I have found that the plastic bobbins tend to "dance" around in the
so many, many years ago I changed to metal bobbins. And
when I got my Brother ULT embroidery machine in 2003, I just continued
using the metal bobbins in it as well. When I use the metal bobbins I do
make sure that I disengage the scissors/cutter on the embroidery
machine. It is just something that I have always done to ensure that no
"mishaps" occur. It is probably not necessary, but this is what I have
always done and have never had any problems with the metal bobbins.
If you look at the bottom of the bobbin case (bobbin holder or race -
whatever you want to call it(lol)),
you will see it has a magnet. When you use metal bobbins, this
magnet ensures that the bobbin remains securely in the bobbin case
whilst the machine is stitching.