Thread cutting: metric thread and the correct core hole

Thread is one of the most common details on a part and also one where small errors become expensive. If the core hole is drilled incorrectly the thread is either too loose or so tight that the tap breaks off inside the part — and a broken hardened tap is difficult to get out. Being able to choose the right thread, the right drilling dimension and the right method is therefore basic knowledge that you use again and again.

§The metric ISO thread

The most common thread in metalwork is the metric ISO thread indicated with M followed by the nominal diameter — for example M8. Each size has a standard pitch (coarse thread): M5 has pitch 0.8 mm M8 has 1.25 mm and M10 has 1.5 mm. The pitch is the distance between two thread peaks and determines how 'coarse' the thread is. There are also fine threads with smaller pitch where finer adjustment or greater holding power in thin walls is needed.

§The core hole: the rule and the table

Før du kan skære et indvendigt gevind, skal der bores et kernehul (gevindbor). En brugbar tommelfingerregel er, at boredimensionen ≈ den nominelle diameter minus stigningen. For M8 (stigning 1,25) giver det cirka 6,8 mm, for M10 (1,5) cirka 8,5 mm og for M5 (0,8) cirka 4,2 mm — værdier, der stemmer med de officielle gevindtabeller. Bruger du de rigtige tabelværdier, får gevindet fuld bæreflade uden at overbelaste tappen. Bor for lille, og tappen sætter sig fast; bor for stort, og gevindet bliver svagt.

§The methods for cutting the thread

Internal threads are usually cut with a thread tap external threads with a thread cutter (die set). On the lathe external and internal threads can be thread turned with a thread tool in multiple chips. On milling and machining centres thread milling is increasingly used where a small thread milling tool screws itself in a spiral — it reduces the risk of broken tools in the hole and can make both right and left-hand threads plus different diameters with the same tool. Thread forming (without chips) is a fourth option in soft formable materials.

§The classic pitfalls

Most thread errors have few causes: wrong core hole, lack of lubrication, misaligned start or too little chip breaking when tapping blind holes where chips pack together and break the tap. In blind holes you must use a tap that feeds chips upward and pull back along the way to break the chip. With thread turning the start angle and clean, correctly ground thread steel are critical. Good lubrication is cheap insurance in all methods.

• 01Choose bore dimension from table — check with 'diameter minus pitch'
• 02Always lubricate when tapping and threading in metal
• 03In blind holes: use chip breaking and pull back to clear chips
• 04Thread milling reduces the risk of broken tools in the hole
• 05Check thread with a thread plug or thread gauge

“A thread is only as good as the hole you cut it in.”