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Why Does My Spiral Point Tap Keep Breaking? 

Wondering why your spiral point tap keeps breaking? Learnn the top causes, real workshop failure data, and proven solutions to stop repeated tap breakage in CNC tapping.
Spiral point taps (also known as gun taps) are the most popular tools for machining through-hole internal threads in mass production. They eject chips forward, offer fast cutting speed, and deliver stable thread quality. However, many CNC shops face a frustrating issue: spiral point taps keep breaking unexpectedly, causing high scrap rates, downtime losses, and rising tool costs.
Most tap breakage is not caused by poor tool quality. Based on 2026 CNC workshop failure statistics, 92% of spiral point tap breakage comes from incorrect parameters, wrong pilot holes, and improper chip evacuation. This arrticle breaks down the core reasons with real production data and practical fixes.

1. Undersized Pilot Hole (No.1 Cause of Breakage)

Many operators use standard drill sizes without adjusting for material hardness. A slightly smaller pilot hole increases ta pping torque sharply, overloading the spiral point tap’s flutes and shank.
Verified production data (M6×1.0 tap):
Using 4.8mm drill (undersized): torque increases by 38%, tap breakage rate jumps to 7.1% Using standard 5.0mm drill: breakage rate drops to 0.3%
For aluminum and mild steel, keep standard drill sizes. For stainless steel, use a +0.05mm oversize pilot hole to reduce torque load.

2. Chip Compaction & Forward Clogging

Spiral point taps push chips forward out of the hole. In deep through holes (depth ≥2.5×D), long chips twist and compact at the hole exit. Chii ps cannot discharge freely, locking the tap and causing sudden fracture.
A metal parts factory reported frequent breakage on M8 deep through holes in 6061 aluminum. Inspection showed 67% of failures were caused by tangled forward chips.
Fix: Reduce cutting speed by 20% for deep holes and apply high-flow tapping oil to break chip continuity.

3. Excessive Cutting Speed & Dry Tapping

Spiral point taps run fast, but over-speed generates extreme heat and tool wear. Dry tapping without lubrication leads to material adhesion, flute blockage, and thermal cracking.
Best speed benchmark data:
  • Aluminum: 80–100 m/min
  • Mild steel: 50–60 m/min
  • Stainless steel: 25–35 m/min
Running stainless steel above 40 m/min increases breakage risk by 4x.

4. Misalignment & Spindle Runout

Spindle runout over 0.02mm creates lateral force during tapping. Spiral point taps have thin flute structures and cannot withstand eccentric cutting force. Even minor misalignment causes fatigue breakage after 200–500 holes.

5. Inappropriate for Blind Holes

This is a common mistake. Spiral point taps eject chips forward. If used on blind holes, chips accumulate at the hole bottom with no exit space. Never use spiral point taps for blind holes — use spiral flute taps instead.

Real Workshop Case Study

A automotive hardware manufacturer suffered continuous spiral point tap breakage on M5 through-hole steel parts, with 5–8 broken taps per day.
Root causes: Undersized pilot holes + no speed reduction for deep holes.
Solutions applied:
  • Updated pilot hole size to standard tolerance
  • Reduced tapping speed by 15%
  • Switched to dedicated sulfur-based tapping oil
Result: Tap breakage rate dropped from 6.8% to 0.2%, tool lifespan increased 3.2x, and daily tool cost reduced significantly.

Quick Checklist to Stop Tap Breakage

  • Use correct pilot hole size matched to material
  • Avoid spiral point taps on blind holes
  • Reduce speed for deep holes and stainless steel
  • Check spindle runout regularly
  • Use professional tapping lubrican
Spiral point taps do not break randomly. The main triggers are wrong pilot holes, chip clogging, over-speed cutting, and misapplication. By standardizing your tapping parameters and operation rules, you can eliminate almost all tap breakage issues, lower scrap rates, and maximize tool service life.
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