Next figure (Fig. 7) shows the microphotos of a chip pro- duced during X63CrMoV51 (tempered) machining.   When

Fig. 5. Regions of cutting speed for milling a different workpiece material.

vc = 50m/min (Fig. 7a), the chip-formation mechanism is the same as in the case of Ck15 and X63CrMoV51 (an- nealed, Fig. 5a) machining. Obviously, this is the   conven-

tional cutting-speed region. During machining with a cutting speed of 150 m/min (Fig. 7b) the chip is segmented and pos- sesses a typical saw-tooth shape.

In contrast to the machining of this annealed steel grade with a cutting speed of 1500 m/min, Fig. 7b, the so-called white layer appears, indicating thermal softening process dur- ing chip formation. Generally, saw-toothed chip formation is an inherent feature when machining steels in their hardened state. It can be related to the brittleness of the material and to the generation of high compressive stresses on the workpiece during cutting. Instead of the material flowing plastically dur- ing cutting, a crack will begin on the workpiece surface. This crack releases the stored energy and acts as a sliding plane for the material segments, allowing the chip segment to be forced out from between the parting surfaces. Because of the high local temperature the white layers of martensite are formed. Therefore, the thermal softening of material becomes in- creasingly important and more and more influential on the plastic behaviour of the material in comparison to the ef- fect of the strain hardening. Strain hardening is the dominant process during machining of this type of steel with a cut- ting speed of 50 m/min, but when the cutting speed reaches 150 m/min the formation of an initial crack and the thermal softening process are dominant. This is especially visible on

Fig. 6. Cross-sections of chips produced during X63CrMoV51 (annealed) machining.

photographs of the chip produced at cutting speeds of 300 and 1500 m/min (Fig. 7c and d). When the cutting speed increases more obvious layers and a thinner chip with smaller segments appear simultaneously. It is clear that during machining of X63CrMoV51 (tempered, 629 HV, 52 HRC) the high-speed region is entered when the cutting speed is 150 m/min, which can be concluded on the basis of the chip shape.

5. Quick tool-making

5.1. CAM concept by manufacturing approach

Successful HSM is always related with appropriate CAM software. We should use the same CAD–CAM system for de- velopment (modeling of a product), design (tool design) and process planning (using CAM program). In this way data transference is not complicated since their formats are not different. It is important that CAM module includes milling simulation, which enables virtual check before actual ma- chining. Simulation usually shows metal removing  dynam-

Fig. 7. Cross-sections of chips produced during X63CrMoV51 (tempered) machining.

ics and tool path. A CAM role in manufacturing depicts Fig. 8.

5.2. Choosing appropriate procedure in tool-making

In regards to product and tool design we choose one of the following machining procedures:

• HSC,

• EDM,

• combination of both procedures.

Machining of different dies made of various steels and hardness is usually done in the next sequence:

a. Machining sequence includes rough and semi-rough ma- chining followed by heat treatment and final machining of HSC, which ensure excellent accuracy and high surface quality.

b. Machining of a smaller dies made of alloy tool steels in their hardened state up to 62 HRC has become a cost-effective manufacturing process. HSC in combina- tion with various cutting-tools (hard-metal cutters, drills, plan and angle cutters) is cost and time saving procedure.