Steel machining can be a bit challenging, but it’s achievable with the right preparation.
What you need is a good machining tool, optimal process control, and the right grade of steel to perform the desired machining operations.
Machining steel can be performed by using a professional-level CNC mill at moderate speeds and feeds with a good flow of oil-based lubricant to minimize the risk of overheating during the machining process. Generally, it is recommended to use free machining steel for machining projects, due to its good machinability.
This article discusses steel machining in detail by going through its optimal parameters and discusses the machinability of different grades of steel.
In the end, the article also discusses the tools suitable for machining steel and sheds some light on techniques to improve the quality of machined steel.
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Steel Machining Guide: 3 Key Parameters
Steel is a tough material that can pose challenges during machining operations.
Getting these three parameters right is key to successfully machining steel.
| Parameters | Optimal Value |
|---|---|
| Speeds and Feeds | Moderate speed and feed for balanced machining. Slow speed and feed for entry-level machines. |
| Cutting Tool | 2 or 3-flute end mill is preferable |
| Cutting Fluid | Oil-based cutting fluid for better lubrication |
Speeds and Feeds for Steel Machining
The speeds and feeds should be regulated to ensure a smooth cut and minimal heat generation during the machining of steel.
Generally, machining steel with moderate feed and high-speed results in a high surface finish at the cost of tool life.
On the other hand, high speed with a high feed rate is recommended for roughing cuts, where high material removal rate is desired over a smooth surface finish.
Therefore, a moderate feed with slow cutting speed is recommended for applications that require high-quality cuts with good tool life and decent material removal rate at the cost of machining time.
However, a slow feed and slow speed are recommended for DIY applications that involve machining steel on entry-level machines.
This is because slow feed and speed minimize the risk of vibrations, thereby maximizing the surface finish and accuracy of the cut.
Cutting Tool for Steel Machining
Steel is generally ductile in nature and can produce long stringy chips, which can adhere to the cutting tool and result in sipping the tool.
Therefore, a 2- or 3-flute endmill is recommended for machining steel.
This is because fewer flutes provide easy chip clearance, eliminating the risk of clogging the cutting tool, which can often result in breaking the tool.
Apart from that, a higher helix angle is preferred for cutting pockets, as it facilitates upward clearance of chips, whereas a lower helix is optimal for machining through holes, pushing the chips downwards.
Cutting Fluid Suitable for Steel Machining
Machining steel results in high frictional heat generation, which can increase tool wear and damage the workpiece.
Hence, it is recommended to use a cutting fluid to regulate the cutting temperature while improving tool life and enhancing the surface finish.
Apart from that, a high flow of cutting fluid also helps in chip clearance by flushing the chips out of the cutting area, thereby reducing the chances of chip re-cutting, which can otherwise damage the cutting tool.
Maintaining a continuous flow of cutting fluid allows you to machine steel at comparatively higher cutting speed, greater depth of cut, and faster feed rate.
Generally, it is recommended to use a soluble oil cutting fluid (1:20) when using a cemented carbide tool to machine steel at high cutting speed.
On the other hand, sulfochlorinated mineral-fatty oil is ideal for operations such as broaching, threading, and reaming of high-strength steel and tool steels.
However, WD-40 sprays also provide good lubrication for small-scale DIY operations where relatively small workpieces are to be machined.
Machinability of Steel: Ease of Machining
The machinability of steel refers to how easily you can remove the material from a steel workpiece while ensuring a good surface finish and less tool wear.
Physical properties of steel, such as its ductility, hardness, and thermal conductivity influence machinability.
Carbon steels, with a high percentage of carbon, provide excellent strength and hardness with poor machinability.
Hence, high-carbon steels are comparatively more difficult to machine and require strong cutting force for material removal.
On the other hand, free-machining steel is re-sulfurized and re-phosphorized carbon steel that provides excellent strength, hardness, and abrasion resistance, along with good machinability.
The addition of sulfur and phosphorous facilitates chip breaking, resulting in small chips that are cleared away easily during machining, thereby enhancing its machinability.
The machinability of steel is also significantly influenced by factors like microstructure, chemical composition, grain size, heat treatment, yield, and tensile strength.
Machinability index
The machinability index can be used to compare the machinability of various grades of steel to standard steel.
Generally, the higher the machinability index, the better the machinability of the material.
Machinability index (%) = (Vi÷Vs) x 100
Vi = Speed of machining the steel under consideration for 20 minutes of tool life
Vs = Speed of machining the standard steel (SAE 1212) for 20 minutes tool life
Types of Steel and Their Machinability
| Steel Type | Machinability index (%) |
|---|---|
| Carbon steel | 40 – 140 |
| Stainless steel and super alloys | 19 – 70 |
| Alloy steel | 50 – 80 |
| Tool steels | 27 – 42 |
Steel is available in different grades that offer different properties suitable for various applications.
But which type of steel offers the best machinability?
Free machining steel offers the best machinability and is, therefore, recommended for machining applications.
On the other hand, tool steel has a poor machinability index, making it difficult to machine. This type of steel is generally used for matching cutting tools for machining other metals.
Generally, tool steel is shaped by implementing different types of forging techniques or casting.
Carbon Steel
This type of steel contains up to 2% (by weight) carbon. More broadly they are further categorized based on carbon content as low (< 0.30%), medium (0.3 – 0.5%), and high carbon steels (> 0.6%).
Generally, the machinability of carbon steel decreases with the increase in carbon content.
Apart from that, adding lead as an alloying element to carbon steel enhances its machinability, making it suitable for various applications where good strength and machinability are required.
Free machining steel, a type of carbon steel is best suited for machining applications as it provides the best machinability index when compared to other grades of steel.
Stainless Steel
In addition to carbon, stainless steel contains about 11% chromium and showcases excellent corrosion resistance. But can stainless steel rust?
Yes, it can rust under certain circumstances. However, it is more resistant to corrosion, stains, and rust than ordinary carbon steel, making it suitable for food processing equipment and accessories, medical equipment, and the aviation industry.
A small addition of sulfur slightly reduces corrosion resistance but increases machinability which makes it suitable for making fasteners, bearings, bushings, and other smaller components, which require more accurate machining due to the close tolerances.
Alloy Steel
Alloy steels are iron-based carbon steel with one or more elements other than carbon.
Each component contributes its unique attributes to produce a desired physical property or characteristic.
The variety of elements to achieve a range of improved characteristics includes-
- Chromium, molybdenum, vanadium, and tungsten improve appearance.
- Manganese, nickel, silicon, and copper enhance strength.
- Nickel and copper to improve the corrosion resistance.
- Lead, selenium, bismuth, and tellurium increase machinability.
Tool Steel
Tool steel possesses high strength and durability, making them difficult to machine. As a result, these tools are used for making hand tools, that require durability and reliability.
Due to poor machinability, tool steel is generally shaped by other metal processing techniques, such as drop forging, die casting, stamping, and sometimes machining.
Some of the common applications of tool steel include the manufacturing of chisels, dies, drills, cutters, hummers, and shear blades.
Tools for Machining Steel
| Tools | Accuracy | Cutting Time | Finish of Cut | Cost |
|---|---|---|---|---|
| Hand Tools | Good | Slow | Good | Low |
| CNC Mills | High | Moderate | Good | Moderate |
| Water Jet / Laser Cutter | Very High | Slow | High | Comparatively High |
| Plasma Cutter | Good | Quick | Good | Moderate |
The type of steel used and the application involved have a significant impact on the tooling selection for machining steel.
CNC mills are best suited for machining steel where intricate cuts with good accuracy and good surface finish are required.
Whereas laser cutters and water jet cutters are best suited for cutting thin workpieces of steel with extremely high accuracy.
On the other hand, plasma cutters are best suited for cutting steel sheets with thicknesses up to 20mm.
Generally, stainless steel is the least machinable grade of steel due to the presence of alloying elements like chromium, and the higher the percentage of alloys, the lower its machinability.
Therefore, when using a CNC mill, it is critical to use tools and inserts made specifically for stainless steel to ensure better tool life and appropriate chip load.
Carbide and high-speed steel tools are recommended for machining steel due to their high strength and abrasion resistance, allowing for faster cutting and longer tool life.
Furthermore, cutting tools with Calico Aluminium Titanium Nitride (AlTiN) coating is preferred for roughing/semi-finishing operations.
On the other hand, Titanium Aluminium Nitride (TiAlN) coatings are recommended for dry high-speed finishing and general machining.
How to Improve the Quality of Steel Machining?
Minimizing the Vibrations in the Machining Setup
Machining steel, generally involves high-speed rotating too that comes in contact with the workpiece to remove the material.
These high-speed rotations can induce strong vibrations in the machining setup, which thereby reduces the surface finish of the machined steel.
Furthermore, an entry-level CNC machine consists of a comparatively less rigid chassis than an industrial CNC machine.
This weak chassis design leads to flexing of the machine when delivering strong cutting forces, and results in a poor surface finish with reduced tool life.
Therefore, it is important to ensure no or minimal vibrations in the setup when machining steel.
Apart from this, vibrations can also arise due to poor work holding, worn-out milling chuck, etc.
The following steps can be taken to minimize the vibration:
- Proper selection of cutting edges and angles as per the requirement of the machining process
- Avoid the long overhang of the cutting tool
- Choose a small nose radius for turning and boring operations
- Use rigid tools and tool holders.
- Upgrade your machine with a more rigid and sturdy machine
- Ensure proper clamping of the workpiece
- Boltin the machine to the shop floor to minimize vibrations
Heat-treatment of Steel
Heat-treatment processes such as annealing, tempering, normalizing, etc., involve heating the workpiece to a certain temperature and allowing it to cool.
This alters the internal structure and modifies certain mechanical properties of the steel workpiece.
Generally, annealing of steel is performed to relieve internal stresses, and enhance ductility, making the workpiece suitable for machining.
While annealing a steel workpiece before machining makes it suitable for machining, annealing it post-machining relieves internal stresses and makes it suitable for further processing.
Using an Adaptive Cut Approach
When machining steel on a CNC mill, the plunging of the cutting tool into the surface of the workpiece can result in a high stress, which can damage the workpiece and the cutting tool.
Therefore, an adaptive cut approach is recommended to minimize the impact load during the cutting process, leading to a smooth surface finish and better tool life.
Generally, a ramp or helix toolpath is recommended for steel machining. In these adaptive cuts, the depth of cut increases gradually, thereby reducing the impact load on the cutting tool.
Post-processing to Improve the Quality of Machined Steel
After completing the machining operation, there are various surface treatment processes to enhance the surface finish and corrosion resistance of steel.
Generally, it is recommended to perform a grinding operation to further enhance the surface finish of steel workpieces.
Although grinding stainless steel can be challenging, it can be performed by using the appropriate tool and ensuring good process control.
Apart from that, you can also opt for chemical treatments like anodizing to enhance the corrosion resistance of the workpiece.
However, the cost of anodizing depends upon the size of the workpiece and the type of coating, therefore, it should only be performed when demanded.
An alternative to anodizing steel workpieces is to perform hot dip galvanizing.
Hot dip galvanizing provides better corrosion resistance and is more suitable for steel workpieces, when compared to anodizing.
Final Thoughts
Although steel is a tough material, it can be easily machined, provided that you follow the proper procedure and set the optimal machining parameters.
Generally, free-machining steel and alloy steel are best suited for machining applications, whereas stainless steel is one of the least machinable metals.
While hand tools can be used for cutting steel workpieces, a CNC mill is preferred for cutting and engraving detailed shapes and patterns.
For high accuracy and cleaner cuts, it is recommended to use a laser cutter, waterjet machine, or plasma cutter.
Furthermore, when machining steel on a CNC mill, it is advised to use a professional-level CNC mill that provides good rigidity and power to remove the material from the workpiece.
Frequently Asked Questions (FAQ)
Which grade of stainless steel is best for machining?
The grade stainless steel best suitable for machining is 400 series stainless steel, whereas austenitic stainless steels are difficult to machine.
Can we use a CNC lathe to machine steel?
Yes, steel can be machined on a CNC lathe. However, the selection of a good CNC lathe, proper cutting tool, and appropriate lathe coolant is important.
Can we use an angle grinder to machine steel?
Yes, you can use an angle grinder to machine steel. However, angle grinders are only suitable for cutting and grinding thin sheets of steel.
