Renishaw's Polished cranial plates
Renishaw's Polished cranial plates
Renishaw’s Medical and Dental Division has created a solution to dramatically reduce the amount of time it takes to finish cranial implants using Rösler's readily available technology.
Renishaw's 3D printed patient-specific implants; in particular, cranial plates are much lauded the CT scan-to-CAD software that was used to create them is award winning and they featured on the front cover of a 2016 edition of this very magazine. Much has been written about the CT scan-to-CAD step and how LPW's titanium powders were used in Renishaw's Additive Manufacutring (AM) machines to print these life-saving implants but little has been said about the exceptional quality of the surface finish.
The original commissioning neurosurgeon, Bartolomé Oliver required the surface to be satin-like in order to match the patient's cranial contours accurately. It was Renishaw Applications Engineer, Andy Wescott's job to create a repeatable and streamlined process to finish the cranial plates top both satin and highly polished finishes from the as-built condition.
"Traditionally, post-processing these parts to a low surface roughness value, was very manual and time-consuming," said Wescott. "The post-process time for a large cranial plate was up to five hours. We needed to reduce that time and the amount of manual input because if you have an operator working on a part and they lose concentration for one second, burning a hole in your part then you've just made one expensive bit of scrap."
Cranial plates come in all different shapes and sizes.
Cranial plates come in all different shapes and sizes.
The solution was not a major reinvention of the wheel merely a slight adjustment of one, namely a Rösler tumbling machine. Metal 3D printed parts are now of such quality that they can be treated like any other metal part. And Rösler knows a thing or two about surface finishing metal parts having been in the post-processing game with its vibratory finishing and shot blasting divisions for more than 80 years. The fact that most metal parts come out of the AM machine with a relatively rough surface means Rösler can apply years of learned knowledge to AM. As such Wescott’s division decided to purchase a Rösler High Energy Centrifugal Disc Finishing Machine FKS04 to finish their parts.
Now, after a cranial plate comes out of a Renishaw AM250, it undergoes only a small amount of manual operation to remove supports witnesses and slightly improve the surface using a carbide burr and flap wheel. It is then placed into the Rösler tumbling machine were it automatically goes through a three-step process to produce the impeccably smooth finish, reducing manual operating time to under an hour.
A Renishaw Medical and Dental Implant inside Rosler tumbler
A Renishaw Medical and Dental Implant inside Rosler tumbler
The clever part, which ensures repeatability on each and every part, comes from a little bit of Renishaw engineering know-how as Wescott outlines:
"We've designed a tool to hold our parts in a particular orientation (inside the tumbling bowl). Rather than just throwing parts into three media types and coming out with a sort of fixed part, our parts require precision. Certain features require protection (from the tumbling media) so we've invented a tooling method that keeps parts face down into the media only applying the surface finish where needed."
Although there isn’t yet a one-size-fits-all solution for finishing Renishaw has shown that it needs not to be a gruelling experience. Much like the additive manufacturing process itself, your finishing technique depends on your application and you should adapt finishing products to suit your needs.
"Finishing is still a relatively weak link with AM, but I think the risk is thinking of additive as just one technology all by itself," says Marketing Manager for Renishaw's Medical and Dental Division, Ed Littlewood. "Take milling, for example, we've been doing that for years and it is taken for granted that you have deburring operations next to it, you have anodising operations and all other removal processes. So just as with milling, we need to develop reliable processes that go alongside AM."
