Fronting up to the groove

Higher chip volume with trochoidal and plunge milling

You don’t always need the end milling cutter or right-angle milling head to mill open grooves. The new trochoidal and plunge milling strategies, combined with the specially developed trochoidal milling tools from Seco Tools, enable even low-performance machines to achieve a considerable chip volume.   If grooves with a high depth of cut are to be milled on weaker machines, the depth of cut and the feed speed must be drastically reduced, as the machine is otherwise overloaded and vibrates. The result: much longer machining times. This can be avoided with trochoidal milling. In this process, the continuous rotation of the milling cutter is combined with a linear feed movement. In this way, the required groove width is created with a high depth of cut in a “slab milling process”. Shell end mills with inserts and cylindrical carbide milling cutters are used here, among other things, whereby the tool diameter is smaller than the groove width to be created. The low radial infeed leads to a clear reduction in tool and machine load. By increasing the cutting speed and the feed, machining is more economical than with conventional groove milling.

Axial distribution of force through plunge milling

In conventional milling, where one layer is always removed each time, radial forces are mainly applied to the spindle. In the case of high projection lengths and high shear force in particular, this results in considerable bending moments. If large volumes of material then have to be removed on thin-walled workpieces, the feed and cutting speed must be reduced, otherwise the vibrations are too great. Plunge milling offers one solution here. In this procedure, machining does not take place at the perimeter of the tool but on the front side. Machining is carried out through the offset plunging of the tool. The load is mainly applied vertically on the spindle and workpiece clamping and, therefore, only in the direction of the greatest machine rigidity. As a result, higher cutting values are possible, in particular for unstable machines.

"Open groove" cycle for ShopMill and ShopTurn. Together with Seco Tools, Siemens has developed the “open groove” cycle, which enables automatic path generation for trochoidal and plunge milling once a few parameters have been entered

Cycle for trochoidal milling on open grooves

Previously, the NC programs for complex path movement in trochoidal milling were generated by CAM systems. With ShopMill, this milling strategy is now available directly on the control system. The dimension and position of the groove is queried on the screen. ShopMill uses the parameter details, including the groove width, milling diameter and machining type (rough-work or smoothing) to automatically determine the path movement of the milling cutter. Same direction, counter-direction and – for maximum chip volume for roughwork – a combination of same and counter direction can be selected as the milling direction. Material is removed in both directions of the oscillating movement of the milling cutter. The cutting length of the milling cutter can be set using the infeed depth. As a result, the optimum cutting length is always used.

Cycle for plunge milling on open grooves

Plunge milling is ideal for creating deep cavities and grooves with unstable machines and workpieces. In plunge milling, the milling cutter enters the material vertically and pulls back again once it has reached the maximum plunge depth. This upward movement is carried out if possible at less than 45 degrees with a wrap angle for the milling cutter of less than 180 degrees, or otherwise vertically upwards. The plunge operation is continued on alternate sides along the groove. For plunge milling, as in trochoidal milling, the machining type can be rough-work, smoothing of edge and/or base and pre-fi nishing. If a lot of residual material remains on the groove walls if rough-work is used, it is benefi cial to mill the walls to the fi nishing overmeasure using pre-finishing. Chamfer machining is used to break the edges after the groove has been milled.