In principle, machining large parts involves the same cutting action and chip formation process as for small or mid-size parts. However, large dimensions demand a specific approach to machining, and manufacturers need to plan technological processes and choose more effective cutting tools in order to produce heavy parts that take up a great deal of space. Article by Andrei Petrilin, Technical Manager, ISCAR.
Transporting a part inside a shop floor, mounting the part in a machine tool and clamping it properly, and machine setup are major challenges. Workholding massive and large parts is not an easy task, and often requires non-standard solutions. Machining large parts involves removing a lot of material that may cause significant deformations due to unrelieved stresses. Another factor, which leads to dimensional problems, is thermal expansion caused by heat generation during cutting: the large sizes make it much more sensitive comparing with “normal-in-size” workpieces. The necessity to remove a significant material stock requires appropriate chip evacuation to prevent the chip re-cutting, which negatively affects the applied cutting tools.
The key for overcoming the difficulties lies in technology, based on effective process planning and utilizing the most suitable machine tools, optimal workholding, and minimal part relocation. Single setup machining represents an absolute ideal for machining a large part, and producers from fields such as power generation, aerospace , railway, die and mold making, and heavy industry make every effort to approach this ideal. And cutting tools play a meaningful role towards reaching the target.
A distinct feature of these industries is their substantial consumption of large heavy-duty tools, mostly indexable, intended for productive removal of large quantities of material, especially in rough and semi-rough machining operations.
Large part manufacturers expect the same from cutting tools as any other producer using metal cutting technologies: excellent performance, good tool life, and high reliability. The latter two are especially essential because the large sizes lead to increased machining time, but replacing a worn tool in the middle of a pass and unpredictable breakage of the tool during cutting are totally unacceptable. In order to maximally meet the requirements of large part manufacturers, cutting tool producers provide various solutions, based on both standard and special designs. As a leading company in the cutting tool industry, ISCAR’s years of accumulated knowhow and experience have proved to be advantageous in developing efficient solutions to these challenges.
It is hard to machine a large part without face milling operations. Rough and fine machining of free and bounded planes and preparing datum surfaces require various indexable face mills. ISCAR’s standard face mills possess nominal diameters up to 315 mm (12″), while special tailor-made tools might feature higher values. The inserts are mounted in face mills and vary in cutting geometry as they are intended for machining different groups of material. Significant removal of machining stock by milling is primarily an issue for the production of large parts from steel and cast iron and, slightly less, from titanium and aluminum.
ISCAR’s line of standard face mills includes many tool families for large part manufacturing. HELITANG T465 features cutters with a 65° cutting edge angle and carrying tangentially clamped inserts. The robust design enables productive machining with a depth of cut up to 19 mm (.750″). The HELIDO 890 family features 89° face mills with lay-down square double-sided inserts (Fig. 1). These efficient mills, which are truly indispensable in milling a plane near the shoulder, offer an important economic advantage: the square inserts provide eight indexable cutting edges for depth of cut up to 9 mm (.354″).
Extended Flute, Extended Effect
Indexable extended flute “long-edge” cutters are considered as winning tools for productive rough milling. In manufacturing large parts, they excel in machining deep shoulders and cavities. Extended flute cutters are also utilised in “edging” – milling wide straight edges, an operation which is common for various processes from machining slabs and ingots to primary contouring.
ISCAR’s line of indexable extended flute cutters varies in design configuration, integrating a shank- and arbour-type mounting method and a radial or tangential insert clamping principle. These tools work in hard cutting conditions and experience significant mechanical and thermal loading. Intensive material removal requires the appropriate volume of a tool chip gullet to ensure effective chip evacuation. The situation can be dramatically improved by applying ISCAR’s extended flute cutters carrying inserts with chip splitting geometry to divide a wide chip into small segments. As a result, cutting forces are reduced, vibrations are stabilized, and thermal problems are eased.
Although 90° tools are the most commonly used cutters, machining large parts also requires rough milling of inclined and 3D surfaces, for which ISCAR provides a family of tapered extended flute cutters with 22.5°- 75°cutting edge angles. In some cases, particularly in die and mold making, combined rough and shoulder milling is needed. The DROPMILL 3 extended flute ball nose mills were designed specifically for such applications.
Producing large-size aerospace components from hard-to-machine titanium alloys is an extremely metal-intensive process with a significant buy-to-fly ratio. The eventual weight of a part may be only 10%, or even less, of the original weight of a workpiece. The XQUAD extended flute cutter family, one of ISCAR’s newest products, is intended for high-efficiency milling of deep cavities and wide edges in titanium parts. These tools (Fig. 2) are suitable for machining with high pressure coolant supply, which significantly increases productivity and improves tool life. The tools have already proved themselves: for example, component producers have achieved a 700-1000 cm³/min (43-61 in³/min) metal removal rate (MRR) by using an 80 mm (3”) diameter XQUAD cutter.
In railway engineering, combine mills are used to ensure simultaneous machining on several areas of the part. These mills incorporate an extended cutting edge, formed by a set of successively mounted indexable inserts.
Productive fast runner
High efficiency machining by indexable extended flute cutters and large-diameter face mills can be likened to the work of a heavy excavator digging sand with a big bucket. The full sand bucket, operated by a powerful engine, slowly moves a large volume of waste material. At the same time, there is an alternative method for efficient excavating. Imagine a more compact track trencher with a rapidly moving digging chain. Each link of the chain removes a small volume of sand but does it fast. In metal cutting, this trencher is a high feed mill, which machines at shallow depths of cut but with a feed per tooth that is far higher than the usual rates – millimetres as opposed to tenths of millimetres.
Fast feed mills are applied mainly to rough machining of plane faces, cavities and 3D surfaces (Fig. 3). These tools are more typical in manufacturing large parts from steel and cast iron, although high feed milling (HFM) titanium and high temperature alloys is not uncommon today.
ISCAR has a wide choice of fast feed mill families, intended for cutting various materials in different applications. The “world” of ISCAR’s HFM cutters encompasses tool families in diameter ranges of up to 160 mm (6.3″) that can meet the requirements of the most demanding customer.
High feed milling requires machine tools with high-speed feed drive. Large part manufacturers often have heavy, powerful but slow machines that are not suitable for high feed face milling. For these customers, ISCAR developed moderate feed (MF) cutters. Compared with fast feed mills, moderate feed cutters feature a higher cutting edge angle; they move slower but machine at higher depths and need more power to make them suitable for applying to heavy machines.
Large parts are often made from difficult-to-cut materials such as hard and high wear-resistant steel or cast iron. The welded part structure and the process of repairing worn parts by spraying fillers or soldering, add materials that are not easy-to-machine either. High speed milling (HSM) resolves these issues. Originally applied in die and mold making, high speed milling was developed as a productive method of milling hard steel that led to decreasing a part relocation, lessening setup, minimizing manual finish and polish, and, as a result, reducing cycle time. High speed milling features a small-in-diameter tool that rotates at high speed and mills material at shallow, light cuts.
The most suitable HSM tool is a solid carbide endmill and ISCAR’s MULTI-MASTER family of assembled endmills, which carry cemented carbide exchangeable heads, also represents a viable option. ISCAR’s line of solid carbide endmills offers various multi-flute tools in diameters of up to 20 mm (.750″), intended for high speed milling materials with hardness up to HRC 70. Decreasing machining allowances due to the production of more accurate workpieces for large parts, for example by using precise casting or molding, opens up new opportunities for high speed milling.
Exchangeable Heads Change The Dynamics
In many cases, manufacturing large parts is small-volume and even individual. In this context, minimizing machine tool downtime has critical importance. Intelligent process planning to considerably reduce setup time can help solve this issue. Each time a worn cutter is replaced, additional measuring and CNC program corrections are required, which increases downtime.
ISCAR’s families of rotating assembled tools with exchangeable heads – MULTI-MASTER mills and SUMOCHAM drills (Fig. 4) – enable substantial decreases in downtime. Face contact between a head and a tool body ensures that the head overhang is within strict tolerance limits, resulting in high dimensional repeatability of the assembly. Replacing a worn head does not require additional setup operations or removal of the tool from a machine.
U-Turn With Turn Milling
Turn milling, which is the method of cutting a rotating workpiece by a face milling cutter, is a good option for machining heavy rotary parts. In turning, the cutting speed is a function of rotating velocity. If the main drive of a machine tool does not allow rotation of large masses with the required velocity, due to limitations of its working characteristics, then the cutting speed is far from the optimal range and turning performance will be low. Turn milling offers an effective solution to the above difficulties. When turning large eccentric parts like crankshafts, off-centre masses of the parts cause unbalanced forces that adversely affect performance. Turn milling features low rotary velocity of a part, which prevents this negative effect (Fig. 5).
The majority of ISCAR’s indexable face-milling cutters are suitable for turn milling. The success of their application depends on cutter positioning with respect to the machined part, choosing optimal geometry of inserts, and cutting data calculation. ISCAR’s specialists in the field studied turn-milling kinematics and developed an appropriate methodology for defining these parameters.
Machining large parts is a time-consuming process, during which the tools cut material for a long period, and this means that tool reliability, stability, and predictable wear are high priority issues. A sudden tool failure may seriously damage the part and even cause its rejection. A cutting tool manufacturer has a limited choice of instruments for improving reliability, including advanced tool design, progressive cutting material, and technological development. Effective utilization of these instruments is the key to successful large part machining and ISCAR’s recently-introduced range of new tools and carbide grades provides that key.
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