Advanced workpiece manufacturing technologies—such as metal injection moulding, 3D printing, investment casting and close-tolerance forging—innovative machine tools, and a quantum leap in digitizing of manufacturing will increase the needs for finishing complex surfaces with minimum machining stock. Article by Andrei Petrilin, ISCAR.
Endmills featuring a cutting edge that is actually the segment of a large-diameter arc were introduced approximately 25 years ago. As the cutting-edge shape of these endmills is reminiscent of a barrel profile, terms such as ‘barrel milling cutters’, ‘barrel endmills’ or, in shop talk, often simply ‘barrels’ soon became common when referring to these types of endmills.
At first, the use of these barrel-shape mills was limited more or less to a few specific applications, such as machining 3D surfaces of complex dies and turbomachinery components. However, advances in 5-axis machining and in CAM systems have significantly expanded the boundaries of barrel endmill applications.
At the same time, the design principle of a cutting edge as the segment of a large-diameter arc has been realized successfully in other types of milling cutter—the tools for high feed milling (HFM), also referred to as ‘fast feed’ (FF) milling. The concept provides a toroidal cutting geometry that ensures productive rough machining at extremely high feed rates due to a chip thinning effect. Unlike high feed milling tools, barrel endmills are intended not for roughing but for finish and semi-finish machining of 3D surfaces with low stock removal.
Traditionally, ball-nose and toroidal cutters perform these machining operations. However, the large-diameter arc of the endmill cutting edge results in a substantial reduction of the cusp height generated between passes machined by a ball-nose or toroidal cutter. Another advantage of this type of cutting edge versus ball-nose and toroidal cutters is a significant increase in the distance between passes (a stepover or a stepdown, depending on the direction of a cutter displacement after every pass)—at least five times more without degradation of the surface finish parameters! (Figure 1) This means that the number of passes and, subsequently, machining time can be noticeably reduced. Increasing the distance between passes also improves tool life and, therefore, diminishes tool cost per part.
The classical barrel shape in endmills has undergone some changes to make these cutters more versatile. Combining a ball-nose tip with peripheral large-arc cutting edges creates a multi-purpose ‘cutting oval,’ which facilitates the use of a barrel endmill as a ball-nose milling tool.
How to say no to vibrations in machining? Find out more in this article by Andrei Petrilin, Technical Manager at ISCAR.
Figure 2: The FINISHRED series of solid carbide endmills features chip-splitting geometry coupled with variable pitch flutes.
Vibrations in machining are generally an unavoidable part of the metal cutting process. They have a forced or self-excited nature and always accompany a cutting action. Machining vibrations are referred to as “chatter,” highlighting their specific nature, which inheres in every processing where chips are formed. Even if cutting is considered as stable, it does not mean that vibrations do not take place. In this case, the vibrations simply remain on a level that provides the required machining results, and is considered as a “no vibration” operation.
In fact, vibrations in cutting are a damaging factor that reduces performance. Manufacturers make every effort to diminish vibration and, ideally, bring them to a level that does not affect machining results. Chatter is a subject of serious research that has already provided manufacturers with ways to model vibrations in machining which, despite their complexity, can be very effective in finding a way to reduce chatter. However, this modelling takes time and requires various input data, including sometimes additional measurements. In most cases, when manufacturers face vibrations during machining, they only have a few tools at their disposal for a real-time response to decrease the chatter. The most common practice is to vary cutting speed and feed, which usually leads to productivity reduction. Therefore, any effective method of diminishing vibrations that does not adversely affect machining operation productivity will be attractive to manufacturers.
Vibration reduction in machining requires consideration of a manufacturing unit as a system comprising the following interrelated elements: a machine, a workpiece, a work-holding device, and a cutting tool. While the influence of each element on total vibration reduction is different, improving a vibration characteristic of one element may have a significant impact on the system’s overall dynamic behaviour. Most efforts to protect against vibrations focus on developing more rigid machines with intelligent sensors and computer control, and advanced vibration-dampening tooling. Can a cutting tool, the smallest – and probably the simplest – system component, dramatically change the vibration strength of a manufacturing unit? Even though producers might not have great hopes for the role of cutting tools in decreasing chatter, in certain cases a correctly selected tool can simply stop vibration without any adverse effect on productivity.
Figure 3: The SUMOCHAM-IQ family of HCP exchangeable carbide heads.
The right tool geometry makes cutting action smooth and stable. The geometry strongly influences cutting force fluctuations, chip evacuation and other factors, which are connected directly with vibration modes. ISCAR’s tool design engineers believe that the cutting geometry can considerably strengthen vibration dampening of a tool and have developed interesting solutions accordingly.
ISCAR’s various indexable inserts, exchangeable heads, and solid carbide tools feature chip-splitting cutting edges. Such an edge may be serrated or have chip-splitting grooves. The chip splitting action causes a wide chip to be divided into small segments, resulting in better dynamic behaviour of a tool during machining, and vibration is stabilised. In rough machining, extended flute milling cutters remove a large material stock and work in heavy conditions. Significant cutting forces acting cyclically generate vibration problems. When using chip-splitting indexable inserts, it is possible to tackle these difficulties. Mills with round inserts, a real workhorse in machining cavities and pockets, particularly in die and mold making, are often operated at high overhang that affects rigidity and vibration resistance of a tool. Problems with cutting stability occur when the overhang already exceeds 3 tool diameters. Applying serrated round inserts with a chip-splitting effect redresses this situation and substantially improves robustness (Figure 1).
A skilfully defined tooth pitch is an effective way of taking the dynamic behaviour of a cutting tool to the next level. ISCAR’s CHATTERFREE family of solid carbide endmills (SCEM) was designed on the basis of a pitch control method. The family features a variable angle pitch in combination with a different helix angle. This concept ensures chatter free milling in a broad range of applications.
The FINISHRED series of solid carbide endmills features chip-splitting geometry coupled with variable pitch flutes (Figure 2) that provide surface finish when machined according to rough machining data.
The principles of vibration-proof cutting geometry, which demonstrated their effectiveness in solid carbide endmills , have been applied to the design of exchangeable multi-flute milling heads made from cemented carbides in the MULTI-MASTER family.
Figure 4: ISCAR’s ISOTURN WHISPERLINE family of anti-vibration cylindrical bars.
Chatter in drilling leads to poor surface finish and accuracy problems. In ISCAR’s SUMOCHAM family of assembled drills with exchangeable carbide heads, the double margin design of QCP/ICP-2M heads substantially increases tool dynamic stability.
If vibration occurs when a drill enters material, it may cause serious damage and even breakage of the drill. The SUMOCHAM-IQ family of HCP exchangeable carbide heads (Figure 3), intended for mounting in the bodies of standard SUMOCHAM tools, can ensure reliable self-centring capabilities. The key is an unusual concave profile for the head cutting edge reminiscent of a pagoda shape. This original cutting geometry enables high-quality drilling holes of depths of up to twelve hole diameters, directly into solid material without pre-drilling a pilot hole.
The “magic pagoda” features another ISCAR innovation: the LOGIQ3CHAM family of latest-generation drills carrying exchangeable carbide heads with 3 teeth to ensure higher productivity. The steel drill bodies have 3 helical flute that weaken the body structure when compared with a 2-flute assembled drill of the same diameter. In order to improve the dynamic rigidity, the flute helix angle is variable. This design principle in combination with the pagoda-shaped cutting edge provides a durable chatter-proof solution for stable high-efficiency drilling.
Tool Body Material
An assembled cutting tool comprises a body with mounted cutting elements such as indexable inserts or exchangeable heads. Choosing the right body material presents an additional option for forming a chatter-free tool structure. Most tool bodies are made from high-quality tool steel grades, for which the material stress-strain behavior is similar. However, in some cases tool design engineers have identified successful material alternatives to improve vibration strength.
The MULTI-MASTER, an ISCAR family of rotating tools with exchangeable heads, provides a range of tool bodies, referred to as shanks, produced from steel, tungsten carbide or heavy metal. A steel shank is the most versatile. Tungsten carbide with its substantial Young’s modulus provides an extremely rigid design, so carbide shanks are used mainly when milling at high overhang and machining internal circumferential grooves. Heavy metal, an alloy containing around 90 percent tungsten, is characterised by its vibration-absorbing properties, and heavy metal shanks are most advantageous for light to medium cutting operations in unstable conditions.
Anti-vibration Tools for Deep Turning
A typical tool for internal turning or boring operations comprises a boring bar with a mounted insert or a cartridge carrying an insert. The bar is the main factor in the dynamic behaviour of a tool. Stiffness of a bar is the function of the bar overhang to diameter ratio, and large ratios may be a reason of tool deflection and vibrations, affecting dimensional accuracy and surface finish during machining.
ISCAR has developed three types of boring bar to cover a wide range of boring applications: two integral (from steel and solid carbide) and one assembled, having a vibration dampening system inside.
The steel bars enable stable machining with the overhang up to four diameters. Exceeding this value can induce vibrations due to steel’s elasticity characteristics. Changing the bar material from steel to a more rigid solid carbide ensures efficient vibration-free boring with the overhang of up to seven diameters. However, further increasing the boring depth is also limited by the material stress-strain behaviour. In order to clear this overhang barrier, ISCAR developed the ISOTURN WHISPERLINE family of anti-vibration cylindrical bars. The bars carry interchangeable boring heads for indexable inserts of different geometries and have inner coolant supply capability. The main element of the bar design is a built-in vibration-dampening mechanism to provide “live” vibration damping during machining. This enables effective boring with the overhang from seven to 14 diameters (Figure 4).
A vibration-dampening unit is used also in ISCAR deep grooving and parting tools. The unit is in a tool blade under the insert pocket. Each blade is pre-calibrated by ISCAR for optimal performance for a wide range of overhangs, but end-users can complete fine tuning calibration themselves if needed.
Cutting tool manufacturers have a limited choice of means in the abatement of machining vibrations, with only tool cutting geometry, tool body material, and maybe a cutting tool with built-in vibration-damping device at their disposition. Considerable skill and ingenuity are required to make a chatter-free tool with these limited resources. It is feasible, however, and ISCAR’s solutions highlighted in the above examples affirm the possibilities.
Following the steady increase in the processing of nickel-based high temperature superalloys (HTSA) such as various grades of Inconel, Incoloy, and Haynes, amongst others, in the aerospace industry, and the demand to decrease production costs, ISCAR has launched solid ceramic endmills that enable increasing the cutting speed by up to 50 times when compared to carbide tools, drastically saving machining hours and reducing production costs.
Available in two configurations—E3, with three flutes for shouldering applications, and E7, with seven flutes, feed mill style for rough applications—the new endmills can also be successfully applied to productive roughing of cast iron and graphite.
The solid ceramic endmills are produced from two ceramic grades: IS6, designed specifically for machining HTSA, and IS35, intended for cutting mainly cast iron and graphite. They are available in 6mm to 20mm diametres.