高速切削加工组合机床英文文献和中文翻译(2)
It is very difficult to balance the tool perfectly, so for the optimal balance is usually accepted the point beyond which further improvements in tool balance do not improve the accuracy or surface finish of the workpiece. The tool unbalance acceptable for the process is determined by the cutting forces in the cut, the balance condition of the machine, and the range in which these two aspects of process affect each other. 5 Machine tools for HSM – requirements Even though high speed spindle options for conventional machining centers has been available for some time, it is only recently that machine tool designers and engineers have been developing the machines for HSM. As it was noted previously, HSM requires fitting of many parameters specified in Table 4, which are connected with the machine tools. Below are some typical demands on the machine tool and the data transfer in HSM (ISO/BT40 or c omparable size, 3 – axis). Table 4. Demands on the machine tool and the data transfer [2]. ?? Spindle speed range <= 40 000 rpm ?? Spindle power > 22 kW ?? Programmable feed rate 40 – 60 m/min ?? Rapid travels < 90 m/min ?? Axis dec./acceleration > 1g (faster w. linear motors) ?? Block processing speed 1 – 20 ms ?? Data flow via Ethernet 250 kbit/s (1 ms) ?? Increments (linear) 5 – 20 ?m ?? Circular interpolation via NURBS (no linear increments) ?? High thermal stability and rigidity in spindle – higher pretension and cooling of spindle bearings ?? Air blast/coolant through spindle ?? Rigid machine frame with high vibration absorbing capacity ?? Different error compensations – temperature, quadrant, ball screw are most important ?? Advanced look ahead function in the CNC 6 Machining methods for the die and mould manufacturing Practically, HSM is used to reduce the costs of workpiece production. Such a case takes place when machining press dies or moulds. As it is known dies consist of cavities in various shapes, with the dimensions and numerous radii sizes of corners. As an example methods for machining of a cavity below are described. Based on experience, or other production information, the surface machined, can be split up in segments. Each segment can be machined with one set of insert edges. This technique can be used both for roughing and finishing. It gives several benefits, namely [2] : better machine tool utilisation – less interruptions, less manual tool changing, higher productivity, i.e. easier to optimise cutting data, better cost efficiency – optimisation vs. real machine tool cost per hour, higher die or mould geometrical accuracy, which means the finishing tools can be changed before getting excessive wear. 6.1. Methods for machining a cavity There are several methods used for machining the cavity. One of them is to pre-drill of a starting hole. Corners can be pre-drilled as well. This method is not recommendable, because a special tool is needed. When the cutter breaks through the pre-drilled holes in the corner, the variations in the cutting forces and temperature appear negative from a cutting point of view. When using pre-drilled holes the re-cutting of chips also increases, (Fig. 5). In the second method a ball nose end mill is applied. Thus it is common to use a peck-drilling cycle to reach full axial depth of the cut and then mill the first layer of the cavity. This is repeated until the cavity is finished as shown in Fig. 6 [2]. One of the best methods is linear ramping in X/Y and Z to reach a full axial depth of the cut [2]. The inclination can start both from in to out or from out to in. It depends on the geometry of the die or mould. The main problem is how to evacuate of the chips in the best way. Down milling should be done with a continuous movement and continuous cutting. It is important to approach with ramping movement or even better with even circular interpolation, during changing to a new radial depth of cut. (Fig. 7)
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