place considerable emphasis on choosing the appropriate group of commands, focusing on why a particular command is used.

Set up gCADPlus for ease of use

The installation process for gCADPlus normally leaves a desk top shortcut to the program. An appropriately configured shortcut can help you get much more out of your gCADPlus software. We will deal with that in a later session.