How is turning between centres done?

Turning between centres makes it possible to machine parts that demand high accuracy. To carry out this type of work it is important to follow a series of recommendations and strict steps, as well as to use suitable tools and an operator who is experienced in this turning technique.

Features of turning between centres

To carry out turning between centres, also known as centre turning, the workpiece must be placed between the centre points of the machine tool, in this case the lathe. One of them is fitted in the main spindle (known as the dead centre and reducing sleeve); the other must be mounted in the tailstock spindle and can be either a dead centre or a live centre.

Dead centres are lathe accessories that are hardened and have mounting surfaces, and they conform to a Morse taper. The other end is finished in a 60-degree cone with a sharp point.

When is turning between centres used? This type of machining can be used for long workpieces that tend to bend or deform, produce vibration in the machining area and may come loose from the clamping. In other words, turning between centres provides greater stability when you want to machine parts with particular characteristics; for example, when soft metals are used or when the parts are of a size that could, simply because of their own dimensions, deform.

Advantages and disadvantages of centre turning

Why do machine shops turn to turning between centres? This technique makes it possible to work on parts in series, always on their outer surface. In addition, mounting the workpiece to be machined is quick and very secure, which is a benefit for operators. It is worth stressing that the main purpose of this system is to machine long tapers whose angle is not very steep, because the travel range of the machine's head is not wide.

Once the technique is mastered, the results are perfect straight tapers. That is why this method is the most widely used, for its reliability and, of course, for the excellent results obtained. However, this process has the drawback of producing unwanted taper when the tailstock has been poorly set up. On the other hand, when dead centres in the tailstock are used, the part of the cutting tool that rubs against the workpiece, as well as the workpiece surface, must be lubricated. When lubrication of the workpiece and the tool is lost, the centres deform.

To work with this technique successfully, it is important to avoid a build-up of heat in the workpiece. When the part overheats, its thermal expansion can tighten the fit between the centres.

One of the limitations of this technique, as we mentioned earlier, is that it only allows external machining of parts. In addition, this type of machining throws off the lathe's axis of symmetry, which means that a percentage of the workpiece's strength is lost during machining.

Stages of machining between centres

This technique, which as its name indicates can only be carried out on lathes, can be divided into two parts or stages. In the first phase the preparation of the workpiece must be carried out, that is, support centres must be made in it. To complete this part of the machining it is essential that the overall lengths of the workpiece have already been set.

Once the workpiece has been prepared, it is time to get the lathe ready. Here you will need to set up the dead centre, the live centre, the reducing sleeve, the driving plate and the machine's reducing bush. Once these stages have been completed, you can start the machine and begin machining. The sequence is routine and consists of a few simple steps:

• Fit the lathe dog on the workpiece to be machined.
• Fit the driving plate and the dead centre. The dead centre must have the taper and the live centre.
• Mount the workpiece on the machine.
• Check that the fit between the centres is correct.
• Check the manual rotation and that the set-up works properly.
• Check the safety margins.
• Start the machine and begin the machining process.

As general recommendations for this type of work, it is advisable to check that the lathe dog sits as close as possible to the plate and that it has free movement without restriction. You should also pay attention to speed, because although the cutting speed is normal, it may be subject to variables that cause it to drop.

Finally, for turning between centres it is best to leave the job in the hands of an experienced lathe operator. Although the technique is simple and easy, it requires the operator's skill to apply the right speed.