Monday, September 15, 2008

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 90ยบ 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.

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