Gimp,
I've been doing this with Thunderheads and Muzzys, and you can swing them pretty hard without it letting go.
The trick is to get a strong magnet, like the kind that are 1" square blocks that take some effort to pull off ofwhatever they are stuck to.

I like BobCo's device too.
With his, itwould be easier to diagnose the actual direction of error, and would provide a quantifiable magnitude of error by the path the tip traced on the board. Very nice.

The magnet trick is just a very quick way to see if they spin true, and verify before loading your quiver.

Enjoy. The other night I checked 6 arrows in about 90 seconds, and I only got cut 3 times. Just kidding.

I love it.