Press C while in knife mode to enable Angle Constrain.
Note that it snaps to angles relative to the view. For cuts on faces which are not aligned to any axis, you can:
Press ShiftNumpad 7 to align the view to the face
Press K to enter knife mode, then C to enable Angle Constrain:
If you grab a recent build of Blender of r58453 or greater (2.7x up), you can use Shift + S > Selection to Cursor (Offset)
The official commit can be found here. I could not find any documentation but it's pretty intuitive.
AutoMerge Editing might be what you are looking for. It can be toggled on and off by clicking the entry.
EDIT: I've added the image posted as a comment by username: gandalf3 since he pointed out that there is also toggle button in the header bar which makes this feature easier to access.
If you hold ⇧ Shift while snapping, then it will snap in smaller increments:
Or if you view from an aligned view such as top ortho (Numpad 7), the snapping increments will be equal to the grid subdivisions (which change with zoom level):
You can use C while using the Knife Tool to enable Angle Constrain. This will enable you to cut at the following angles (with reference to your current view):
At 45° angles
Also, notice that when you start to use the Knife tool, the 3D View Header bar changes to show you all the available Hotkeys to perform all ...
You can snap to the grid or the cursor by pressing ShiftS, however I'm not sure that is the fastest way in this case.
Transform snapping and a guiding plane.
Add a plane underneath the object
Enable Face Snapping:
Press GZ0 with the object selected and your cursor over the plane:
Works in object mode and edit mode
Requires a new ...
Select one cube, then enable the snap during transform option (click on the magnet icon), change the snap element to Vertex and snap target to Closest. Move the cube (G) along X,Y and Z axis until it snaps to the proper place.
Snapping will use the resolution of the displayed grid, so you have to change the scale of that.
You can do that in the Properties Shelf (press N). Open Display rollout and change the scale of the grid. (For example if you use Metric system, type 0.01 and Blender will snap things to centimeters).
When set to median and face as target an objects median (center) will snap to a face.
This example shows how snapping works with different objects and snapping an edge of the same object.
When you use closest as snapping target then objects will snap to the top of the face.
Snapping doesn't work by pressing Enter by itself.
Using Face|Closest|Project ...
Try this way:
Select both sides, W - Bridge Edge Loops. Enable Merge, set Factor to 0 (or 1);
Keep current selection, V to rip, then RMB;
Mesh > Edges > Edge Slide ( or GG) to enable Edge Slide. E to toggle Even, if the current result is in a wrong way, then F to flip it (those are options shown on 3D View header).
Move to the expected position, then LMB or ...
You can use Vertex Snapping.
You can enable it by clicking on the magnet icon or If you wan't to use it temporarily, hold down Ctrl.
I would recommend setting the second option to active:
You use it by moving your object in edit mode Tab:
You can CtrlAltQ to toggle Quad View, then click to the nearest grid crossing to place 3D Cursor there, then ShiftS > Cursor to Grid to snap the 3D Cursor in two of the side views (to snap it precisely to the grid dimension as you want). Then, ShiftS > Selection to Cursor (Offset) to offset the selection to the 3D Curor, without collapsing:
NOTE: Snapping ...
You can use a helper object as the target location agent:
Select the target face you want to snap to, CtrlH -> Hook to New Object, which will create a new empty object at the face center;
select the target part and keep the target face active, be sure to use Acive Element as Pivot point;
Turn on Snap tool, set Target Element to Vertex, and Snap Target to ...
Blender is unfortunately very limited for use in precision modeling and CAD like operations.
It's snapping tools and precision transforms are very lacking. Dupligroup handling is one of it's many shortcomings, this is a know limitation of the system, you can't really control what to snap to and from with empties.
You can still snap to the linked geometry, ...
It's stated pretty clearly in the Blender manual, but if someone omits this source it'll be here:
The Cursor to Selected option is also affected by the current Pivot Point. For example:
With the Bounding Box Center pivot point active, the Cursor to Selected option will snap the 3D cursor to the center of the bounding
box surrounding the objects’ ...
For the vertex you want to snap, in the Snap options, you have the choice between Target > Closest / Center / Median / Active. To point out the vertex you want to snap to, use your cursor.
For example Active will snap the last vertex you've selected.
The easiest way to do this in blender 2.68 is to open the properties panel (N), and copy-paste the values from the 3D-cursor location fields to the transform median fields by hovering the mouse over them and hitting CtrlC and CtrlV.
This does the same thing as the new snapping with offset that's implemented in the current trunk, as stated by iKlsR. My answer ...
You can enable and use the Drop to Ground addon, it's under the Testing category. After, that just select the object then the ground plane and press Drop to Ground. No need to change your object centers or any cumbersome tricks with the 3d cursor.
With the current snapping tools (this will hopefully be better one day), you have to join the objects first.
After joining the two objects CtrlJ, and entering edit mode you will be able to snap one edge (or vertex) to another.
Select all of one of the pieces L, and then make the edge you want to snap the active edge.
With snapping set to Edge and the Snap ...
With cursor positioned as desired and vertices / edges selected scale it while choosing axis which you'd like to affect on scaling (in this case Global X):
Select vertices, set Pivot Point to 3D Cursor.
Press S to scale, then X to scale to 3D Cursor by X axis axis.
Enter amount of effect of scaling. In this case it will be 0.
Blender cannot snap to mid points unfortunately, only the knife tool exclusive snapping system can snap to midpoints as far as I know.
As a workaround you can use Blenders Mark Multiple Snap Targets feature to simulate a midpoint.
You will have to make sure snapping is forcefully turned on from the active magnet icon, and also set to Vertex and Closest
With the top face deleted and vertices selected ...
Set Pivot to Median
Set snap to Vertex and Active, and set it to influence Scale. Check 'Project on to Self'.
With cursor close to Active (white) vertex Extrude E Scale S Limited Axis ShiftZ and snap the active vertex to the one below it.
(Holding Ctrl while dragging toggles the 'Snap' state.)
Press ShiftS>Selection to Grid.
As an expansion of Leon Cheung's answer:
Snap the cursor to the selection:
Snap the cursor to grid
Snap the selection to the cursor with the Offset option
The selection is now aligned with the grid, so you can snap using the normal increment snap.
You can hit CTRL+ALT+Q to toggle quad view mode, showing your model along all major axes.
From there, you can rotate with r for manual corrections. For finer control, hold Shift while you rotate.
Edit: An alternative method to ensure a symmetric mesh when you're done:
1 to view from front
Tab to enter edit mode
z to display wireframe
b+box select the ...
I know this is an old question, but I've been having the same issue and managed to figure it out:
In Edit Mode, select an edge or a face that you want to use as a custom transform, this works also in object mode for objects.
Press Shift + Numpad 7, this will align the view to selected in orthographic view.
Press Ctrl + Alt + Space bar, in order to create a ...
Unfortunately, you can't use solely a modifier for a shortcut, plus there already is a shortcut to toggle this: Shift + Tab.
If you still want to, you can change this in the User Preferences.
If you open the user preferences with Ctrl + Alt + U, and switch to the Input tab, you can search by shortcut (Click the dropdown next to the search box and select ...