Precision turning – precise dimensions and the highest-quality surfaces

The name says it all: precision turning is all about giving turned components the final finishing touch and producing very delicate precision-turned parts. This method uses special turning tools to ensure the shape and dimensions of the turned part are perfectly accurate and achieve particularly high-quality surfaces. The advantage of this method is its high accuracy, which generally negates the need to rework the surface afterwards. Precision turning is used whenever the component has to meet particularly high requirements. Examples include precision parts such as gear wheels or shafts for clock mechanisms, implants and surgical instruments for the medical technology sector, and bearings and holders used under extreme conditions in the aerospace industry.

Compared with conventional turning, precision turning enables you to achieve surfaces of even higher quality. Roughness values of just a few micrometres are possible. When working on lathes, you can achieve this level of precision by using the precision turning tool with a low feed rate and small cutting depth. This article explains what that looks like in practice.

Choosing the right precision turning tool

The main difference between precision turning tools and conventional turning tools is their minimal edge rounding and the fine cutting edges. Depending on the specific requirements, these edges may also be manufactured from particularly wear-resistant materials. Carbide is commonly used, as well as other options such as ceramic cutting materials, MCD or PCD. The tools are specifically designed to remove only a minimal amount of material.
When choosing which precision turning tool to use, you first need to bear in mind that there are tools for internal machining and external machining. A precision turning tool for internal machining, for example, can be fed in very accurately to the inner surface of bore holes. The tools are designed for low cutting speeds and can achieve roughness values from Rz 1 to 5 μm. Similarly, there are precision turning tools for external machining, which enable you to optimise the surface quality of turned parts or machine flat and shaped surfaces, for example. This can eliminate the need for downstream machining steps such as grinding.

When choosing a suitable precision turning tool, ensure that all the components are compatible with one another and suitable for the application in question (internal or external use, type of machining, material).

After choosing your precision turning tool, you need to select a suitable clamping toolholder. Precision turning requires a particularly precise, low-vibration holder as even the smallest vibrations can have an adverse effect on the surface quality and dimensional accuracy. Moreover, precision turning tools often require special holder geometries which permit short overhangs and tight tolerances in order to achieve a more accurate result. Tip: thanks to the quick-change system, indexable inserts can be exchanged in next to no time if the cutting edges exhibit too much wear.

Step 3: Clamp and align the precision turning tool
When clamping the tool, also pay attention to the condition of the cutting edges – they should be sharp and in good condition. First, insert the precision turning tool in a designated, sturdy toolholder. Precisely set the tool to the necessary height and correct alignment. The cutting edge must generally be positioned level with the rotational centre of the workpiece in order to ensure a neat cut. Then securely clamp the tool in place using screws or clamping mechanisms. Apply pressure evenly here to prevent it shifting or twisting.

Step 4: Set the lathe
Set up the lathe for precision turning. Set the rotational speed and feed rate according to the requirements for the workpiece and the precision turning tool. Precision turning requires lower feed rate speeds and higher rotational speeds to ensure a high level of precision. However, when setting the rotational speed, it is essential to observe the maximum speed of the lathe chuck you are using.

Step 5: Remove the material
Carefully move the cutting head up to the workpiece and set the infeed. With precision turning, the cutting thickness is very small because only a small amount of material should be removed. Nevertheless, take care here: if the cutting thickness is too small, this may result in unwanted crushing of the material. Please see the box for details of how to avoid this. In the case of micro-turning tools, the infeed can also be less than the corner radius on account of the minimal edge rounding.

Start the precision turning process by machining the workpiece slowly and evenly. Ensure that the tool removes the layer of material evenly without applying too much pressure. It is advisable to machine the workpiece in several smaller steps to avoid overloading the tool and to maximise the surface quality.

Step 6: Monitor and adjust
Check the progress regularly during the precision turning process. Measure the tolerances and the surface quality of the workpiece to ensure they meet the requirements. If required, adjust the machine parameters or the precision turning tool to achieve the desired results.

Step 7: Final steps and reworking
If the dimensions and shape are sufficiently accurate and the surface quality meets your expectations, remove the workpiece from the lathe and carry out a final quality check. You can then rework the workpiece further if required. For example, you could deburr, polish, apply a matt finish and round off the precision-turned part using particularly gentle vibratory finishing.