Need Help!- plunge cut along ruled wireframe

[riverracer]

hi all,

i'm trying to plunge cut some vents on an existing cavity's shutoff taper using a 6mm cutter.
i have drawn the arcs, and can get it to cut to full depth, example 20mm.
problem lies in the 'z' depth limit.
as you can see by the attached photo, to achieve 10mm depth i have to do it in the 'y' axis.
if i try to do it in 'z' axis it goes way past the depth!
why is that?

only have hle at home, so can upload the file if needed 2morrow

thanks

x4 - solids