Pre-drill core hole for threading

Anyone who wants to drill or cut an internal thread must pre-drill a core hole. Since the later threads change the diameter, the size of the hole must be precisely pre-drilled. It is slightly smaller than the size of the screw, which is to be screwed later.

Rule of thumb and ISO list

The selection of the correct drill size determines the future thread. The core hole must have exactly the diameter that will later have the inwardly directed Windungskanten. Many artisans calculate the drill size with the rule of thumb to multiply the thread size by 0.8.

A safer way to determine the exact diameter for pre-tapping before tapping is the so-called ISO table. It assigns a drill size to each thread size. The dimensions do not always match the rule of thumb.

Gradient and nut height

The calculation factor is the thread pitch in the metric ISO thread table. The size is the way a thread turns. It is also known as pitch. In addition, the table indicates the required nut height for a subsequent screw connection. This can be an important piece of information when choosing the diameter for pre-drilling. If the height projection of the nut is limited for technical reasons, a smaller thread diameter can be selected.

Further dimensions in the metric ISO table according to DIN 13-1 are helpful for the planning of thread cutting. They play a subordinate role for pre-drilling and home improvement.

Thread size, core hole drill and nut height

M1 / 0.75 mm / 0.8 mm
M2 / 1.6mm / 1.6mm
M3 / 2.5mm / 2.4mm
M4 / 3.3mm / 3.2mm
M5 / 4.2mm / 4.0mm
M6 / 5.0mm / 5.0mm
M8 / 6.8mm / 6.5mm
M10 / 8.5mm / 8.0mm

A total of 21 thread sizes are listed in the complete ISO metric table.

Special drill sizes

The crooked drill sizes are not included in a normal drill set. They are often sold as special sizes under the name taps in retail sale in hardware stores. After determining the appropriate drill bit size, pre-drilling may begin before threading.

Pre-drilling with drill press

1. Fix workpiece

If possible, fix the workpiece in such a way that it can be reached with a pillar drill. For freehand pre-drilling, a vice is suitable. Grain the drill hole point.

2. Apply drill

Place the drill bit on the grained cavity of the workpiece and start the pre-drilling at a slow speed.

3. Do not tilt

If you are not using a pillar drilling machine, make sure that you drill straight ahead and do not tilt the drill.

4. Cool the hole

Drill bored milk, cutting milk or oil into the drill hole. In addition, the drill is facilitated milling.

5. Lower the edges

If you slightly widen the top edges of the core hole after pre-drilling with a countersink, then threading is easier.

Tips & Tricks

On slide calipers of good quality, the most important ISO dimensions are usually engraved. Thus, the corresponding drill size for pre-drilling can be read off immediately using a measured bolt thickness.

Video Board: How to tap threads using a drill