Smooth rolling on a lathe – high-quality, wear-resistant surfaces

Crankshafts, axles, piston rods, bearing seats – these are just some of the many examples of metal components that have to withstand high dynamic loads in practice. What all these components have in common is that they don’t just need a highly accurate fit in order to work properly, they must also be wear-resistant and resilient. To make these types of components ready for practical use, the production method known as “smooth rolling on a lathe” is an excellent choice. The method can be used on both conventional and CNC lathes and achieves the desired combination of a particularly smooth surface and high wear resistance.

Smooth rolling is often also referred to as smoothing. But be careful – smoothing refers to a group of several methods with different objectives. The box below tells you when which method is used.

Before we discuss the practical aspects of smooth rolling, let us briefly explore the theory behind the forming method. In particular, we need to understand the forming process that takes place on the workpiece. During smooth rolling, a special tool – usually a roller – acts on the component by applying pressure. This results in compressive stress at the point of contact. When the material’s ‘yield strength’ is reached, a plastic, i.e. permanent, change in the workpiece’s shape occurs. The yield strength is the stress value up to which a material will be deformed elastically, i.e. will return to its original shape once the load is removed. In the case of heat-treatable steel 42CrMo4, this value is usually 650 to 800 MPa (treated condition, 28-32 HRC). The deformation improves the surface quality while also optimising the component’s mechanical properties. The roughness is used as a measure of surface quality here: it expresses how many “peaks and valleys” the surface has, i.e. the extent to which the surface profile deviates from the mean line on average. The roughness is often expressed as the average roughness Ra in the unit μm.

The following step-by-step instructions explain how to use it successfully in practice.

During smooth rolling, it is essential to prepare the workpiece well and then roll it evenly in a controlled manner. You should choose a moderate rotational speed and feed rate in order to prevent surface defects. We recommend working through the steps gradually, particularly if you are trying out this process for the first time.

1. Prepare the workpiece
Good preparation is half the battle. The workpiece should already have a smooth surface without any major unevenness. An Ra value of 1.6 to 3.2 μm is commonly used in practice. If the values are higher it may not be possible to compensate the unevenness; if the values are lower the smooth rolling process may leave behind unsightly marks. Tip: make sure that the workpiece is free of dirt, damage and scoring. The better the turning quality, the more effective the smooth rolling process will be. Before starting to work on the component, thoroughly remove grease, chips and other contamination from it as foreign particles may impair the surface quality during smooth rolling.

2. Select the smooth rolling tool
Smooth rolling requires the use of a special tool, consisting of a hardened roller or a spherical head which is clamped in a plain shank and connected to the toolholder. When choosing the tool, pay attention to the workpiece diameter and the material properties. Whereas a robust, wear-resistant rolling tool, e.g. one made of carbide, is suitable for hard materials such as hardened steel, a smooth rolling tool made of steel can also be used for softer materials such as aluminium.
• Tip: before using the tool, ensure that it is in good condition and that the roller is not damaged. A damaged roller can severely clog the surface.

3. Set the rotational speed
During smooth rolling on a lathe, you should tend towards a moderate to low rotational speed. The reason is simple: if the speed is too high, there may be vibrations on the workpiece that adversely affect the surface quality. Tip: as a rule of thumb, 200 to 400 rpm is a good starting value. However, depending on how the first test passes go and the specific workpiece, you can also increase this value to improve efficiency.

4. Set the feed rate
You also have to find the happy medium for the feed rate. If the feed rate is too fast you will not achieve the desired smoothing effect; if the feed rate is too slow you may even damage the surface. Tip: the feed rate should be uniform and moderate, typically around 0.05–0.2 mm per revolution.

5. Smooth rolling on a lathe
Now you can get going: carefully place the tool on the surface of the rotating workpiece and apply pressure evenly. Make sure that the tool is making even contact and does not apply too much pressure that could deform the material. Tip: take a gradual approach when performing smooth rolling. Start by applying a small amount of pressure and increase it depending on the results you obtain. For soft workpieces such as those made of aluminium or copper an initial pressure of 100 to 300 MPa is recommended, whereas hardened steel will require 600 to 1,000 MPa or even higher values.

6. Check the surface
When making the passes with the tool, don’t just check the surface visually – if necessary also use a measuring device to determine the Ra value. Depending on the specific requirements and the type of component, the target value should be below Ra 0.2–0.4 μm. If you do not achieve your desired roughness in the first pass, you can gradually work up to the target value with additional passes.

7. Final steps and reworking
Once you have achieved your desired smooth finish, remove the workpiece from the clamping set-up and clean it thoroughly. Depending on the requirements, you can then also polish the workpiece to achieve a mirror-smooth surface.

Additional practical tips for smooth rolling

• Material: Smooth rolling works best on malleable metals such as steel, aluminium and copper. If it is used on relatively brittle materials, the surface may break. Please note that the higher the degree of hardness, the more brittle the material. For hardened steel with an HRC value as from around 45, the material will tend to form cracks or break and smooth rolling will then become increasingly difficult.
• Cooling: If required, use a suitable coolant to reduce the build-up of heat and the wear on the tool.