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RU:Общее представление о CAD
This topic section is a place to link to knowledge about specific CAD ideas. Many ideas are universal across different CAD software packages, and some ideas are universal to design as a discipline.
Entities are graphical objects in a CAD system. Typical entities which are supported by most CAD systems are: points, lines and circular and elliptical arcs. More complex and CAD specific entities include polylines, texts, dimensioning, hatches and splines.
Attributes: Every entity has its attributes - such as its color, line type (continuous,dashed etc..) and line width.See example image below.
A basic concept in computer aided drafting is the use of layers to organize a drawing. Every entity in a drawing is on exactly one layer and a layer can contain lots of entities. Typically entities with a common 'function' or common attributes are put on the same layer. E.g. you might want to put all axes in a drawing on a layer named 'axes'(see Figure 1). Layers can have their own attributes also (colour, line width, line style etc...). Each entity can have its own attributes or have its attributes defined by the layer it is placed on. In the latter case for example you can change the colour of all the entities on the "axes" layer by setting the colour (red for example) for the layer 'axes'.
In traditional manual drafting, a similar approach was used. Whether for Engineering, Architectural or Construction drawing etc...Layers were used to show different aspects of a drawing -for example this could be a layer set up for showing centre lines on an engineering drawing or to show different building systems, such as wiring and air conditioning. The layers were often drawn on separate transparent sheets of paper. These sheets were then overlaid one on top of another to produce final drawings.
A Block is a group of entities. Blocks can be inserted into the same drawing more than once with different Attributes and at different locations and at a different scale and rotation angle (see image below) . Such an instance of a block is usually called an Insert. Inserts have attributes just like entities and layers. An Entity that is part of an Insert can have its own attributes or share the attributes of the Insert. Once created, Inserts are still linked to the Block they instantiate. The power of inserts is, that you can modify the Block once and all Inserts will be updated accordingly.
A block becomes part of the drawing file in which it was created and saved to.
Creating and saving blocks is a useful way of inserting frequently used symbols e.g windows and doors in a house plan.
Blocks can contain uesful text, dimensions and reference notes.
Some block insert examples
Drafting in CAD
In many ways, CAD is similar to traditional manual drafting. For example when drawing a plan or a view of an object on paper, you would use tools such as a pencil and a ruler to draw lines. In CAD systems there is a variety of tools available to achieve the same goal. The big advantage of using a CAD system is the fact that you can change every entity of your drawing easily after you've created it, very often in seconds! This is probably the most difficult thing to get used to and learn when making the move from "manual" drafting on paper to using a CAD system.
When working with a CAD system you will very often create temporary construction lines, reference points etc... to aid your workflow, that do not necessarily need to be on the final printout. Some lines may not have the correct length and will need to be trimmed later - and so on (see Figure 2).
A common mistake for CAD beginners is wanting to create the final drawing straight away. The preliminary auxiliary construction lines, reference points etc...Are an essential part of the process of working with a CAD system and are in fact encouraged! You will find in LibreCAD and most all CAD programs these modification tool sets to "clean" up and modify your drawing before the final printout.
The coordinate system used in most CAD applications sets the main reference point (called the "origin" or "zero"; at coordinate 0,0) in the lower-left corner of your worksheet. In LibreCAD this is represented by a red + mark, visible in the lower left corner of a new drawing.
You can set the unit of measure for the coordinate system points;
For the Current drawing:
From main menu -> Edit -> Current Drawing -> Units.
For the 'Default 'unit of measure (affecting all new drawings):
From main menu -> Edit -> Application Preferences -> Defaults -> Units.
Selecting the unit of measure tells LibreCAD how much "1 unit" is equal to (without defining this, +1 unit to a line could mean "add 1 millimeter" or as easily as "adding 1 mile").
More details: coordinate system
Object and Grid Snapping
When you want to specify and select a coordinate of an object/entity in LibreCAD, you can use the 'snap' tools which allow you to precisely 'snap to' grid points or significant points on existing objects: endpoints or midpoints of lines, center points, middle points and intersections etc...
More Details: Snap Tools
Entities can be inserted, selected, and, once selected, can be deleted, transformed or duplicated. To insert an entity means to draw it by selecting the appropriate drawing tool, such as line, arc, etc. from the main CAD toolbar (see image below), and by locating points that define the object to be drawn, such as the endpoints of a line.
1.Show line tools
2.Show circle tools.
3.Show polyline tools.
7.Show modify tools.
9.Show arc tools.
10.Show ellipse tools.
12.Show dimension tools.
14.Show info tools.
15.Show selection tools.
An entity must be selected before it can be deleted, duplicated, or transformed. Entity selection is one of the most basic of CAD operations. There are a wide variety of selection tools available to use in LibreCAD to quickly select groups of entities, entities within a range, connected entities, etc. (See image below).The selection tools are available from either the main menu - 'Select' or from the show toolbar 'Select' tools (no. 15 above).
Alternatively you can use the keycodes:
Ctrl-A - Select all
tn / Ctrl-K - Deselect all
Selection tools image to follow.
Deleting an entity means to totally remove it from the drawing. In LibreCAD The deletion options 'delete' and 'delete selected' are available via the main menu > modify. The 'delete' option is also available from the 'modify' toolbar and by entering the keycode 'er' in the command line. See example images below.
Basic modifications (changes) of an entity in a CAD system include translation, rotation, reflection, and scaling. These operations do not alter the characteristic geometry of the selected entity - however there are some CAD modifications that do - such as break, trim, extend or stretching of selected entities.
The modification tools are available from either the main menu > Modify or from the modify toolbar or by entering keycodes in the command line - see here: http://wiki.librecad.org/index.php/Commands_and_tools#Focus_on_commandline.
Required sizes of objects within a drawing are conveyed through the use of dimensions. Dimension 'distances' may be shown with either of two standardized forms of dimension - Linear and ordinate.
linear dimensions use two parallel lines - called “extension lines,” which are spaced at the 'required' distance between two given points.A line perpendicular to these extension lines is called a “dimension line,” with arrows at its endpoints. The numerical indication of the distance is placed at the midpoint of the dimension line, adjacent to it or in a gap provided for it!
Ordinate dimensions use one horizontal and one vertical extension line to establish an origin for the entire view. The origin is identified with 0,0 placed at the ends of these extension lines. Distances along the x and y axes to other points on a drawing are indicated using additional extension lines with numerical information placed appropriately.
Sizes of circular features are indicated using either diametral or radial dimensions. Radial dimensions use an “R” followed by the value for the radius; Diametral dimensions use a circle with forward-leaning diagonal line through it, called the diameter symbol, followed by the value for the diameter. A radially-aligned line with arrowhead pointing to the circular feature, called a leader, is used in conjunction with both diametral and radial dimensions. All types of dimensions are typically composed of two parts: the nominal value, which is the “ideal” size of the feature, and the tolerance, which specifies the amount that the value may vary above and below the nominal. Architectural Dimensions
Scale and Viewing
Unlike in manual drafting, there is no need when using a CAD program to first determine and work-out in advance the sheet size and drawing scale. When drafting with CAD, all sizes and distances are specified using their full-scale values,for instance drawing a 10 meter object in CAD is drawn as a 'real size' 10 meter object.It is only when it comes to printing that the drawing scale needs to be determined and adjusted based on sheet and drawing size.
On the screen (in model space) the user can adjust the current visible area of the drawing by using the 'Zoom' tools, for example: zooming in to view more detail or zooming out to view a wider extent!
Auto Zoom , will zoom all entities/window to full extents
Window Zoom , option to define a window to zoom!
2.Another important viewing operation in CAD is the use of 'panning' - panning allows us to to see another portion of the drawing without changing the display scale, a user pans to it by “moving” a rectangular display window until it's over the desired spot.
Zoom Panning, click and drag to pan zoom - a small hand icon will show when using this function.
In addition by pressing the middle 'scroll' wheel on most computer mouse devices will do the same thing here!