CNC milling and die-sinking EDM in toolmaking: processes, differences and influence on component quality

1. why modern manufacturing processes are crucial in toolmaking

The demands on toolmaking have risen continuously in recent years. Components are becoming geometrically more complex, tolerances tighter and development cycles shorter. At the same time, customers expect high and consistent quality with the shortest possible delivery times. In order to meet these requirements, modern and efficient manufacturing processes are indispensable in toolmaking today.

In-house toolmaking, from design using modern CAD/CAM systems to mechanical production, is increasingly developing into a key strategic competence.

Figure 1: CAD system at GUME GmbH

It enables a high degree of flexibility in the realisation of customer requirements, shortens the sampling phases and enables a rapid response to design adjustments. Development times can be significantly reduced, particularly in early project phases, if tools are not outsourced but can be manufactured or modified directly in-house. In this way, suitable tool concepts can be developed in parallel to the customer's product development process and influence can be exerted on subsequent manufacturability. In addition, direct control of the manufacturing process enables better scheduling and planning reliability.

Two manufacturing processes play a central role here: CNC milling and die-sinking EDM. As the main machining process, CNC milling forms the basis for the production of precise contours and basic geometries. Die-sinking EDM supplements where machining reaches its limits due to complex geometries, intricate details or areas that are difficult to access. Together, the two technologies form the foundation of modern, high-performance toolmaking.

2 What is CNC milling?

The abbreviation CNC stands for „Computerised Numerical Control“ and describes the computer-aided, numerical control of machine axes.

According to DIN 8580, the milling manufacturing process is categorised in the main group Disconnect, subgroup Machining with geometrically defined cutting edge assigned. This means that the shape of the cutting edge is geometrically clearly defined. During milling, chips are removed using rotating tools.

Figure 2: CNC milling machine at GUME GmbH

Depending on the machine concept, machining takes place in three, four or five axes. While only linear movements are used for 3-axis machining, rotary axes are also used for 4- and 5-axis machining. A distinction must be made between indexed machining (3+2) and simultaneous 5-axis machining. In so-called 5-sided machining, the rotary axes are only used to position the workpiece, while the actual machining continues to take place in the three linear axes. Simultaneous 5-axis machining, on the other hand, utilises the continuous interpolation of all axes during the milling process. This allows complex free-form surfaces, undercuts and contours that are difficult to access to be produced efficiently and precisely.

Possible applications of CNC milling

In practical application, a distinction is made between different removal strategies. During roughing, as much material as possible is initially removed efficiently with the allowance without making any special demands on dimensional accuracy or surface quality. Subsequent finishing with minimal material removal is used to achieve the desired dimensional accuracy and finer surfaces. These strategies are supplemented by processes such as high-speed milling (HSC), high-feed milling (HFC) or hard milling, which can also be used to machine hardened materials directly. These modern milling processes are made possible by the use of high-performance cutting materials and coatings based on carbide, PCD or CVD diamond in combination with specially designed cutting edge geometries.

The advantages of CNC milling lie in the high precision and repeat accuracy, the great geometric flexibility and the economical production of individual and series tools. CNC milling therefore forms the basis of modern tool production.

In mould practice, this flexibility is particularly relevant when adjustments to geometry or tolerances are required at short notice and can be implemented directly in mould making.

In addition to the actual moulds, metal inserts for rubber-metal composite parts are also manufactured using CNC milling in the area of moulded rubber parts. CNC machining enables the efficient production of precise metal parts that fulfil a strengthening function in the subsequent component as well as providing connection geometries for the surrounding structure. The surface structure of the inserts plays a decisive role in the secure and durable bond between metal and rubber.

3 What is die-sinking EDM?

Die sinking, also known as EDM (Electrical Discharge Machining), is a thermal removal process and, according to DIN 8580, is categorised as a manufacturing process in the main group Disconnect, subgroup Removal through thermal energy (eroding). In contrast to machining, the material is not removed mechanically, but by electrical discharges between a moulding electrode and the workpiece.

Figure 3: Die-sinking EDM machine from GUME GmbH

Very high temperatures are generated locally, causing the material to melt and vaporise on the surface of the workpiece. EDM machines are also CNC-controlled machines, whereby the tool carrier is moved relative to the workpiece in a process-controlled manner.

Materials for electrodes: copper vs. graphite

Copper or graphite are predominantly used as electrode materials. Copper electrodes are characterised by high dimensional accuracy and good surface quality, but are comparatively expensive to manufacture. Graphite electrodes enable higher removal rates and are more economical for complex geometries, but have higher electrode wear. Processing takes place in a dielectric bath, usually with special erosion oil or deionised water. This insulates the process electrically, cools the process zone and reliably flushes the removed particles from the erosion point. In toolmaking, die-sinking EDM is generally used downstream of milling. This downstream machining in particular requires close coordination between the milling and EDM processes, as changes to electrodes or cavities often only occur during fine machining. While CNC milling produces the basic geometry and easily accessible contours, EDM is used wherever machining processes reach their limits. Typical applications include very deep or intricate contours, sharp internal corners and undercuts that could not be produced by milling or only with disproportionate effort. Another advantage is the ability to machine hardened materials without additional pre-treatment.

The particular strengths of die-sinking EDM are the completely non-contact material removal and the associated minimisation of cutting forces, distortion and mechanical stresses in the workpiece. In addition, geometries can be realised that cannot be produced with rotating tools. The high dimensional and detail accuracy makes die-sinking EDM an indispensable addition to CNC milling in modern toolmaking.

4 What role do CNC milling and die-sinking EDM play in toolmaking?

CNC milling and die-sinking EDM complement each other perfectly in modern mould making and each perform specific tasks. Typical applications include the production of mould plates, mould inserts, cavities and cores as well as parting lines, contours and fine details. The basic structure of the moulds is mainly created by CNC milling. This includes not only the main geometry of the mould, but also infrastructural elements such as sprue systems, vents, flood grooves, tear-off edges, simple mould cavities and fixing points. At the same time, milling is used to efficiently remove material so that subsequent EDM processing can be carried out precisely and efficiently.

Die-sinking EDM is primarily used where the highest level of detail accuracy or the machining of areas that are difficult to access is required. Typical examples are the finest contours or undercuts in mould cavities that would be difficult or impossible to reach with milling tools. EDM is usually carried out in selected mould cavities, while the main machining is still carried out using milling technology. This interplay between the two processes enables complex tool geometries to be produced efficiently, ensuring both high dimensional accuracy and excellent surface quality. The prerequisite for this is a tool shop in which both processes are closely interlinked in terms of organisation and technology.

5 What is the significance of CNC milling and die-sinking EDM for the quality of the end product?

The quality of the finished mould has a direct influence on the end product. CNC milling and die-sinking EDM in particular make a decisive contribution to the dimensional accuracy and surface quality of the moulded parts. Precisely machined cavities and contours ensure that the dimensions of the rubber or plastic parts correspond exactly to the specifications and can be reliably demoulded.

The service life of the mould also depends heavily on the quality of the machining and the material used. Aluminium moulds are lighter and well suited for small series or prototypes, but wear out more quickly. Standard steels offer high stability and a medium service life, while hardened steels that can be machined by EDM or hard milling are particularly durable - ideal for series production with high quantities in injection moulding systems.

The relevance is particularly clear in the rubber and plastics sector: high-quality tools ensure uniform surface structures, clean edges and reproducible moulded parts, which reduces rejects and minimises rework. This makes it clear that the choice of manufacturing processes and the quality of their execution directly influence the performance and cost-effectiveness of the entire production process. Equally decisive is how quickly tool-side adjustments can be implemented on the basis of component feedback.

6 CNC milling and die-sinking EDM: Two processes, one goal - precise toolmaking

CNC milling and die-sinking EDM form the indispensable pillars of modern tool production. Each process has its specific strengths and limitations, and only their interaction enables the efficient production of complex tools with high dimensional accuracy. The right choice of manufacturing process is a decisive factor for quality, cost-effectiveness and process reliability.

Looking to the future, automation and digitalisation are becoming increasingly important in toolmaking. Intelligent process controls and the digital mapping of production help to shorten processing times, minimise rejects and further increase flexibility, e.g. in terms of component changes. This means that toolmaking remains a key success factor in the production of rubber, plastic and other moulded parts, even in the face of growing demands.

The efficiency of these processes depends not only on the machine technology used, but also to a large extent on the integration of toolmaking into the overall development and production process.