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CNC simplifies complex medical milling.

CNC simplifies complex medical milling

Making metal parts to work with the human body is a very precise, delicate process. Just ask Randy Dull, shop supervisor of Danninger Medical Technology Inc, Columbus, OH. For several years, his machine shop has made small stainless-steel plates for spinal implants and parts for a therapeutic motion machine known as a CPM. For the implants, high accuracies and excellent craftsmanship are vital because the patients wear them for a lifetime. The implant details have tolerances of [+ or -] 0.005", and some parts made have tolerances of [+ or -] 0.0007".

The small implant plates, 3/16" thick X 5/8" wide, range in length from 1 3/4" to 15 1/4". They have several tiny slots with chamfers on each side and seats for small screws. The plates are made of 316 LVM grade stainless steel, which was chosen because of its corrosion resistance to body fluids.

Recently, as the design of implants and CPM machines became more advanced, milling the small, complex parts proved to be so difficult on manual machines that Danninger decided to upgrade the operation. The firm bought a Journeyman[R] 325 CNC mill from Tree[R] Machine Tool Co.

Danninger found that the new CNC mill could handle the complex jobs, as expected, but also provide time and cost savings. The result has been greater business.

"Jobs that were just about impossible to do on a manual mill can be run easily on the CNC mill," says Dull. "For example, the plates have slots that vary in number from one to ten in half-slot increments. With the CNC, we write one program for a ten-slot plate and easily cut it down to the other increments. It would be very hard to do this on a manual mill."

Plastic, too

Another success story involves a molded-plastic electronic enclosure for the CPM. Dull says there is a large radius curve and a small notch with little clearance. "It would be difficult to machine this part on the manual mill, because we'd have to set up a rotary table or get a huge (impractical) cutter. Also, it would take extensive setup time. But with the Tree, we've made one fixture to hold these parts - and mill two at a time."

He points out, "You're almost guaranteed to have more consistent parts with higher accuracy. You don't have to worry about how the workpiece is put on the vise each time, or how tools are set. The Journeyman mill's repeatability of [+ or -] 0.000 15" ensures that the extra machining speed does not compromise accuracy."

Dull adds, "With the CNC, we can test prototypes to make sure they work before we invest thousands of dollars in tooling for castings. Programming speed and system versatility enable us to run small batches and still be profitable."

The mill's DynaPath[R] System 20 CNC control allows the first part to be "proved" by graphically simulating the machining operation on the screen, showing the sequential machining steps from blank to finished workpiece. If the operator programs beyond the mill's capabilities, the control gives an error message.

Once it's programmed, the job is retained in CNC storage. Then, the next time that program is needed, the operator simply calls up a part number. All steps are remembered and the machining cycle is repeated. No reprogramming is necessary.

For more milling-machine information, contact Tree Machine Tool Co Inc, 1600 Junction Ave, Racine, WI 53403 or circle 419.
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Title Annotation:computer numerical control
Publication:Tooling & Production
Date:Jan 1, 1990
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