Clean threads start with the tapping drill hole

What is a tapping drill hole and how do you calculate the size?

The tapping drill hole is the pre-drilled hole into which the internal thread is subsequently cut. The tap cuts the thread flanks out of this hole. To ensure that the finished thread runs cleanly and holds reliably later, the diameter must match the thread size and pitch exactly.

For metric ISO threads, a simple rule of thumb is useful in everyday workshop practice:

An example:

• Thread: M10 x 1.5
• Nominal diameter: 10 mm
• Pitch: 1.5 mm
• Calculation: 10 mm minus 1.5 mm
• Tapping drill size: 8.5 mm

The correct tapping drill size for M10 x 1.5 is therefore 8.5 mm.

The complete thread designation is important. M10 is not automatically the same as every other M10 thread. An M10 coarse thread usually has a pitch of 1.5 mm. An M10 x 1.0 fine thread, on the other hand, has a smaller pitch. This also changes the tapping drill size: instead of 8.5 mm, it is approximately 9.0 mm.

You should therefore always check the following briefly before drilling:

• Which thread size is required?
• Which pitch is specified?
• Is it a coarse thread or a fine thread?
• Is there a drawing, table or tool specification?

The rule of thumb is particularly practical when time is short. However, it does not replace the tapping drill table in every case. Especially for fine threads, inch threads, pipe threads, special threads, series production or safety-critical components, you should always work with the appropriate table values.

So that you do not have to search through a table yourself, we have stored all values in our tapping drill calculator. Simply select the thread system and size – whether metric coarse or fine thread, inch threads such as UNC and UNF, or special threads such as pipe or PG threads – and the correct tapping drill size appears immediately.

In short: For simple metric coarse threads, the rule of thumb provides good guidance. As soon as greater precision, a special thread or higher load capacity is required, the tapping drill table is the reliable basis.

Which tapping drill sizes are common for metric coarse threads?

Some tapping drill sizes are particularly common in practice. These include:

These values apply to metric coarse threads. For fine threads or special threads, the appropriate tapping drill table should always be used.

Too large or too small: What happens when the tapping drill size is wrong?

The tapping drill size must match the thread. Even small deviations can cause problems later. A tapping drill hole that is too small is not the only critical case. A hole that is too large can also weaken the connection.

If the tapping drill hole is too small, the tap has to cut more material. The torque increases, chip formation becomes more difficult and the tool is subjected to greater loads. This can quickly become critical, especially with blind holes, stainless steel or tough materials.

Typical consequences include:

• high force required during tapping
• poor chip evacuation
• rough or damaged thread
• increased wear on the tap
• tool breakage inside the component

If the tapping drill hole is too large, the tap often initially runs more easily. However, the problem becomes apparent later in the finished thread. The thread flanks become shallower, the screw engages less effectively and the connection loses load-bearing capacity.

This particularly affects components that are screwed together repeatedly, exposed to vibrations or required to withstand high forces.

In short: A tapping drill hole should not be approximately correct. It must be selected so that the tap can cut cleanly and the finished thread will hold reliably later.

How do you prepare the tapping drill hole correctly?

For a clean internal thread, the diameter is not the only important factor. The preparation must also be correct. The tapping drill hole should be straight, clean and drilled with a suitable drill. The hole is then lightly countersunk so that the tap starts more easily and the beginning of the thread is cleaner.

For blind holes, the depth is also important. The tap needs space for the chamfer, chips and the required usable thread length. If the hole is drilled too shallow, complete thread turns will be missing at the end.

Clean preparation consists of:

• the correct tapping drill size
• a clean and straight hole
• a light countersink
• sufficient drilling depth for blind holes
• a suitable cutting fluid
• a tap positioned straight

This makes the tapping process more controlled and reduces the load on the tool.

Cutting tap or forming tap: Why the tapping drill size is not the same

A cutting tap removes material. A forming tap displaces the material and shapes it. The two processes therefore require different tapping drill sizes.

Anyone who simply uses the tapping drill table for cutting taps when working with a forming tap risks excessive forces, poor thread quality or tool breakage.

The rule is therefore: Before machining, it must be clear whether the thread will be cut or formed. Only then can the correct tapping drill size be selected.

Drilling the tapping hole and cutting the thread cleanly with VÖLKEL

The tapping drill hole is a good example of why thread cutting does not begin with the tool itself. A clean-running and reliable internal thread is only created when the hole, thread size, material and tool are correctly matched.

We offer taps for different applications, materials and thread types. From hand taps to machine taps, the important point is to select the right tool for the machining situation.

This is particularly important for:

• repairs
• individual parts
• series production
• blind holes
• fine threads
• difficult-to-machine materials
• components with highly loaded threads

The advantage is reliability: Correctly preparing the tapping drill hole and working with a suitable tap reduces tool breakage, improves thread quality and creates a connection that will hold reliably later.

FAQ about tapping drill sizes for taps