The 'magic' of 118 degree drill geometry - why?

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rfdes

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I'm curious whether anyone understands the 'why' regarding the drill bit point requiring the 118 degree included angle? I guess the stock answer would be it cuts the best, but there must be some scientific reason as to why this is the optimum. Anyone understand this so that it could be explained?
Thanks
Jim
 
I don't know why they chose that angle either. At one time there was a spiral pointed drill bit. Supposedly was able to reduce burrs. Back to your question, if both lips are equal in length the angle doesn't matter. Just make sure the area behind the lip clears so the bit doesn't drag. I've ground several bits both freehand and using a fixture. Anything under a 3/16" in my opinion is to pitch it out.

I don't even want to know how they grind those small bits. They have to use a magnifying glass.
 
118 degree is able to center better than say a 140 degree but is less efficient at cutting, I don't think there is anything scientific to it, just tried and true practise.
 
Because of the way drills are produced you can not change the DP angle without bowing the cutting edge. If you grind too much relief, the cutting edge will become convex, too little relief and it will go concave. Same thing with the point angle.

118 degree is able to center better than say a 140 degree.
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If you are using a chisel point bit without at least using a punch mark to guide it, all drill points will wander on the surface. You can narrow the chisel width with a hand grinder and cutoff wheel (like dremel tool) and create a split point drill. SP drills do not wander, pick up center punch marks easily, and take much less downward force to cut.

 
It is a compromise to get reasonable cutting and centering properties. The angle can be 180 degrees to if the set up and drill is stable like a pin mill.
 
Micke S said:
It is a compromise to get reasonable cutting and centering properties. The angle can be 180 degrees to if the set up and drill is stable like a pin mill.
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What is a pin mill?
 
Split points will walk maybe not as far but they will. The split point is to thin the web.
 
Andre said:
Because of the way drills are produced you can not change the DP angle without bowing the cutting edge. If you grind too much relief, the cutting edge will become convex, too little relief and it will go concave. Same thing with the point angle.
Click to expand...
You lost me here Andre, Are you saying that I cant regrind my drills to a different angle than what the manufacturer originally produced?

Cheers Phil
 
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