Drilling for threads

How to use the calculator

Select the standard (ISO Metric, UNC or UNF), thread size, tool type and material - the calculator will calculate the diameter of the drill bit for the thread and the degree of profile filling. For custom threads, use manual entry: enter "M12x1.75" or "1/4-20".

Two modes:

• Cutting tap—classical formula Ø = D − P. Gives ~75–80% filling in steel. Universal for most operations.
• Roller (forming) - larger hole, less filling (~55–65%). No chips, higher thread strength, but requires lubricating coolant and precise hole diameter.

What is % fill and why is it important?

Fill rate is the proportion of the theoretical thread profile height that the tap actually cuts. At 100% filling, the cutting moment is maximum - the tap breaks. At 60%, the shear strength of the thread is ~95% of 100%, and the torque is reduced by 40–50%.

Rule of thumb: for steel and cast iron keep it at 70-80%, for stainless steel and heat resistant materials 60-68%, for aluminum up to 85%.

Tap or roller: when to choose which

The tap is suitable for any materials, through and blind holes. In blind holes, spiral flutes are required - they throw chips upward, rather than compacting them at the bottom.

The rolling tool is used in ductile materials (aluminum, mild steel, copper) where there is no room for chips or where increased thread strength is needed. Requirements: lubricating coolant with EP additives, rigid clamping, exact hole diameter ±0.02 mm from the calculated one.

Frequently Asked Questions

Why did the tap break at a high percentage of filling? The cutting torque increases disproportionately with filling. When moving from 75% to 85%, the torque doubles. In stainless steel or heat-resistant ones, this is a guaranteed bummer. The solution is to increase the drill Ø by 0.05–0.1 mm.

Is it possible to drill with the same diameter for a tap and a rolling tool? No. For a rolling tool you need a hole 0.3–0.6 mm larger than for a cutting tap with the same thread. Otherwise, the rolling tool will not fit in or will break during the first pass.

How does Metric differ from UNC/UNF in calculation? Step only: Metric sets the pitch in mm, UNC/UNF - in TPI (twists per inch). The calculator automatically converts TPI to mm (P = 25.4 / TPI) before calculating the diameter.

We supply taps, rolling tools and tools for thread cutting from the catalog. We select according to the material and tolerance range 6H/6G.