My DIY stirplate is gonna be the death of me! Please help...

So I'm on my 2nd fan. Went from a 3in to a 4in. Thinking that was the problem that my "found" stir bar won't start spinning.

Do I need an actual stir bar? I'm using a threaded post that u would find in an electrical box in the ceiling.

Now I'm thinking my hard drive magnet isn't strong enough?

Or I haven't got the polarities right on the fan?
My instructions say to break the magnet in half then glue the pieces to a piece of cardboard that you've glued to the fan. Making sure to get the polarities right so the bar will attract. But either way I do it my bar attracts to it. So that makes no sense to me.
I have it wired to a 12V cell phone charger and the fan has 3 speeds on it.

So basically I can't get my scavenged stir bar to attract and stir with any consistency.
Please Help.

Possibly the power supply you're using is not strong enough. Maybe I'm missing something, does your fan work ok?

Yes, you need a stir bar. The ends of the bar are magnetized and have polarity to match up to your magnets on the fan. A screw will not work.

Yeah the fan works fine.
Looks like I'm ordering a stir bar. I had to idea that was a magnet too.

My first starter used a homemade stirbar. A cut ferrous nail inside a vinyl tube that was burnt shut on both ends. It worked fine.

You say that your stirbar won't start spinning. I would say that your magnets are too close to your motor or you don't have them shielded enough. Move them away from your motor about 3/4" (pvc, old tape roll dispenser, anything to provide distance).

With a magnetic stirbar you will need to have the polarities correct otherwise one end will attract while the other detracts, and you'll constantly be throwing the stirbar.

After the post from stpug I put a threaded screw from a electrical box in a cut down plastic straw. I have it spinning in 1L of water as a test. It seems to work pretty good it's been spinning and drawing a vortex for 30 minutes now.

Well damn. I already ordered a stir bar. So its on the way.
But damn. I can't figure this thing out.

While your waiting for it to arrive you can play with the screw, I just put it in a straw to protect the glass and test stpugs method. It seems harder to get a non stir bar centered. Having it in clear liquid makes it easy to watch the screw and make adjustments. Once it starts it works but is more of a PITA to adjust. I think it would be even more frustrating to get it rotating if you couldn't see the screw. Play with the spacing as suggested above and if you can get the thing to work it will be easier with your stir bar than with the screw you are currently using.