Nine Common Reasons for Tap Breakage – Insights by Duyang CNC
A tap is a tool used to cut internal threads. It can be categorized by shape (spiral flute vs. straight flute), usage (hand taps vs. machine taps), measurement system (metric, imperial, UNC), and origin (imported vs. domestic). Taps are one of the most essential tools for machining internal threads in manufacturing. Below, we break down the nine most common reasons for tap breakage:
Inferior tap quality is one of the primary causes of breakage. Factors such as material quality, tool design, heat treatment, machining accuracy, and coating quality can all lead to failure.
Design flaws: Sharp transitions in cross section or lack of rounded corners cause stress concentration, leading to fractures.
Heat treatment issues: Improper quenching (e.g., lack of preheating, overheating, or premature cleaning) can cause microcracks.
This is also a key reason why some domestic taps underperform compared to imported ones.
For materials with high hardness, use high performance taps such as cobalt highspeed steel taps, carbide taps, or coated taps.
Different tap designs (e.g., number of flutes, flute angle, and size) are suited for different applications. Choose based on specific needs.
As new and difficulttomachine materials emerge, selecting the right tap for the material is critical. The "most expensive" or "imported" tap is not always the best-the most suitable one is.
For example, when cutting an M5×0.5 thread in ferrous metal, a 4.5mm drill should be used for the pilot hole. If a 4.2mm drill is used instead, the tap will undergo excessive cutting force and likely break.
Always select the correct drill size for the tap type and material. If an exact match isn't available, choose the next larger size.
Impurities in the workpiece, such as hard spots or air pockets, can cause sudden imbalance and break the tap.
The machine tool and holder must meet the precision requirements of the tap. Poor concentricity-misalignment between the spindle axis and the pilot hole-causes excessive torque during tapping, leading to breakage.
Lowquality cutting fluids or lubricants can result in poor thread quality (e.g., burrs) and significantly reduce tool life. Many companies using imported tools and machines have realized this through experience.
Too slow: Reduces cutting efficiency and increases surface roughness, leading to poor thread accuracy and burrs.
Too fast: Increases torque and causes breakage.
Recommended cutting speeds:
General steel: 6–15 m/min
Hardened steel: 5–10 m/min
Stainless steel: 2–7 m/min
Cast iron: 8–10 m/min
Smaller taps allow higher speeds; larger taps require lower speeds.
Many operational issues require judgment and feedback from operators. For example:
In blind holes, failing to reduce feed rate as the tap approaches the bottom.
Forcing the tap when chips are not cleared properly.
Improving operator training and responsibility is crucial.
As shown above, tap breakage can stem from various factors: the machine, fixture, workpiece, process, tool holder, or the tap itself. Identifying the true cause requires onsite investigation and analysis. A skilled and responsible operator must observe, analyze, and respond to realtime conditions to prevent tool failure.
