Groove turning: produce turned parts with clear edges and depths

There are various grooving methods that you can use to produce components such as shafts and axles, rings and bushings, and turned parts for machines and motors. Grooving, groove turning and parting off may be used depending on the component in question.
This guide briefly examines the different variants and provides practical tips and rules of thumb

Overview of groove turning methods

Grooving, groove turning and parting off are all variants of turning. They differ in how the tool moves and the aim of the machining work. During grooving, a tool is positioned perpendicular to the workpiece axis and penetrates the material in order to produce slots or grooves. Groove turning expands this movement – the tool also moves axially to machine specific contours or complex geometries along the length of the workpiece. By contrast, parting off is used to separate a workpiece from the bar of raw material or to detach components. In this case the tool is precisely guided transversely to the workpiece axis. The two methods of grooving and groove turning are summarised below.

Grooving

A key aspect of grooving is the direction from which the tool performs the grooving movement. Grooving along the axis of rotation is referred to as “longitudinal grooving (radial grooving)”, whereas grooving perpendicular to the axis of rotation is called “transverse grooving (axial grooving)”.

In addition to this classification, grooving is further differentiated into internal grooving and external grooving.

• Internal grooving: in the case of internal grooving, the tool is introduced into the inside of a bore hole to produce things like slots or contours on the inner face. The method is ideal for producing internal slots for seals, retaining rings or bearing seats.

• External grooving: in the case of external grooving, a tool penetrates the surface from the outside, perpendicular to the workpiece axis, in order to produce slots, grooves or shoulders on the external cylinder face of the workpiece.

As a result, internal and external grooving differ in the accessibility and alignment of the tool. With internal grooving, the material is removed inside the workpiece, e.g. in bore holes or cavities, which necessitates the use of smaller, slimmer tools. By contrast, external grooving is easier to access and enables larger tool dimensions and more stable machining conditions.

Parting off

During parting off, the tool is guided vertically into the workpiece until it cuts through the material and a part is cut off. This method produces components such as rings or discs.
As parting off continuously reduces the workpiece diameter, the cutting speed also continuously falls if the rotational speed remains constant. You can easily see the reason for this by the formula in the info box.

In practice, the decrease in cutting speed causes an increase in the cutting forces and a higher risk of tool breakage or irregularities in the cut. For this reason, it is worth progressively increasing the rotational speed as the diameter decreases.
Tip: also reduce the feed rate by approx. 50% when moving towards the centre, in order to reduce the cutting forces and extend the tool life.

Practical guide: how to succeed with the various grooving variants

When grooving, the steps for preparing and performing the work depend heavily on which grooving variant is being used, and to what end. For this reason, we have provided some method-specific tips below.

Step 1: choose the tool

Choosing the right grooving tool is the key to success. Choose a tool that is suitable for the workpiece material. When working on stainless steel and hardened metals, you should choose particularly robust cutting edges to achieve even wear and a high-quality surface.

When choosing the tool parameters, you can take the following practical tips as a guide:

Grooving:

• To negate the need for a new tool every time you perform grooving, choose a cutting edge with a width that is smaller than the desired width of the groove. This will enable you to achieve the target width gradually by performing the grooving several times.
• When handling complex grooving geometries, it is advisable to use the same tool for all of the machining work.
• Face grooving: the external diameter of the blade carrier is the maximum grooving diameter; the internal diameter is the minimum grooving diameter for the first groove cut. It is essential to observe these parameters
• Internal grooving: to minimise vibrations, choose an overhang length that is no larger than 2-2.5 times the tool diameter. Tip: to increase the possible overhang while still keeping vibrations low, you can also use a boring bar stabiliser.

Parting off:

• The groove depth should not be more than 8 times the width of the insert. This restriction is important to keep the tool stable and avoid unnecessary wear. The choice of suitable toolholder is also influenced by the desired groove depth
• At the same time, the insert should be as slim as possible. A smaller insert width will minimise the loss of material and reduce costs. However, the insert must still be wide enough to achieve the required groove depth.
• The use of a neutral setting angle (κ = 0°) for the insert will enable a high surface quality and a precise, perpendicular cut. In addition, tools that have a neutral setting angle are generally able to withstand higher feed rates. If the user does not mind burrs or pips, the use of neutral inserts is often a better choice.
• A smaller corner radius will ensure smaller pips and enable better chip formation, which is advantageous when working with small feed rates. By contrast, a larger corner radius will make the tool sturdier and enable higher feed rate speeds.

Step 2: align and secure the tool

Correctly aligning and securing the grooving tool is essential if you want to machine the workpiece precisely and achieve a long tool life. Here are some practical tips to help you achieve optimum results when grooving and parting off:

Tool alignment

• Correct position: the tool should always be aligned exactly perpendicular to the workpiece axis to ensure an even load on the cutting edge and prevent unnecessary wear.
• Tool height: the cutting edge must be positioned exactly at the centre of the workpiece. If the tool is set too high or too low, this can lead to increased wear, poor surface quality and even tool breakage.

Practical tip 1: use a gauge or centring aid to quickly and easily check the height. This will enable you to prevent deviations and ensure a more stable process.
Practical tip 2: make sure the clamping face is clean before securing the tool. Even small chips or impurities can adversely affect the alignment and make the tool less sturdy.

Step 3: the turning operation

Now things get exciting: during the turning operation, the rotational speed and feed rate should be precisely coordinated with one another to avoid overloading the tool and produce neat chips. Follow these practical tips and you’ll be sure to succeed with groove turning:

Grooving:

• Optimum cutting speed: maintain a constant cutting speed and adjust it to suit the material and groove depth. As the tool penetrates the workpiece in a specific location, tend towards a moderate speed to avoid too much heat building up. Tip: start with a relatively low cutting speed and increase it gradually until you find the perfect balance between material removal rate and tool life. On machines with a constant rotational speed, use the average rotational speed. On machines with a constant cutting speed, the machine will make adjustments itself.
• Prevent vibrations: when producing deep slots, the tool is exposed to a high lateral load. This may result in vibrations which impair the surface quality and make the tool wear down faster. Tip: minimise how far the tool protrudes and reduce the feed rate speed if vibrations occur.
• Check the cutting edge: regularly check the cutting edge on the tool as the lateral contact made when grooving the workpiece can lead to increased wear. Parting off:

1. Constant feed rate speed: A constant feed rate speed is crucial for keeping the cutting process stable and reducing the risk of pips. Tip: keep the feed rate as constant as possible, particularly at the end of the parting off process when the workpiece is less stable. Reduce the feed rate only slightly to prevent tool breakage.
2. Provide coolant to control the temperature: Cooling is particularly important during parting off as the friction and build-up of heat increase towards the end of the cut. Tip: use sufficient coolant to keep the temperature low and protect the tool. A targeted jet of coolant will also help to improve chip evacuation.