Drill Point Calculations Made Simple

Hanging out on the shop floor we see a lot of programmers / operators "struggling" with finding the correct dimension for either countersinking or drilling through a workpiece. So we thought it a good time to put some simple formulas out there to remind our fellow machinists / programmers how the calcs are done and what factors effect those calculations.

What factors effect the calculations? There are basically two  :

• Diameter of the drill
• Angle of the drill point

Once those values are known ... you can easily calculate the length of the drill point using one of the choices below :

The Complicated ... But Accurate Method :

1. Find the radius of the drill
2. Take the angle of the drill point and divide that by 2 ... a normal twist drill angle is 118 degrees.
3. The final formula is drill radius / Tan(angle from step #2)

Obviously this method will work and provide an accurate answer for any angle drill and countersink.

The "Rule of Thumb" Method for STANDARD drills :

Here is the common "rule of thumb" method for standard 118 degree drills. Obviously if you are dealing with countersinks or non-standard tip drills ... you need Method #1 above. But for a down-and-dirty calculation for standard drills ... just multiply the drill diameter by .300 ... inch or metric, doesn't matter.

Here's where that formula came from ... it assumes the standard drill point angle of 118 degrees :

1. 118 degrees / 2 = 59
2. 90 -59 = 31 ( using the angle opposite that of the one above )
3. The tangent of 31 is .6009 / 2 = .3004 ... or rounded of to .3

Hope this helps the next time you are calculating the depth of a counterbore ( in which case method #1 should be employed ) or how to deep to drill through a workpiece ( where the rule of thumb method should do just fine ).

If you're looking for a software application that gives formulas like this as a handy reference ... check our our KipwareTB® - Machine Shop Toolbox Software ... just click the link below.

Until Next Time ... Happy Chip Making !!