Drill bit applications

[twstoerzinger]

Can one (or more) of you experience machinists give me a quick, practical summary on the pros and cons of 118 deg vs 135 deg angle twist drills?
I know 135s are supposed to be self-centering, and 118s are supposed to cut faster, but I suspect there is more to it than that.

I am looking to bolster my stock of drill bits, and I want to make some practical choices.
Mostly I work with mild steel and cast iron. Occasionally I work with aluminum and brass / bronze.
These will be used both for drill press work and for tail stock drilling on the lathe.

I think I am looking at all HSS at this time.

Thanks,
Terry S.

 

[tarmo120]

As a rule of thumb, the harder the material the bigger the angle.
Soft metals 90 to 118 degrees.
Steel and stainless 118 to 135 degrees.

But usually I run most bits at around 118 degrees.
Works really good. Just have to watch the drilling speed.

 

[Tom Griffin]

It's the grind on the point that makes a drill self centering, not the angle. If you want a self centering drill, look for drills with a split point grind, available in 135º and 118º (not as common). As already stated, if you commonly drill harder materials like stainless or tool steels, go for the 135º point, for aluminum, brass and mild steel, 118º is the way to go.

Tom

 

[Ray C]

Related question here... Is it safe to assume that most people grind their bits 60* per side (i.e 120 total) because it more or less splits the difference between 118 and 135? That's what I usually do.

Also, whenever I have a hole that needs a higher degree of placement precision, I always use a properly sized centering bit to pre-drill a starter hole. Longer bits will walk all over the place and end-up way off the mark otherwise.

Ray