**4.4 Machine limits**

562 Mechanical Engineering

(a) effective radius (b) contact tool-surface

Another weakness of this process is the surface roughness. First, the cutting speed is zero in the extreme centre of the cutting tool. Therefore, when the centre of the tool is in the cutting area, the tool smash and do not cut the material. This phenomenon affects negatively the surface roughness. The ball mill also leaves a material remain between the cutting passes, known as cusp high. The cusp high depends upon the axial (ap) and radial depth per cut

Fig. 8. Cusp high in a planar surface and contact tool-surface in free form milling

2 2 ( ) *R R R ap ef* (2)

Fig. 7. Effective tool diameter and contact tool-surface in free form milling

Where:

Where:

Ac= cup high [mm]

R = tool radius [mm]

ae = radial depth of cut [mm]

*Ref* = Effective radius [mm]

*R* = Tool radius [mm]

*ap* =axial depth of cutting [mm]

(ae), tool diameter and surface curvature (see Figure 8).

High frequency spindles, in combination with high feed rates are a more precise characterization for HSC (High Speed Cutting), especially for die and mould manufacturing. Literature mention HSC operations using feed rates as high as 20.000 mm/min. However, the feed rate varies severely along the path. These feed rate oscillations depend on two limitations: mechanical (inertia) and electronic (CNC control). In the first case, the machine does not achieve acceleration and deceleration as fast as required by a specific programmed feed rate. In the second, CNC limits the feed rate to a maximum value according to its processing capacity, considering a tool path is described by small linear segments. Feed rate oscillation happens even in the most updated CNC machines and it drastically affects machining time and surface quality. The ordinary CAM software estimates the machining time considering that the feed rate is kept constant as set in the NC program; the software does not consider the CNC and machine's limitations. Therefore, the machining time estimated by CAM becomes very inaccurate [COELHO et al, 2010].
