Most objects contain a variety of smooth and hard edged surfaces.
Although there are exception, mainly in organic modelling, most models contain a combination of both.
Further explanation and screenshots can be found here - Blender Wiki Smoothing & Doom 3 Smoothing Groups.
Solid and Smooth Objects
You can change smoothing type of the object in the Tool Shelf, under the shading panel.
- Select the object and either set to "Flat" or "Smooth".
Emulating Smoothing Groups
Smoothing groups is a way of grouping faces together into either hard or soft faces.
Blender uses per face smoothing instead of smoothing groups, like 3DS Max.
However this does not mean that it cannot be achieved in Blender.
You should first clean your mesh of double vertices
- Edit Mode: Select all vertices(A), then Remove doubles (Tool Shelf, Remove Panel)
I will discuss two method, both of which achieve the same out come.
The first method goes through the process, the second automates the process.
This method is where you physically break up the model manually into groups of smooth regions.
In Edit Mode: Face Select mode
- Select the faces you want to form part of a grouping.
- Split the from the mesh using (Y).
- Repeat for each smoothing group you need.
This approach can be rather tedious and prone to error.
Edge Split Modifier Method
This is my preferred approach.
- Set the entire object smooth.
Under the Modifier Tab of the Properties Panel.
- Add the Edge Split Modifier.
- Untick the Edge Angle options.
Visibly the model hasn't changed, we need to mark which edges will be hard.
In Edge Mode:
- Select the edges, Ctrl+E to bring up the Edge Menu then Mark Sharp
- The edges will turn blue to show the are Sharp, the edges will now appear Sharp
As we are using the modifier, these changes are not permanent.
- Removing the modifier will remove the Sharpening.
- Applying the Modifier will split the Mesh into seperate objects.
- Change the edge marking is reflected immediately in the 3D View Window.
Subsurfacing will make your model smoother by adding polygons; you can use the crease function (Shift-E) to change the smoothing radius on selected edges.