Robotic lathe boosts vehicle component production

A robotically-loaded MHP lathe from Geo Kingsbury forms part of an automated cell for machining bells at GKN Driveline, Birmingham. It is 55% more productive than similar, manually-loaded lathes

Over the past few years, first-tier automotive supplier, GKN Driveline, has replaced pairs of external, angle-approach grinders with single hard-turning lathes from the MHP division of Geo Kingsbury for machining critical features on an extruded steel component known as a 'bell'.

It is the outer race in the constant velocity joint that forms part of front-wheel and four-wheel driveshafts.

GKN Driveline's third MHP 65/500 lathe for hard-turning bells was installed in the autumn of 2002, but unlike the first two, the latest machine has a robot loader and forms part of a largely automated cell for bells production.

"Compared with two similar cells that comprise all manually-loaded machines and produce 450 components per shift, the automated facility produces 700 in the same time - an increase of 55 per cent," says Paul Daniels, the engineer responsible for this cell at the Birmingham factory.

This increase in productivity applies to all the auto loaded machines in the cell, not only the MHP lathe but also a vertical CNC mill and a special-purpose, twin-spindle vertical turning lathe that replaced five internal grinders.

Other operations include soft turning on MHP lathes, spline rolling and induction hardening.

A further advantage is that, whereas five operators are needed to run a manual cell, only three are required for the automated cell, reducing labour costs significantly.

Manning is an expensive part of production and as the hard turning cycle is relatively short at around 32 seconds, if the lathe operator has to manually load and unload the machine he is not able to do much else.

Likewise, if he goes away from the lathe, it stops.

The success of the first automated cell prompted GKN Driveline to invest in four more robot-loaded MHP lathes and again, each replaced two grinders, this time in flow lines for bells production.

There are now only two, relatively new grinders remaining for bells production on the Birmingham site.

These grinders will be redeployed for machining the inboard joint - a gearbox seal face that cannot be hard turned - at which time they will be replaced by a further MHP lathe and the changeover from grinding to hard turning of bells will be complete, a process that will have taken seven years, according to Mr Daniels.

It is GKN's policy to replace as many grinders as possible with hard turning, as each time a lathe is installed there is a saving in capital investment of several hundred thousand pounds.

Additionally, it takes two hours to change over an external grinder for a new batch compared with 10 minutes for a lathe, so production downtime is less by a factor of 12.

GKN Driveline is also committed to automation and is considering retrofitting robots to the MHP lathes in the two manual bells cells, probably during 2004.

At the outset, the company considered gantry-loaded lathes but decided that they were too inflexible to change over easily to machine a new component, should it be necessary.

Another consideration was the extra factory space a gantry needs at the ends of the turning machine, whereas a robot can be installed within the footprint of the machine.

Each MHP lathe is fitted with an ABB robot at Geo Kingsbury's Gosport factory and supplied, after pass-off, to GKN Driveline as a turnkey package, complete with programs.

The MHP lathe in the automated cell produces seven variants of bell.

Components are fed from the previous operation via a slat band conveyor in front of the machine.

A gate releases one part to a pick-up position from where the robot's double gripper handles it to a laser and sets it at the correct datum position by scanning an internal track.

The robot then enters the machining area of the MHP lathe, removes the machined part from the spindle and inserts the new part in a known orientation to coincide with the C-axis position.

Driving the original change from grinding to hard turning were difficulties with grinding the hub support diameter of the bell and more particularly a shoulder called the back location face, both of which are pre-hardened to 58 - 62 Rc.

The main problem used to be cracking of the location face during grinding, caused by overheating owing to difficulty in projecting sufficient coolant between the grinding wheel and the vertical face.

The earlier grinding operation and the dry hard-turning sequence that has replaced it includes in-cycle machining of a third OD in an unhardened state of 45 - 48 Rc, namely a seating diameter for an ABS pulse ring.

The availability of modern CBN (cubic boron nitride) tooling materials has been instrumental in allowing ferrous metals to be turned in their hardened state.

Paul Daniels uses Sandvik Coromant's six-edge CBN inserts, which have a PVD coating for easy wear identification and a 0.8 mm radius.

A parts counter on the lathe ensures that the inserts are indexed or replaced after every 500 components have been machined.

The ABS diameter is turned in-cycle using a normal cemented carbide insert.

Every 10th part passes to an inspection tray for off-line, manual checking of the diameters.

If this is not done by the time another 10 have been machined, there will be no place for the 20th on the tray and the machine will stop automatically.

As all the turned features need to be concentric with six internal ball tracks, drawing tolerances are tight: +/- 0.013 mm for the hub support diameter and +/- 0.015 mm for the ABS diameter, surface finish for both being 1.6 æm.

As regards the back face, 0.15 mm of material is machined away to leave a 0.8 æm surface finish.

The Ra figures achieved in practice are significantly lower than these.

Process capability achieved is Cpk 3.66 for the hub support diameter and Cpk 2.65 for the ABS diameter - significantly better than the required Cpk of 1.67 (5 Sigma).

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