Metal properties for machining | HEULE

COFA Tool deburrs bore edge on metal block in the CNC machining area

Key points at a glance

Fundamentals: Metals are chemical elements with characteristic properties that significantly influence their machinability. Around 80 per cent of all elements in the periodic table are metals.
Structure: The metallic lattice, with its freely moving electrons (electron gas), is responsible for the typical properties of metals, such as ductility. This property has a decisive influence on chip formation and burr formation during machining. The metallic bond holds the positively charged atomic nuclei together.
Relevance to machining: Properties such as hardness, toughness, thermal conductivity and chemical reactivity determine tool wear, cutting parameters and the achievable surface finish. Understanding these relationships is key to optimising every manufacturing process and ensuring machining is carried out with the correct tools.

What are metals? Definition and basics

Metals make up the majority of chemical elements and are found on the left-hand side of the periodic table, below the dividing line from boron to polonium. This group comprises around 80 per cent of all known elements and plays a central role in modern manufacturing. From aluminium components in lightweight construction, through copper components in electrical engineering, to high-strength steel girders – metals are everywhere.

The term ‘metal’ is derived from the Greek ‘metallon’, meaning ‘mine’. These materials are characterised by specific physical and chemical properties that clearly distinguish them from non-metals and semimetals. In the periodic table, they form a clearly defined group, although the transition to the semimetals is gradual.

Metallgitter mit Elektronen und Elektronengas

The Structure of Metals: The Metallic Lattice as the Key to Machinability

The unique structure of the metallic lattice explains all the characteristic properties of metals. In this structure, positively charged atomic nuclei are arranged in a regular pattern, whilst the valence electrons can move freely between the atoms as what is known as an ‘electron gas’. These freely moving electrons are the key to understanding the physics of metals and their machinability.

The metallic bond arises from the electrostatic attraction between the positively charged atomic nuclei and the negatively charged electron gas. This cohesion ensures mechanical stability, whilst the mobility of the electrons and the lattice structure determine the unique functions and behaviour of the metal when subjected to force.

Materials with higher ductility, such as copper or aluminium, require specially adapted tools to overcome the challenges of burr formation and built-up edge formation. Our tools are designed to offer the highest precision and durability for these materials.

Physical properties relevant to machining

The physical properties of metals are a direct result of their atomic structure. However, not all of these properties are equally relevant to machining.

Ductility and chip formation

The flexibility and ductility of metals are among the most important properties for machining. They result from the specific nature of the metallic bond. When subjected to mechanical stress from a tool cutting edge, the atomic layers in the metal lattice can be displaced relative to one another without the bond breaking.

Tough materials (e.g. copper, stainless steel): The high ductility leads to the formation of long, flowing chips and a pronounced burr. This places high demands on process control. By using tools specially adapted for these applications, these chips can be efficiently removed, for example using deburring tools from HEULE.
Brittle materials (e.g. cast iron): Lower ductility results in short breakage chips and less burr formation.
Gold (Au) is the prime example of extreme ductility and can be rolled into wafer-thin sheets.

Hardness and mechanical properties

The hardness of metals has a significant influence on tool wear and the cutting forces required. Whilst pure aluminium is relatively soft, hardened alloys such as chromium-vanadium steel or chromium-nickel steel achieve extremely high degrees of hardness. Tensile strength and yield strength are key parameters that define how a material reacts to the forces exerted by the tool’s cutting edge.

Thermal conductivity

Closely linked to electrical conductivity is the ability of metals to conduct heat. This property is critical in machining:

Good heat conductors (e.g. aluminium, copper): Process heat is rapidly dissipated away from the tool into the chip and the Workpiece.
Poor heat conductors (e.g. titanium, stainless steel): The heat concentrates at the cutting edge of the tool, leading to extremely high wear. Targeted coolant is essential here.

Chemical properties and their influence on machining

Chemically, metals tend to lose electrons. This reactivity has a direct impact on machining.

Base metals such as iron, zinc or aluminium react readily with other substances. When machining aluminium, this can lead to the formation of built-up edges, where material becomes welded to the tool cutting edge.

Precious metals such as gold or platinum exhibit very low reactivity. They are less prone to chemical reactions with the tool material. Whilst extremely reactive elements such as sodium (Na 2 – this was probably a typo for Na) play no role in machining in their pure form, the reactivity scale is crucial for the selection of tool coatings.

Classification of metals for the workshop

Metals are classified according to various criteria. In practice, classification based on density and corrosion resistance is particularly relevant.

Light metals vs. heavy metals

A density of 5 g/cm³ is taken as the limiting value.

Light metals: Aluminium (2.7 g/cm³), magnesium (1.74 g/cm³) and titanium (4.5 g/cm³) are typical examples. They are often easy to machine, but may present specific challenges such as high thermal expansion (aluminium) or low thermal conductivity (titanium).
Heavy metals: lead (11.34 g/cm³), copper (8.96 g/cm³) or gold (19.32 g/cm³). Their high density affects handling, but their machinability is determined by other factors.

Precious metals vs. base metals

This classification based on corrosion resistance often correlates with chemical reactivity during machining.

Precious metals: gold, silver, platinum. They are generally soft, ductile and prone to forming heavy burrs.
Base metals: iron, zinc, aluminium. Their tendency to oxidise is less relevant to machining than their tendency to undergo chemical reactions under pressure and at high temperatures at the cutting edge.

Alloys: Targeted adjustment of machining properties

Alloys are produced by mixing different metals or by adding non-metals. This composition makes it possible to tailor the properties specifically to certain applications and to improve machinability.

Steel: Produced by adding carbon to iron. The production of different grades is the main application.
Chromium-nickel steel (stainless steel): Combines corrosion resistance with high toughness, but is difficult to machine due to its poor thermal conductivity.
Chromium-vanadium steel: Has an application for tools, where it combines high strength with toughness.
Free-cutting steels: Targeted modifications with lead or sulphur improve chip breaking, which makes machining considerably easier.

Specific examples of metals and their machinability

The diversity of metals is reflected in the different challenges they present during machining.

Close-up of a cleanly deburred aluminum workpiece, showcasing a perfect edge finish by a HEULE tool.

Aluminium

As a light metal, aluminium is generally easy to machine and allows for high cutting speeds.

Challenge: It is highly prone to the formation of built-up edges and burrs. The long, flowing chips require effective chip management.
Solution: Very sharp cutting edges, polished tool surfaces and high cutting speeds are required.
Suitable tools: Aluminium can be machined very effectively using all HEULE systems, such as the COFA deburring tool, the SNAP chamfering tool or the BSF backspotfacer. Optimal blade coating reduces material adhesion. Dull tools are a critical issue – they quickly lead to slippage and uncontrolled burr formation.

Copper

Copper is the metal with the second-highest electrical conductivity after silver. However, it is challenging to machine.

Challenge: Extreme toughness and ductility result in long, tough chips and a very high level of burr formation, which is difficult to remove.
Solution: Sharp tools with a special geometry are required to cut the chip cleanly and minimise burr formation.
Suitable tools: Copper requires very sharp cutting edges and low machining forces. HEULE’s tools for rear-side bore machining (e.g. COFA, DL2 and DEFA) all feature defined cutting edges. They are therefore ideally suited for machining copper workpieces.

Precision-deburred steel component with a smooth, uniform chamfer created by an automated HEULE tool.

Iron/Steel

As the most commonly used metal, iron forms the basis for the various grades of steel.

Pure iron: Relatively soft and ductile, similar to copper in terms of machinability.
Steel alloys: Machinability varies dramatically. Unalloyed structural steel is easy to machine, whilst high-alloy, stainless steels (e.g. chromium-nickel steel) are among the most challenging materials due to their toughness and low thermal conductivity.
Challenge: High cutting forces and heat generation lead to tool wear and can compromise component quality.
Solution: Robust cutting materials, stable processes and optimised cutting parameters are crucial.
Suitable tools: Steel is an ideal application area for HEULE’s tooling solutions. Thanks to specially coated blades, these tools deliver consistent deburring (COFA, DL2, COFA-X), chamfering (SNAP, DEFA, GH-K) or counterboring (BSF, SOLO) even under high loads. Uncoated tools or blades, on the other hand, would pose a problem – they wear out quickly and lead to poor results.

Titanium

Titanium is a high-performance material that combines very high strength with low weight.

Challenge: The low thermal conductivity leads to a build-up of significant heat at the cutting edge. At the same time, there is a risk of stress cracks forming in the material.
Solution: Very high-quality cutting materials and controlled cutting conditions are essential.
Suitable tools: Titanium requires robust tools with high cutting control. COFA ensures reliable deburring, SNAP enables precise chamfering, whilst the BSF backspotfacer, with its carbide cutting blades, delivers clean counterbores even under high loads.

Cast iron

Cast iron is a typical cast material with uneven surfaces and tolerances.

Challenge: Irregular edges and differences in height make it difficult to achieve uniform deburring.
Solution: Tools must be able to respond flexibly to geometric deviations.
Suitable tools: This is where systems with a movable blade really come into their own. This is because the Blade only begins deburring upon contact with the bore edge. This means, for example, that castings – with their typical tolerance variations – can be machined reliably and with consistent deburring results. All HEULE solutions, such as the COFA deburring tool or the SNAP chamfering tool (see video), therefore ensure consistent results. Rigid tools are unsuitable as they cannot compensate for tolerances.

Inconel

Inconel is one of the most challenging materials to machine.

Challenge: Extreme heat resistance, high strength and the formation of long chips lead to rapid tool wear.
Solution: Special cutting materials and optimised process control are required.
Suitable tools: Inconel causes high tool wear and produces long chips. COFA enables reliable deburring even on difficult edges. DEFA is ideal for heavy burr formation, as it is specifically designed for interrupted cuts. SNAP also ensures clean, defined chamfers in a single operation.

The right tool for every Material

The properties of metals play a decisive role in the success of your machining process. Whether you are machining tough stainless steel, soft aluminium or abrasive castings – there is an optimal tooling solution for every Challenge to remove burrs and achieve a perfect surface finish.

Knowledge of materials is an integral part of any good process design. Particular attention must also be paid to health and safety, as extreme processes such as high-speed machining without coolant can generate very high local temperatures, which could theoretically lead to the formation of metal vapours.

We’d be happy to provide you with consulting services.

Our application engineers analyse your material and your specific application to identify the ideal HEULE tool for a reliable and efficient process. Benefit from our decades of experience.

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The description of machinability describes how well a material can be machined. It is influenced by hardness, toughness, thermal conductivity and the tendency to form built-up edges. Good machinability means low tool wear, good chip formation and high surface quality with low energy consumption.

Burr formation is a direct consequence of ductility. Ductile materials such as copper or stainless steel are not cut cleanly by the tool at the exit edge, but are plastically deformed and pushed aside. More brittle materials, on the other hand, break more easily and produce fewer burrs.

It determines how the heat generated during the process is dissipated. If thermal conductivity is poor (e.g. titanium), the heat concentrates on the cutting edge of the tool and quickly damages it. If thermal conductivity is good (e.g. aluminium), the heat is distributed more evenly, which protects the tool.

The metallic lustre is caused by the reflection of light off the freely moving electrons. Whilst this is not relevant to machining, it is a characteristic property that stems from the same atomic structure which also determines the material’s malleability. The colour of the lustre depends on the electronic structure.

Iron, in the form of steel, is by far the most important structural material. Aluminium dominates in lightweight construction and in castings. Copper and its alloys (brass, bronze) are indispensable for electrical engineering applications and plain bearings. Access to detailed information on specific types of metal is crucial for any manufacturing company.

Most metals can be recycled extremely well, often without any loss of quality. Aluminium can be recycled an infinite number of times, requiring only 5 per cent of the energy used in primary production. Steel is the most commonly recycled material worldwide. Precious metals are recovered almost entirely. Recycling can only become complex in the case of heavily contaminated or mixed materials.

Do you need help choosing the right material? We’d be happy to provide you with consulting.

Our materials experts will help you find the ideal metal for your specific application. Benefit from our many years of experience in materials consultancy.

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Properties of metals: A guide to machining

Key points at a glance

What are metals? Definition and basics

The Structure of Metals: The Metallic Lattice as the Key to Machinability