I want to see the whole photo about the design dovetail clamp.

 

Stress release or slight vibrations when the tool reach that area? Ave you tried already to lower the speed of the process ?

I guess this component is elongated.

 

i am sure these are machining marks, not from aluminum casting, aluminum6061 material is ok.

 

http://en.allexperts.com/q/Metallurgy-2280/2010/6/Stretcher-Strain.htm

Maybe?

 

Vibration while machining. Clamp it better. Cool it better