X-Head Porting....

Once the burr is spinning, it will tend to stay straight, but you need a real tight grip on the grinder when it starts. Any stuttering or runout in the bearings or collet will destroy the burr and can hurt you too. A loose grip can give it just enough play for it to whip and bend.
A soft start can help prevent it too, but not sure what it would take to make one for a motor.

I was trying my hand at some cylinder head porting on an old 4cyl alum head using an electric grinder. I nearly lost my fingers! Between the burrs biting and bouncing around the port, the massive weight of the armature, and the abject fear of the burr helicoptering (which did finally happen after about 2 hrs) when I switched the damn thing on, I gave up and decided it just wasn't for me. I had better luck with coarse stones and atf as a cutting fluid, but those electric grinders still make it tricky. Spool up too fast and spool down too slow.