Choosing a machine tap: blind holes, through holes, and the real causes of tap breakage

The tap is one of the most frequently broken, most time-costing tools in any shop. Most breakages don't come from the quality of the tap itself, but from choosing the wrong type, the wrong hole diameter, or poor chip evacuation. This article is here to make that choice clear.

Blind hole or through hole? Where will the chip go?

The first question in tap selection isn't the hole type — it's where the chip will go.

• Spiral-point (gun-nose) tap: Pushes the chip forward, into the hole. Used only in through (open) holes. Because the flutes are straight, the body is strong, allowing higher feed rates and longer tool life.
• Spiral-flute tap: Pulls the chip back, out toward the operator. The only correct choice for blind holes. The flute helix angle varies by material: around 15° for short-chipping materials, 40–45° for ductile steel and aluminum.
• Straight-flute tap: Collects chips in the flutes; works in blind holes too for powdery-chip materials like cast iron and brass.

Running a spiral-point tap into a blind hole compacts the chip at the bottom of the hole and breaks the tap. This is the most common mistake.

HSS-E or HSS-E-PM?

Property	HSS-E	HSS-E-PM (powder metallurgy)
Toughness	Good	Very good
Wear resistance	Good	Superior
Cutting speed	Standard	20–30% higher
Typical use	General steel, aluminum	Stainless, high-strength steel, serial production
Cost	Lower	Higher, usually more economical per tool life

For operations beyond a few hundred threads per year, a PM tap is generally cheaper once tool-change time and breakage risk are factored in.

Tap drill diameter: the most neglected parameter

For a standard M thread, hole diameter = thread diameter − pitch (example: 8.5 mm for M10×1.5). This gives roughly 75% thread engagement, not 100%, and that's correct — demanding 100% engagement doubles the tapping torque without meaningfully adding strength.

• Hole too small → torque increases, tap breaks
• Hole too large → thread is incomplete, bolt runs loose
• For ductile materials (stainless, aluminum), choose the hole diameter near the upper tolerance limit

Six rules that prevent tap breakage

1. Correct type: blind hole = spiral-flute; through hole = spiral-point.
2. Correct hole: pick the drill diameter from the chart, leaning toward the upper tolerance for ductile materials.
3. Tap length and bottom clearance: in blind holes, leave at least 2–3 pitches of clearance below the thread length for the tap's chamfer.
4. Synchronized tapping: use rigid (synchronized) tapping on modern machines; a floating tap holder is only needed on older machines.
5. Cooling and lubrication: tapping is a high-friction process; if emulsion isn't enough, use tapping oil or an internally-cooled tap.
6. Coating: TiN for general purpose; TiCN and TiAlN for stainless and hardened steel; for aluminum, prefer an uncoated polished tap or TiCN.

Conclusion

Tap breakage is more a selection problem than a quality problem. When hole type, material, and hole diameter are matched correctly, even a standard HSS-E tap delivers a long tool life. Get in touch to work out the right tap type and parameters for your application.