As manufacturers introduce products with greater functionality and efficiency, the components that comprise the products become more complicated and difficult to manufacture. In addition to featuring complex contours and tighter tolerances, the parts are often made of high-performance materials that provide strength and reliability but negatively affect machinability.
Toolmakers continuously develop new tools engineered to maximise machining efficiency. Progress in milling cutter technology is one example of this effort. Solid carbide cutters have traditionally provided high metal removal rates and excellent accuracy. However, the one-piece nature of the cutters necessitates that they be reground when worn. Changing tools for regrinding interrupts production and consumes time and money. Applying a reground cutter requires changing NC program parameters, and producing a different part contour may mean choosing a different cutter with a more appropriate tooth geometry.
As a cost-efficient alternative to solid cutters, toolmakers have developed milling cutters with indexable, replaceable inserts. When cutting edges wear, inserts are simply indexed or rotated. Worn out inserts can be quickly replaced with the tool body still in the machine, and different insert geometries can be fitted to the same cutter body to create a variety of contours.
Negative vs. positive
The geometries of indexable inserts have evolved as well. Many indexable inserts have cutting edges on both top and bottom sides. Turning such a double-sided insert over after indexing doubles the number of available cutting edges. However, the design of double-sided inserts does not include space behind the cutting edge for a clearance angle, and thereby does not permit the tools to be applied in a positive-rake approach.
In a positive rake tool, the bulk of the insert body is tilted back from the cutting edge, enabling the edge to shear the workpiece material. On the other hand, in a negative-rake tool the bulk of the insert body tilts toward the workpiece and the tool essentially pushes the material being cut. Because of its design, the cutting edge of a double-sided insert must approach the workpiece from a negative angle.
Negative-rake cutting tools do offer certain benefits. They are strong; they can withstand heavy chip loads and cutting forces and thereby can handle rough milling, interrupted cuts and tough, abrasive workpiece materials such as cast iron. The availability of multiple cutting edges results in tool cost savings that can make double-sided negative inserts an economical choice in simple 2-D milling of easy-to-machine workpiece materials. However, because negative rake tools push the work material instead of shearing it, they generate high cutting forces and heat. If the machine tool’s power is lacking or if the entire machining system is not sufficiently rigid, final accuracy will suffer. The cutting forces generated by a negative-rake tool, for example, can distort a thin-walled part.
Conversely, a positive-rake tool is free cutting, which minimises cutting pressure, and often can cut tight contours that a negative tool cannot reach. Positive insert geometries provide a large degree of flexibility to perform a variety of operations including slotting, contouring, helical interpolation and ramping. The tools can help stabilise machining on older and/or less rigid machines. And minimising cutting forces is crucial when machining tough materials such as titanium, Inconel, and many stainless steels.
To provide indexable inserts that can cut in the positive mode, tool manufacturers offer one-sided inserts with clearance angles behind the cutting edges. Although the inserts cannot be turned over they can be rotated in the holder, providing multiple useful cutting edges.
The free cutting nature of positive-rake tools reduces cutting forces and heat generated in the cut, which enhances tool life. That is important because a one-sided insert must have three times more tool life per edge to be cost effective, compared with double- sided inserts.
Among the available high-performance, one-sided, positive-rake inserts developed for milling are the XO.X10 tools developed for the Turbo 10 series of milling cutters from Seco Tools. The inserts are engineered to minimise cutting forces and control chips while providing the flexibility to cut a wide variety of part contours and materials.
Beyond the basic distinction between positive and negative cutting edges, tool manufacturers have developed a variety of edge treatments aimed at maximising productivity in different workpiece materials. For example, up sharp, uncoated polished edges work well in softer materials such as free-machining aluminium alloys. For more difficult-to-machine materials such as steels, cast iron, or stainless steels, honing or chamfering the tool edge is necessary to protect it from chipping. Inserts engineered to machine cast irons, for example, often feature edge treatments including both a negative chamfer and an edge hone in the order of 30-35 microns. Inconel and titanium, on the other hand, require a freer-cutting geometry with no chamfer and an edge hone in the range of 20-25 microns.
Grades and coatings
Much of a cutting tool’s performance is determined by combining substrate characteristics with wear-resistant coatings. Insert substrates generally strike a balance between toughness and hardness. A tough grade has impact resistance to handle interrupted cuts, certain difficult workpiece materials, and less-than-rigid machining setups. Wear-resistant grades, on the other hand, have the hardness required to machine abrasive workpiece materials.
Coatings are intended to further fine-tune tool performance by providing additional resistance to wear and heat. Basic coatings include general-purpose materials such as TiN (titanium nitride), which is cost-efficient and good for ferrous metal applications not involving high levels of heat. A more advanced coating material is AlTiN (aluminium titanium nitride). It is more expensive than basic coatings but possesses good resistance to high temperatures, making it excellent for use in challenging applications such as milling of titanium, nickel-base alloys, and cast irons. In some situations, the coating’s high heat resistance makes dry machining possible. Each tooling manufacturer develops its own variations on basic coating concepts; in Seco’s Duratomic® coating, for example, the atomic structure of the aluminium oxide outer layer is controlled to maximise surface smoothness, tool life, and high speed capability.
Square shoulder and helical cutters
Milling cutters generally are configured to handle specific machining situations. Square shoulder cutters, with a single row of inserts located radially around the cutter periphery, are appropriate where axial cutting depths are less than the length of a single insert’s cutting edge. For greater depths of cut, helical cutters, with staggered rows of inserts arranged axially, can be used for slotting, pocketing and ramping as well as circular milling and helical interpolation.
A milling cutter’s pitch, or the spacing between the inserts it holds, will influence its performance. Coarse pitch (large spacing) cutters are appropriate when machine power and rigidity are limited, or when the cutter is mounted on a long extension or applied in a deep axial cut. Fine pitch cutters, on the other hand, provide maximum metal removal rates when used in rigid setups on powerful machines. If machine power and rigidity are lacking, aggressive application of a fine-pitched cutter can result in vibration.
The Seco Turbo 10 selection of milling cutters includes both square-shoulder and helical examples. With the appropriate insert geometry, the square-shoulder cutters can mill corner angles to 90 degrees. The cutters are offered in two pitches, with four- and five-tooth versions. The recently-introduced Turbo 10 helical cutters are available in diameters from 32 to 80 mm, for cutting depths from 0.8xD to 2.0xD. Both square-shoulder and helical cutters are available in a variety of mounting systems. The Turbo 10 system’s flexibility is underlined by a choice of 13 carbide grades, three PCD tipped grades and five geometries suited for a variety of materials including steel, stainless steel, cast iron, hardened materials, high-temperature alloys, and nonferrous materials.
In regard to accuracy, solid carbide tools generally have less runout than indexable tools and normally can achieve a better surface finish. As such, in many situations, it is appropriate to rough and semi-finish with an indexable inserted cutter and create the final surface finish with a solid endmill. On the other hand, some cutter bodies, including Turbo 10 cutters, feature precision-ground pockets to minimise mismatch between insert levels and thereby boost machining accuracy as well as tool life.
Advanced tools must be applied correctly to take full advantage of their increased capabilities. A typical case of application error is the use of an incorrect grade, such as employing a hard, wear-resistant substrate in a cut where a tough, impact-resistant tool would be more appropriate. However, the most common error in the application of advanced milling tools is the use of cutting parameters that don’t fully exploit the tools’ performance potential. Many users run the advanced tool at the same parameters as they employed with the lesser tools they replaced, often using too slow of a cutting feed, speed, or both. The key to productivity is to machine more aggressively. For some tools, it is necessary to increase the speed, while other geometries require increased feed rates. Users should be sure to consult the cutting data toolmakers provide when they introduce new products.
Many industries, especially aerospace, power generation and defence, are developing new products that feature complex, high-precision components and utilise new workpiece materials that are more difficult to machine. More and more parts will feature contours that demand five-axis machining technology, and some components that formerly were assembled from separate parts will be machined as a unit.
Those trends will further spur development of new milling tool technologies. Free cutting, high-performance tools that can machine exotic alloys with low cutting forces and high accuracy will be increasingly in demand. The evolution of milling tooling will continue unabated.
Tapio Alatalo, Global Product Manager Square Shoulder/ Helical Milling, Seco Tools
Headquartered in Fagersta, Sweden and present in more than 50 countries, Seco Tools is a leading global provider of metal cutting solutions for milling, turning, holemaking and toolholding. For more than 80 years, the company has provided the technologies, processes and support that manufacturers depend on for maximum productivity and profitability. For more information on how Seco’s innovative products and expert services bring success to manufacturers across all industry segments, please visit www.secotools.com.