Hi all, I made a shaft via revolve boss. I needed a rectangular profile keyway on the top of the shaft so I drew a rectangle on the top plane and planned to extrude cut that rectangle. I need the extrusion cut to be 2.5mm depth from the top plane but somehow solidworks doesn't show the keyway at all when I blind extruded by 2.5mm. The cut is only visible when I extrude cut by a much larger depth like 10mm. Is this because the cut starts from the centre of the shaft instead of the top surface? How do I rectify it?  

Please help me, I've been trying to solve this for hours but to no avail. Below are two images that show my problem. First one shows the preview of the extrude cut, 2nd one shows the result, where the cut cannot be seen at all.  

Thanks so much!