Kernlochbohrer-Rechner für Gewinde
Metrische Regel- und Feingewinde, UNC, UNF und NPT Kernlochmaße mit exakter Gewindetragtiefe (50%-85%).
Thread Selection & Parameters
Select thread standard, size, tap process, and custom engagement percentage
75% engagement delivers full thread proof strength while keeping tap torque moderate.
Nearest Standard Drill Bit Equivalents
Closest fractional, letter, number, and metric sizes if exact drill is out of stock
| Drill Size | System | Decimal (mm) | Decimal (inch) | Engagement % | Delta |
|---|
The blue zone represents the engaged flanks of the internal thread. 75% height engagement provides 100% of the bolt tensile failure strength without excessive tap friction.
The Science of Thread Engagement Percentage
Most machinists are taught the traditional rule of thumb targeting 75% thread engagement. However, empirical tensile testing proves that any engagement above 60% provides over 95% to 100% of the bolt's ultimate tensile fracture strength. The bolt will snap in tension before the internal threads strip!
Conversely, forcing an internal thread up from 65% to 80% engagement increases the tap cutting torque by roughly 60%. In tough or work-hardening metals like Inconel, 316 stainless steel, and titanium, this extra torque is the primary cause of tap breakage.
Recommended for titanium, tool steels, 304/316 stainless, and hole depths exceeding 1.5× diameter. Minimizes tap friction and breakage risk.
The standard industrial choice for mild steels, free-cutting brass, structural steels, and rigid production setups.
Best suited for low-shear materials like Delrin, nylon, magnesium, or cast aluminum where maximum shear area is needed.
Cut Tap vs. Roll Form (Fluteless) Tap
Cutting Taps
Cutting taps have sharp cutting flutes that slice out chips as the tap advances. They are universal and work in virtually any material, including brittle cast iron.
Formula: Tap Drill = Major Dia - (1 / TPI) (Imperial)
Roll Form (Cold Forming) Taps
Form taps have no flutes or cutting edges; they cold-form the metal by plastic deformation. This creates stronger grain structures and zero chips, but requires ductile metals (aluminum, brass, mild steel) and a larger pre-drill hole.
Formula: Form Drill = Major Dia - (0.0068 × % / TPI) (Imperial)