Hard turning and grinding cells

At Turner Powertrain Systems, part of Caterpillar's transmission business unit, an increasing number of features on drive shafts for off-road vehicles are being hard-turned rather than ground.

At the Wolverhampton factory of Turner Powertrain Systems, part of Caterpillar's transmission business unit, an increasing number of features on drive shafts for off-road vehicles are being hard-turned rather than ground.

The latest lathe for hard turning is a Traub TNA 500 supplied through UK agent, Geo Kingsbury Machine Tools, Gosport.

It is the third machine on site from this German manufacturer and was installed in November 2004 in a rapid, five-month lead time to cope with sharply increasing world demand for shafts used in backhoe loaders, telehandlers and wheeled loaders, both within and outside the Caterpillar group.

Production volumes at Wolverhampton were up 20 per cent in 2004 over the previous year to 58,000 shafts across 13 variants.

The move towards hard turning of complex shafts in carburised steel of 58 - 62 Rc started in the mid-90s when a larger Traub 600 lathe was installed, although simple parts such as gear blanks were already being hard turned on other lathes.

At the time, Turner Powertrain Systems was finding it difficult to maintain form on the critical seal grooves of geared shafts and in addition, grinder burn and heat cracking used to occur on the component.

The problems were solved by hard turning the grooves to the required 0.3 micron surface finish, following which the company moved on to hard turning features on the flange.

Said operations development manager, Stephen Cooper, "The first Traub was a great success.

It hit the ground running, proved very reliable, produced good-quality parts within tolerance without heat cracking, and was quick to set between batches.

"We were a bit worried initially that hard turning would cause lathe vibration, leading to loss of accuracy and high tool wear, but early trials on the TNA 600 proved these concerns to be unfounded.

The results were testament to the build quality of the Traub and its strong tailstock." The latest TNA 500 is compatible with the earlier model in terms of operability, programming and tooling, allowing shafts to be swapped between machines.

With its introduction, Mr Cooper resolved to extend further the proportion of hard turning on shafts at the expense of grinding.

He conducted trials to turn other previously ground hardened features on the shafts, such as seal and bearing diameters, in addition to the grooves and flange.

Results achieved have been impressive.

For example, on the drawing for the seal diameter, required surface finish is 1.25 microns whereas 0.58 - 0.70 microns is achieved.

Figures for run-out and cylindricity are 0.007 mm and 0.005 mm respectively.

Mitsubishi Carbide is advising on the optimum CBN indexable inserts with wiper chipbreaking technology to achieve best results, including when taking intermittent cuts.

A Neidlein driver in the main spindle pushing the component against the tailstock is used for clamping in preference to a chuck, avoiding the latter's longer changeover time and the possible introduction of inaccuracies.

Renishaw probing ensures correct orientation of the shafts prior to turning, allowing for any distortion during heat treatment.

Continued Mr Cooper, "We used to plunge grind the flange and bearing diameters but there was always a compromise on speed, as they are of different diameters, and it was difficult to hold corner definition for subsequent electron beam welding.

Hard turning the features eliminated the problem.

"So too does CNC grinding with a narrow wheel rather than plunge grinding, as wheel speed can be matched to the component diameter.

The need for dressing is eliminated and the technique is more accurate, as large plunge grinding wheels can taper across their width.

"We need to find best practice in both areas of metalcutting - turning and grinding - as we use both on the shop floor, balancing the cycles and exploiting the strengths of each process." To this end, two new cells are being set up based on the TNA 500 and TNA 600, each of which will be partnered with two angle-approach grinders.

One operator will attend each cell, producing finished shafts rather than having batches of 100-off shafts waiting around to be finished.

Implementation of the cells will be July and October 2005.

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