Tap drill hole explained
A tap drill hole is the cylindrical pilot hole that is technically necessary in order to cut or form an internal thread in a workpiece. Its diameter (the tap drill diameter) is always smaller than the nominal size of the thread, and it largely determines the flank engagement, the process reliability and the pull-out strength of the resulting bolted joint. The rule of thumb for metric coarse threads is: nominal diameter minus pitch.
What is a tap drill hole?
The tap drill hole is the cavity produced by machining (drilling) in solid material or in a profile cross-section, which serves as the basis for creating an internal thread. Technically, the diameter of the tap drill hole corresponds to the minor diameter of the female thread.
In the context of aluminium construction profiles (slot 8 or slot 10 systems, for example) the tap drill hole has a particular relevance: the central bores (core channels) of the profiles are often extruded to a tolerance that lets them act directly as the tap drill hole for standard threads (usually M8, M10 or M12). The drilling step is then frequently unnecessary; the thread is cut or formed straight into the channel.
Purpose & key questions in practice
Designers, manufacturers and buyers regularly face the following questions:
- Exactly how large does the hole have to be?
A hole that is too small leads to tool breakage (the tap jams because of excessive friction or chip build-up); a hole that is too large reduces the load capacity of the joint (risk of the thread stripping). - Cutting (with chips) or forming (chipless)?
Is the thread to be cut or roll-formed? This changes the required tap drill diameter significantly. - How deep does the hole have to be?
In blind holes the tap drill hole has to be deeper than the usable thread, to leave room for the lead of the tap and for the chips.
Relevance in an industrial context
Getting the tap drill hole right is not a detail; it is a decisive factor for machine safety and economy.
- Standard conformity: compliance with the tolerances of DIN 336, especially on safety-relevant parts such as guard fences.
- Economy: faulty tap drill holes cause scrap, for example through taps that break off in profile frames which have already been machined.
- Joint stiffness: on aluminium profiles exposed to dynamic loads, fatigue strength depends directly on the quality of the tap drill hole.
Key components / technical elements of tap drill holes
Determining the correct diameter follows fixed physical and geometric rules.
- The formula (thread cutting):
- The countersink: before tapping, apply a 90° countersink to the nominal diameter (+10 %) to avoid a raised burr.
- The tool angle: standard twist drills 118°; for materials that are hard to machine, sometimes 135°.
Thread table for coarse threads (selection):
| Thread (M) | Pitch P (mm) | Tap drill (mm) | Typical application (aluminium profile) |
|---|---|---|---|
| M4 | 0.70 | 3.3 | Light fixings, handles, sensors |
| M5 | 0.80 | 4.2 | Connector plates slot 5/6 |
| M6 | 1.00 | 5.0 | Standard connectors slot 6/8 |
| M8 | 1.25 | 6.8 | Standard for the central bore, slot 8/10 |
| M10 | 1.50 | 8.5 | Heavy-duty connectors, levelling feet |
| M12 | 1.75 | 10.2 | Central bore slot 10/12, butt connectors |
Factors influencing selection & application
The following parameters determine how the tap drill hole is made:
| Factor | Effect on the tap drill hole |
|---|---|
| Material | Short-chipping aluminium: a standard drill is sufficient. Long-chipping aluminium (wrought alloys): sharp drills and emulsion are needed to prevent smearing. |
| Type of hole | Through hole: the chips fall out downwards. Blind hole: the chips have to be carried out along the flutes, and the depth has to provide a reserve. |
| Surface | On anodised profiles the surface is hard and electrically non-conductive. The drill has to cut cleanly through the anodised layer. |
| Tolerance | Run-out or a worn drill chuck produces out-of-round tap drill holes, and the thread will not pass the gauge. |
Benefits for companies & engineering teams
- For companies: standardised tap drill holes secure quality and reduce scrap. Longer tap life lowers tooling costs.
- For design and production: faster selection of the fasteners, and no extra design work when the profile's core bore already fits.
Practical tip for design & purchasing
- Mind the difference between cutting and forming: aluminium is soft, and forming work-hardens the material by displacing it. A larger tap drill hole is required.
- Example M8, cutting: Ø 6.8 mm
- Example M8, forming: Ø approx. 7.40 mm
- Check the profile manufacturer: the central bore is often a compromise (e.g. Ø 7.0 mm) and may need to be drilled out for high loads.
A current example (2024–2026)
In an automation cell (2025), threads were stripping on heavy-duty profiles with a central bore of Ø 10.5 mm for M12. The solution: switching to thread forming for M12. The material was compacted, producing a high-strength thread, and the pull-out forces rose.
Future trends
In 2026, sensor-based in-process monitoring (AI) measures the torque during drilling. Deviations – a tap drill hole that is too small or too large – lead to an automatic stop or to a flag in the digital twin. The error rate on pre-machined profile kits drops accordingly.
Conclusion
The tap drill hole is the foundation of a stable bolted joint. Respect the tables and tolerances and you save cost and increase safety. Check your profile system, or contact Maunsystem for technical advice.
