Working out Drill Depth

When a drawing asks for a drill to be taken to a certain depth of full diameter, e.g. 12mm diameter hole 25mm deep, then you always have to program the drill to go deeper than the 25mm specified because of the drill point. As a machinist you don't want to take the drill too far past what is drawn to create the full diameter hole.

Using trigonometry you can work out how far to take the drill. The standard drill point angles are 118, 135 or 140 degrees.

If the drill point is:

118 degrees then multiply the diameter of the drill by 0.3

135 degrees then multiply the diameter of the drill by 0.207

118 degrees then multiply the diameter of the drill by 0.182

For example, if the 12mm drill, mentioned earlier, was 135 degrees and the drawing asked for 25mm of full diameter then:

12mm * 0.207 = 2.484mm

Add this onto the 25mm depth = 27.484mm.

So, take the drill 27.484mm deep to produce 25mm of 12mm diameter.