ISO 9001:2015 Certified Manufacturing
Pilot Hole Design for Metric Thread Formers
For a metric roll-forming (thread-forming) tap the pilot hole is d minus 0.0068 x % thread x P. At 65% thread an M6 x 1 needs a 5.56 mm hole, M8 x 1.25 needs 7.45 mm and M10 x 1.5 needs 9.34 mm. Enter the diameter and pitch below.
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Calculators & Converters
Maximizing Thread Performance through Displacement
Thread forming differs from traditional tapping because it displaces material rather than cutting it away. This process creates a stronger internal thread by improving the grain flow of the metal. Because no chips are produced, the success of the joint depends entirely on the precision of the pilot hole.
If the pilot hole is too small, the torque required will spike, likely breaking the forming tool. If it is too large, the crest of the thread will not form completely, resulting in a weak joint with insufficient engagement. Calculating the exact hole diameter based on your material’s ductility is the only way to ensure a reliable assembly.
The 65-75% Rule
For most industrial applications using thread formers, a 60% to 70% thread height provides the best balance between pull-out strength and tool life.
Interactive Pilot Hole Calculator
Select your nominal metric size and desired thread percentage to determine the correct drill size for your application. Use these values to prevent tool breakage and optimize joint strength.
Pilot Hole Design (Metric Formers)
Calculate recommended starting hole diameters for cold-forming (roll) taps.
Recommended Pilot Hole (65% Thread)
Millimeters
65% Thread Hole
5.60
75% Thread Hole
5.55
85% Thread Hole
5.50
Cutting Drill Ref
5.00
Pilot Hole (Scientific - Meters)
0.00e+0
*Values based on roll tap (forming) standards. Unlike cutting taps, forming taps displace material, requiring a larger starting hole.
Formula: $D = d - (0.442 \times P)$ at 65% thread (hole = d - 0.0068 x % thread x P, Machinery's Handbook).
How to Use the Thread Forming Pilot Hole Calculator
- Enter the nominal diameter d in millimeters (M8 = 8).
- Enter the thread pitch P in millimeters (M8 coarse = 1.25).
- Read the recommended hole at 65% thread, plus the 75% and 85% options and the cutting-tap drill for comparison.
- Drill to the nearest available size and check the formed thread with a gauge on the first parts.
Formula and Worked Examples
Hole = d – 0.0068 x % thread x P (the forming-tap rule in Machinery’s Handbook, written for millimeter pitch). A forming tap displaces metal instead of cutting it, so its hole is larger than a cutting tap drill (d – P).
| Thread | 65% thread | 75% thread | 85% thread | Cutting tap drill |
|---|---|---|---|---|
| M6 x 1 | 5.56 mm | 5.49 mm | 5.42 mm | 5.00 mm |
| M8 x 1.25 | 7.45 mm | 7.36 mm | 7.28 mm | 6.75 mm |
| M10 x 1.5 | 9.34 mm | 9.23 mm | 9.13 mm | 8.50 mm |
| M12 x 1.75 | 11.23 mm | 11.11 mm | 10.99 mm | 10.25 mm |
Around 65% is the usual target for formed threads: the cold-worked thread is stronger than a cut thread of the same percentage, and higher percentages raise forming torque sharply. Forming works in ductile materials (steel up to about 35 HRC, aluminum, copper alloys, stainless), not in cast iron or brittle alloys.
Impacts of Pilot Hole Precision
Frequently Asked Questions
What size hole for an M6 forming tap?
5.56 mm at 65% thread (5.5 or 5.55 mm drills are the common choice). A cutting tap for M6 x 1 uses a 5.0 mm drill.
Why is a forming tap hole bigger than a tap drill?
A forming tap pushes material into the thread crests instead of removing it, so it needs extra metal left in the hole wall. Using a cutting-tap drill with a forming tap overloads and breaks the tap.
What thread percentage should I use for roll tapping?
About 60 to 70% for most work. A formed thread at 65% is typically as strong as a cut thread at 75%, with much lower tapping torque.
Can I roll form threads in cast iron?
No. Forming needs a ductile material. Cast iron, some die-cast alloys and hardened steel above about 35 HRC should be cut-tapped.
Next Steps
- Tap Drill Size Calculator: drill sizes for cutting taps, metric and inch
- Thread Engagement Calculator: how deep the thread must be
- Tap Drill Size Chart: every UNC, UNF and metric size
- Request a quote for the fasteners in your joint.
Related Calculators
More free tools for the same job:
- Tap Drill Size Calculator: Find the tap drill size before tapping a thread.
- Metric Thread Profile Calculator: Work out metric thread dimensions for correct fit.
- Plastic Screw Pilot Hole Calculator: Design the boss for screwing directly into plastics.
- Thread Engagement Length Calculator: Find the minimum thread engagement for blind and through holes.
Troubleshooting Your Tapping Process?
Our engineering team can evaluate your material type and application to recommend the ideal pilot hole specs for your production line.