Producing Internal Threads

The entire racing powertrain, from the engine, through the clutch to the transmission, is littered with threaded fasteners. Studs and bolts are responsible for holding together everything from the most mundane unstressed cover plate to the most highly stressed cyclically loaded fasteners such as those found on con rods, crankcase main bearing studs and cylinder head studs. For every bolt there is a corresponding female thread, and for every stud there are two female threads.

Greater attention is paid to the production methods for male fasteners, as they are generally more apt to fail than their female counterparts. Machine-cut threads on highly stressed cyclically loaded male fasteners are generally frowned upon, as their durability compared to rolled threads is notoriously inferior. For many years, when it came to producing female threads in metals, there really was only one game in town, and that was to use a conventional thread tap. Before tapping, a hole is drilled at the size of the minor diameter of the internal thread, and the ’space’ into which the fastener is screwed is created by the tap, which essentially has the same form as bolt, but with multiple cutting edges.

There are two more modern methods which each have their own advantages. The first is internal thread milling. A milling cutter with the correct thread form and a given pitch, used in a CNC milling machine, is capable of producing any diameter of thread and either hand of thread of that specific pitch. The range of threading tools required to cover many different sizes of thread is therefore smaller than the number of taps required for the same job. It is also possible to get the full thread form closer to the bottom of a blind hole using thread milling than is normal with a conventional tap.

The second method is internal thread forming, which is closer to the conventional tapping process than thread milling. It requires a tool for each combination of diameter and thread pitch, as is the case with conventional thread tapping. However, the tool has no cutting edges and there is no cutting of metal involved in the process. The pilot hole for the thread forming process is larger than for conventional tapping, as the material has to ‘flow’ inwards to produce the full thread form. The process forms a characteristic ‘claw’ shape at the internal diameter, but it is at the major diameter of the internal thread that the advantages arise. The grain flow here is favourable, strengthening the material and making the internal thread more resistant to fatigue loads.

Forming internal threads (often called roll tapping) has some advantages in terms of tool longevity, but there are also some disadvantages. The process is less suitable than conventional tapping for materials with limited ductility, although this has improved with tooling design over recent years. Polymer materials which exhibit high elastic strains also tend not to perform well with thread-forming taps. The method also tends to displace material axially at the entrance to a hole. Countersinking or counterdrilling of roll-tapped holes is generally necessary to remove the displaced metal at the hole entrance, although thread-forming taps are available with features which automatically remove the offending material.

Written by Wayne Ward