Creating a Sketch

The first sketch we're going to make of the table is its plan view. A plan view is usually made as though we were looking down on the object from above. Of course, terms like "down" and "above" are totally arbitrary in mechanical drawing. In general they mean what you'd expect them to, but to give them a more concrete definition, a plan view (also called the top view) is the view of an object in the first octant of a three-dimensional Cartesian coordinate system as seen when looking in the direction of the negative Z axis. Additionally, the direction of gravitational attraction is taken be in the negative Z axis direction. The figure below shows the projection of the top of the object on the sketch plane.

Our plan view is going to be very simple. We're only going to draw the outline of the top of the table. To do this, go to the Tools menu and select Rectangle. When you move the mouse cursor into the main graphics display window, you'll see that it has an indication that we're in rectangle creation mode (). Create the rectangle by pressing the left mouse button when the mouse is in the main graphics display window and dragging the mouse (with the button held down) until the rectangle is approximately 20" by 20" square and then release the left mouse button. You should have something similar to the figure below.

Although this operation is simple, there a few things to observe:

• When you enter rectangle creation mode, the user interface status area of the status line indicates the mode with the text "Construct Rectangle". Both the cursor and the user interface status area return to normal after the rectangle is complete.

• As you drag out the rectangle, dimension lines are drawn to indicate its current size. Do not be concerned about creating a rectangle which is exactly 20" by 20". It is very easy to adjust the size of a rectangle after it is created. Similarly, do not be concerned about the grid lines on the sketch. The actual position of the rectangle on the page is unimportant. (The grid lines will be more useful when there are objects other than rectangles to create.)

• After releasing the mouse button, the rectangle is activated. In this state, the rectangle is drawn with highlighted edges and eight resize handles. (They're supposed to look like little blocks of wood.) Hover the mouse cursor over each block and the cursor will change to indicate the direction in which the rectangle can be resized. Press and hold the left mouse button over a resize handle and you can reshape the rectangle. Clicking (away from a resize handle) and dragging an edge will cause the rectangle to move. If you click somewhere in the window but not on the rectangle itself, the rectangle will be deactivated. Here is a short table of cursor types and their meaning.

• Notice that one corner of the rectangle has an X under it. This is the origin of the rectangle. Ordinarily, the origin is not of interest to the user; however, certain resizing operations on the whole rectangle will move the corner opposite of the origin.

• If you botched the rectangle by releasing the mouse button too soon and ended up with a 0" by 0" rectangle, you can simply undo the rectangle creation command. You can either choose the menu option Edit|Undo, click the Undo tool bar button, or press Ctrl+Z. Alternatively, if the rectangle is still activated, you can delete it with the menu option Edit|Cut, or by clicking the Cut tool or by pressing Ctrl+X. Finally, if you picked the wrong Tools menu option altogether, you can press Escape to cancel the operation. Escape works both before you press the left mouse button to start the drag and during the drag.

• There are no (theoretical) limits on the size of the page. You can pan (scroll) the page by pressing and dragging the left mouse button. You can also zoom in and out by using the mouse wheel. There are also equivalent keyboard inputs.

(Note that these commands are for the two-dimensional pages. The three-dimensional pages have different bindings.)

To return to the default view orientation, press and hold the right mouse button in the main window. This will bring up the context menu for the current page. Select Restore View to return to the default view. The context menu for a Sketch page is show below.



• Note that there are three other toggle buttons in the Sketch context menu: Wireframe, Solid and Texture. These choices select the default drawing style for geometry on the current page. The default mode is wireframe, equivalent to drawing the outline of an object. Solid draws geometry as filled polygons using the same color as the wireframe. Texture is just a simple gloss on solid which draws filled polygons using a default texture map. (No attempt at realism is made in this case.)

The final size of our top is actually 19 ½" by 16". Point the mouse at the dimension value on the width dimension at the top of the rectangle. You will notice that the dimension value, the dimension's arrows and its extension lines will be highlighted. Additionally, two square, translucent resize handles will be highlighted. Because the dimension has not been activated yet, the resize handles are not active yet. Double click on the dimension; this will bring up the dimension information dialog.

This dialog shows us all the detail of the dimension. Its name is x1; that is, it is the dimension between the two edges of the rectangle, x0 and x1. It has a specified size of 20 1/4". It is Above its reference edge (i.e., above the top edge of the rectangle). It has a horizontal orientation.

[There are a number of shortcomings exposed by this dialog. Chiefly, x1 is not the name of the dimension. Dimensions actually don't have names. If you change this name, you will change the name of the corresponding edge of the rectangle. Also, the Imported dimension size is currently not available, so each dimension is a stand-alone entity. In other words, you can't make parametric relations between dimension's yet. A later version of lignumCAD will incorporate a better distinction between dimensions (and constraints, in general) and the objects which refer to them.]

Adjust the specified offset to 19 ½". You can do this by clicking the arrow buttons in the Specified spin box or by typing in the input field. Note that to type this dimension, you must type 19b1/2 (that is, literally, "1" "9" "Space" "1" "/" "2"; lignumCAD will take care of the units abbreviation). (Note that it is also OK to type 19.5.) Apply your change by pressing Enter (or clicking OK).

Modify the height dimension to 16" in the same way.

Next, double click on the rectangle itself. This will bring up the rectangle's information dialog.

There is a lot to see here (and a rectangle seems so simple!). You can see the dimension values which we changed earlier. You can also see where the origin of the rectangle is located (in this case at the Cartesian coordinate (12 5/8", 23 1/8")). The dimensions are on the Above/Right sides of the rectangle and the rectangle is drawn in the default style. For now, we just want to change the name of the rectangle to Top. So, navigate to the Name field (try pressing Alt+N), type in Top and press Enter (or click OK) to apply the change.

Next, we want to add a text message to the page to serve as a reminder of what this object is. Select the Tools|Annotation menu option. The mouse cursor will become the create annotation cursor (). Click and drag out a small box below the top's rectangle. When you release the mouse, the annotation information dialog will appear. Navigate to the main text box and enter Top. The annotation information dialog will let you modify the style of the text if you choose.

Applying the text styles works in the usual way. If you select some text and then change the property widgets (e.g., click Bold), the new properties will be assigned to the text. If no text is selected and you change a property, then any subsequent typed text will have that property. To complete the annotation, click OK or press Alt+O (this is one of the places where just pressing Enter will not dismiss the dialog). If you made your annotation box too small, you can resize it with the resize handles. You can also click anywhere inside the annotation to drag it around.

So, we've completed the creation of the plan view of our table. Now would be a good time to save the model. You can either select the File|Save menu option, click the Save tool button, or press Ctrl+S.

Next, we want to work on the front view of the table. This is usually called an elevation. To create a new Sketch, you can can do one of three things:

1. Select the menu option Insert|Sketch. I'd call this the usual way.

2. Use the Insert Sketch button on the toolbar: .

3. Use the context menu on the tab bar (press and hold the right mouse button anywhere on the page tab bar) and select Sketch. The tab bar context menu is shown below.

   

Note that from the menu tab bar context menu, you can create a new page of any type. You can also rename the page and delete a page. Note that both the Rename Page and the Delete Page options apply to the currently displayed page, not the one whose tab might have happened to be under mouse when the menu was invoked. These options are also available from the Edit menu.

After you create the new Sketch, name it Front Elevation.

On the Front Elevation page, create a rectangle near the top of the window and set its size to 19 ½” by ¾” and set its name to Top. The is the side view of the table top. [In a later version of lignumCAD, you will be able to reference the width of the Top figure on the Plan sketch; for now, you have to enter the width again.]

Next, we want to create a rectangle for the left leg for the table. In creating the rectangle, we want to take several criteria into account. First, the top of the leg should be flush with the bottom edge of the top. Second, we want the left edge of the left leg to be offset from the left edge of the top. These requirements are called constraints in lignumCAD. The program can describe both of these constraints. To create such a constrained rectangle, activate the Rectangle tool and position the mouse cursor along the bottom edge of the top and near the left side. The bottom edge of the top should be highlighted: it will be colored a brighter color than the rest of the rectangle and translucent resize handles will appear at the ends of the edge.

Press the left mouse button and drag out the rectangle. lignumCAD will now create three dimensions for the rectangle: the width, the height and the offset from the left edge.

Again, very simple, but there are a few observations to make:

• The top edge of the leg is now coincident with the bottom edge of the top. This is indicated by the row of green and magenta arrows. The magenta arrows point to the primary edge (that is, the edge which is constrained; namely, the top edge of the leg); the green arrows point to the secondary edge (that is, the edge to which the primary edge is constrained; namely, the bottom edge of the top). This constraint has the effect that what ever position the bottom edge of the top has, so too will the top edge of the leg.

• The left edge of the leg is now offset from the left edge of the top. (In the program, this is called an offset constraint.) If you hover the mouse over the dimension, you will see that it, too, has a magenta arrow and a green arrow. As in the coincident constraint, the magenta arrow points to the primary edge and the green arrow points to the secondary edge. In this case, if the left edge of the top is moved, the left edge of the leg will move along with, always separated by the dimensioned offset.

• There are no resize handles along the top edge of the leg. Once an edge is made coincident, it is not possible to move it independently of its secondary reference. (Note that there is an option to delete constraints if necessary.)

When you created the left leg, you probably ended up with some overlap between the offset dimension and the width dimension. You can modify the position of the dimension line by activating the dimension, and then dragging it. In the case of the figure above, the 2” width dimension was adjusted to a higher position than it initially had. You also have some additional control over the dimension positioning. In the dimension information and rectangle information dialogs, you can choose the side of the reference on which the dimension is to appear. In the case of rectangles, the width dimension can appear above or below the rectangle and the height dimension can appear either to the left or to the right of the rectangle. An individual dimension can also appear above or below its references.

Complete the left leg by bringing up the rectangle information dialog and naming the rectangle Left Leg, setting its size to 1 ½” by 26”and shifting the width dimension to the Below position. Double click on the offset dimension and set its size to 1 ¾” and the Below position.

To create the right leg, we could follow the same procedure, but instead, we'll take a slightly different tack. First, create a rectangle in roughly the position of the right leg. Don't try to create the coincident constraint when you create the rectangle; that is, make sure the bottom edge of the top is not highlighted when you press the mouse button to start the rectangle. Next, we will create the coincident constraint by grabbing the top resize handle of the right leg (i.e., position the mouse cursor over the handle so that it becomes the vertical resize shape and pressing the left mouse button). Holding down the mouse button, move the mouse until the cursor is over the bottom edge of the top. The bottom edge of the top will highlight when the mouse is in the right place. As you drag the mouse, the right leg rectangle will resize itself; however, when you release the mouse button, the right leg rectangle will snap to the bottom edge of the top and a coincident constraint will be created. Note the order of this operation: the dragged edge becomes the primary constraint object and the highlighted edge becomes the secondary constraint object.

To create the offset from the right edge of the top, select the menu option Tools|Dimension. The mouse cursor will become the create dimension shape (). Start by highlighting and clicking on the right edge of the right leg rectangle. The cursor will then become the “add another edge” shape, (). Now highlight and click on the right edge of the top.

Complete the right leg by bringing up the rectangle information dialog and naming the rectangle Right Leg, setting its size to 1 ½” by 26” and shifting the width dimension to the Below position. Double click on the offset dimension and set its size to 1 ¾” and the Below position.

The last piece of geometry we will add to our elevation view is a skirt. What are the criteria for the skirt? It should be flush with the bottom of the top and the inside edges of both legs. We could make the appropriate constraints as the rectangle is constructed or by dragging its edges afterwards; however, we take another slightly different tack again. Create a rectangle under the top and between the left and right leg but without coincident constraints. Pick the menu option Tools|Alignment; the mouse cursor will become the create alignment shape (). Highlight the left edge of the skirt and click the mouse button. The cursor will become the “add another edge” shape (). Click on the right edge of the left leg and lignumCAD will create a coincident constraint between the left edge of the skirt and the right edge of the left leg. Do similar actions to constrain the top edge of the skirt to the bottom edge of the top and the right edge of the skirt to the left edge of the right leg. Open the rectangle information dialog for the skirt rectangle and name it Skirt and set its height to 5 ½”. Also, adjust the height dimension so that it is “inside” the skirt. Your elevation view should now look like the figure below (with the skirt activated).

This could complete our work on the table sketches, however, there are a few aspects which are not optimal. For example, ordinarily, we would expect both table legs to be the same length. As we created the legs, we explicitly assigned the same length to both leg rectangles. However, if we wanted to change the height of the table, we would have to remember to change the height of both legs. While this is obvious for this simple model, its easy to create a model where it's not so clear. As a first attempt to repair this deficiency, could simply align the bottom of one leg to the other. So, for example, activate the right leg, grab the resize handle in the middle of the bottom edge and move the mouse so that the cursor highlights the bottom edge of the left leg. Note that nothing will change (substantially) on the right leg until you release the mouse. At that point, the height dimension of the right leg will disappear and the bottom edge will be made coincident with the bottom of the left leg.

Now there is only one dimension which controls the length of the legs. For example, change the height of the left leg to 23” and both legs will be shortened.

While this arrangement is more convenient, it is still not exactly what we want. What is really of interest is the height of the table itself, not the length of the legs. Thus, what we need is an indirect way to specify the length of the legs relative to the overall height of the table. This is purpose of reference lines.

To introduce reference lines, we will create a “floor” for the sketch. Select the Tools|Reference Line menu option. The cursor will become the create reference line cursor (). Move the mouse to just below the bottom edge of the legs and press the mouse button but don't release it just yet. If you drag the mouse a short distance, you will see that it becomes the rotation cursor (). Reference lines can be either horizontal or vertical; drag the mouse up and down and left and right to see the different orientations of the reference line. Release the mouse button when the reference line is horizontal. (The default is to create horizontal reference lines, so really all you have to do is click and release the left mouse button.) Double click on the reference line to bring up its information dialog and rename the reference line to Floor.

Before we create the constraints from the legs to the “floor,” we need to delete the coincident constraint between the bottom of the right leg and the bottom of the left leg. Select the Tools|Delete Constraint menu option; the cursor becomes the delete constraint cursor (). Highlight the bottom edge of the right leg and click the left mouse button. The coincident constraint will be removed.

Next, drag the bottom edges of both legs to be coincident with the “floor.” Then, create a dimension from the top edge of the top to the floor (i.e., select Tools|Dimension, and then pick the top edge of the top and then pick the reference line). Set the height of the table to 27”. Our front elevation should now look like the following figure.

[You might wonder now how helpful this version of our sketch is. After all, when you go to mill the legs, you do have to know their true length. Later, when we create construction drawings, we'll be able to recover the exact dimensions of the legs. (In fact, if you bring up the rectangle information dialog for the legs, you'll see that they have a height of 26 ¼”.) In general, the goal of Sketch pages should be to create the important dimensions and relations of the parts of the model. The details will be worked out on later pages.]

Consider that aesthetics generally favors symmetry in a design (as a counter-example, perhaps, see www.nakashimafoundation.org). In order to describe symmetry, lignumCAD supports a centerline notation.

Select the Tools|Centerline menu option and the mouse cursor becomes the centerline cursor (). Click somewhere between the vertical edges of the top to create a vertical centerline. (Note that you can rotate a centerline like a reference line if you want a horizontal centerline.) Next you want to create a centered constraint. You do this by selecting Tools|Dimension and clicking first on the centerline. Now you need to select two edges to be centered about the centerline. Select the two vertical edges of the top. Your page should now look like the figure below.

(When you create a centered constraint, the dimension lines tend to be jumbled somewhat together. If you grab a dimension by its value (i.e., by the number text), you can drag the value from side to side as well as up and down.)

Now, you can adjust either the width or the half-width of the top and the entire table will remain centered on the centerline. Note that this is not especially meaningful for such a simple model; however, centerlines do become more useful in complicated models.

As the last step of creating the Sketches, we want to print our work. Indeed, if the project is very simple, this may be all that we really need to do. Before printing, however, we need to set up some configuration information. Select the menu option Edit|Preferences; this will invoke the application default preferences dialog.

Under the Identification tab, fill in the Name and Location fields with suitable values for yourself. Note that for the logo, which is optional, you can use either a raster image file or a Scalable Vector Graphics (SVG) file as the logo. Both types of files will be scaled to fit in a box along with the name and location information. Click OK to apply the changes and exit the dialog.

[Note: as of lignumCAD version 0.2, only very simple SVG files can be printed. StarOffice 6/OpenOffice can output reasonable SVG files, however, you might need to edit the width, height and viewBox attributes of the svg tag to increase the precision. For example, I made the tree above as 1” by 1” Drawing but SO wrote the size in the SVG file as 25mm instead of 25.4mm and the viewBox dimension as 2500 instead of 2540.]

Next, select the File|Print menu option. The exact dialog that appears is system dependent, but you can usually select the printer and paper size. Note that your pages are printed exactly at the scale and position each page has in the user interface. In particular, the absolute spatial position of the lower left corner of the screen is the position of the lower left corner of the printed image. [Eventually there will be a print preview so you can see exactly what will be printed and where.]

Here is an example of the second page our notebook.

(Note that this image is actually from the Enhanced MetaFile (EMF) generated from the File|Export menu option.)