CNC Milling

Why and how should die casts be milled?

Die casts of aluminium, zinc and other alloys used in die casting technology frequently require an additional process of machining. Strict requirements related to narrow tolerances and finishing of the item surface result in a need to use precise machining techniques. In this article, we will try to present the subject of CNC milling for products delivered by us as a die casting company.

When we talk about milling of die cast products, we mean machining of product surfaces. Referring to a definition of milling, it is a situation when a rotating tool removes material from a processed item. The tool itself moves along a specified path in relation to the processed item. Blades of the machining tool remove material in form of shavings.

Die cast design

Due to technological limitations of the die casting, or economic restrictions associated, for example, with a scale of production, operations such as milling, drilling or threading are necessary. In many cases, it is not possible to obtain directly from a mould a die cast that would meet all requirements specified in the documentation.

The die cast design should take into account technological drafts, possibilities to open moulds, excess material for processing, subsequent finishing of visible surfaces, requirements concerning, e.g., galvanic coating, and other requirements associated with the die casting technology or requirements for the finished item.

A technology for production of a finished product may vary in individual cases. However, it will always start with removing die cast from the mould cavity. Then the feeding and the venting systems are removed. Already at this stage, traces of a division are removed, and at the next stage the item can be delivered to the CNC table for necessary drilling or milling operations. After this processing, barbs created during machining must only be removed and the item can be transferred for a final quality control.

Fig. 1. A model without angle drafts at an opening (top), and a model with an angle draft and an excess material for processing (bottom)

Documentation versus technology

The simplest example where processing is required are threaded or fitted holes.
To release a die cast from the die mould, drafts must be incorporated. While in the case of threaded holes it is not always necessary to drill openings formed by mould cores, then in the case of fitted holes, drafts and excess material left for machining need to be removed. When it is possible to obtain a hole dimension for threading directly from a mould, with tolerances maintained, e.g., in the case of M6-6H, the hole diameter for a screw tap ranges from a minimum of 4.134 to maximum of 4.334, thus threading can be performed without drilling the hole. In the case of deeper holes, where the angle draft determined on the basis of a tolerance of a hole for the screw tap will be too small to release the core, the obtained hole needs to be drilled first, and then threaded.

Certain elements of the item have a specific location, which cannot be obtained from the mould when tolerances are narrow. For example, when we have a hole with a position specified in relation to a certain surface of the item. In that case, the base surface and the hole need to be processed.

Die cast products are frequently used as housings of various mechanisms. Sometimes such housing consists of several cast items screwed together. When the mechanism installed inside requires lubrication, then contact surfaces of die casts and between cast item surfaces and elements of the mechanism protruding outside need to be sealed. The documentation will specify a tolerance for flatness of such surfaces, which cannot be achieved in the die mould due to stresses occurring in the casting process. In certain cases, traces left by pistons or the feeding system need to be removed. By planning the die cast surface, we can meet requirements specified in the documentation.

Fig. 2 Flatness tolerance of 0.3 mm on the entire surface, and of 0.1 mm at a section of 20 mm.

Fig. 3 Hole diameter of Φ78 with fitting H7, and tolerances for roundness and alignment.

Micro and macro scale for the die casting

It is impossible to list all cases when milling on the CNC machine is required. However, the above description provides a certain picture for the product designer, emphasises limitations of the die casting technology, and shows possible solutions. On CNC machines, machining can be performed with a micronic precision. With a correctly selected processing technology we can easily achieve dimensions of IT5 grade or lower.

Machining ensures not only a required shape of an item. It ensures that the required surface finish is achieved, impossible to be obtained directly from a die mould. Stiffness of CNC machines allows to achieve very low roughness, so grinding is not necessary in many cases, including both flat surfaces, surfaces of holes, and formed surfaces.

Modern CNC machines ensure high accuracy, fast processing, and a possibility of performing operations at multiple axes. Therefore, in many cases all necessary operations can be performed for one mounting, and this in turn translates into accuracy and repeatability related to limiting errors in the item setting, occurring when an item is mounted several times. The obtained results are determined by the accuracy offered by the machine and a correctly selected processing technology. Different details require different approach to processing. We do not need a 5-axis processing centre in every case. Frequently, a machine with three axes suffices, even when a few mountings have to be used.

CNC Machine, holder, tool

Die casts frequently have a complex shape, so they cannot be mounted directly in a vice. Then accessories for processing, special holders or seats are required to enable repeatable mounting of items or placing a larger number of them on the processing machine table. From the economic point of view, we should attempt to place the largest possible number of items, to minimise the time when the machine is not in operation. A device used to mount items should be simple and enable easy setting and basing of an item.

The actual processing is not possible without correctly selected tools. In this short article it is not possible to list all examples, but we will try to focus on those that are most important or interesting from our point of view. Catalogues of tool suppliers are very extensive, but we are mainly interested in tools used for aluminium processing. In such tools, blades have different angles than in tools for processing of hard steel. They also differ in material of which they are made, coating, geometry of shaving breaking, etc. The tool storage in the CNC processing centre will be equipped with drill bits, cutters, chamfers, screw taps, reamers, countersinks, boring bars, and milling heads. When there is not enough space in the tool storage, special tools, such as twist drills, can be considered, allowing drilling of two coaxial diameters during one drilling operation.

A correctly selected tool will not only ensure the desired effect in form of a high quality item, but also lead to savings resulting from longer and more effective service life. Therefore, by ensuring optimum conditions for processing, the required economic result will be achieved.