The Value of Swiss-Types Milling Rectangular Medical Parts

Offering more than 15,000 products, Boston Scientific is a market leader in stents, catheters, pacemakers, defibrillators, ablation devices, monitoring equipment, and spinal and brain stimulation equipment. 

Recently, the company experienced issues machining an insertable cardiac monitoring battery component at its Arden Hills, Minnesota facility. Larry Hennig, manufacturing engineering specialist there, explains that the year-round production of this small, rectangular part involved numerous milling machines and required multiple milling operations. In addition, each machine required an operator to tend it at all times.

Parts were produced using two milling operations, op10 and op20, on two different machines. Both operations used multi-part fixturing devices. Light hand deburring after op10 machining was required for each part as they were prepped and fixtured for op20 work. Total time to complete a part was 7 minutes.

Hennig and his team members determined that a Swiss-type machining platform could be more beneficial as it could eliminate manual deburring and reduce process time to approximately three minutes. In addition, one operator could tend multiple Swiss-types with the ability for the process to run unattended while auto-segregating particular parts for in-process dimensional inspections.

This reduced the number of employees required to tend machines for the previous process by six while the Swiss-types are able to run 12 hours unattended holding the part’s targeted 0.0002" profile tolerance.

Additionally, there was a significant improvement in material savings, Hennig notes. “We were using 0.300" of stock per part for the milling process, while the new process only requires 0.100" of stock material per part.” In addition, the new process proved to be more stable while yielding better surface finishes and nearly triple the tool life. 

The Marubeni Citizen-Cincom Swiss-type machines used for the new process, Citizen L12x models, were purchased through the local Minneapolis Citizen distributor, Productivity Inc., partnering with Andy Walser and its tooling representative, Terri Farrell. Boston Scientific now has 17 such models.

The story might end here, except for a key component in further improving this part’s production process. Farrell introduced Boston Scientific to Preben Hansen, president of Platinum Tooling Technologies, who in turn introduced a Heimatec speed increaser (1:4 ratio) specially designed for the company’s Citizen L12x machines. 

“While this component is not a round part, it is small and requires small tooling,” Hennig explains. “We were not getting enough rpm to match the tool diameter and necessary surface feet per minute, causing long process times and reduced tool life due to the tool not meeting the necessary rpm requirements. I have worked with many types of electric spindles and many brands of speeders. With the electric spindles, we had some success on light drilling and small-diameter high-speed milling. But the obstacles we faced involved load on the tool that would cause the electric spindle rpm to fluctuate, therefore leaving tool marks across the material or even alarming-out the electric spindle when load was applied.”

As a result, the company needed to rebuild all the bearings and seals on its high-speed electric spindles annually, additionally rebuilding the previous manufactures’ gear-driven speeder heads multiple times during the year while running high-production volumes of parts. 

Hanson’s idea was to use Heimatec 1:4 speeder heads and tooling attachments to provide additional live tool positions and spindle speed. Maximum spindle speed is 48,000 rpm.

Hennig says that with these additions, the company achieved a superior surface finish when drilling, reaming, profiling and face milling, which improved tool life. In addition, it realized a major reduction in cycle time by using these speeders compared to the 1:3, 1:2 and 1:1 standard tool provided by the OEM or brand X.” Hennig reports the Heimatec speeders provided by Hansen have been in operation for over two years and have not required a rebuild. He says, “They still feel like the day I brought them into the process.”

“We helped Larry convert the process for production of these components by enabling a single operator, a more stable machining operation, improved cycle times with the higher rpm and proper feed, plus the big advantage, an optimum surface finish, so critical on these parts,” Hansen says. “The deburring issue is simply no longer an issue.” 

Hennig notes that the company was also able to develop some new macro features with this latest project. Examples include rotation macro-offsets for alignment from the main spindle to subspindle and safety crash macros to help prevent accidents into toolholders due to limited clearance between the subcollet and a face of the part. The latter was important because only 0.006" of the part is chucked during part pickoff/transfer and there is limited clearance on the subspindle side. Crashes can now be prevented into the cutoff or subspindle tooling, which otherwise could happen with incorrect setups or tool offsets.

“Together with our team’s tribal knowledge, we designed custom tools that spot, drill, ream and chamfer all with a single tool, keeping the chamfers concentric, as well as the custom deburring tools providing burr-free product,” Hennig says. “These were all conditions that we struggled with on most on the mills with their need for manual deburring and several tools intersecting the same feature.” 

Hennig also explains that, based on the company’s experience, tooling specs rarely work in Swiss applications due to the lack of work holding compared to mills or even conventional lathes. “For example, when programming a high-volume product, I usually start at 50% of the tooling manufacturer’s recommendations and then run 15 to 25 test cuts at various speeds and feeds to find the optimal conditions,” he says. And when milling on Swiss-type machines, he has learned that performing climb milling for rough cuts and using a conventional mill spring or finish pass has helped tremendously with knocking off any burrs, resulting in quality surface finishes and extended tool life. 

“With any medical or implantable device, Boston Scientific is tasked with much higher standards compared to most manufacturing facilities when holding surface finish, dimensional requirements and creating no burrs,” Hennig explains. “We hold every single part to the highest standards cosmetically, dimensionally and functionally. Our number one priority is quality and providing the best patient impacting components, regardless of expense. That’s one thing I really value about this company.”