A region is a two-dimensional shape with a unique property. It behaves like a 3D object. You could think of it as a surface. Regions can be edited with Boolean commands like SUBTRACT and UNION just like 3D shapes and surfaces. Regions can be hatched and have an area properties similar to polylines. Unlike polylines, regions also have physical properties such as centroid and moment of inertia. You can determine the value of these properties using the MASSPROP command. You can apply materials to regions and use them in renderings. Regions can be extruded into 3D solids, and they can be positioned in 3D space. A region can be used to mask objects that are behind them. Regions are created from any 2D geometry that creates a closed loop, but doesn't overlap. We're going to be creating the profile for a spur gear using regions. I have three circles and a couple of arcs. I'm going to start by creating a region using the inside of the two arcs as my boundary. I'm going to switch to the region layer using the layer drop-down list on the ribbon. I can select the region tool on the draw panel of the ribbon, or I can use the boundary command. I'm going to use both methods in this exercise so you can see how they each work. I type Bo for boundary, I set my object type to region. This means I will be creating a region. I choose the "Pick" tool on the "Boundary" dialogue and pick inside the two arcs. I press "Enter" and a region is created. If I hover over the object, AutoCAD displays it as a region. Notice that the region was created on the current layer. I'm going to use polar array to array the region around the center of the circles. I select the "Polar Array" tool from the ribbon. I select the region. I press "Enter" to indicate I am done selecting objects to use for the array. The command prompt asks me to select a center point for the array. I select the center point of the circles. I change the number of items to 12. Notice on the ribbon, I can enable the objects to rotate around the circle. If I disable that option, notice how the preview changes. Obviously, I want to rotate the region. So I'm going to re-enable the rotate option. I have associative disabled. Associative groups the elements of the array together so the array can be modified. I'm going to be performing a Boolean operation with this regions, so I'm going to keep them as non-associative. I press the green check on the ribbon to close the array command. I'm going to freeze the region layer to turn off the display of regions to make it easier to create the next region. In order to do that, I'm going to switch back to the spur gear layer and make that layer current. Remember, you can't freeze the current layer. Once I have the spur gear layer as current, I can freeze the region layer. I'm going to delete the two arcs because I don't need them anymore. I want to convert the circles to regions. I select the region tool from the drop panel on the ribbon, I select the circles, and press "Enter." If you look in the command prompt area, you see the three regions were created. When I hover over the circles, they highlight as separate regions. I am going to thaw the region layer using the "Thaw All" tool on the ribbon. This tool thaws any layers that are frozen. It's a little faster than using the drop-down list. I'm going to make the region layer current. Next, I want to combine the red region group with the yellow region middle circle to create my spur gear. To bake it a little faster, I'm going to group all the red regions together into a single group. I right-click and select Quick Select. I set my object type to region, I set the properties to layer, and set the layer to region. This will select any region that is on the layer called region. I press "Okay." All the red regions will highlight and are selected. I select Group from the groups panel on the ribbon, and unnamed group has been created. Now, if I select any element that is part of the group, all the elements will be selected. If I wanted to name the group, I could use the Group Manager from the ribbon, but this group is not going to be around for very long. It's going to be used in a Boolean operation. I type Union and select the yellow middle region. I select the red region, and notice all the red regions are selected because they were made into a group. I press "Enter." We now have a new region that's combined from the two selected regions. Notice the new region is on the spur gear layer even though the region layer is current. This is because the first item we selected for the union was on the spur gear layer. We currently have three regions in our drawing; the outside circle, the combined region, and the small circle. I want to subtract the inner region circle from the combined region. I type Subtract. Look at the command prompt. This area is telling me what to do as I walk through the operation. It asks me to select the element to subtract from. It also lists which object types can be used for Boolean operations; solids, surfaces, and regions. I select the combined region as the object to subtract from and press "Enter." Now I am prompted for the elements to be subtracted. I select the inner circle region as the element to be subtracted and press "Enter." I delete the outer circle. Now I see my spur gear. If I select it, notice it is a single object. If I use the grip to move it, notice that it moves as a single object. If I want to find out the properties of my spur gear, I type MASSPROP and select the region. MASSPROP gives me the option to save the data to a file. I press "Enter," to select No. I don't need to save that information. Remember, you can always press "F2" to expand the command prompt window to review any of that information. The MASSPROP command provides you with the area, centroid, moments of inertia, and more. We use the region tool to create a spur gear. We also used the commands Polar Array, Group, and Quick Select. In addition, we did some Boolean operations like SUBTRACT and UNION. Boolean operations can be used with regions, solids, and surfaces.
