Stainless Steel Turning – All You need to know

What is turning?

As already mentioned, turning belongs to the category of so-called metal-cutting manufacturing techniques. The aim of the process is to bring the workpiece into the required shape. A lathe is used for this purpose. A main feature of the technique is that the workpiece moves during machining, while the tool remains still. This represents a major difference from milling.

When steel is turned, the workpiece rotates around its own axis while being pushed forward at the same time. In the process, chips are removed in thin layers with the help of the stationary tool.

Different turning processes

Stainless steel can be machined using various turning techniques. These are the most commonly used:

• Longitudinal rotary turning is the most important and widely used variant.  Here, the tool moves parallel to the turning axis. Many different tools can be used in this method. These are specially adapted for the respective requirements.
• In transverse facing, the tool moves perpendicular to the axis of rotation. The method usually represents the first operation. By means of the technique, a reference surface is created in the axial direction, which is important for the further machining of the workpiece.

Other processes that are used are longitudinal turning and transverse turning.

What problems can occur?

Turning stainless steel is not always without difficulties. Although steel is the most frequently machined material, its machinability depends to a large extent on the alloying elements, the previous heat treatment and the manufacturing process. Normally, machinability decreases with the alloy content.

Stainless steel generally has a high hardness. For this reason, attention must be paid to heat development during turning. It is therefore common to use a special cooling lubricant, without the workpiece would deform or warp. By using a very small setting angle, it is possible to avoid excessive burr formation on the workpiece.

To make machining possible, special super-hard cutting materials are used for some materials, such as hardened steel. The cutting edges of the tools must be very stable, otherwise damage to the surface of the workpiece may occur.

What must be considered during the process

Whether a material can be turned depends to a large extent on the carbon content, the heat treatments previously carried out and the alloying elements of the material. These properties determine the machinability. However, other factors also play a role:

• To obtain an ideal result, the stainless steel should be short chipping.
• The lathe used must have specific cutting forces between 1800 and 2850 N/mm².
• Some stainless steel alloys, such as 303, add sulfur to improve machinability.
• The alloying elements nickel and molybdenum increase the temperature resistance of stainless steel. However, their content also reduces machinability.
• The carbon content determines the microstructure constituents and thus the machinability.
• For ferritic and martensitic stainless steel grades to be machinable, they must have a Rockwell hardness of at least 55 HRC.
• The largest diameter of the machined workpiece determines the cutting speed of the tool. The feed rate is also determined by this.