spiral pattern, everyone has an opinion, i've used cross dilled rotors on drag cars , turned them blue , stopping a 3000 lb car with small front wheels and 15x 33 goodyears on the back, from 165 mph tends to make some serious heat, show me some of these cracks that some of you are seeing, I,ve not had the opportunity , :wink: :wink:
As a former structural integrity guy, I will tell you that holes and slots are areas of stress concentration...stresses around holes can be at least 3 times the amount of the stress far from the holes. (I'm not sure what the stress concentration factor is for slots, but if you leave sharp corners in the slots, you're begging for trouble.) You can minimize this by making the holes large enough and also making sure they're spaced far apart enough. Where I work, we use 4 times the diameter of the hole + 0.03", which looks like from the pics you far exceed that...that's good. The holes also need to be flawless, any little scratches in the holes are where cracks will initiate.
Chamfered is where an edge is filed down so that you don't have a sharp corner. For example, if you had a part that had a square cross-section, chamfering would make the cross section look like an octogon. The edges are less sharp and less likely to chip off and create a crack initiation site.