Blender has several options for you to correct the orientation of normals. Before you proceed, make sure that your model is manifold and that there are not any duplicate vertices. As both of these can confuse CAD/CAM software.
Fixing Normals:
Go into Edit mode
{tab}, then, on the bottom menu bar, look for the button called
{Mesh}. Click it and go up to
{Normals} then over to
{Recalculate Outside} or just
{CTRL+N}. Additionally, if you have already
'Walled' your model.
(In other words, it has an inside surface as well as an outside surface.) You can select the inner mesh and perform the same function, except Recalculate it as the inside. In most instances this will fix issues with normals.
However....
If that doesn't work, Blender has another trick up its sleeve for you to fix Normals. Blender can also calculate an entire surface area based on the angles of adjacent Faces. This can be a bit tricky until you have done it a few times.
In edit mode (
{tab} If you are not already there). Change to Face Select mode By hitting in sequence
{Ctrl+Tab+3}. In this mode, you will select parts of the model based on Faces instead of Vertices. You want to select a face that you consider the outside wall of your model. Then hit the following keys in sequence.
{Ctrl+Alt+Shift+F} This will prompt you to provide the minimum angle that Blender will calculate to be part of one contiguous surface area. Leave the default setting of 135 and click the
{OK} button. If the outside of your model has sharp edges, you may have to perform this several times until you have the entire 'Outside' of your model selected. (Just select another part of the model with
{Shift+RIGHTClick} and repeat the above steps.) Once you have it all selected you can recalulate the direction of the normals by hitting
{Ctrl+N} Now, to fix the 'Inside', You can select the inverse of the current selection with
{Ctrl+I}. Now that you have the 'Inside' surface of the model selected, you can recalculate the normals with the command
{Ctrl+Shift+N}.
If going through this doesn't work, you will have to take more drastic measures and start dissecting you model. That will be a REALLY detailed thread if needed.