Engineering Insight

 

In 2001, Castleberry set up an 8 × 15-ft machine shop, housing just a manual lathe and knee mill. But the company quickly expanded and outgrew its modest in-house machining setup. Castleberry relocated and upgraded the shop to CNC capability with a VF-2 vertical machining centre from Haas Automation (Zaventem, Belgium; www.haascnc.com). After outgrowing two more locations, the company’s machine shop now has six experienced machinists and new CNC equipment from Haas. The machines include a VF-2, two Mini Mill and two Super Mini Mill vertical machining centres, all equipped with 4- or 5-axis HA5C indexers. There are also two TL-1 Toolroom lathes and also a Super Mini Mill 2 fitted with a TR-110 trunnion.

 

Haas vertical machining centres use a single-mount, multi-axis positioning tool to perform complicated tasks in a single operation.

“‘Cheetah Speed’ is the motto we often have to work by and we have about one-and-a-half machines for every machinist,” Castleberry notes. “We rely on the Haas VMCs fitted with rotaries to give us the full 4- and 5-axis capabilities we need. But, importantly, they save us time by letting one single-mount, multi-axis positioning operation handle jobs that would otherwise require multiple operations and multiple setups. Working with ‘always configured’ multi-axis Haas machines means that there is little time lost to setups,” Castleberry says.

 

“None of our work is simple, because virtually everything the shop produces will be used to slide in and out of a human body, and is handled by surgeons wearing easily damaged latex gloves,” says Castleberry. He explains that sharp edges must be avoided, requiring the surfaces to typically have 30-µm or better finishes. “We carefully configure our CAM postprocessing to take advantage of the 15K spindle and high-speed look-ahead capabilities of our Haas VMCs and trunnions. Getting a smooth finish right off the machine reduces post-finishing,” Castleberry adds.

 

NuVasive does a lot of work with 17-4 stainless steel, which is heat treated in-house and then machined hard to get a clean finish and predictable final dimensions. To prevent potentially harmful reactions within the human body, the stainless steel and titanium pieces are passivated. “Our tools often go as small as 0.020-in. ball mills,” Castleberry explains. “Machining titanium with such small tools is especially tricky as pieces tend to distort when heated. Therefore, we must produce large chips to carry away the heat.”

 

Although NuVasive standardises equipment and procedures as much as possible, almost all of its products are unique. “Engineers are used to ‘drop that, go with this’ requests,” Castleberry concludes. “Having exactly the right tools—reliable multi-axis machines configured and ready to go—makes our type of work possible.”