Doing the work of two

Chris Law, project leader at the Huddersfield factory of the VTL Group says that the ability to hob in the main spindle of a Traub TNL18/7 sliding head lathe while reverse end machining in the counter spindle results in a one hit cycle time of 60 seconds to produce an automotive spline shaft for a Toyota gearbox.

During comparative cutting trials, before the lathe was purchased in June 2013, other types of slider took twice as long to complete the part.

A second 20mm Traub sliding head bar auto, a TNL18P, was sourced at the same time from UK agent, Geo Kingsbury, to produce two simpler automotive shafts, also in very efficient cycles involving turning and cross drilling. According to Mr Law, capital cost for the two machines was 15% less than would have been needed to purchase three sliding head lathes of another make to cope with the production volumes.

The mild steel spline shaft, part of a shift lever assembly for the Toyota Yaris and Avensis gearbox, is produced from 18.5mm diameter, 4m bar fed into the TNL18/7 by an LNS Express 220 magazine.

Balanced OD roughing and finishing in the main spindle along 80% of the shaft length is followed by centre drilling the end, drilling a cross hole and countersinking both sides, spline hobbling and subsequent milling away of a single tooth.

Meanwhile, the previous front end-machined and parted-off workpiece, which has been synchronously picked up in the counter spindle, undergoes simultaneous operations on the reverse end. The remaining 20% of the OD is turned, an undercut is machined and a hexagon is milled around the end. Tightest dimensional tolerance is 60µm total on two of the ODs.

The Traub TNL18P sliding head lathe is of slightly lower specification in that there are fewer CNC axes on the upper turret. Nevertheless, the machine can still produce the spline shaft in an 85 second cycle, which includes simultaneous hobbing and reverse endworking. The lathe therefore provides back-up for the TNL18/7.

However, the main role of the TNL18P is to produce the other two, smaller shafts which form part of a selector lever assembly for the same Toyota gearbox. Cutting trials showed that these mild steel components are also produced faster on the Traub than on other sliders. At 32 and 20 seconds respectively, the cycles are 18 and 29% faster, representing a significant productivity advantage.

Key to the speed of TNL lathes are the fast kinematics and the design of the top tool turret, which allows machining to be carried out with its unusual Z-axis movement and overlapped with additional operations.

Another design feature of the Traub lathes is that they can be used in either sliding head mode with the bar supported by a guide bush or in fixed head mode with the bar gripped in a spindle collet. The spline shaft definitely requires a sliding head cycle, while the shorter of the two simpler parts could be produced a little more efficiently in fixed head mode. However, to avoid changing over the TNL18P and consequent downtime, albeit short, this machine is also used permanently as a sliding headstock lathe.

Forming part of a £1.5 million investment by VTL in a new cell to produce these automotive assemblies for Toyota, the two Traubs are joined by a Bauromat automated welding machine, a Studer CNC grinder and an automatic assembly machine from Expert Automation. When it enters full production the cell will run 24/5. Meanwhile, the sliders are busy producing sample shafts for commissioning the other machines.

Geo Kingsbury Machine Tools
www.gkholdings.com