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Y-axis turn-milling halves medical equipment production times

Mills Manufacturing Technology : 05 January, 2005  (Application Story)
Three Daewoo Puma 2500SY lathes, installed over a 12-month period to November 2004, have been key to improving productivity at Finsbury Orthopaedics, which specialises in the design, development and manufacture of implants for hips, knees and other joints.
University spin-off company, Finsbury Orthopaedics, was formed in 1978 by the present managing director and owner, Mike Tuke, with just £1,000 of capital. It specialises in the design, development and manufacture of implants for hips, knees and other joints, as well as the stainless steel instruments used for inserting them.

Production volumes have increased significantly in the past couple of years, prompting the company to expand its factory in Leatherhead and install three Daewoo Y-axis lathes from Mills Manufacturing Technology.

The healthcare industry, like other areas of manufacture, is constantly seeking to drive down costs but Finsbury is not prepared to make savings by compromising on product quality. Instead, manufacturing techniques have been optimised to reduce costs and improve product performance. The company invests 20 per cent of turnover in development every year and has recently made strides in reducing the linear wear rate of implant heads and cups to less than 2 microns per year by minimising variations in sphericity, surface finish, clearance and metallurgy.

The three Daewoo Puma 2500SY lathes, installed over a 12-month period to November 2004, have been key to achieving the improvements. By that time they were producing 1,500 components per month and there were well over 100 part numbers. The first two machines are devoted to manufacturing instrument parts and also to turning implant-grade plastic for the production of impaction caps that protect the surface of a hip implant as it is inserted in the patient. The third Puma, however, produces a particular type of implant called a modular head used for full hip replacement.

The modular head has a cone at one end that is inserted onto the femur component and a cup at the other which fits into the pelvis. To ensure that a minimum of bone is removed from the patient, there are many varieties of head ranging in diameter from 38 to 58 mm diameter in 2 mm increments, and with four different neck lengths. Previously manufactured in four successive operations on a 2-axis CNC lathe and a carbide wheel cut-off machine, they are now produced in one hit on a Puma 2500SY.

A casting is inserted manually into the main spindle and the spigot is parted off. The sphere is rough and finished turned. Fitted with special jaws to suit the diameter of head being produced, the sub spindle advances to grip the component for a sequence of reverse end operations including facing the neck to length, slot drilling, rough and finish boring of a taper, and the use of a mini boring bar to produce a flat face at the bottom.

Cycle time is 12 minutes, and is becoming faster as the program is refined, compared with 20 minutes total machining time previously, to which must be added manual handling within the 2-axis lathe and between the machines. There is a big saving in setting time as well, bearing in mind that batch sizes are relatively small at around two dozen; and work-in-progress has been eliminated. The process on the Puma is also more consistent, it being easier to hold the required +5, -0 seconds of arc accuracy on the taper and 0.1 mm positional tolerance between the taper and front face of the neck.

The C-axis, Y-axis, driven tooling and bar magazine on this latest lathe are not required for machining modular heads, but were included in the specification so that the machine can be switched to instrument manufacture as production schedules demand. The other two Pumas, also fitted with Hydrafeed short bar magazines, use all of their functionality for the manufacture of complex instrument components 24 hours a day, including a ghost shift from 7.00 pm until the next morning.

Each surgical instrument is an assembly of between four to 20 or so separate components made from 17-4 or 316 stainless steel. A typical example is a cup alignment aerial part produced from 316 stainless bar of 41 mm diameter. In the main spindle, the bar end is faced and rough turned to leave a 10 mm diameter boss which is subsequently finish turned, faced and chamfered, followed by skim turning of the main OD. A 5 mm diameter hole is drilled through the centre of the boss. Then the lathe¹s Y-axis in conjunction with the X-axis, fixed C-axis and driven tooling come into play, milling two opposing flats 21 mm across.

With the component still in the main spindle, four 6 mm diameter, blind cross-holes are drilled into the remaining segments of the OD. Two holes are drilled through the flat and a central, 8 mm wide cavity is opened out using a 3-tooth slot drill in combination with the Y, X and C axes.

Only then does the sub spindle synchronously rotate with the main spindle and pick up on the boss, followed by part-off, and rough and finish turning to create a spigot on the reverse end, which is drilled and tapped (M6). A plunger acting through the sub spindle pushes the component into a parts catcher which deposits it onto a conveyor.

Cycle time is around half an hour, whereas the previous method of manufacture on a lathe and a vertical machining centre fitted with a dividing head would have taken more than an hour, plus the time needed for handling and setting up both machines for each new batch.
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