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Ans:

About using tool cutter with 12mm to cut 30mm big width keyway which down width is narrower , Hereby our Engineer reply it as below: :
If we use 12mm cutter cut 30mm keyway by 3 cuts , The keyway width at down is narrower ,  You said that Top width is good , So controller X axis position is no problem.

The possible reason is  :

1. The slotting bar rigid is not enough , normal rigid Ratio is 3-5 , For example Diameter is 40 , the Length is 120-200mm , Your keyway width is 200mm , Slotting bar out length need min.230-250.   So your slotting bar diameter at least should be 50mm  ,  If the slotting bar too weak (flexible) , The slotting bar at top OK. but it bend at bottom Period . 
2. Besides, you also need to check the perpendicularity between table surface and Ram slide well. 
3. The third possible reason is you don’t add support or fixing fixture such as fixing ring to let work-piece no moving during cutter cut at bottom range , Because you just clamp work-piece at top but lower part don’t fix , While slotting tool forced cutting at lower range , the work-piece may be escape (move out) , So happen the problem.