End mills and HERMLE

As you can imagine, end mills are vital equipment in any HERMLE machining centre. But what factors determine the quality of an end mill? Why are some end mills made of HSS and others of tungsten carbide? What is tungsten carbide anyway? Are there even harder cutting materials available that are even more robust? What about applications like drilling, face milling, profiling, chamfering and deburring? Or smoothing and roughing? What mills are best for which applications and when? Read our article below for answers to these questions and more. 

End mills: the basics

So let's begin with the fundamentals. What is an end mill exactly? It's a tool – a mill, of course – used in metalworking and in numerous industrial applications, as it is at HERMLE, although it can be used for processing wood and plastic too. It can be deployed in any application where a workpiece needs to be stripped of excess material, forming chips in the process, and then cut into shape. Unlike side milling, where the cutting tool moves perpendicularly to the surface of the workpiece to remove material from its side, the cutting tool in end milling rotates around its axis and moves axially along the length of the surface of the workpiece to remove material from its end. Generally speaking – or at HERMLE, at least – the end mill is clamped in a machine such as a CNC milling centre.

So what does an end mill look like? It consists, firstly, of a cylindrical shaft that can be clamped into the chuck or milling spike on the milling machine and, secondly, a milling head that can have different shapes, bits and cutting edges depending on the requirements and the task in hand – whether slot milling, pocket milling, face milling, contouring or other milling applications. But more about that later.

While square end mills are the most commonly used type, end mills come in not only different shapes and with different cutting edges but also in different sizes and designs capable of handling the tasks and requirements outlined above on either a large or small scale. Just to give you some idea of the scale we're talking about here, one HERMLE C 62 / C 62 U MT can machine workpieces with a mass of up to 2,500 kg, while the C 12 can be used for finishing cubic workpieces of up to 100 kg. So these factors also determine the size of the mills and their diameter – and, of course, the material they are made of. Potential materials include high-speed steel (HSS), carbide (tungsten carbide) or one of a variety of special cutting materials such as ceramic, which is both extremely hard and wear resistant. Because the challenge is ultimately to find the right fit, the material used for the end mill depends on the material of the workpiece to be machined. It has to be harder than the material that it's supposed to shape – an old wisdom of the metallurgy industry.

Different sizes? Different designs? Old wisdoms of the metallurgy industry? This sounds all a bit vague and 'rule of thumb', a world away from advanced, high-precision, CNC-programmed machining technology. So for all those who are curious: yes, there is a standard for end mills with cylindrical shafts – it's called 'DIN 844' and defines all the relevant dimensions and technical delivery conditions.   

End mills and their applications

What about the areas of application? End mills are used wherever complex shapes need to be formed repeatedly and to high precision by means of milling. After all, quality isn't about achieving the perfect result once and once only but, among other things, doing this again and again. One-off perfection is nice, mass-produced perfection an obligation. Always. And this is impossible without mills and milling – after all, nothing that is produced exists in completely ready-to-go form without needing some final touches first. And that's exactly why, in the metalworking industry, mills – or end mills – are such a vital, if not indispensable, part of the tool fleet, whether in the automotive, aviation and mechanical engineering industries or in any of the countless other arenas where precision machining is called for.

End mills are the perfect choice for a diverse range of machining tasks. Below is a list of the most common tasks and applications – including, for the sake of completeness, some that HERMLE does not provide for.

Drills, roughing cutters and lots more

Drilling: Perhaps not the most common application, but one that's entirely viable because end mills are available on the market that can also be used for drilling, say, through-holes or recesses – one example here being in foundries where through-holes are drilled into manhole covers. Unlike a drill, which drills only in the axis of rotation and straight down into the material, an end mill generally drills in a perpendicular direction or at an angle to the axis of rotation; in other words, it's designed for sideways cutting. But an end mill fitted with blades that project beyond the centre point can also be used for drilling.   

Chamfering and deburring: End mills can be used for chamfering edges or deburring sharp edges or workpieces.

Slot and groove milling: End mills can also be used for milling slots and grooves into workpieces. This is common practice in woodworking, for example to create tongue-and-groove joints.

Thread milling: Special end mills are available for milling threads in workpieces. One example of this can be seen in the manufacture of drills with exchangeable heads using the HERMLE C 32 U. Learn more about this below.

Contour milling: This is an important area of application, especially in metalworking. An end mill can be used here to create complex contours and shapes in workpieces – and, as mentioned above, to an extremely high degree of precision.

Face milling: This involves the use of face cutters – in other words, special end mills that create flat surfaces on workpieces.

Profile milling: Profile mills are used to create flat surfaces on some workpieces and specific profiles or edges on others. Special end mills sculpt these workpieces into the desired shape.

Smoothing and roughing: Smoothing is a precision machining step and roughing a coarse grinding step – but what they have in common is that they both involve removing material from the workpiece to achieve the desired dimensions and surface properties. And end mills are available for both tasks.

Looking at this list of potential applications, you can see that there are numerous different tasks requiring numerous different end mills. But choosing the right end mill depends not only on the specific machining task and not only on the material to undergo machining (see above), but also on the results you want to achieve. In short, end mills are versatile tools used in myriad industries in myriad applications in order to perform repeatable, high-precision machining operations, whereby different milling heads and cutting materials exhibit different properties and open up a wide range of machining possibilities.

Roughing cutters and other milling head types

You can tell which mill is best for which job by looking at the shaft and blade. The number of blades, for example, tells you whether a mill can be deployed as a roughing cutter or is better suited to precision tasks. End mills can also be differentiated according to the type of milling head:

1. Two-bladed: This is the universal end mill. Its large chip flutes make it ideal for machining non-ferrous metals. It is also commonly used for perpendicular machining into the material.
2. Four-bladed: This type of end mill ensures a much better surface finish because the four blades, which are distributed evenly around the milling head, make it ideal for more precise machining. Four-bladed tools are suitable both for smoothing and milling complex shapes and creating smooth surfaces.
3. Eight- and twelve-bladed: Eight- and twelve-bladed mills are also available, although these are generally used only for specialist applications requiring extreme precision and that often require correspondingly powerful milling machinery.

Milling heads, too, come in all different shapes and sizes – usually specialist end mills designed for specialist applications:

1. Ball cutter: This end mill features a hemispherical cutting edge that is perfect for performing 3D milling operations and creating rounded contours and surfaces.
2. End mill with radius: This milling head might sound similar to the one above, but it has its very own field of application. Its rounded cutting edge is used for creating radii and rounded edges.
3. T-slot mill: This is used for milling T-slots. That's what it's designed for – and what it looks like, too. The clue is in the name. It's especially common in wood processing, although we haven't got time to cover that here too.
4. High-feed milling cutter: Designed for fast feed rates and high productivity, this is used for roughing and the efficient removal of material. Though designed for coarse grinding, it has little in common with the two-bladed mill.
5. Micro-miller: The same applies to the micro-miller, which is specially designed for small-scale, high-precision machining tasks.
6. End mill for aluminium: There are almost no limits to the depth of specialisation. This mill is one example of this because its special cutting edges and chip geometries make it the ideal choice for machining aluminium.
7. End mill for plastic: Plastics can undergo milling too, albeit with special equipment. This is because milling generates significant heat that can easily melt the plastic, and also the rotational movement can easily cause the plastic to break. The end mill for plastic is designed to prevent exactly these problems.  

So that brings us to the end of our review of the different applications and milling head types. Given that even more end mill types are available on the market, this list was nothing more than just a selection. All the end mill categories outlined above are fundamentally distinct from other, similar equipment such as cylindrical cutters, indexable insert cutters, gear cutters, lathes and so on. 

What is an end mill made of?

Now let's take a quick look at the materials typically used to make end mills:

• HSS mills: These mills are made of high-speed steel. 'High-speed' refers to the fact that this steel enables higher cutting speeds because it has a higher alloy content than standard tool steel. While standard steel can start to soften at temperatures as low as around 200°C, HSS steel can withstand temperatures as high as 600°C. This makes HSS mills ideal for machining soft metals and plastics. Another benefit is that they are comparatively inexpensive.
• Tungsten-carbide mills: These mills are extremely hard and durable, so the perfect choice for machining hard materials such as metals and composites. As such, they represent the next most highly stress-resistant category of milling cutter.
• Coated end mills: To improve the durability, service life and performance of end mills, versions with special coatings – e.g. titanium-nitride (TiN) or titanium-aluminium-nitride (TiAlN) – are also available.

End mills for carbide in HERMLE applications

Following this quick tour of the world of end mills, let's take a closer look at some of the examples we gave. As we said earlier, choosing the right end mill depends on a range of factors such as the material, machining task, desired surface finish and machine capacity. After all, high-quality outcomes are possible only if you choose the right mill for the job. Our examples will, of course, focus on the machining of metal, solid-carbide metal, aluminium – and copper pieces.

Let's start with the production of a bottle stopper using HERMLE's C 12 U machining centre. The first step here is to remove material from the surface of the workpiece in a planar motion and then to chamfer and deburr the edges. The profile and face are milled to a high degree of precision. The cutting depth is maintained exactly, and machining is fast but incredibly smooth. And the angle? Perfect.

When the task is focused more on machining contours and highly complex shapes, our reference model – the carnival mask – is an impressive example of what can be created when an end mill machines a single block of aluminium. Sure, this is not something you would normally see in a mass-production context, but it's a fantastic demonstration of precision machining, smoothing and 3D profiling. It shows what can be done. That this is possible with copper, too, is demonstrated in our sailing boat project, in which the copper deck and roof surfaces were exposed using the mill. Anyone interested in both precision machining with end mills and in additive manufacturing would be well advised to take a closer look at this project.

One traditional field of application is tool manufacture. Take, for example, the manufacture of a drill with exchangeable heads using, say, HERMLE's C 32 U machining centre. Here, the drill thread is milled smoothly and with precision and the excess material gradually removed. In the field of fluid mechanics, removing material and shaping the profile of a propeller using HERMLE's high-performance C 52 / C 52 U MT line is a far more complex undertaking.      

End mills: the most important details in review

Let's summarise all this information once again. End mills are available in many different versions, made of different materials, each with different diameters and different blade counts – and not to mention with a variety of milling head types. The number of blades is an important indicator of the surface quality that can be achieved – whether coarse or fine, flat or textured, chamfered to a specific angle or with the edges deburred. The more blades, the finer the finish. We already covered the details and looked at some examples. If you're keen to learn more about end mills and what products they are especially suited to, please don't hesitate to get in touch. Nothing beats a one-to-one chat. And best of all, we'll provide you with a demonstration of specific applications directly on one of our machines.