Quick way to estimate how deep to drill.

A quick tip to avoid drilling too deep is to use a very basic property of trig. While not completely precise it is quick. Most drill bit that are used are 118 degrees, which is quite close to 120 degrees. A 120 degree drill bit would form a 30-60-90 triangle. As you can see from the diagram above you can figure out how deep to drill by figuring out what x is. Most machinists keep a pocket calculator handy (or should) which will make calculating your approx. drill depth quick and easy. Well x*sqrt(3) = .5*d with d being the diameter of the drill.

So You want to drill X Deep...

Step 1) Divide the diameter of the drill by 2.
Step 2) Divide that number by the square root of 3. (Just divide it by 1.732)
Step 3) Add this to how deep you want your hole at the full diameter to be. The resulting number is how long the tip of the drill is.