Need Help!- FACE GROOVE

[nascad]

HELLO EVERYBODY, i hope this is the right palce to post this i have to machine a face groove with an angle of drft in the wall of 66 deg. (looks like a dovetail in the section view)we have to do it in the vmc because the cnc lathe is to small the grove is about 15 in dia. so i have tried with an end mills shaped in the form of the groove ,for roughing i used cobalt but for finish i used carbide but i broke 4 of them already and only one good part, i started to think the problem is the end mill because it has an angled flute, and since basically the operation seems to call for a key cutter with straight flutes i am planing to ask my boss(who is not a machinist) to change the tool from end mill to a shaped key cutter high speed material since from my experience is more forgiving then carbide (with the required angle and radious)so i can machine first yhe OD part of the groove and then the ID wall of the groove with .1 in axial depth of cut and about .015 of radial step over, 400 SFM .002 in FPR ,the material is aluminum casting, can anyone please helpme with this problem, is killing me and a good way to hold down the alluminum ring because it tends to lift,since the tolerance in the groove hight is +,- .002
thank you very much for your help any ideas and thoughts are appreceated thank you all. i am posting the drawing is the deatail F