Hi Ben,
The Wrench was mirrored, and the path direction was not the same for both parts. In Mesh> Spline> Reverse Direction, one can select a segment of the Spline and reverse it, then it can be joined as one Spline; If not Trouble like that happens.
In point mode, the color of the Spline has a gradient. It should be one gradient, from white to pale-blue.
The wrench Spline was not closed.
The Cogwheel was set to B-Spline, while a typical cogwheel is converted from a Primitive to a spline object as Bezier. I set it to Linear.
The Axis of the Cogwheel was way off, as long as the spline points are on one “plane” it should be fine. However, since all points have a child-parent relation to the object axis, I would always get this sorted out.
Here is your file back
https://www.amazon.com/clouddrive/share/nmbKAfVlqo0pemoUUBLFvWqNk3yK7UHfNJ7uRKW4QrV
All the best