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Inside economical milling: an expert looks at all the angels. (Cover Story).

Optimization and improvements of milling operations are characterized by various approaches, including the design of the cutting tool, its connection to the machine tool, the design of the insert geometry and the selection of the cutting tool material. In addition to the tool design and machining conditions, the use of high-speed machining, hard milling, dry cutting, high-performance cutting and other measures to improve performance can be mentioned. All of the above leads to much higher productivity, more economical milling and reduction of the overall machining cost per part.

The development of new inserts with more cutting edges, new tool geometries and new cutting tool materials leads to better efficiency, stability, accuracy and tool life. Using new tool-design methods like finite element modeling (FEM) enables the optimization of shape and application even before producing the tools themselves.

For economical milling, high-performance, all parts involved in the milling process should be selected and optimized. Not only the milling tools and connections to the machine tool, but also the machine tool itself, the workpiece material and shape as well as the machining conditions should be optimized.

The optimal insert geometry--including the outer shape, the cutting edge configuration, the rake face and the clearance face--can improve performance in all industrial milling applications. In the past, simple flat inserts with straight cutting edges were used. Today, more cutting edges have a helical profile in order to improve accuracy, tool life, surface quality, machining stability and cost efficiency.

The use of standard positive flat SPKN, TPKN or SEKN inserts, as well as screw-clamped flat inserts, shows continually decreasing tendencies. Older cutter types with wedge or top-clamped inserts have been replaced gradually by cutters with screw-clamped inserts with molded chipformers.

The latest versions shown in the illustration above are helical inserts with non-parallel upper and bottom surfaces. These inserts developed during the last decade possess a rake angle larger than the inserts' axial rake. The concept of the helical-cutting-edge geometry with the non-parallel surfaces, as explained in the illustration on page 57, enables also a strong cutter body and high rigidity.

The helical cutting edge is the intersecting line or curve between the cylindrical outer surface of the cutter and a plane which contains the cutting edge itself. When rotating inserts with this cutting edge around the center line of the cylinder, the resulting machined surface is flat and perpendicular to the frontal face. These inserts solve the main geometrical disadvantages of a straight-cutting-edge insert--the lack of workpiece side-wall straightness, flatness and perpendicularity. The inserts thus save additional finishing steps. In addition, the helical geometry of the rake and clearance faces provides constant rake and clearance angles on the mounted inserts for better tool life.

Utilizing forces

When using inserts with the helical configuration, each cutting edge penetrates gradually into the workpiece with a step-wise force increase, reaching a maximum value that is lower than that achieved with a straight cutting edge. When cutting edges overlap each other, the resultant cutting forces are lower, the tools are always under load, stability is improved, vibration is reduced and the required machine power during the milling operation is lowered. Because of the use of larger radial rake angles and the unique geometry, the chips are easily deformed and the side walls remain perpendicular.

The ADKT rhombic-type inserts (in the illustration) with sloped-cutting-edges and with thicker frontal ends act to strengthen the cutter body and enable machining with higher loads and higher feeds. While the frontal edge is at maximum strength, the rear side has a smaller insert wedge angle, but at the same time is subjected to lower cutting forces. The inserts are also equipped with wiper flats, which act on the frontal machined surface to improve surface quality. The above illustration also shows square, multipurpose helical inserts for 90-degree shouldering, facing and slotting for improved economy and performance. These square inserts combine the helical cutting edge on all four edges with an additional wiper to perform for 90-degree sidewall milling operations. The typical insert designs have helical edges at the central part of each cutting edge, while sloping downwardly or upwardly in the wiper area. The unique design with the wiper insures high-surface quality and stable machining.

The multipurpose inserts are clamped with a relatively large axial angle on end mills, shoulder cutters, slotting cutters, face mills and also on milling cutters with 45 degrees for chamfering operations as well as on drill mill cutters. The unique features of the square insert design offer the possibility of using each of the four corners for machining flat, straight and perpendicular shoulders.

Clamping the inserts on heavy-duty cutters with multiple inserts provides full-effective helical cutting edges and the possibility to machine at large depths. The latest developments are the unique square inserts designated QDMT or QPMT, mainly for machining shoulders, slots, and deep walls. Each of the new inserts with four helical cutting edges can be used for right-and left-hand milling directions, reducing stock requirements significantly. The inserts have reinforced cutting edges and cutting corners due to a unique corner design. Each corner is equipped with a wiper for right-and left-hand applications. A deep recess on each of the four clearances faces is designed to mount the insert in a selected position on the cutter periphery being able to select cutter width. Square inserts are normally available in 6-, 10- and 16-mm square shapes and corner radii of 0.8 to 3.2 mm.

Octagonal inserts

New octagonal inserts (shown at right) offer eight cutting edges, each with 45-degree lead angles, for more economical milling applications.

Inserts combining helical cutting edges, a large axial angle, and a rib-type rake face design offer a self-balanced, stable tool for cost reduction and high performance in semi-finishing and finishing. The sloped, unstraight cutting-edge inserts make it possible to use fine-pitch cutters with more inserts on the same body. This enables machining with higher loads and higher table feeds.

The special protruding wiper flat design on each of the eight cutting edges offers, at the same time, good surface finish. Inserts are normally available in the economic M-pressed-and-sintered versions or with additional ground wipers for even higher surface quality. The arrows on the rake face indicate the indexing direction to insure maximal usage of all eight cutting edges at small cutting depths (up to 2.45 mm). At larger depths the number of effective edges is less than eight. Therefore, the highest economical advantage is expected in the finishing range.

The insert is equipped with a sequence of depressions to reduce the contact area between the chip and insert rake face to reduce heat penetration into the insert, to reduce friction, and to improve tool life. The positive rake angle in combination with these depressions also reduces cutting forces and machining power. The positive rake face of these octagonal inserts meets the tendency to machine smaller workpiece oversizes with less stock removal on low-powered machines.

The illustration also shows a variety of possible machining options. Both octagonal and round inserts may be mounted on the same cutter bodies. Operation A is facing at low forces and a small depth of cut, providing a good surface quality. Operation B is a deep plunging operation in which the frontal, side and 45-degree cutting edges are all active at the same time. Operation C shows plunging and facing and Operation D demonstrates machining of a slot with a round or shaped profile. Operation E is a few, successive 45-degree slotting steps, and operation G is a side shouldering application. The final operation, F, is a chamfering operation making use of the 45-degree cutting-edge configuration.

The basic octagonal OFMT type of insert was developed for more economical applications with several modifications having different rake face geometries. (See illustration at right.) The preferred geometry depends upon the workpiece material and application. The basic OFMT 07T3-AER-76 insert with its helical cutting edge, and a flat wiper on each corner, has a depression-style rake-face design as shown. The economical pressed, as-sintered style, OFMT 97T3-AEN modification is designed for use in right-and left-hand machining directions. This type, without ribs or depressions, is very useful for most milling applications on steel.

To improve surface quality of the finished product, a wiper-type ground insert designated OFCT 07T3-AER for right-hand machining has been added to the range t)f inserts. Generally, only a single insert with this wider wiper configuration is used on an octagonal cutter while all of the other inserts on the cutter have one of the other standard configurations.

For machining aluminum and similar soft materials, a ground octagonal insert type OFCR07T-AEN with high positive rake angles and sharp cutting edges is recommended. For heavy-duty profile milling applications, the RFMW 1905 can be used. This insert is designed for heavy-duty applications, interrupted cuts, and similar operations. The round, RMT 1905-LM-76 insert with depressions on the rake face is more positive, used for softer cuts and covers most milling operations.

For even higher economical milling applications, especially in face machining of engine blocks and very large surfaces, new negative, octagonal inserts with 16 cutting edges, or a positive octagonal insert with eight cutting edges. have been developed.

High surface quality

In finish milling applications, one of the essential requirements is the high surface quality of the machined surface. It results from the wiper, which means from the frontal cutting edge and the corner design geometry, in combination with the machining conditions. Wiper inserts are used in order to save additional fine milling or grinding operations, saving cost as well as improving productivity and efficiency resulting in significant cost saving.

One of the inserts on the cutter for finishing applications (as seen in the accompanying photo) can be a wiper insert to achieve the high surface quality. Each wiper insert has two wiping edges, one on each side and can be mounted in the same pocket as the other octagonal inserts. Typically, one wiper insert is enough to provide the required surface quality.

The advantage of a wiper-type insert OFCT 07T3-AER in comparison with other options is shown below. In milling alloy steel with an 80-mm cutter diameter, with five inserts, at feet of 0.25mm/tooth and depth of cut varying between 0.5 to 1.5mm, an O.5[micro]m Rcla average surface roughness value could be achieved. When using the same cutter and the same machining conditions without a wiper insert, values around 1.5[micro]m and above were measured. On the other hand, there is no need and no advantage in using two wiper inserts in one cutter since similar roughness values as with one wiper insert were obtained. The resultant good surface quality can even save additional finishing operations, improving the economy of milling operations.

Also, when using the ground OFCR 07T3-AEN inserts in finish milling soft materials like aluminum, stainless, and titanium a very high surface quality can be reached ranging between Rcla= 0.27 and 0.30[micro]m. In addition, the octagonal ground OFCR insert, with very high positive rake angles showed excellent tool life results in semi-finishing conditions.

Heavy-duty applications

Higher removal rates can be achieved by using larger depths of cut, higher feeds or applying higher cutting speeds. In normal facing, shouldering and slotting of high carbon and alloy steel, the use of very high cutting speeds [HSM] is rather limited by tool life. As the depth of cut is normally small due to the near-net shape production, the best option for higher removal rates is to use higher feeds.

HPM--High Performance Machining

The limitations for using higher feeds are insert strength, chipping of the cutting edges as well as cutter toughness and rigidity.

Recent research activities succeeded in developing the M1LL2000 tool system with high-strength helical cutting edge inserts. The inserts with an overall dovetail shape are safely screw clamped on the cutter body. The dovetail shape improves insert stability in seat even at very high loads, feeds and depths of cut. To achieve proper clearance angles, the insert is mounted above the centerline. This unique design improves also cutter strength and rigidity, enabling using more than 50 percent higher feeds compared with standard tool designs. Two insert sizes, the AXKT13 and AXKT20 with 13 and 20mm cutting edge lengths, are available for large depths of cut.

A wide range family of end mills, face and shoulder cutters as well as extended flute cutters is offered today. In larger cutter diameters a replaceable shim is used to protect cutter body against damage. The unique design of the negative shaped insert causes the forces to be directed into the cutter body, eliminating chipping or breakage, causing more uniform force on the seat or shim beneath the insert, but at the same time keeping cutting forces relatively low due to the positive rake angle. As a result almost unlimited feeds and depths are possible.

Ballnose-type end mills

The development of ballnose-type end mills is characterized by the introduction of new cutting tool materials, new geometries, new clamping systems and the use of simulation and evaluation methods like the FEA.

For economical profile milling various HSS, brazed tools, solid carbide and more and more indexable-insert end mills are used. Normally, up to about 8-mm diameters, all tools are solid, while larger diameter end mills are made more and more with indexable inserts. Small solid end mills are ground, either as square 90[degrees] shapes with a small corner radius or a chamfer, or as ballnose with a full radius. Rake angles are very positive to reduce forces, improve stability, and improve chip flow.

New end mills with larger diameters are equipped with helical cutting edge inserts, and unique chipformer geometries, as shown below. The frontal centerpoint is located in the center of rotation, and the cutting edge moves upwardly describing a full radius. By using the helical shape, the maximum cutting force values are reduced, and the force gradients at entrance and exit are smaller. Smaller-sized tools normally have a single cutting edge, and tools with larger diameters have two ground edges for balancing and high surface quality.

When a high superfinish surface quality is required, the two effective-cutting-edge insert (CRF) should be preferred. The cutting edges are ground, sharp, the rake face is positive, and the frontal centerpoint is slightly below the tool center to ensure soft cuts. The results with these CRF ground inserts can produce very high surface quality with about Rcla 0.1-0.2[micro]m.

The use of the CR negative-insert configuration with a helical side-cutting edge positioned above center provides the necessary relief angle. This unique insert design is clamped with a torx screw in the dovetail-shaped seat with sloped side walls in order to provide safe and secure holding.

For high-speed machining at higher speeds and feeds solid carbide end-mills are recommended. In some cases, especially at medium speeds, normally screw clamped or blade-type inserts can also be used.

Designing of cutting tools, insert geometry, clamping devices, and chip-formers has been achieved in the past using trial and error procedures along with some physical and mechanical basic models. During the last few years more effort has been devoted to the use of analytical models using FEA--finite element analysis--for the development of milling tools with optimal performance.

Solid carbide end mills

The development of sub-micron carbide substrates with excellent mechanical properties is the main reason for the replacement of more and more high-speed steel end mills by carbide tools. In addition, the improvement and optimization of the new TiAIN PVD coating provides the optimal solution for hard milling, for dry cutting and for high-speed machining.

Therefore, almost all modern carbide end mills are made of sub-micron substrate with improved toughness and hardness, better resistance to cracking or chipping, and high wear resistance. This combination of substrate and PVD coating properties provides excellent, high-performance in a wide range of applications. New end mills, uncoated or with the unique PVD-coating of TiCN or TiAIN, have sharp cutting edges and high resistance to impact and interrupted cutting action.

The use of indexable inserts for finishing operations may be limited due to surface quality requirements, run-out of the tools, adjustment limitations, cutting edge configurations, and the very small feeds and depths of cut used, Furthermore, when high side-wall flatness and accuracy is required, the use of extended flute cutters with multiple inserts is limited. In all of these cases, endmills, either solid or with single indexable inserts with various configurations can be used. The solid end mills in the past were mainly made of HSS, coated HSS, and today more and more of sub-micron carbides, CBN or PC]D tips brazed on carbide bodies ground to final dimensions.

New technologies

Economical and performance improvement in milling is possible also by using balanced tools which enable higher machining conditions and longer tool life. When combined with new adaptors or clamping systems which improve stiffness and rigidity of the cutting tools, higher feeds, larger depths of cut, and higher speeds can be used.

Recently, unique plunging tools were introduced for rough machining of deep cavities, slots, and deep walls as shown below.

Plungers are very effective and economical in deep machining of dies and molds and when very large tool overhangs are required. Radial or side forces on the tools are minimized and mainly axial forces should be considered. The loads on the machine tool and workpiece are thereby optimized and bending forces on the tool are decreased.

The shown tools are equipped with unique tangential inserts with four cutting edges. This strong, double-sided insert with a roof-type slope can be screw clamped from both sides of the insert in any of the seats on the cutter. In addition the same insert can also be clamped in the peripheral position using the short cutting edge for the facing and the long cutting edge for the boring, thereby using much higher plunging feeds.

Machining of large deep cavities can be accomplished by a sequence of axial plunging operations covering the whole area by overlapping between two or three nearby holes to provide a complete surface. The reduction of machining time, and therefore, machine cost combined with the use of only a few tools for various complicated shapes and cavities reduces tool stock, offers a wide range of machining options with high removal rates.

Cuts above others

High-speed machining (HSM), the machining of hardened steel (HC) or dry cutting (DC) are the main methods to improve overall cost efficiency and to fulfill various environmental requirements.

Furthermore, the goal of a more economical machining process influences not only the machine tool, the clamping devices, and the machining conditions, but also the necessity to use coolant. If the operation can be done without coolant, a significant cost saving is achieved.

The question of high-speed machining compared with conventional speeds depends, first of all, on the workpiece composition and properties. While cutting speeds for machining plastic, aluminum and other nonferrous materials reach today about 1,000 m/min, the use of high-speed cutting is aimed to reach more than 3,000 in/mm. Machining cast iron by using, for example, silicon nitride, reaches today nearly 1,000 in/mm in milling and 800m/min in turning. Machining high-temp alloys like titanium and nickel alloys is still done at low cutting speeds of less than 80 or 40m/min, respectively. Machining hardened steel (50-60 HRc), which was done at very low speeds, can now be doubled or more by using new cutting tool materials under optimal machining conditions.

For high-speed face milling of aluminum and other soft materials new balanced cutters with high precision have been introduced.

The inserts, with two cutting edges and with very high positive rake angles, are available in uncoated carbide grade ISO K-10 and with PCD brazed tips. The inserts are screw-clamped and additionally secured on the cutter body. The milling cutter is balanced to enable cutting speeds up to 3,900 in/mm. Excellent surface finish is achieved eliminating any additional finishing steps.

The development and introduction of new insert and tool geometries for economical milling is an on-going process improving performance, tool life, and surface quality. Iscar Ltd., Tefen, Israel, or circle 450
COPYRIGHT 2001 Nelson Publishing
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Copyright 2001 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Wertheim, Dr. Rafi
Publication:Tooling & Production
Date:Sep 1, 2001
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