Richard Turner reports that misconceptions about sliding head automatic lathes mean companies are missing out on the productivity and cost benefits of multi-turret fixed head machines.

In the past few years we have seen more and more parts in the smaller size range produced on 25 to 32mm capacity sliding head machines. These machines are designed to produce long slender components - and for that type of part nothing can beat them. But people have run away with the idea that they are designed for all types of small turned parts.

For most parts above 20mm in diameter, and for many smaller parts too, a fixed head, multi-turret machine may be able to make them in half the time with less waste.

The reason for this is that, with one or two exceptions, the fixed head machine manufacturers have neglected this size range and left the field wide open to companies offering sliding head machines.

In the last couple of years Nakamura has moved to redress the balance by introducing fixed head machines specifically designed for the highly productive manufacture of small components.

Nakamura initially introduced the WT-100, a 42mm capacity machine specifically designed for producing parts down to less than 10mm.

It has two turrets, a Y-axis on the top turret, twin spindles for simultaneous machining of first and second operations - and a lot more power than a sliding head machine.

With up to 7kW on the driven tools and 11kW on the spindles we can use modern tooling technology to its utmost potential and really attack cycle times - literally halving them in many cases.

More recently, the company introduced the Super NTY3 twin-spindle machine - with three driven-tool turrets, each with Y-axis machining.

This means that up to 96 tools are immediately available, including 36 driven tools, for simultaneous cutting with three tools.

You've not just got two turrets that are truly flexible to either spindle, you've now got a third turret that can work between the spindles as well.

So with three Y-axis turrets you have a unique and absolutely flexible capability.

The cycle time improvements are not the only reason for choosing fixed head machines.

The beauty of the sliding head machine is that the part is supported by a bush, so the tool is cutting right up against the bush, which means it is very rigid when you are producing shaft-like parts.

But once you have fed the material through the bush you can't go back to it - so you generally have to produce the finished diameter in one pass.

That means that you have to compromise on the cutting conditions.

If you are going from 32mm down to 5mm, yes you can do it, but you may have to use lower feed rates and this will lead to problems with swarf.

This is why the fixed head machine is so much better on the majority of turned parts.

Another cost advantage of the fixed head machine is that there is no need to use high-quality bar as raw material because it is being held in a conventional chuck rather than passing through a precision bush.

There is also a lot less waste.

On a fixed head machine the waste at the end of a 3m bar is literally the length of the component and the grip length of the collet, whereas on a sliding head machine you can lose up to half a metre of material per three metre bar.

That's maybe not such an issue at the smaller sizes, but it can be on larger diameters, and when you are working in expensive materials such as brass.

Given these lower operating costs, combined with the lower capital cost of the Nakamura machines, 'UK turning' can add substantially to its 'bottom line'.

* About the author - Richard Turner is with Turning Technologies UK - distributor for Nakamura-Tome high precision turning centres and CNC lathes.