General   (General discussion, talk about anything.)

No, lower spool cannot be fixed to a spline, as to make a stitch the upper thread has to pass around it.

A the needle descends, the tension arm drops, so there is now no tension in the top thread. As soon as the needle begins to rise, with no tension, and nothing to support the weight of the thread it will form a loop. You will see a hook on the mechanism which houses the lower spool, engage with this loop. As this structure partially rotates, it pulls this loop all around itself, resulting in the thread from the needle now having passed under the lower thread. The needle and tensioning arm then rise to remove any slack and pull the stitch into position.