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Laser & Waterjet Profiling
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A number of different tool criteria need to be fulfilled in order to meet the increasingly stringent cost and quality demands associated with the successful machining of aluminium aerospace components. Until now, solid carbide tools have been deployed to perform several of these router type operations, but recent innovations from Sandvik Coromant have brought indexable insert tools into the equation as Solutions reports.
Solid carbide tools have traditionally fared well largely because of their sharp edges and geometries that offer the low cutting forces beneficial for finishing aluminium, as well as ample space for unobstructed chip evacuation.
Additionally, carbide is very stiff – the deflection of solid carbide cutters is approximately only a third of that exhibited by indexable insert tools under the same load. A further advantage of solid carbide milling cutters is the presence of a helix to give ultra smooth entry and exit cuts, as well as smooth chip flow. These factors help minimise cutting force variation as a potential source of vibration.
Indexable insert tools have been used effectively in the rough machining of aluminium when medium to large diameter cutters (25mm to 100mm) can be deployed. Here, regrinding is eliminated and the versatility, security and metal removal capacity of end mills with inserts have already provided aluminium machining with unsurpassed capability. With finishing processes however, indexable inserts have traditionally fallen short of the required quality but now a new solution from Sandvik Coromant is said to addresses this issue. CoroMill 790 features new edge, insert, seating and clamping technology to provide a new option for aerospace manufacturers.
In the process of developing a new end mill concept for aluminium machining, several parameters were acknowledged as being crucial to a breakthrough for radial milling with indexable inserts. Among these were good chip formation, a smooth cutting action, high metal removal rate, low power consumption regarding quantity of material removed, high surface finish with minimised mismatch and strong tool security at rapid spindle speeds.
Conventional indexable insert edges have tended to be comparatively blunt for aluminium machining, leading to a ploughing effect, particularly when cutting thin chips in finishing operations. The entry of the edge into the cut has also tended to be abrupt, leading to sudden gains in cutting force.
Combined, these properties can lead to excessive tool deflection and power requirements. The problem is compounded further by the need for an edge that must be both positive and sharp for finishing and capable of high removal rates when roughing. Subsequently, there is a real demand for a fresh approach to the indexable insert concept that concentrates on resultant cutting forces, edge entry, chip formation, stability, insert location and clamping.
Cutting edge developments
The abrupt impact of a milling cutter’s edge when it engages with a workpiece will create tool vibration. The resulting cutting forces are very much dependant on the thickness of the uncut chips, which is proportional to feed. The initial tool vibration has a tendency to alter this thickness, which then might continue increasing as fluctuations in force feed larger vibrations back into the system. Cutting force direction and amplitude largely determine vibration tendency. This form of regenerative vibration is more commonly known as chatter, and if unaddressed, force amplitude can rise, leading to poor surface finish. In some cases even the cutting edge, tool and machine spindle can be put at risk. Cutting force amplitude has to be dampened at the start and again later if it begins to grow during the cut. However, in most cases this has to be achieved through insert geometry design.
The development of a model that calculates and predicts cutting forces accurately was one of the principal foundations for a new patented insert geometry design from Sandvik Coromant. Innovative FEM simulation helped provide answers to the combined design of edge line, rake angle and chip former, as well as the creation and optimisation of a new edge feature on the insert clearance face – a precision primary relief land.
The promised land
It is a well established fact that in cast iron milling, the formation of flank wear on the clearance face of the cutting edge offers some dampening of vibration. The worn ‘land’ begins to grind against the machined surface, absorbing energy and modulating the vibration amplitude. Logically, it should be possible to apply this effect to dampen vibrations in other types of milling. The challenge is how to apply a designed flank wear land effectively as a primary relief. It must be at a precise angle, width and extent to the cutting edge to offer the correct dampening and complement other insert design features.
If applied correctly, the primary relief land forms a buffer, breaking up any increases in deflection amplitude and thus controlling radial cutting forces and chip thickness. Using 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, combating any amplitude growth that occurs during machining. The result is a constant steadying effect that is part of the cutting action. The short, intermittent contact between the primary relief land and workpiece has no impact on performance, wear development or burr formation and the upshot is considerably less variation in radial cutting force.
Typically, for a 25mm end mill, 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 – a material rated with good machinability. Aluminium features a specific cutting force of about a third of steel and a melting point of 625°C, which is sufficiently low that the temperature in the cutting zone will not rise above this level regardless of cutting speed. Cemented carbide inserts can resist far higher temperatures before enduring excessive wear and loss of performance at the cutting edge.
However, a common problem in machining aluminium at high speeds is the requirement for sufficient machine power, leading to a sometimes disadvantageous ratio of material removed per power unit. From a tool viewpoint, tangential cutting forces have significant influence on power requirements. Reducing the power needed per volume of material removed has a positive effect on aluminium milling, typically through higher productivity. As well as determining the ease of cut, the rake angle also affects tangential cutting force. By increasing this angle to offer a more positive insert, but also aligning it with the rest of the geometry, resultant cutting force can be minimised. Sandvik Coromant’s new CoroMill 790 insert design results in a considerable lowering of the power requirement.
The cutting edge entry into the material when milling must be gradual as this will affect the rate of growth, magnitude and direction of radial cutting forces. It will also affect tool deflection and the amplitude of any form errors on the workpiece.
Sandvik Coromant discovered that by designing the edge of the new CoroMill 790 insert geometry to be higher and more extended, it offered a prolonged and advantageous entry effect – lessening the shock effect substantially and leading to minimised mismatch on radially milled faces. What’s more, the axial cutting force is also reduced, so that the pressure exerted by the tool on the machined surface is less – a critical 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 chips and how they are ejected from the insert pocket – out and away from the cutting zone and workpiece surface. The new geometry also creates a smaller contact area between insert and chip, which means reduced friction, enhanced cutting action and the capacity for larger depth of cut.
Despite the insert cutting edge seemingly being weakened by a sharper edge and deeper chip forming geometry, stress levels are no greater than in comparatively less sharp cutting edges. A more systematic approach in tandem with advanced calculations, simulations and testing has led to a more intelligent insert structure that not only performs better, but is equally secure and strong.