Graphite and CFRP machining

High speed machining is common practice in many workshops generating electrodes through high speed, which are then used in SEDM. Cutting speeds are so as high as the machine can, using very high speed spindles (40.000 rpm) although with little power.

Graphite is very soft (he’s the pencil mogul) yet its character is abrasive thus we recommend using diamond coated tools achieving a very high duratioon. Another problem is the dust generated and must be aspirated effectively (specially problematic in CFRP, carbon fibber reinforced plastics), otherwise one can infilter CNC, and other electrical elements causing short-circuits. Plus he has an abrasive effect which can lead to damage of the machine carriage.

Fig.(Left.) High speed milling of a graphite electrode. (Right.) Image of same. Centre Detail of introduction in an electroerosion machine.

Machining of thermo-resistant alloys

Thermoresistant alloys are also called superalloys, two examples are nickel (Inconel 718) and cobalt (Haynes 25). Regarding their machining, they are even worse than titanium, A very high cutting machine speed for these is 80 m/min. Therefore, their machining is performed on conventional machines, and is by no means a high speed process. The term conventional does not mean a low quality machine, but rather it does not have spindle over 6000 rpm nor does it work with feeds over 4 m/min.

The added value of components manufactured in these alloys is very high and the machines used usually have a very high precision, therefore high range machines and a lot of precision. If you will allow a simile, i.e. if one could choose between a high speed car of “I want and can’t have’ or a high-end German one, which would prefer ?. Machining of superalloys was the purpose of the study using hybrid machining techniques, as shown in Fig. and will be commented in the corresponding.

Fig. Cobalt alloy machining at the Univeristy of the Basque County with plasma asistance.

Machining of titanium alloys

With coated hard metal tools machining at cutting speed around 200 m/min is currently common, against the 40 m/min applied 8 years ago. Thus, it is not truly a high speed machining but a much ‘quicker machining’. Diameters of tools used are between 4 and 20 mm, therefore the milling machines do not require spindles over 6000 rpm.

Titanium alloys present several problems:

- They have very low thermal conductivity, and therefore heat concentrates in the cutting area.

- High temperatures in the contact area between tool/chip and the high chemical reactivity of the titanium alloys with most tool materials, are the main causes for the rapid crater wear.

- The low elasticity module of these alloys causes flexions in the part, particularly on thin walled parts. This causes large inaccuracies on the finish and enables machining instability

Fig.11 Aeronautic engine components (photo courtesy of Volvo Aero).

Iron casting machining in the die stamping sector

In this sector HSM is located exclusively in the superfinish operation of the matrizes pursuing a specific object, i.e. reduction of maximum roughness ( Rt) of surfaces with values of 10 microns or less. As feeds can reach 5 to 10 times higher than conventional machining, it offers the possibility of increasing the number of passes to the same extent for the same finish time. The result is a better quality surface, reducing subsequent manual polishing tasks, which might imply almost 30% of the total mould manufactuer.

Machines used are gate type with 5 axes. However, they do not usually machine with the 5 axes simultaneously, but simply orientate headstock and machine after that. As they are superfinishing operations with ball-end tools and over-thicknesses of 0,2 mm we would be talking about a machining process similar to that of tempered steels regardig their physical nature. Superfinishing times with HSM are very long, 39 o 40 hours a big die. This is why it is important for the process to be highly reliable and the tool unlikely to break or wear during the operation. The iron castings used like the globular type GGG70, are easy to machine. Cutting speed reaches 400 m/min with coated hard metal tools and up to 1000 m/min with the PCBN. In Fig. 10 we can see a machine in progress and a partially mechanised boot.

Fig. Milling a die and aspect of this die prior to the operation.

Manufacture of forge dies and recovery thereof

This sector has traditionally used shinking electrodischarge (SEDM) for the manufacture of forging dies in treated steels of approximately 40 HRC of hardness. Nevertheless, the simplicity of the cavities, wide tolerances and roughness requirements to be reached have made this one of the high speed milling star applications.

In the last eight years there has been a real migration from (SEDM) to HSM. In this sector, development times of new series are critical given the high competition between forgings, thus the reduced HSM process times is a highly important factor. In Fig. we can see two die cases, one medium-sized and the other small. Both are high speed machined in under 2 hours from treated steel (already hardened) to over 40 HRC. High speed milling also allows dies to be remachined for their recovery. Once used in the forging process some parts of the die are worn out, so welded material is added. After this process it is remilled at high speed, in this case a somewhat uneven material which co-exists with the original, i.e. additions and even over-tempered areas. The final result is a die ready for forging again.

Fig.9 Forging die of securing element and a suspension bearing.

Machining treated or tempered steel in mould sector

The entry of high speed in this sector has perhaps been a decisive factor in the rapid spread of this technology, since number of users is high yet company size small. I.e. there is a very varied demand which for machine tool manufacturers represents a clear target sector and numerous. If HSM had continued being exclusively for the aeronautical sector it would have had its heyday in the second half of the 1990s. The die and mould sector means talking about numerous plastic injection companies of parts of all sizes and applications, companies dedicated to aluminium and zamac injection; and finally those dedicated to forging. There seems to have been a reciprocal effect between cutting process development and machine tool performance.

In the 1990s tools were developed enabling tempered steel cutting conditions to be increased (@ 50HRC) beyond those considered conventional. These tools were and are of submicrograin hard metal, coated in TiAlN, undoubtedly the kings of machining today, or the PCBN tools PCBN (Polycrystalline Cubic Boron Nitride). Using these tools cutting speeds can be increased 4 and 5 times. This cutting speed obliges the machine to have a spindle capable of spinning at high speed (> 15000 rpm). This spindle rotation speed, together with feeds per tooth recommended for tools implies the machine must maintain working feedrates higher than usual, i.e. greater than 5 metres/minute. Moreover Numerical Control must control axes which are interpolated with sufficient precision. Therefore a machine with very high performances is required, called “ high speed machine ”. This machine has a high speed spindle, a CNC capable of governing spatial movements at high feeds and be very rigid to achieve good precision.

Fig. Small plastic injection mould machined in HSM.

Growing industrial demand for these machines has led to rapid development of different machinery aspects and subsystems like electro-spindles, the axis drives new structures equipped with greater robustness, etc. Thus, machines with superior technology to the conventional have appeared. The new machines also open new application possibilities and substantial improvements in the process, like greater cutting stability, greater contour precision, possibility of machining on 5-axis simultaneously, etc.

Fig. Moulds machine on 3 and 5 axes machines.

n conclusion, if the ‘egg’ came first, the possible new cutting speeds in the ‘proces’, subsequently was the ‘chicken’ i.e. the ‘high speed milling machine’ equipped with features highly superior to the conventional. And at the same time, these new features enable greater process performance, opening new perspectives. Therefore, we find ourselves in a spiralling improvement process aimed at seeking ‘global machining solutions’ with greater productivity and precision, not to mention capable of generating greater added-value for the user.

The high speed milling of tempered steel moulds is centred on the finishing operation with a ball-end mills. In this phase excess material of 0.2 or 0.3 mm is eliminated. Cutting geometry is reflected in Fig. 8. Due to the complex geometry of the cavities ball-end mills whose diameter should not exceed 20 mm., must be used. If one bears in mind the slopes of the shapes to be generated vary between 0 and 90º inclination, one can conclude reaching an effective cutting speed of 300 or 400 m/min (at point A of the figure) requires head rotation speed to exceed 15,000 rpm. However, effective cutting speeds have a value of 200 to 400 m/min, not 4000 m/min as Solomon claimed (see Fig.). Therefore, the chipping process at these speeds is similar to conventional without variation of basic phenomena.

Fig. Cutting speed ratio with axial depth and slope.

In conclusion, high speed milling of hardened steels is ‘a conventional process’ from a thermophysical viewpoint, but performed on a ‘high speed machine’, which machines small chip thicknesses much quicker than the conventional one. Nevertheless, in this case we should forget numerous theories (like Solomon’s) which are found in many informative articles which may lead to confusion.

Another aspect to highlight is that it is currently becoming difficult to clearly separate a high speed machine from a conventional one for the mould world. Some industrial solutions have even appeared like spindle machines with direct coupling of motor and spindle which reach 12,000 rpm., being a cheaper and more robust option for multiple sector applications.

Machining magnesium alloys

These alloys are even softer than the aluminium ones and so easier to machine. The most widely known is AZ91, which is cast and given its lightweight is used in manufacturing parts previously made from aluminium.

The main problem posed is the inflammability of the chips and problems of possible explosion of stored chips, therefore it is a question of safety. Cutting speed may be higher than for aluminium.

Machining aluminium alloys

Undoubtedly this kind of machining is close to the physical high speed concept, since cutting speeds can reach a value of 2000 m/min, or higher when using milling plates whose diameters exceed 50 mm. At this speed chip generation is different from conventional, mainly because almost all the heat generated by the deformation energy inherent in the chipping process is evacuated with the same, which is highly positive for both tool and part.

We could say lightweight alloys is the most traditional field of high speed machining, known since the 1970s and applied to the fuselage component machining sector. In fact the first systematic HSM studies were in the aeronautical field.In the case of aluminium alloys, there are two different cases as per alloy type:

- Aeronautical alloys, particularly the 2000 or 7000 series, called malleable or wrougth. Easily machinable, used in component construction obtained by eliminating a large amount of chips from an initial prismatic block. A small component example is shown in Fig. 5 (left).

- Cast alloys (series 3xx), used mainly in car engines (blocks and pistons), are highly abrasive because they contain silica. The typical operation is planing and finish of a cast part very similar to the final one, with little chip volume. An example is the block shown in Fig.

Airframe component. Aluminium Aluminium engine block.