Conceptual design of machine tool interfaces for high-speed machining

High-speed machining is a promising technology to drastically increase productivity and product quality. However, conventional machine tool design often cannot meet stringent requirements needed to overcome various problems brought about by high-speed rotations. One particularly critical issue is how to hold an exchangeable tool rigidly during high-speed rotations. For this purpose, several new standards of tool interfaces between the toolholder and the spindle nose have been proposed. However, it has become increasingly difficult to choose among various competing interface standards or even different variations within a common standard. This issue is further complicated because many machine tool builders also offer various proprietary modifications to the common standards. Furthermore, there is also a need to develop new tool interfaces or modifications if existing standards are not satisfactory. The objective of this paper is to provide a systematic approach that can assist in the conceptual design of machine tool interfaces for high-- speed machining. This conceptual design methodology can also be used to select appropriate interface designs from various competing alternatives that exist today. Two embodiment concepts for high-speed end milling are developed as a case study based on the proposed conceptual design methodology. It is shown that the proposed design methodology can assist systematic design decision making and sound reasoning of the design choice.Machine tools such as machining centers have spindles that consist of the arbor, the toolholder, and the tool, with interfaces and separation points in between (Ronde 1991). Toolholding is essential for machining processes to establish precise relative positions between tools and workpieces (Agapiou, Rivin, Xie 1995; Aronson 1994a, 1994b). As machining speed and precision requirements become higher, the need to hold tools rigidly at high speeds and under high loads continues to grow in importance. Tool interfaces are pairs of mating geometries that are standardized to permit exchangeability. This paper deals with the interface between the toolholder and the spindle nose. In general, the interface includes three major couplings: the mating surfaces of (1) the toolholder and the arbor, (2) the toolholder and the tool, and (3) the gripping chuck and connections to the toolholder, which are used for tool ejection and optional transmission of cutting fluid. Figure 1 illustrates the location of the interface of interest and the related components.

7/24 Taper Shank

Conical shanks without keys were the earliest shapes of tool interfaces. The basic principle is a conical toolholder that is drawn into an equally shaped hollow cone with a constant force that is usually generated by spring or hydraulic pressure. The conical surface centers the tool, determines its axial position, takes all external forces and bending moments, and transmits torque. To transmit torque, it is important that friction between interface surfaces is sufficient during operation.

One radius-to-length ratio of the cone was chosen as 7/24, which became a standard both nationally (ANSI B5.18-1960, ANSI B5.18-1972) and internationally (ISO 7388). For automated tool change, a circular groove with a V-shaped cross section was added, which is known as a V-flange tool shank (ANSI B5.50-1978). The 7/24 steep angle taper is the most common interface type for machine tools with large inventories in industry (Beier 1994, Wang and Horng 1994, Weck and Schubert 1994).

Various gripping mechanisms are available for the 7/24 toolholder. Most often, a retention knob is added at the tail of the taper, where it can be clamped by levers from the outside and drawn axially into the spindle nose. Another common option is a threaded bolt built in as part of the spindle that can be activated to screw or unscrew the toolholder (Effenberger 1987).

Many problems, however, are associated with 7/24 toolholder interfaces. One problem is that tool-- holders from different companies are not always exchangeable because this toolholder standard leaves too many aspects unspecified (Eckle 1986), and many companies have developed their own proprietary modifications.

Another problem with the 7/24 steep angle taper is its comparatively lower radial stiffness for the same nose diameter (Scheer 1986). The standard 7/24 taper is defined to guarantee a clearance between the end face of the spindle and the tool-- holder flange. The lack of end surface contact can result in lower radial stiffness. This problem can be explained by a stiffness cone concept as shown in Figure 2. An imaginary cone can be drawn, with the cone base defined by the most outward contact points. The length of the stiffness cone provides a comparison for permissible external radial loads without exceeding a given deflection. The angle of the imaginary stiffness cone can be chosen arbitrarily but must be held constant for comparison of alternatives. As shown in Figure 2, for two conical shanks of the same diameter, the one with a flange (Figure 2b) has a longer stiffness cone and thus suggests a higher radial stiffness than the one without (Figure 2a). Of course, proper end-surface contact with the flange is assumed for this comparison. However, it is not a trivial issue to achieve proper end-surface contact. The precision requirement for the relative positioning between the taper and the end surface is very high. This issue is critical and must be addressed.