Helix cutters

On the basis of the data given in Table 1.6, the operative receptance for the particular cutting conditions considered is presented in Figs 1.18(a) and 1.18(b) for the slot and the helix cutter respectively. 

By using the theory and method discussed in the literature," the stability charts shown in Fig. 1.19 (rows I) are obtained. It must be stressed that these are valid for a particular machining process specified by the geometry of the tool/workpiece and the material properties, as specified in Table 1.2. Both relate to the bonded machine. Fig. 1.19(a) for machining with a slot cutter and Fig. 1.20(a) when a helix cutter is used. 

Fig. 1.20. Theoretical and experimental stability chart ofbonded machine with helix cutter, (a) Theoretical stability chart (kx = 0 63 X 10" N//im//im C = 0 0021 Zc = 1). (b) Experimental stability chart.

Results reported so far correspond to the slot cutter specified in Table 1.6, and used in previous work. However, tests performed by machine-tool markers generally follow the recommendations of the Machine Tool Industry Research Association (MTIRA), and these involve the use of a helix cutter. As pointed out, with the slot cutter the limiting factor of machine performance is the onset of chatter, while with the helix cutter it is both the onset of chatter and the available driving horsepower. It is clearly of practical interest to establish whether the theory of chatter allows the prediction of the stability for both types of cutter. 

For the calculation of the stability chart for the helix cutter a depth of cut ratio of d/R = 0-2 was chosen. The number of cutting teeth in continuous contact varied between = 1 and = 2-0 though, for reasons previously explained," the maximum was z<, = 1-1. It was thus taken to be z = 1. 

With these data, and those in Table 1.6, the stability chart was calculated and it is presented in Fig. 1.20(a), row I. Note the very much greatervalueof IFmoi = 24-8 mmandlF o2 = 3-5 mm compared to those obtained in the case of the slot cutter (Fig. 1.19(a)). However, the two stability charts are not comparable since they correspond to different values ofz and d/R (Table 1.6). As far as the d/R ratios are concerned, the very much smaller value of the helix cutter was necessary since otherwise the available drive power would have been exceeded, as will be seen from the experimental results. 

Fig. 1.22. Specimens for cutting tests, (a) Slot cutter, (b) Helix cutter (large width), (c) Helix cutter (small width).

The experimental stability charts for the bonded machine, determined with the slot and the helix cutters, are presented in Figs 1.19(b) and 1.20(b), respectively. A comparison with the theoretically predicted charts shown in Figs 1.19(a) and 1.20(a) indicates good correspondence, both as far as the variation of the width of cut and that of the chatter frequency are concerned. Note that the existence of two sets of unstable lobes is confirmed. Similarly, the chatter frequency of these varies as predicted. 

Figure 1.20(b) shows that the helix cutter loads the machine so much that between zN = 750 and 820 the maximum available horsepower is exceeded. This explains why in that speed range no chatter tests were carried out. 

Finally, at zN = 2335, point Z, only the higher frequency mode, i.e. unstable lobes at the higher speeds, is unstable. Thus, as Fig. 1.23 shows, even when chatter is fully developed it can consist of the superposition of two unstable modes. Similar results for the helix cutter were not possible to obtain because of the stalling of the drive motor. 

Figure 1.17(b) shows the direct and cross receptances of the cast-iron machine. From these the operative receptances are found, presented in Fig. 1.18(c) for the slot cutter and Fig. 1.18(d) for the helix cutter. The particular conditions to which these refer are specified in Table 1.6. 

Fig. 1.25. Theoretical and experimental stability charts for cast-iron machine, (a) Theoretical chart (slot cutter), (b) Experimental chart (slot cutter), (c) Theoretical chart (helix cutter), (d) Experimental chart (helix cutter).

As a further illustration, a sample of the cut obtained by the helix cutter, on both the bonded and cast-iron machines, shown in Fig. 1.26, is illustrated in Fig. 1.27, showing large width of cut at which the machine starts to chatter, as compared with the cast-iron machine. 

The extension of the 2D examination to the 3D milling cutter is shown in Fig. 15. Due to the helix angle, each point of the cutting edge is at a different rotational position on the tool and on a different height. So for an analytical calculation, the cutter has to be divided into height increments... 

A staggered-tooth side-and-face cutter, also having teeth on the periphery and on both sides, is designed for deep-slotting operations. In order to reduce chatter and provide maximum chip clearance, the teeth are alternately right-hand and left-hand helix, and each alternate side tooth is removed. Fig. 11.8(d). Staggered-tooth cutters are available in the same sizes as the plain side-and-face type. 

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