At its factory in Mannheim, Germany, John Deere assembles 40,000 tractors each year for sale around the world. It also produces a wide range of gearbox components and housings, and the company is constantly seeking ways of optimising the manufacturing techniques it uses for these products.

Cutting time

John Deere wanted to reduce the time taken to machine the sealing grooves in a cast iron gearbox housing. These were taking 11 minutes to machine using conventional circular milling techniques, so precision machining company MAPAL introduced interpolation turning to replace the milling operations and, in conjunction with custom tooling, this cut the machining time to 3.6 minutes – a saving of almost 70%. A component that presented particular challenges was a differential housing manufactured from GG25 cast iron, which requires five sealing grooves to be machined. With conventional milling, even at the highest achievable cutting rates, the time needed for machining the grooves could not be reduced below 11 minutes, and this was a significant constraint on productivity.

Interpolation turning

MAPAL suggested that a better approach for machining the grooves would be to switch to interpolation turning. With this process, the tools are moved helically by means of interpolation of the axes of the machining centre. At the same time, the cutting edge is guided by the position control of the drive axis so that the radial contact position and angle of the cutting edge to workpiece remain constant throughout each revolution. This technique not only allows faster machining times to be achieved than are possible with circular milling, but also produces a better surface finish. For the John Deere differential housing, five sealing grooves must be produced but by using customised tools, MAPAL has made it possible for this to be achieved with just two tools. The first tool is fitted with a single indexable insert and machines a sealing groove with a depth of 3.5mm and an inside diameter of 185mm. During this process, which requires 35 helical revolutions with a pitch of 0.1mm, two chamfer transitions are produced. The axes move at 50m/ min, which corresponds to a cutting speed of approximately 140m/min.

Two grooves – one step

The second tool, a new MAPAL development, has two indexable inserts and machines two sealing grooves in one machining step. The sealing grooves are opposite each other on an outside diameter of 244.2mm and an inside diameter of 324mm. Both grooves are cut simultaneously over 33 helical revolutions.

Initially, the process engineers at John Deere were sceptical about the interpolation turning process proposed by MAPAL, but were sufficiently interested in its potential benefits to sanction a trial. The results of the trial were good enough to convince the engineers this process’ effectiveness, to the point where they now looking at applying this technology to other machining challenges on the site.