Threading in the automotive industry

Boosting output with the optimum machining technique

In the machining of automotive components, thread manufacturing would appear to account for a comparatively small share of the entire volume of cutting work involved. Under closer scrutiny, however, it becomes clear that, with the quantity of threads to be manufactured,  selecting the right machining techniques can deliver considerable savings.

To manufacture internal threads on the engine, on its attachment parts and on the drivetrain, any of a variety of techniques could be employed. Making the right choice depends on the batch size and material, the machine used and the requirements for the fastener, such as whether the thread should finish close to the bottom of the hole or whether the start of the thread should have a full profile turn.

Summary of comparisons

One alternative to the conventional tapping process is thread milling. Both of these machining techniques involve cutting the material. Another option is thread forming, a process in which material is displaced rather than removed. As a result, there are no chipremoval problems and there is no costly cleaning of components to remove swarf. The threads are produced without interrupting the grain flow of the material being machined. The thread surface is compacted and a smoother finish is achieved than with tapping. Under static load, the strength of the formed thread is identical to that of a cut  or milled thread but greater under dynamic load. For this reason, thread forming - if feasible given the ductility of the material used - has proved to be a popular technique for fastening points that will be subjected to high load.

Tapping and forming are supported by all machines. Milling, however, requires a CNC machine capable of helical interpolation. During this process, the threads are created in a circular motion, with the rotating tool simultaneously moving axially at a rate equal to the thread pitch. In the process, the thread mill plunges into the core hole exactly down to the programmed thread depth. There is none of the overrun typical of conventional tapping and, in contrast to taps and thread formers, thread mills do not have a lead chamfer. This means that thread milling can produce a thread down to the bottom of the hole without the need for any secondary operations. In addition, different tolerances can be machined using the same tool or different dimensions with the same lead. No chip root is left inside the finished thread and a higherquality surface is achieved by comparison with tapping. If the machine is able to offer high rotation speeds, considerable time savings can be achieved in the machining of large threads or thread milling in aluminium. Short chips and instantly adjustable and repeatable thread depths help to ensure a reliable threading process.

Another advantage is that relatively large threads can be milled in cored holes directly. No finish machining of the core hole is required. Virtually all materials can be machined and the tools are regrindable.

Frequently used

With large batch sizes, components are often machined on transfer lines, which restricts thread manufacturing options to tapping and forming. Here, there are usually basic feed units in the individual machining stations linked in series or in a loop. While HSS taps were used in the past, today they tend to be made from solid carbide. These taps have a substantially longer tool life, which enables cost savings to be realised not least because fewer tool changes mean shorter machine downtimes.

Other tools that are highly attractive from a design point of view include thread formers, such as the JEL®  MOREX  variants with brazed carbide inserts. With the JEL®  MOREX R, the carbide inserts are even indexable. These are popular not only for the qualities described above, but also because they feature an HSS-E shank. This is able to compensate for minor alignment errors, while the carbide material with its hard compression edges provides an extremely long tool life. In cycleoperated special purpose machines, where offset problems tend to occur, tool lives can be increased many times over.

Current challenges

In contrast to mass production, component machining, particularly in the automotive industry, has become ever more marked by considerable model variety and faster innovation cycles. This, in turn, has led to decreasing batch sizes. At the same time, the desire to reduce machining times has generated demand for ever faster machines with higher rotation speeds. In addition, modern component designs are placing increasing demands on the properties of threads and the way in which they are produced. As wall thicknesses become thinner, effective thread depths must increase or threads must be manufactured without the typical entry burr. There are the special challenges presented by specific materials or machining techniques, such as cutting with minimum quantity lubrication or dry machining. The trend, therefore, is towards production on CNC machining centres, which can be used as standalone machines or linked to form a chain. With these machines, it is possible to implement and exploit superior thread milling technology and all the advantages it has to offer. The prerequisite to this, however, is that the machine must support helical interpolation. Nowadays, this feature is available on the majority of machines and, in isolated cases where it is not, it simply needs to be activated. These machines also enable the use of combination tools, which can perform not only axial but also circular operations. They provide scope for accomplishing as many tasks as possible with a single tool.

Premium class of thread manufacturing

The KOMET GROUP offers two types of thread mill: JEL® MKG thread mills (without countersink) and JEL® MGF thread mills (with countersink). As solid carbide tools, they are suitable for working all materials. With brazed PCD cutters, they are optimised for machining aluminium. JEL®  TOMILL  (GWF) thread mills are designed for larger threads from 20 mm upwards. Carbide combination drill and thread mills produce a complete thread in a single operation, drilling the core hole with 90° countersink and then cutting the thread. The thread is milled in a circular motion, with the tool simultaneously moving axially at a rate equal to the thread pitch.

For use with various materials, combination drill and thread mills are available in solid carbide, uncoated or coated or PCD versions.
The tools can also be designed for compatibility with minimum quantity lubrication (MQL). A threeflute variant is available specifically for machining holes cored in aluminium.

The DBGF has a specialised geometry and is a purely circular milling tool that creates both the core hole and the thread turn by turn simultaneously. With its larger core cross section and special combination of drilling and threading cutting edges, the DBGF is ideal for deep threads down to 3xD in the 6 to 12 millimetres diameter range.

As part of general technical developments, machinists are faced with increasing proportions of aluminium materials. To machine these materials, cutting edges should preferably be tipped with PCD. The polycrystalline diamond tip enables highspeed cutting (HSC), which also creates great potential in the further development of machines in terms of increasing the maximum rotation speed available.

Combination drill and thread mills are often used as the central tools in JEL®  VABOS  tooling systems (variable drilling, countersinking and thread milling system). These modular tooling systems are suitable for multiple uses and, in addition to thread milling and combined drilling and threading, offer combinations for spot facing and chamfering in the same pass. The systems comprise a mount with a central tool and an arrangement of indexable inserts.