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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

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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