Threads / Process guide
Length and height
answer different questions.
A 75% tapping reference is not 75% engagement length and is not a prediction of joint strength.
Axial engagement length
The length along the axis over which the mating threads engage. Establish it from the assembly and drawing, allowing for incomplete threads and chamfers.
Example: 10 mm axial engagement remains a length, whatever the tap-drill chart’s percentage.
Theoretical thread height
The percentage used in the cited tapping-hole equations describes thread height. Changing the pre-tap hole changes this quantity; the chart is not an axial-length calculation.
Example: the 75% column selects one theoretical target. Drilling runout and the exact tap affect the result.
For a blind tapped hole
Record required full-thread depth, tap lead, drill-point geometry, bottom clearance and chip space separately. The tap-drill diameter tool does not derive drilling depth or joint capacity.