I just got my machine back from it's annual. They had put my metal bobbin in a baggie with a note. It said to use only the #15 plastic bobbins. I don't know why but will ask them the next time.

I was told that the metal ones will wear out the race more quickly than the plastic ones. I've got metal bobbins too and that's what I use in my Brother as well and 6 years later it is still OK. (You can tell when it needs an oil very quickly as well).
The stitches on this Brother are the best of all my machines.

DON'T DO IT !!!! USE WHAT YOUR SUPPOSED TO. take a good look at the plastic ones they are tapered not flat like the metal ones. why risk damaging your machine. for once listen to the woman !!!

I have a Brother machine too and it will only work with the plactic bobbins. The best thing is to talk to the dealer where you bought the machine. IMHO, if you want to get the best you can out of your machine, then you need to get in touch with your feminine side and read the manual. LOL

I had many metal bobbins, but was told only use the bobbins for your machine. It could do some real damage to your machine.

When all else fails, read the destructions!

Check your book. It may tell you in there.

I have a Brother too and use only plastic bobbins. I just read this on another quilt board:

If your machine came with plastic bobbins, that's what you should stick with. The metal class 15s are intended for use with either top- or front-loading vintage mechanical machines and newer front-loading machines with metal bobbin cases.

Your machine probably has a magnetic hook, and the metal bobbin will interfere with its performance.

Also, just my guess, but the cover over the bobbin is probably see-through plastic, which allows you to see how low your bobbin thread is. Using a metal bobbin makes this more difficult.

Hey David - don't throw out the metal bobbins - remember the Vintage Shoppe - you may have a need for them sometime. I use metal in my old Kenmore and old Singer and plastic in my Brothers.

Then you're taking a BIG chance on ruining your new machine. No kidding, bobbins have to be correct or your stitches won't have the correct tension. If your new machine has a 'bobbin almost out of thread' sensor, it won't work unless you're using plastic bobbins.