Getting creative with carbide tooling

Solid carbide tools have largely taken over from high speed steel (HSS) end mills for aluminium cutting due to their especially sharp edges and geometries that provide low cutting forces and plenty of chip space for unobstructed material evacuation. But, as tooling manufacturer Sandvik Coromant explains, the technology can be taken even further.

The benefits of using carbide have made it the dominant technology in the market. For example it is three times stiffer than HSS, reducing deflection to third of that demonstrated by indexable insert tools. Carbide tools can also be given a helix to provide a very smooth entry and exit into and out of cuts and yield a smoother chip flow. This all amounts to a minimised cutting force variation and vibration.

Indexable insert tooling still has a place in the rough machining of aluminium when medium to large diameter tools are used. Tool regrinding is eliminated and the security, versatility and metal removal capacity of end mills with inserts provide aluminium machining with unequalled capability. Finishing, however, has in many cases been problematic, but this has now been addressed by Sandvik Coromant's CoroMill 790 tooling, which features new edge, insert, insert seating and clamping technology. A new approach Conventional indexable inserts have tended to be comparatively blunt for aluminium, often leading to a ploughing effect, cutting thin chips when finishing. The entry of the edge into the cut has also tended to be abrupt, leading to a sudden rise in cutting force magnitude. Together, these properties result in cutting forces that introduce excessive tool deflection and power requirements. The issue is made more complicated by the need for an edge that needs to be both sharp and positive for finishing, and capable of high removal rates when roughing. Consequently, there is a genuine requirement for a new approach to indexable inserts, focusing on resultant cutting forces, edge entry, chip formation and stability, as well as insert location and clamping. The establishment of a satisfactory model that calculates and predicts cutting forces accurately was one of the main foundations of the new insert geometry. Advanced FEM simulation presented many of the answers to the combined design of edge line, rake angle and chip former, as well as the development and optimisation of a new edge feature on the insert clearance face - a precision primary relief land - based on calculations of vibration wave shapes from measured modal parameters. Vibration moderation It is a well known phenomenon that in cast iron milling, the formation of flank wear on the clearance face of the cutting edge leads to some dampening of vibration. The worn ‘land' starts to grind against the machined surface, absorbing energy and resulting in the vibration amplitude being modulated. With the CoroMill 790 tooling the challenge has been to apply this principle by designing a flank wear land suitably as a primary relief. Correctly applied, the primary relief land acts a buffer, breaking up any growing deflection amplitude and thereby controlling chip thickness and radial cutting forces. With the new patented Sandvik Coromant design, as the insert deflects from the workpiece, the land makes momentary contact with the arising machined curvature of the component as it reflexes, thus countering any amplitude growth during the course of machining. This ensures a constant steadying effect as part of the cutting action. The short, occasional contact between the primary relief land and workpiece is slight enough to have no effect on performance, wear development or burr formation. Larger diameters do not require the same dampening and feed rate also ceases to be a major influential factor, with the magnitude of the radial cutting forces only varying slightly between feeds for the cutter (typically 0.25mm and 0.35mm per tooth). Typically, for a 25mm endmill, the land can be 0.1mm wide and angled at 1° to follow the curved cutting edge precisely between certain points, with a rake of 20° for aluminium. The balance of power

Cemented carbide inserts can withstand far higher temperatures than aluminium's low melting point of 625°C before encountering excessive wear and loss of strength at the cutting edge. However, a frequent problem of machining at high speeds is the need for high machine power, leading to a sometimes disadvantageous ratio of material removed per power unit. For this reason, any reduction by the tool in power requirement is of great advantage. From a tooling perspective, tangential cutting forces have a decisive influence on power requirement. Lowering the power needed per volume of material removed has a positive influence on aluminium milling applications, either in the form of higher productivity per operation or on the demand for machine capability. As well as determining the ease of cut, the rake angle also affects tangential cutting force. By making this angle larger, the CoroMill 790 design gives a more positive insert and aligns it with the rest of the insert geometry, resulting in minimised cutting forces. Easy entry To counter the generation of sudden rises in cutting forces, the entry of the cutting edge into the material needs to be as gradual as possible when milling, as this will affect the rate of growth, magnitude and direction of radial cutting forces. Consequently it will also affect tool deflection and the amplitude of any form errors on the workpiece. Sandvik Coromant's engineers have found that by designing the edge of the new CoroMill 790 insert geometry to be higher and more extended, it provided a prolonged and advantageous entry effect – reducing the shock effect substantially and leading to minimised mismatch on the radially milled face of the component. Furthermore, the axial cutting force is also lowered considerably, which means that the pressure exerted by the tool on the machined surface under the tool is less. In addition, the new chip forming geometry on the rake face of the insert has been deepened to reduce cutting forces and to optimise the formation of the chip and the way it is ejected from the insert pocket – out and away from the cutting zone and workpiece surface. The new geometry has also led to a smaller contact area between insert and chip, which means less friction and better cutting action. Additionally, it means that the new insert has the capacity for a larger depth of cut. In spite of the insert cutting edge seemingly being weakened by a sharper edge and deeper chip forming geometry, the stress levels are no higher than in comparatively less sharp cutting edges. A more systematic approach, sophisticated calculations, simulations and testing have all led to a more intelligent insert structure that not only performs better but is equally secure. www.sandvik.coromant.com