True Position Calculator
Use this true position calculator to assess whether the bore and shaft axis positions are within the tolerance limit.
The formula
countersink depth = (Dcs − Dhole) ÷ (2 tan(angle ÷ 2))
Four hole problems, one page
A countersink’s depth follows from simple trigonometry: the cone narrows from the countersink diameter to the hole diameter, and how fast depends on the half angle. Note that 82° is the imperial standard for flat-head screws while metric uses 90° — mixing them leaves the head either proud or sunk.
True position is the one people get wrong most often. GD&T expresses it as a diameter, so the deviation is twice the radial error: being 0.003 out in x and 0.004 in y is a 0.005 radial error but a 0.010 true position deviation.
The clearance and rivet figures are conventions rather than laws, but they are the conventions drawings assume: 1.5 diameters to an edge, 3 diameters between fasteners.