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### Countersink Spot Drill Depth Formula and Calculator

**Countersink and Spot Drill Depth Formula and Calculator**

Preview: **Countersink and Spot Drill Depth Calculator**

The following formulas are for countersinks and spot drills that have been sharpened to a point, allowances on tool depth will have to be made if the countersink or spot drill is not sharpened to a point.

For countersinks and spot drills that have been sharpened to a point

To find the tool travel “Z” required for any sharp-point countersink or spot drill angle, use either of the following formulas:

Z = ( D / 2 ) [ 1 / tan ( α / 2) ]

or

Z = ( D / 2 ) cot ( α / 2)

α = countersink angle

D = countersink diameter

Z = depth of tool travel required from surface of part

To find the tool travel “Z−a” required for any truncated countersink or spot drill angle, use either of the following formulas:

Z - a = ( D - T ) / 2 · [ 1 / tan ( α / 2 ) ]

Z - a = ( D - T ) / 2 · cot ( α / 2 )

a = allowance for truncated countersink tool

d = diameter of through hole

T = diameter of small end of truncated tool

The table below is a quick reference for some predefined To calculate the tool travel “Z”, multiply the factor by the countersink diameter.

Countersink Angle α, degrees |
Factor (Imperial or Metric Units) |
Countersink Angle α, degrees |
Factor Imperial or Metric Units) |

60 |
0.866 |
118 |
0.3004 |

82 |
0.5752 |
120 |
0.2887 |

90 |
0.5000 |
135 |
0.2071 |

100 |
0.4195 |
142 |
0.1722 |

Related:

- Drill and Counterbore Sizes for Socket Head Cap Screws per. ASME B18.3
- Rivet Application Tables and Installation Practice per. MIL-R-47196

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