Steel milling is a process of forming elements made of an alloy of iron and carbon (sometimes: with other additives). It is performed with machining tools – traditional (like routers) and CNC mills.
That way of forming steel is applicable for many types of that alloy; from stainless steel (the most popular in CNC milling machining), through tool steel, to cast steel. The process of milling steel elements allows the production of a wide range of elements of almost any shape while delivering high precision of production.
It is worth mentioning – milling combined with numerical control (called CNC milling) is currently one of the industry standards due to the ability to produce high-quality elements fast and with keeping negligible tolerances.
Steel milling (stainless and other types)
Machining a block of steel with a milling machine is called the loss method. It means the shaping process is based on removing excess material from the block. It is done with a special cutter inserted into the spindle rotating at high speed.
Steel is one of the materials that can be formed that way and just like other alloys (e.g. aluminum) its machinability is defined by the contents of carbon and other additives (chromium, molybdenum, etc.).
The steel itself is an alloy that combines iron and carbon (less than 2%; over that amount is called cast iron) and other elements. They can provide additional features for a particular alloy (e.g. Cr-Mo 4130 steel, with additives of chromium and molybdenum).
Steel stands out as a hard material, with high tensile strength and elasticity. Thus – that alloy is more fragile, but many types of additives can moderate that problem to some extent.
Stainless steel is the most popular type of that alloy. It is used in the production of a vast variety of parts and elements: from small bearings, through tools, up to big construction elements. Many types of that metal can be divided by their physical and mechanical characteristics.
The most popular categorization of steel is:
- constructional steel (used in steel constructions and some types of parts),
- tool steel (used for tool production),
- special steel (magnetic, heat resistant, and the most popular one: stainless).
On the other hand – stainless steel can be categorized as:
- ferritic steels,
- austenitic steels (70% of stainless steel production),
- martensitic steels,
- ferritic-austenitic steels are also called "duplex steels".
Steel milling - CNC and manual – what to know?
The steel milling process (CNC and traditional) can be divided into 4 key stages:
- initial machining – the bulk of the material is removed from the block of steel,
- medium precision machining – the element takes shape close to the final,
- high precision machining – the element is finally formed and has its final shape,
- finishing machining – focused on sanding and polishing the surface of the
element.
Elements machined that way are ready for further treatment: e.g. hardening or coating surfaces with additional layers.
Like aluminum milling, stainless steel milling requires many adjustments for machining
blocks of metal.
For the machinability of stainless steel, the most important factor is the carbon content. Lower, the whole process is more challenging, due to the softness and stickiness of the shavings. It can generate significant problems by sticking to and blocking the spindle.
With the higher content of steel, shavings get more and more fragile, thus, cut from the
block of machined steel shader. All of that makes machining steel challenging, especially with the CNC mills.
During manual milling, the operator can see the spindle the whole time and will notice problems with high amounts of shaving. While machining steel with a CNC mill the operator needs to be aware of the limitations and requirements of a particular type of alloy. Mainly: to program the whole process with the right parameters. In other words: milling steel with a CNC machine requires the operator a lot of experience with numerically controlled equipment, but also with metallurgy and characteristics of metals.
Steels with the content of carbon of approx. 0,25% are considered the most machinable (especially without any thermal treatment). It is worth mentioning, other additives can improve or worsen the machinability of steel. For example: molybdenum, chromium or nickel are improving the mechanical durability of alloy and also improve its machinability.
Another important parameter of steel milling is a balance between hardness and milling velocity. The higher the hardness of an alloy, the lower the cutting velocity should be.
During milling steel you always need to consider the selection of the right:
- cutters – due to the hardness and overall machinability of the material,
- rotating speed of the spindle,
- the velocity of milling – that means: what will be the velocity of every pass of the milling spindle,
- effectiveness of the cooling milled area. Due to significant (thermal) energy transfer, machined elements can overheat very fast. Poor heat dispersion can lead to overheating and cracks in the structure of machined elements. Also: can damage the cutting tools.
Although important, these are areas of much higher significance for professional CNC operators. Their knowledge allows them to speed up the whole steel milling process and ease the overall process.
The support from companies focused on CNC machining of steel makes the whole process much easier. You just need to describe the expected part and then order it – professional CNC operators will produce these elements for you.
Types of steel and its machinability
There are many types of steel – with different characteristics and abilities for the machine. Selection of the right type is one of the most important elements of the production of high-quality steel elements.
Although many types of methodologies for the machinability of steel, there is a strong emphasis on the correlation between carbon content and ease of milling steel.
It means that:
- low carbon steels, with carbon, containing below 0.15%, are hard to machine. Milling such material with CNC routers can be problematic. If usage of that type of steel is necessary, its machinability can be improved by cold forging block of steel,
- steels of carbon contain approx. 0.25% – has one of the best machinability potentials of all steels.
Further machining and processing of steel-milled parts
Machining steel with a mill or a router (CNC or automated) rarely is the last step of production of that element.
There is a vast variety of further processes. They can improve the hardness of the surface and increase elements inside structure. This process is categorized due to the type of impact of the milled steel parts:
- thermo-chemical processing of steel,
- thermal processing of steel.
For the thermo-chemical processing of steel, you can include the enriching surface of an object with elements during heat treatment. Such a surface has better resistance to the outside elements and can be more aesthetically pleasing.
The most typical types of thermo-chemical processing of steel are:
- chroming – coating steel part/element with chromium; improves heat transfer and corrosion resistance. Also: makes the surface more aesthetically pleasing;
- nitriding steel (nitro-coating) part/element with nitrogen oxides. It provides better corrosion and tear resistance,
- sulfuring – coating steel part/element with sulfur oxides to improve its wear resistance. Often chosen to improve the surface of milling parts.
Heat treatment is focused more on the improvement of properties for the whole part/element. That process focuses more on heat management – heating and then cooling elements in a controlled environment.
Among the heat treatment methods for improving milled steel parts it can be mentioned:
- hardening – the most recognizable steel-improving process. It focuses on heating steel elements and then rapidly decreasing their temperature, e.g. by immersing elements in cold oil. That type of process improves the internal structure of the part,
- annealing – similar to hardening. But the cooling of heated-up elements is slower and such elements can be kept at high temperatures for a longer time.
Steel milling - contact us
At RADMOT, we can offer CNC milling services, CNC turning services as well as many additional services, including washing, aluminum anodizing, laser marking and assembly.
Contact us and tell us what you need. We have been providing CNC services for almost 40 years. Our quote is completely free. And if you are in doubt about which technology will work best for you, we are able to advise customers from many industries on machining and quickly determine the price of machining the parts you order - our expertise is at your disposal.
