https://www.youtube.com/watch?v=KHl0y3qj64g
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After many test it seems that slow and cool is the way, When using HSS. Here I send a 4.09 mm drill to over 1 inch in Grade 5 Titanium 6AL-4V using a RPM of 70 and flood coolant (Rustslick)
The biggest mistake made when machinint Ti., is excessive speed. Ti., unlike Al, Ti does not easily transfer heat. In fact in some industrial injection molding tools Ti is used as a heat insulator.
With such slow heat transfer, the work generating heat does not freely pass to the chip and escape as it does with Al & Fe. That is why the tip of the drill (or edge of any other HSS tool melts.
When drilling Ti. and tool steel, a drill with an included angle of 135 deg. is better than a 118 deg. included angle. Why? Because the length of the cutting edge is shorter. A shorter length cutting edge requires less pressure to penetrate, thus less work and less heat is generated.
Ti can be turned, drilled and milled all day long by keeping the R's down in a range one half (1/2) that one would use HSS cutters for tool steels such as O1, or D2.
Coolant is most effective when used as a "flood" directly on the cutter edge.Thus, a "peck" drill technique also helps, with or without coolant.
Works for me - been doin it for 40+ years.