One Shop Shares Some Secrets For Working With Platinum.
Johnson Matthey relies on EDM for machining platinum.
The medical-device industry is going platinum as advanced medical procedures require instruments that are made from that precious metal. Because platinum is inert, it will not corrode inside the human body, and patients rarely experience allergic reactions to it. Platinum is also highly conductive, making it the perfect choice for applications such as implantable sensors that are used to control a person's heart rate.
While the increasing demand for platinum in the medical field is saving lives, the same increase in demand for the precious metal spells machining headaches for medical-device-component manufactures such as the Johnson Matthey Medical Products Group (www.jmmedical.com). The shop is a division of one of the world's largest traders of platinum, and it specializes in the production of precision components made from platinum and other exotic materials that are used in micro-medical-devices.
Jyrki Larjanko, the medical products group engineering manager at Johnson Matthey, says platinum is extremely demanding because it is gummy and abrasive, and it does not machine like other materials. In addition, he says the shop must contend with stringent part-finish requirements, workholding issues and all-embracing chip-reclamation procedures.
Johnson Matthey machines most of its micro-medical-device components on Swiss-style screw machines and wire and ram-type EDMs. The high-production screw machines handle the bulk of the primary part operations but, often, EDM is the only way to cut the extremely small secondary-operation details required for many platinum parts. And, for some parts, the shop uses nothing but EDM.
When micromachining expensive platinum on the screw machines, Johnson Matthey always tries to use raw material that is as close to finished part shape as possible. However, since its workpieces are so small — some are only 0.006 in. in diameter — Larjanko says there is a limit to how close the raw material can be made. So, the shop typically runs parts from four, standard-sized raw-material rods that are no less than 0.065 in. larger than finished part dimensions. Anything smaller causes problems for the screw machines.
One such problem is that the shop's cutting tools can break when they gum up or run into an intersecting hole. For the shop's micro-sized parts made of platinum, cutting tools can be as small as 0.008 in. in diameter. A secondary problem is that in machining the parts, burrs can be made that are nearly impossible to remove because the part features are so small.
To accurately cut such features as burr-free as possible and to meet surface-finish requirements, Johnson Matthey uses several EDMs. These include two Edge2 ram EDMs (one of which is equipped with a fine-hole option), a U32J wire EDM, a U32i wire EDM, and an Edge1 ram EDM, all from Makino (www.makino.com).
"With these machines, we produce surface finishes down to 5 microns, and we eliminate, or at least minimize, random tool marks, dings and dents. And there is no foreign material visible at a 10-times magnification on any of the parts coming off the EDMs," Larjanko says. He adds that using the Edge2's fine-hole option helps the shop to exceed the precision micro-component demands of the medical-device industry by maintaining dimensional tolerances from 0.005 in. to 0.0002 in., concentricity to 0.001 in., edge breaks and corner radius/fillets from 0.002 to 0.005 in. and burr tolerances to below 0.0005 in.
When EDMing platinum, shops can not use copper electrodes because they burn inefficiently. "The properties of copper and platinum are so similar in conductivity that they do not burn well against one another," says Scott Breneman, lead EDM machinist at Johnson Matthey. Often, a "reverse burn" occurs.
Instead of burning material away from the part, the copper electrode itself burns, so Breneman uses tungsten or standard brass-type wires for wire EDMing and graphite or tungsten electrodes for ram EDMing when he is machining platinum.
"Platinum is a lot like working with tool steel, as far as wire EDMing is concerned, but on a ram EDM, platinum can be challenging because electrodes tend to wear much faster than the workpiece being burned," says Breneman. To remedy this, he uses special machine parameters he has developed over the years. He is not able to talk about those machining parameters. However, Makino includes starting parameters for platinum in databases it supplies with its machines. Initially, though, Breneman was on his own when he started to EDM platinum.
Although machining platinum is hard enough, Breneman says the shop's biggest challenge is determining how to accurately and efficiently fixture micro-medical parts. Such parts force the shop to design and build its own workholding tools because there are no commercially available systems that will work, Breneman says.
Custom-made fixtures have to secure parts, and they must allow the shop to machine as many parts as possible on a single setup to keep pace with production demands. Johnson Matthey designs its fixtures using solid modeling and tests every workholding concept before it cuts any parts.
"Thankfully, EDMs generate no force against workpieces, so fixturing for them does not have to be that robust. It does, however, have to secure parts enough to withstand EDM flushing forces that may blow the workpiece out of a fixture," says Breneman.
When a shop works with platinum and other expensive materials, it should reclaim as much scrap (chips) as possible from machining operations. Platinum is so expensive that several relatively small chips that could cling to the bottom of a shoe could be worth a lot of money. This is why Johnson Matthey, for instance, reclaims 99 percent of material left from its machining operations, and the shop prides itself on its material-control procedures in which almost every piece of platinum is tracked and accounted for (see Tightening Security).