Understanding scale concept and units in Autocad…

I wanted to write this article, which will help you to understand the AutoCAD’s working logic and learn some concepts like scale and unit, if you have just started using AutoCAD or if you are learning it by yourself.

First of all, I would like make clear that, whichever drawing you are making, area that you are substantiating this drawing is called modelspace. If you are using AutoCAD 2000 or higher versions, you have noticed phrases like “Model / Layout1 /Layout2” over the small tabs under the area which you are making the drawing. Do not forget that you make your drawing in Modelspace.

Just on the lower left corner of Modelspace, there is an icon that shows x and y axes. This is called UCS (User Coordinate System) icon. This icon shows the coordination system according to which you are currently making your drawing. Arrows pointing at x and y directions shows the + direction of Modelspace.

Now we can mention the subject of scale and unit. Coordinate system in AutoCAD Modelspace is cartesian coordinate system. If you take 0,0 point as origin where x and y values are 0 (this is the lower left corner of your screen), values towards right and up will be positive. Accepting 0,0 point as origin, x-axis is 0 degree towards rigth according to degree angle system. Angles will be according to counter clockwise, y-axis will be 90 degrees in + direction, x-axis will be 180 degrees in – direction, and y-axis will be 270 degrees in – direction.

We know that the drawing you make in AutoCAD is called model and the area in which this drawing is made is called Modelspace. This point is very important, drawing isn’t done by scale like it is done on paper, it is done as 1=1 and scaling is made when printing on paper. However, it is important how you accept 1 AutoCAD drawing unit. AutoCAD paper (layout) unit is mm(milimeters). Let’s say, you are drawing a architectural project and you accept 1 drawing unit as 1cm. So, you started your drawing with the assumption of 1 dwgUnit = 1cm. You continue drawing according to certain rules like the doors will be 100 units, height of tables will be 75 units. Thus, you draw everything on the cm basis. When you take print out, if you don’t change the scale, each 1cm will look 1mm as the AutoCAD unit is milimeters(mm). In other words, it will be printed as 1 dwgunit (we accepted as cm) = 1mm, which means 1/10 scale. In order to take print out, you should state in the scale part that your 10 drawing unit (that is to say 100 mm) shall look 1mm on paper, which is 10=1.

I hope it does not look complicated. In order for your AutoCAD drawings to be precise and to take print outs according to any scale at any time you desire you should make an acceptance for 1 drawing unit and you should draw according to one on one scale.

I will be glad to answer if you have questions. I wish you nice works.

Scaling in Autocad drawings and layouts.

As you know, no scale is used in AutoCAD drawings. Drawing models are prepared with real dimensions and scale is given during the print-out. In order to do this, print Plot… command is used from File menu. Plot dialog box will appear (Fig.1). You can take print-outs according to the scale you want by setting correct values in mm ve units boxes inside Plot scale part, which is in Plot Settings tab.
Fig.1
In the example, which is shown in figure, it is set that every 10 units is seen as 1 mm in the print-out, thus 1=10 relation is used. In table.1, you can find plot scale relationships according to several units (m, cm or mm), and scales (1/100, 1/50 veya 1/20) that can be used by the user.
According to the units and print-out scales chosen by the user, in some cases, print-out may not be at the desired quality. In order to correct the print-outs, some command has to be used or some system variable has to be adjusted to some certain values. These commands and variables are explained hereunder.

Text Height
In general practice, all text should be at a definite height. In order to accomplish that, a certain text height values should be used in the drawing model. These values depend on the units and scales chosen by the user. For example, Text height values that has to be used in the drawing model, to get text height of h=3 mm in the print-out, are shown in Table.2.
In order to calculate any other h text height, you can multiply the text height values given in table.2 by h/3.

Hatch scale
When Hatch command is called, Boundary Hatch and Fill dialog box appears (Fig.2)
Fig.2 Boundary Hatch and Fill dialog box.
By using various settings in this dialog box, you can change the hatch properties. However, if the value at scale box is not chosen properly, hatch in the print-out may look too dense or too rare. For various units and scales, suggested hatch scale values are given in Table.3.
Dimscale
As you know, in order to manage the dimensioning operations in AutoCAD, Dimension menu (or Dim command in the command line) is being used. There are several system variables for modifying the various properties of dimension objects. Among these variables, probably the most important one is Dimscale (dimension scale) variable. In order for the dimension entities to look proper in the print-outs, appropriate values for Dimscale variable is shown in Table.4.
While drawing is made, as one of the values are set from the table to the Dimscale variable, other system variables can be also modified according to the user’s demands.

Ltscale
As you know, there are several line types in AutoCAD (dash, dash dot, dot….), if the LTSCALE system variable, which determines how these dashes will look like in the print-out, is not set properly then dashes and dots may look too dense like a continuous line or too rare in the print-out. In order for different line types to show up properly in the print-outs, appropriate values that should be assigned to Ltscale variable are shown in Table.5.
Editor’s Note: Prof. Günay Özmen is my AutoLISP lecturer during post-graduate study in İ.T.Ü. Architectural Faculty. And he is the first person who planted the concept of AutoCAD customization into our brains. I am proud to say that, I owe him everything that I have accomplished till now, and I appreciate him for writing this article. “Orhan Toker”

Coordinates in AutoCAD

Those who have just started working with AutoCAD should first learn about the coordinate system of AutoCAD. When you first open AutoCAD, you see Model Space on your screen and you make your drawing in this space. You can think of the model space as a 3 dimensional paper space. Indeed, if you are going to draw something 2 dimensional and you are looking at the model from top view, then you can think of it as paper directly. Only difference between paper is that model space has no boundaries. If we remember the old days, we used to make a preliminary preparation to align the drawing in the middle of the paper by adjusting the scale also. In AutoCAD, you don’t have to worry about scaling or aligning the drawing in the middle of the paper. You can read my article “Understanding scale concept and units in Autocad” where I have explained these concepts in detail.When you first start drawing in model space in AutoCAD, probably you will mostly encounter with the problem of entering points. AutoCAD will prompt you to enter a point a strating point of a line, corner of a rectangle etc. In this article, I will explain “how to enter a point” as well as the coordinate system.

In how many different ways can we enter a point?

Answer of this question is 3. When you first step into the world of drawing with AutoCAD, for instance when you use LINE command, AutoCAD will ask you to enter a starting point first.

Command: LINE From point:

Here are the answers that you can give to this “From point:” question:

* By entering real coordinates.
* By entering relative coordinates.
* By entering coordinates by using AutoCAD drawing aid tools.

Now, let’s make our first step into drawing and learn about coordinate system by going over these topics.
1. Entering absolute coordinates

Command: LINE From point: -2,1 To Point : 3,4 To point :
Fig.2
As you can see, entering relative coordinates is done by using @ symbol from the last point and entering Δx,Δy values. @ means that we will enter relative coordinates.

3. Polar Coordinates:
Other that cartesian (x,y) coordinates, another coordinate system that you can use to enter points is POLAR coordinates system. It is a coordinate that is reached by a distance and an angle from a certain origin point. We know that angles increase in counter clockwise direction in AutoCAD ( Fig. 3).
Fig.3
After reminding this information, let’s continue to know more about POLAR coordinates. The example given in Fig. 4 explains how to enter polar coordinates by using relative coordinates.
Command: line From point: #0,0 To point: #4<120 To point: #5<30

Fig.3
After reminding this information, let’s continue to know more about POLAR coordinates. The example given in Fig. 4 explains how to enter polar coordinates by using relative coordinates.
Command: line From point: #0,0 To point: #4<120 To point: #5<30
Fig.4
Even though we are not at the topic of relative coordinates, this example is given in order not to skip entering absolute polar coordinates. # symbol at the beginning is used to mention the absolute coordinate entry. Now, let me give an example for entering relative polar coordinates.
Fig.5
Let’s go over the previous example:

Command: line From point: @3<45 To point: @5<285

As you can see, by entering @ symbol, you are saying “relative to last point”. @3<45 style="font-weight: bold; color: rgb(255, 0, 0);">Command: line From point: From point: @5<0 From point: @5<90 From point: @5<180 From point: c
As you can see, entering relative coordinates is very useful to use. Thus, it is the method that is most frequently used. However, relative Cartesians mat also be used for very rare subjects like drawing a rectangle.

Command: rectang First corner: Other corner: @5,3

This example draws a rectangle with dimensions 5×3.

3. Entering coordinates by using AutoCAD drawing aid (dynamic) tools:

If you select a point on the screen by using you mouse cursor, then you cannot enter and exact coordinate. Of course, it would be better if we could have entered the exact coordinate that we wanted just by using the mouse cursor. Since AutoCAD 2000, entering coordinates by using mouse cursor has been improved. And in final versions AutoCAD 2006 and 2007, this has been improved even better. This is called dynamic coordinate entry in AutoCAD. Now, let’s take a look the subject by giving some examples:
Angle by cursor, but distance is dynamic
Fig.6
As you move your cursor and approach to angles that are defined in POLAR settings, then you will see that your cursor is locked on that angle. As the cursor is locked, then we can move as much as we want that along the angle we are locked onto.

Angle locked, distance is dynamic
Fig.7
After selecting the first point to draw a line, if you write <45 style="font-weight: bold; color: rgb(255, 0, 0);">Distance locked, but angle is dynamic:
Fig.8
When we write 200<, then distance will be locked at 200 units and you will be prompted to enter an angle value. Lock is broken at angles specified at POLAR settings.

In future, I will give a second lesson about this subject. That’s all for now about coordinate entries. I hope it will be useful for you.