High cutting speed is a natural attribute of high-speed machining. Understandably, a tool should be also precise; it is required not only by machining accuracy but also by the mechanics of a fast-rotating body. However, in the last years, the issue of tool accuracy has become an additional point to consider. What is the reason? Why is high speed machining penetrating more and more into rough processing? How do cutting tool producers formulate their solutions to meet these new industrial demands? Read on to find out. Article by Andrei Petrilin.
The metalworking industry adopted high speed machining (HSM) in the 1990s. This method was engrained in various industrial branches and caused serious changes in technology and machine tool engineering. The well-known advantages of HSM are repeatedly cited in various books, guides, magazines and other sources of information. Recently, there has been a significant interest in accurate HSM and, more specifically, in precision and other characteristics of cutting tools and toolholding devices intended for this purpose.
Accurate (or precise) machining means maintaining repeatable strict tolerances during cutting operations. The level of such a “strictness” depends on the machining method, for example: milling, turning, or drilling, and the type of operation: rough, semi-finish or finish. Technological advances, especially in producing workpieces that are preformed half-finished products, place special emphasis on accurate HSM.
Precise casting, metal injection molding and 3D printing ensure that the production of workpieces is very close to the final shape of a part. As a result, the need to remove a high volume of material by means of rough cutting decreases. In die and mold making, utilizing HSM as a means of reducing production time has brought a real alternative to traditional methods. In the aerospace industry, machining difficult-to-cut heat-resisting superalloys by ceramic tools at extremely high cutting speeds, in combination with a small stock to be removed, is now common. As for manufacturing aluminum components, here HSM has simply become a daily reality.
Machining operations with low stocks per pass have distinct advantages such as lower power consumption, less heat generation, and better surface finish. Accurate HSM, which features low stock removal, is a logical extension of producing workpieces by precise modern methods. Usually, HSM relates to cutting by rotating tools—mainly milling cutters. In many cases, when a part featuring complex profiles and slots is produced from a solid material, HSM provides productive low-loaded roughing by trochoidal milling. According to this technique, a rapidly rotating milling cutter moves along a complicated trajectory and slices thin but wide layers of the material. This results in pre-shaping the part very close to a final form. The remaining small allowance is removed in the next stage: high speed finish milling. Producing blisks (bladed discs) and impellers is a typical example of the mentioned process that may be defined by the rather oxymoronic term: “accurate roughing”.
Successful HSM relies on a key element chain comprising a machine tool, an effective machining strategy, proper toolholding, and a cutting tool. The low-power multi-axis machine tools designed especially for HSM feature high-torque characteristics, high-velocity drives, effective controllers and intelligent software. They are capable of realizing various machining strategies which were developed for ensuring maximum efficiency. Today, metalworking has in its arsenal highly reliable tool holders designed for secure tool mounting in an expanded range of rotational speeds. Under such conditions the cutting tool—the element that directly contacts a machined part during a cutting operation – can be a limiting factor in maximizing the potential of advanced machine tools. This element is much smaller and less complicated compared to machine tools and holders. Each improvement in the last chain element—the cutting tool—may be crucial. The cutting tool industry is far from stagnation; the branch is on the constant move in developing new solutions to meet the demands of changing metalworking technologies.
Time has not radically changed principal tool requirements: it is expected to be more durable and more efficient when cutting at considerably increased cutting speeds and feed rates. Lowering machining allowances leads to additional tightening tool accuracy parameters. An ideal product is a precise and high-balanced tool that ensures high performance in combination with excellent tool life when cutting at high rotational speeds. ISCAR, staying true to its motto “Where innovation never stops!”, has developed a range of solutions that give new momentum to HSM concepts, and many proposed designs relate to the field of solid carbide tools.
More Flutes, Less Vibrations
Multi-flute solid carbide endmills from ISCAR’s “CHATTERFREE” line were developed especially for vibration-free HSM operations. Their design features a varying helix angle, variable tooth pitch and a specially shaped chip gullet, intended for applications such as semi-finish and finish high speed milling, as well as roughing by trochoidal technique. The CHATTERFREE range comprises several endmill families for different applications. Seven-flute endmills, produced from an ultra-fine carbide grade, are designed for machining hard materials and finish operations. General-purpose multi-flute endmills incorporate an interesting concept, according to which the number of teeth is equal to a nominal diameter in mm. Seven- and nine-flute endmills were designed originally for trochoidal milling complex parts from titanium and today they form the Ti-TURBO family—this name reflects a real “turbo” metal removal rate when milling titanium.
The latest step of the line development integrates chip-splitting grooves (Figure 1) in the endmill design. The new geometry has an unusual appearance because HSM forms thin chips that do not appear to need an additional chip-splitting action. However, the grooves increase vibration resistance and reduce cutting forces, significantly improving trochoidal milling and machining performance at high overhangs. In trochoidal milling, the produced chips are thin but wide. Splitting the chips into narrower segments contributes to better chip evacuation and surface finish, which increases accuracy and effectiveness in rough HSM.
Ceramics that Cut Fast
Milling difficult-to-cut high temperature superalloys (HTSA) by carbide tools necessitates low cutting speeds, normally 20-40 m/min (65-130 sfm). HSM with a small radial engagement, when the width of cut is up to 10 percent of a mill diameter, usually features cutting speeds of 70-80 m/min (230-265 sfm). The metalworking industry is always seeking ways to increase productivity when manufacturing parts from HTSA; and low cutting speed is one of the existing barriers to this goal. A solution may be found in applying cutting ceramics as a tool material for HSM. ISCAR has developed solid ceramic endmills that enable a dramatic increase in cutting speeds of up to 1000 m/min (3280 sfm) when compared to tools made from cemented carbide. The new endmills have a diameter range of 6-20 mm (0.236-0.787 in) and are designed with 3 or 7 flutes. (Fig. 2). Introducing the ceramic endmills in rough milling operations has proved to decrease machining time drastically and to enable fast pre-shaping of a part for further finish operations.
High Speed Master
Long-reach high speed milling operations require tools with long overall length. A solid tool concept is not an economically attractive option. An assembled cutter comprising a body carrying a carbide cutting head is a solution that makes economic sense. Such an approach is at the core of ISCAR MULTI-MASTER—a family of tools with exchangeable heads. A rich variety of tool bodies, heads, extensions and reducers ensures various tool configurations and fundamentally reduces a need for special tools. An important advantage of the MULTI-MASTER line is its no setup time principle, whereby replacing a worn head does not require additional tool measuring or appropriate CNC program adjustment—the insert can be replaced without withdrawing the tool from a machine spindle.
High assembly rigidity, a balanced structure and high geometrical accuracy make MULTI-MASTER suitable for HSM. A typical example of this application is finish milling 3D surfaces of parts produced from hard materials. ISCAR’s “MM HBR” bulb-shape head (Figure 3), featuring a 240°-spherical cutting edge, center cutting ability and strict ISO h7 grade tolerance limits for the head diameter, was developed for this type of operation.
HSM is impossible to perform without the use of reliable, high-grade balanced and accurate tool holders. Thermal shrink chucks are one of the most popular types of tool holder. ISCAR’s line of SHRINKIN chucks includes the X-STREAM family of thermal chucks, featuring coolant jet channels along the shank bore. The new design provides coolant flow directed to the tool’s cutting edges. In the high-speed milling of aerospace components (the aforementioned blisks, for example), a well-directed coolant significantly enhances performance. For deep pockets and cavities, the new chucks with pin-pointed coolant flow have resulted in preventing re-cutting, thereby improving chip evacuation and increasing tool life.
A wet coolant may be a means for upgrading machine tools from low velocity to high speed. SPINJET, a family of compact coolant-driven high-speed spindles (Figure 4), is capable of maintaining rotational velocity up to 55,000rpm and facilitate high speed machining even on the low-speed machines that are still so common in the shop floor.
Changing technologies require new machining concepts: more productive, more economical, more sustainable. High speed machining has already proved itself as a method that meets today’s industrial needs. The progress in producing workpieces by non-machining processes brings in focus low-power high speed roughing. Accordingly, cutting tool manufacturers already feel the growing demands for appropriate products. It is a definite trend, which, no doubt, should be considered.
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