Advances with aluminium

The successful machining of aluminium components used in aerospace applications requires a number of different tool criteria to be fulfilled in order to meet increasingly stringent cost and quality demands. Solutions investigates.

Until now, many of the router type operations have been performed by solid carbide tools, which have taken over from high speed steel (HSS) end mills because they feature sharp edges and geometries that provide the low cutting forces advantageous for finishing aluminium, as well as plenty of chip space for unobstructed material evacuation.

Additionally, carbide is three times stiffer than HSS, which provides further benefits because the deflection of solid carbide cutters is only around a third of that demonstrated by indexable insert tools when subjected to the same load. Another benefit of solid carbide milling cutters is that they can be given a helix to provide a smooth entry and exit into and out of cuts, as well as smooth chip flow, all of which helps minimise cutting force variation as the source of vibration tendencies.

Regarding indexable insert tooling, this brings potential advantages to the rough machining of aluminium when medium to large diameter tooling (25mm to 100 mm) can be used. Tool regrinding is eliminated and the security, versatility and metal removal capacity of endmills with inserts have already proved their worth in aluminium machining applications. Finishing, however, has in many cases not been of the level required, but this has now been addressed by Sandvik Coromant with CoroMill 790, which features new edge, insert, insert seating and clamping technology.

Pushing boundaries

In developing a new endmill concept for aluminium machining, a number of parameters were recognised as being key to a breakthrough for radial milling with indexable inserts. These included a new smooth cutting action, good chip formation, high metal removal rate, low power consumption in relation to volume of material removed, high surface finish (and minimised mis-match), and high tool security at fast spindle speeds.

Conventional indexable insert edges have tended to be comparatively blunt for aluminium machining, often leading to a ploughing effect, especially 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. Collectively, these properties result in cutting forces that introduce excessive tool deflection and power requirements with the issue 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 the indexable insert, focusing on resultant cutting forces, edge entry, chip formation and stability, as well as insert location and clamping. 

At the cutting edge

When the edge of a milling cutter engages with a workpiece, the sudden impact will give rise to tool vibration with the resulting cutting forces being very much dependant on the uncut chip thickness, which is proportional to the feed. The initial tool vibration tends to change the thickness of the uncut chip, which then may continue to grow as the variation in forces feeds larger vibrations back into the system. The cutting force direction, along with the amplitude of the force, largely determines vibration tendency. This type of regenerative vibration is widely known as chatter and if left unchecked, the amplitude of the force can grow, leaving machined surfaces that are poor in finish, and even put the cutting edge and tool at risk as well as negatively affecting the machine spindle.

For this reason, the cutting force amplitude has to be dampened at the start and again later if it should begin to grow again during the cut. However, in many cases this has to be done by design of the insert geometry.
The establishment of a satisfactory model that calculates and predicts cutting forces accurately was one of the main foundations for the development of 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. This was to a great extent based on calculations of vibration wave shapes from measured modal parameters. 

Land of plenty

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. Logically, this effect should be possible to apply intelligently to dampen vibrations in other types of milling. The challenge is how to apply a designed flank wear land suitably as a primary relief. It needs to be at a precise angle, width and extent to the cutting edge in order to provide the right dampening effect and complement other insert design features.

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 that is part of the cutting action. The short, occasional contact between the designed primary relief land and workpiece is so slight that it is has no effect on performance, wear development or burr formation. The result is a substantially smaller variation in radial cutting force. The success of this innovative solution lies in the dimensioning and positioning of the primary relief land in relation to both insert geometry and cutter diameter. FEA with simulation of the cutting process was used to evaluate resulting cutting forces and chip formation as well as the distribution of stress on the inserts.

The diameter factor

Of course, small-to-medium tool diameters are more vulnerable to deflection, whereas large diameters are more stable and not in need of the same dampening. Neither is feed rate 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.

Aluminium is rated as a material with good machinability. It has a specific cutting force of about a third of steel and a melting point of 625°C. The low melting point means that the temperature in the cutting zone will not rise above this temperature regardless of cutting speed. Cemented carbide inserts can withstand far higher temperatures before encountering excessive wear and loss of strength at the cutting edge.

However, a frequent problem in machining aluminium 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 when machining aluminium at high cutting speeds is consequently of great advantage.
From a tool perspective, tangential cutting forces have a decisive influence on power requirements. 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, giving a more positive insert, but also aligning it with the rest of the insert geometry, resulting cutting force can be minimised. The new CoroMill 790 insert design results in a sizeable lowering of the power requirement. 

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 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 – a crucial factor when machining thin walled components.

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 has led to a more intelligent insert structure that not only performs better but is equally secure in strength terms.

Sandvik Coromant
www.sandvik.coromant.com