BIG STEPS FORWARD FOR ALUMINUM MACHINING
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 this patented 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 25 mm end mill, the land can be 0.1 mm wide and angled at 1 deg to follow the curved cutting edge precisely between certain points, with a rake of 20 deg for aluminum – a material rated with good machinability.
Aluminum features a specific cutting force of about a third of steel and a melting point of 625 deg 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 aluminum 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 great influence on power requirements.
Reducing the power needed per volume of material removed has a positive effect on aluminum 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 minimized.
The 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.
Designing the edge of the CoroMill 790 insert geometry to be higher and more extended offers a prolonged and advantageous entry effect – lessening the shock effect substantially and leading to minimized 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 chip forming geometry on the rake face of the insert has been deepened to reduce cutting forces and to optimize the formation of the chips and how they are ejected from the insert pocket – out and away from the cutting zone and workpiece surface. This 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.
Sandvik Coromant, 1702 Nevins Road, PO Box 428, Fair Lawn, NJ 07410-0428, www.coromant.sandvik.com/us.
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