By: Akshay Rajole
In precision machining, tapping is a fundamental operation. But selecting the right tap isn’t just about matching thread size. It’s about fit, form, function, and feasibility.
Today, I faced a real-world issue: an M2 tap requirement where only 11.5 mm of standard thread length was available on the tap shank—not enough for full thread depth in the part. It’s a common frustration for machinists. So let’s break this down systematically.
🔍 What Is a Tap?
A tap is a tool used to cut internal threads in a drilled hole. Taps vary in type, geometry, material, coating, and dimensions. Choosing the correct tap requires understanding the job’s threading depth, hole access, material machinability, and tool holding constraints.
🧰 Types of Taps
Tap Type | Purpose |
|---|---|
| Hand Tap Set (Taper, Plug, Bottoming) | Used for manual or slow machine tapping |
| Spiral Point (Gun Tap) | Pushes chips forward – ideal for through holes |
| Spiral Flute Tap | Pulls chips back – ideal for blind holes |
| Form Tap (Roll Tap) | Cold-forming, chipless, used for soft ductile materials |
| Machine Tap | Designed for CNC or automated tapping |
| Thread Forming Tap | No chip production; stronger threads but needs precise hole size |
📏 Key Tap Dimensions You MUST Consider
Parameter | Importance |
|---|---|
| Nominal Size (M1–M18) | Defines the thread pitch and major diameter |
| Thread Length | Depth of thread – ensure it matches your part’s threading requirement |
| Shank Diameter | Affects tool holding; must match your chuck or collet |
| Square Size | Critical for hand taps or manual tapping devices |
| Overall Length | Impacts machine clearance |
| Thread Entry Type | Tapered (longer lead-in) vs. plug vs. bottoming |
| Material & Coating | HSS, HSS-E, carbide, TiN, TiCN, etc., depending on work material |
| Tolerance Class | Example: 6H is ISO standard fit – tighter or looser fit may be needed |
📘 Standard Metric Tap Data (M1 to M18)
Size | Tap Drill (mm) | Thread Pitch (mm) | Thread Length (mm) | Overall Length (mm) | Shank Dia (mm) | Square (mm) | Notes |
|---|---|---|---|---|---|---|---|
| M1 | 0.75 | 0.25 | 5 | ~40 | 1.8 | 1.4 | Very delicate |
| M1.6 | 1.25 | 0.35 | 6 | ~45 | 2.0 | 1.5 | Thin jobs |
| M2 | 1.6 | 0.4 | 6–8 | ~45–50 | 2.5 | 2.0 | Common for small screws |
| M2.5 | 2.05 | 0.45 | 8 | ~50 | 3.15 | 2.5 | Limited shank length |
| M3 | 2.5 | 0.5 | 8–10 | ~55 | 3.5 | 2.7 | Machine & hand use |
| M4 | 3.3 | 0.7 | 10–12 | ~60 | 4.5 | 3.5 | Widely used |
| M5 | 4.2 | 0.8 | 12–14 | ~65–70 | 5.0 | 4.0 | Watch for chip evacuation |
| M6 | 5.0 | 1.0 | 14–16 | ~70 | 6.0 | 4.9 | Good for form taps too |
| M8 | 6.8 | 1.25 | 16–18 | ~80 | 6.0–6.3 | 5.0 | Standard across industries |
| M10 | 8.5 | 1.5 | 18–22 | ~90 | 7.1–8.0 | 6.2 | CNC ready |
| M12 | 10.2 | 1.75 | 22–24 | ~100 | 9.0 | 7.0 | Needs high torque |
| M14 | 12.0 | 2.0 | 24–28 | ~110 | 11.2 | 9.0 | Not widely stocked |
| M16 | 14.0 | 2.0 | 28–30 | ~120 | 12.5 | 10.0 | Big applications |
| M18 | 15.5 | 2.5 | 30+ | ~130 | 14.0 | 11.2 | Form taps not recommended |
Note: Always verify manufacturer-specific data. Some high-performance taps (like Emuge, OSG, Guhring, YG) may vary in dimensions and tolerances.
⚠️ Real-World Issue: Limited Shank Length
In today's case, we had an M2 tapping requirement, but the standard shank length was only 11.5 mm, which was less than the tapping depth needed. Here’s what you should consider:
✅ Solutions:
- Use Extended Tap: Many brands offer long shank taps (DIN extended series).
- Modify Tap: Risky, but if you're experienced, grinding back the thread relief carefully.
- Use Thread Inserts: Drill a larger hole and use M2 helicoil.
- Use Tapping Head With Depth Control: Useful for repeated tapping with limited clearance.
✅ Pro Tips Before Selecting a Tap
- Always check thread depth in your part vs. tap thread length.
- Use lubrication suitable for the material (cutting oil, tapping paste, etc.).
- Avoid over-tapping – chips can clog the flutes and break the tap.
- For blind holes, spiral flute or form taps work better than gun taps.
- When tapping harder materials, opt for HSS-E (Cobalt) or carbide taps.
- Match tap drill size accurately – too small = breakage; too big = weak threads.
🔧 Final Thoughts
Tapping may look simple, but it’s one of the easiest ways to ruin a part if tap length, diameter, or type are wrong. Always validate your tool data and machine parameters before committing to the cut.
In high-precision or difficult-access jobs (like in die-mould or aerospace parts), standard taps often fall short, and special long-shank or stub-length taps become necessary.
Visit our website at https://www.projectnation.in/services and contact us today to discuss your next project. We look forward to partnering with you and contributing to your success.