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CNC Turning

CNC Turning of die casts

Written by Published: 22.04.2021

When is the cnc turning required?

The process of machining on a machine called lathe is called turning. The previous sentence provides clear information; however, when you take a closer look at currently used CNC machines, some doubt may arise. We talk about turning when the machined item performs only a rotating movement, and a tool (usually, a tool bit) moves in a longitudinal or a transverse direction in relation to a rotation axis. Thus, the simplest lathe is a two-axis machine. But in the era of multiple axis machines, the options are much more extensive. A turning centre can also be used for milling, chiselling, or automatic changing of a mounting position, besides the standard turning process.

Options offered by modern CNC processing machines allow us to completely machine an item on one machine to obtain a finished product, while in the past we needed several machines. One item can be produced on different machines, but eventually we will select one that best suits a given application in terms of economics. Time is money.

CNC turning of die casts

Usually, a die cast item must be machined for several reasons, including removal of excess material, and obtaining a shape and dimensions specified in its documentation. Similarly as in the case of milling, the need for turning of die casts results directly from the requirements specified for a finished product, or from economic reasons (a complex shape of a mould, a scale of the production, etc.).

Die casts to be machined on the lathe usually have a shape similar to bodies of revolution, such as items that are cylindrical or conical on the outside or inside. For example, the turning is useful when surfaces described in the documentation as fitted are to be produced, such as bearing housings or surfaces engaging with gears or clutches. The die cast design may require machining of grooves for sealing, or base surfaces. Many other solutions are possible, specific for each die cast product.

In certain cases, a die cast is processed only to remove excess material or draft. It is especially frequent in the case of central openings, when a formed shape results from the die cast technology.

Fig. 1 Part of die cast with an inner opening, in cross-sections. A shape before and after machining.

Milling on a CNC lathe.

As we have already mentioned, modern lathes, or rather, CNC turning centres, also offer a milling option. Driven chucks can mill along, perpendicularly and at an angle to the spindle axis. This results in a wide range of processing possibilities, and an item removed from a holder can immediately be transferred to quality control.

We talk about milling when a processed item is immobile, and a tool performs a rotating movement and moves in relation to the milled item. When does this apply to a die cast that needs to be machined on a lathe? This situation results from the item shape, for example, transverse holes, grooves, and any other transverse shapes that cannot be obtained directly in the mould.

Fig. 2 A forming insert with an additional division, for production of an internal insert. Wire cutting is necessary to adjust the hole shape to the outer contour of the insert.

Mould cavities are formed on inserts. Frequently, they are additionally divided into smaller inserts. When a shape for insert installation is a hole that is either round or rectangular with rounded corners, the process is easy. It is more complicated when the parting line runs along a casting fin. Such contour can be milled, but wire cutting (WEDM) is a much better and more precise method for its finishing. The machining can easily be conducted in material that was already thermally processed. This way both forming cavities and inserts can be produced.

The wire electrical discharge machining (WEDM) is a very useful technique in many areas of tool construction. Of course, it is not necessary, but has its clear advantages, and in many cases it is indispensable. The costs of processing is significantly higher than in the case of machining, but selection of an appropriate technology should be optimised as far as possible. Frequently, a right balance needs to be struck between the time, the precision, and the price.

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