Two into one will go

 

Turned parts subcontractors often do not to know whether it will be a long or a short component, relative to its diameter, that they will be asked to machine next. Long parts require a true sliding head lathe with a guide bush to support the bar close to the cutting tools, while short parts are more economically produced on a fixed head lathe.
With this in mind, German lathe manufacturer, Traub, has introduced a new CNC sliding head mill/turn centre on which it is possible to select whether to turn a component with the support of a guide bush, or without it. Normally, a manufacturer would need to purchase two separate lathes to have the same versatility.

Designated TNL18, the machine has a capacity of 20 mm. According to Traub, it was launched to acclaim from engineers from all over the world, including 40 from the UK, at an open house held at Traub's Reichenbach factory at the end of 2009. The UK party was accompanied by Geo Kingsbury Machine Tools – the sole agent in the UK and Ireland for Traub turning machines as well as those manufactured by Traub's parent company, Index.
Following the event, three UK visitors spoke very positively about the ‘two in one' sliding head lathe.

Nick Iacobucci, managing director, DKW Engineering

“The first thing that struck me was the enormous working volume of the machine – more like that of a fixed head lathe. It is excellent for setting and programming and gives good visibility all around the machining area.

“Traub says that the machine can be swapped between operating with or without a guide bush in half an hour, but their engineer converted the machine in half that time. It is certainly quick to change over but I question whether many subcontractors would bother. If there are fixed head bar auto's on the shopfloor, it is more likely that shorter parts will be put on those and the TNL18 will be kept in full sliding head mode.

“That said, the lathe would make a good entry level machine, allowing a start up subcontractor to cope with a wide variety of mill/turn work economically with just one machine.

“Admittedly, the price of the Traub lathe is higher than that of an equivalent 20mm slider from some other suppliers, but someone would only need to buy one machine rather than two. Moreover, the quality and robust build of the TNL suggest that the machine will go on forever producing top accuracy parts.” In his comments, Mr Iacobucci draws attention to the TNL18's ability to be operated with or without a guide bush. Bar may be gripped conventionally by a collet and fed by up to 205mm through the guide bush, giving all the classic advantages of Swiss-type turning, including maximum rigidity and accuracy when machining long, thin components.
Alternatively, the bar is clamped by a collet in the spindle and moved 80mm into and out of the cutting zone using the Z axis motion of the headstock travelling at up to 40 m/minute. This set-up is best for machining shorter components as there is no need to bridge the gap between the collet and guide bush, so bar remnants are much shorter and material is saved.

Eliminating the guide bush avoids the need to use expensive ground stock. The process is also ideal for applications where a guide bush is impracticable, such as when producing plastic workpieces and those machined from profiled stock.

Brian Owen, managing director, Precision Products

“Coming from a sliding head cam auto background, on which it was no problem to swap the guide bush in and out fairly quickly, I always thought that CNC sliders were inflexible in their inability to be configured for both modes of operation – until now. The facility to operate the TNL18 with or without a guide bush is undoubtedly an asset, as it covers the application ranges of two machines.

“I like the open front design of the lathe and deep swarf tray for chip management. With a 20mm machine, even on fairly intensive production jobs, swarf is not normally a problem. However, if the material and tooling generate a lot of stringy swarf, the generous working volume could be a real plus point. If Traub decides to introduce a 32mm version of the lathe, the open design will be of even greater benefit.”

Ian Sheldon, managing director, Witon Engineering

Mr Sheldon amplified the above comments about the benefits of being able to operate with and without a guide bush, describing the TNL18 an ideal subcontractor's machine that allows a wide spread of mill/turn work to be carried out in one CNC lathe.

However, he went further: “I can see situations where the machine could be set up in sliding head mode and then the decision is taken to operate it in fixed head mode instead, or vice versa,” he speculates.

“The setter might change his mind as to which will produce the required accuracy in the shortest cycle time or perhaps production has started but the bar is slightly out of round and it is proving difficult to hold tolerance with the stock rotating in the guide bush.

“During the recession, subcontractors have changed their operations drastically, relying on one setter to cover three, four or even five machines. However, if tool changes are frequently needed and the jobs run for only a short time, it can be difficult to cope with all the setting quickly enough, so machines end up standing idle.

“Mill/turn centres like the TNL18 that hold a large number of tools in the turrets are therefore very beneficial, as you can leave the cutters in the machine and they will cover a majority of jobs.”

He continues:“The spacious working area is another plus point, making it easy to access the tooling – even the reverse endworking tools, when they do need changing. I also like the robustness of the backworking tool block and the tool clamping arrangement, which is more substantial than on most sliders.

“Finally, the power of the machine stands out compared with others on the market. European-built machines, especially those from Germany, tend to have higher power, higher torque spindles compared with machines from the Far East and the TNL18 is no exception. Even the power to the driven tooling is greater.”

Mr Sheldon referred to the large number of cutters resident in the new Traub lathe. A singular feature of the machine is the absence of gang tooling and the inclusion of two turrets that are rotated as full NC axes, instead of being indexed and clamped. They may therefore be positioned at any angle through 360°, so multiple toolholders can be used, allowing up to 54 cutters in the working area. All stations can carry either a static turning tool or a live cutter.

An eight station turret is mounted above the spindle centreline, below which is a seven station turret. The latter moves in three orthogonal axes, while the top turret has an X axis plus a simulated Y axis, whereby its rotation is interpolated with the X axis and the main spindle's C axis.

Turret rotation is faster than that of an indexed turret, matching the linear speed of a gang tool post. Chip-to-chip time is under 0.3 seconds for adjacent turret stations, maintaining the same low levels of non-cutting time for which sliding headstock lathes are renowned.

The 3-axis counter spindle is mounted on the same compound slide as the lower turret and swivels independently around the same axis as the turret to offer a parted off component to seven rear endworking tools, three of which are live, for completion of operations on the reverse.

While work is in progress on the back of a component, front end turning and prismatic machining with one or two tools can be carried out simultaneously in the main spindle, allowing up to three tools to be in cut simultaneously.

Geo Kingsbury
www.gkholdings.com