Your best bet is a solid carbide ball end mill with 3 or 4 flutes, running at the slowest speed, then bring the knee up very slowly, keep the quill retracted to ensure rigidity too! Best done with thick brown cutting oil, as it bonds to the cutting edge under pressure and increased temp. Those rails have .09 to .125 thick case hardening, and a soft steel center.
Drill the hole for a shallow thread as mentioned earlier, to make it easier to tap, make the bore through the case about .015 bigger than the tap diameter! |