Machined or lightened....theres a difference.
I class machining as preparing the flywheel for a new clutch....
I would guess you mean lightening, if thats the case then heres a link you may want to give a read.
http://www.geocities.com/motorcity/7177/flywheel.html
As said in the above post, flywheels can and DO disintergrate....personally i've never heard of a K flywheel doing it, but it doesnt mean it can't.