Pre-drill core hole for thread cutting

If you want to drill or cut an internal thread, you have to pre-drill a core hole. Since the later threads change the diameter, the size of the hole must be precisely pre-drilled. It falls slightly smaller than the measure of the screw, which should be screwed in later.

Footform and ISO list

The selection of the correct drill size decides on the future thread. The core hole must have exactly the diameter, which later have the inward winding edges. Many craftsmen calculate the drill size with the fist formula to multiply the threaded dimension with 0.8.

  • Also read - Thread pre-drill is precision work
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  • Also read - thread dimensions in presentation and implementation

A safer way of determining the exact diameter for pre-drilling before taping is the so-called ISO table. It has each thread size a drill size. The dimensions do not always match the rule of thumb.

Pitch and nut height

The thread pitch is added as a calculation factor in the metric ISO thread table. The size quantifies the distance that one turn of the thread travels. It is also known as the pitch. In addition, the table shows the nut height required for a later screw connection. This can be important information when choosing the diameter for pre-drilling. If the height overhang 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 planning thread cutting. They play a subordinate role for pre-drilling and DIY enthusiasts.

Thread size, core hole drill and nut height

M1 / 0.75mm / 0.8mm
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 full ISO metric table.

Special drill sizes

The crooked drill sizes are not included in a normal drill set. They are often offered as special sizes under the designation of taps in retail sale in hardware stores. After determining the correct drill size, pre-drilling can begin before tapping.

Pre-drilling with a drill press

1. Fix the workpiece

If possible, fix the workpiece in such a way that it can be reached with a pillar drill. A vice is suitable for hands-free pre-drilling. Punch the starting point for the drilling.

2. Position the drill

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

3. Don't tilt

If you do not use a pillar drilling machine, make sure that you drill straight as a pin and do not tilt the drill.

4th. Cooling the bore

Drip drilling milk, cutting milk or oil into the borehole. Milling is also made easier for the drill.

5. Lower the edges

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

Tips & Tricks The most important ISO dimensions are usually engraved on good quality calipers. The corresponding drill size for pre-drilling can be read off from a measured screw thickness.

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