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 may mess up with the rotation interpolation, plus the animation won’t take your 3D View Pivot Center setting into account.
The easiest way to do it is to keep your wheel aligned, create your animation, then parent it to an empty, and tilt the empty. The empty parent will somehow be the new world landmark of your object.
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.