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    Furthermore, in common practice, it is difficult to maintain
    all the previous factors under a precise control necessary to
    obtain reproducible results.
    The materials of the parts to manufacture are another
    key element to consider. The actual tendency is to use
    materials, which allow us to obtain an optimum efficiencyfor the moulds, and dies. However, such materials present
    the inconvenience of being difficult to machine [3]. Starting
    from the dimensions and the geometry of the mould or die,
    the tendency is also to perform all the machining operations
    on a harden material with the bulk part or, alternatively,
    perform the last semi-finishing or finishing operations with
    the harden surface [4].
    From what has been said previously, one can see that it
    is necessary to have a deeper knowledge about the optimum
    operation conditions, which will permit us to assure a cor-
    rect dimensional precision and a good surface roughness.
    In order to achieve this, the technique of factorial design is
    to be employed as it will allow us to determine theoretical
    models making it possible to select the optimum operation
    conditions in a particular range of values only doing a small
    number of experiments.
    2. Manufacturing conditions
    The optimum selection of cutting conditions is very im-
    portant in manufacturing processes as these ones determine
    surface quality and dimensional precision of the so obtained
    parts. Thus, it is necessary to know, in advance, proper-
    ties relating to surface quality and dimensional precision by
    means of theoretical models which allow to do some predic-
    tions taking into account cutting conditions such as: axial
    depth of cut, radial depth of cut, feed per tooth (mm/z), and
    cutting speed, etc.
    This study is mainly focused on aspects related to surface
    quality and dimensional precision, which are the most im-
    portant parameters form the point of view of selecting the
    optimum conditions of processes, as well as economical as-
    pects. Functions making it possible to optimise parameters
    related to surface quality in HSM will be obtained by means
    of using design of experiments.
    The considered parameters are: axial depth of cut (Ad),
    radial depth of cut (Rd), feed per tooth (mm/z) (fz), and
    cutting speed (Vc). The material used for manufacturing the
    parts was a W-Nr. 1.2344, hardened steel (50–54 HRC).
    The parts have been manufactured, without coolant,
    in a HSM centre with vertical-spindle using only three
    axis (Deckel Maho DMU 50 evolution with Heidenhain
    control TNC 430 and maximum number of revolutions
    = 18,000 rpm). Fig. 1 shows the HSM centre (left) and the
    part and the tool holder (right) used in the experiments. Af-
    ter manufacturing eight parts, the manufacture was started
    with a new tool in order to assure that tool’s wear does
    not affect surface roughness. A tool holder prebalanced, of
    MST ref. DN40AD-CTH20-75 and a cutting tool of KO-
    BELCO series MIRACLE: (Al, Ti) N-coated micro grain
    carbide, two flute ball end mill VC2SBR0300, diameter
    6mm, have been used as Fig. 2 shows.
    The proposed prototype is shown in Fig. 3. As can be
    observed, this prototype has different slope variations that
    will allow us to analyse the influence of the angle in the
    roughness values. Nevertheless, although these values have
    been obtained by measuring the profile shown in Fig. 4,
    an average roughness value is given in the present work.
    Moreover, in order to analyse the influence of the type of
    milling, that is, climb milling (A) and conventional milling
    (B), each part has beenmanufactured following two different
    strategies, as shown in Fig. 3, where all dimensions are in
    millimetres.
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