I'm trying to rotate the blades of a windmill for an animation but the windmill is already at an angle. When I keyframe it on it's own axis (after pressing control + alt + space) and rotate it the blades spin inwards during the animation before coming back at the end. I've tried locking the other axis but that keeps the blades from rotating back to the right orientation. Also, whenever I rotate the object 360 degrees it turns the 360 into a really small number (0.0001) so it plays forward hallway then backwards (with a 180 degree keyframe halfway). Any help would be appreciated!.
Actually, as your wheel is not aligned with the global axis anymore, Blender will mess up with the rotation interpolation (Quaternion mode will work better than Euler but it's not perfect either). Also, the animation won’t take your 3D View Pivot Center setting into account, it will use the object's origin.
The easiest way to do it is to keep your wheel aligned with the global axis, parent it to an empty, tilt the empty the way you want, and animate the wheel. The empty parent will somehow be the new world landmark of your object and this time it will work fine.
About the rotation value: you should choose 180°, then, in the Graph Editor, shift E Linear Extrapolation so that it understands that it needs to go indefinitely with a constant speed.