BAD VIBRATIONS – SAFELY CONTROLLING VIBRATION DURING MILLING

A common problem when machining moulds or dies with deep or complex cavities is the vibration that builds up in machining tools with long overhangs. Most of the time, this vibration can be controlled by changing a few process parameters. Christian Hämmerle, who is in charge of the industrial mould and die making division at machining specialist Walter, explains the most useful action to take. 

Vibration – a multiple risk for process reliability

In the world of machining, unlike pop music for example, vibration is actually always a bad thing, only some types are even worse than others. The basic principle is to avoid vibration as far as physically possible and economically feasible. But why is vibration in the machining process actually problematic? Basically, vibration is nothing more than motion impulses and forces that have an undesirable effect on some or all elements of the machin­ing process: On the spindle (and therefore directly on the machines to a greater or lesser extent), on the milling cutter with the indexable inserts, and on the component to be machined itself. In the worst-case scenario, vibration damages the tool spindle or parts of the machine tool. However, less extreme damage, such as spindle deflection or tool breakage, is typical of everyday machining in mould and die making. Due to the occurrence of vibration, milling cutters and/or their indexable inserts wear more quickly, but above all unevenly and uncontrollably. This not only increases the tool costs per work-piece, but also leads to poorer-quality results. The surfaces produced are uneven, rippled, rough or look like they are full of streaks. The mould can then either no longer be used for injection moulding or other moulding processes involving plastics, or produces a result that is suboptimal in terms of look or feel. This is why vibration in machining processes must be kept as low as possible and as uniform as possible. 

Vibration cannot be completely prevented, especially when milling deep cavities. Physics takes care of that. Plunging into the material not only exerts the desired axial forces on the tool, which press the tool in the spindle direction onto the machine’s bearing assemblies, thus keeping both stable. Depending on the tool’s lead angle and overhang, which is required due to deep cavities, radial forces also occur, which can be very strong. They in turn push the milling tool away from the contour so that it leaves the cut. It starts to vibrate.

High metal removal rate and process reliability during high-feed milling

One of the conditions that cause vibration, the lead angle of the milling tool, also offers a solution to the problem. If other conditions such as the clamping situation, the rigidity of the machine and wall thickness of the workpiece allow it, a milling cutter with a smaller lead angle should be selected. Numerous companies in the mould and die making industry therefore rely on the Walter Xtra·tec^® XT M5008 high-feed milling cutter for roughing deep cavities. By combining a low depth of cut with high feed per tooth, the milling cutter achieves a very high metal removal rate. With its small lead angle of 10°, the axial cutting forces dominate, enabling a stable machining process. This makes the milling cutter ideal for use with long projection lengths, as it minimises their vibration tendency. For the M5008, Walter offers a comprehensive range of indexable inserts for all material groups that mould and die manufacturers typically encounter – ISO P and H, as well as ISO K and S. The double-sided, rhombic indexable inserts with four cutting edges allow small tool diameters and high numbers of teeth. Tough cutting edges with easy-cutting geometries and the innovative Tiger·tec^® cutting tool materials allow machining with high cutting data and long tool life.

Ultimate concentricity and quick tool changes

The other starting point for minimising vibration when machining deeper cavities is the overhang or clamping fixture itself. In general, the following applies: Only use extensions and compatible adaptors or damping systems once you have exhausted all other options. The first choice when machining cavities or when the component’s clamping setup is somewhat unstable is the use of tools with different, gradually increasing lengths and diameters. However, this approach is not always possible, especially with deep or complex cavities or entire components. In this case, there is no alternative to using extension adaptors. Many mould and die manufacturers also prefer to use shrink-fit chucks for milling tools. This is a proven technology that offers excellent concentricity and strong clamping forces. But especially when milling (and also drilling) in longer clamping lengths, shrink-fit chucks have little damping effect in terms of vibration. 

For almost two years now, Walter has been offering a far more efficient alternative here with the AB019 hydraulic expansion adaptor, and not only for machining processes in which strong vibration is to be expected. The damping effect is the result of its special design: A system of cavities filled with an oil is incorporated in the slim chuck body. The Walter AB019 combines high radial rigidity with high torque transmission, meaning that there is very little loss of machine tool performance in the process. Compared to working with conventional shrink-fit chucks, milling with the Walter AB019 adaptor achieves much better surface quality. Concentricity and indexing accuracy are below a deviation of 0.003 mm. The tool life of the milling cutters and drills used in this way can be increased by up to 40 % in some cases. The efficiency gains in terms of set-up time should also be emphasised. The operator at the machine changes milling cutters or drills about 90 seconds faster with a Walter hydraulic expansion chuck than with a standard shrink-fit chuck. The tool length can be precisely adjus-ted using a screw, and the tool is clamped against an adjustable stop. 

Reliably mastering difficult projection lengths

Vibration becomes particularly difficult to control­ when working with a very long adaptor. In this case, a conventional tool adaptor further amplifies the vibration that occurs at the mill­ing cutter. In order to minimise this effect, low cutting­ data is used. Users achieve significantly better results with much higher cutting data when using a Walter Accure-tec tool adaptor, which has active vibration damping. A radially and axially flexibly suspended flywheel mass in the tip of the tool adaptor compensates here for the vibration occurring at the cutting edges of the milling cutter. The vibration damping­ is preset ex works. The tool adaptors can therefore be used immediately without any specific fine tuning. Especially applications­ where high volumes have to be machined can be completed much more efficiently and quietly with an Accure-tec adaptor – with out­standing surface quality and improved life of tool and spindle. An integrated coolant supply ensures optimum chip removal.