RMS produces up to 20,000 3D printed titanium implants, such as these spinal implants, per month on 30 powder bed fusion machines. All these parts require finishing in a machine shop that the company has set up specifically for additive parts.
As a manufacturer of medical devices, the RMS the objective of the additive division is to provide all the services necessary for the additive manufacturing of implants from âpowder to packagingâ. Much of this ability is machining. âEvery additive part we make is machined,â says Troy Olson, director of operations for the company’s additive group.
âEvery additive part we make is machined,â says Troy Olson, director of additive group operations at RMS.
Before launching its additive division, RMS had solid experience in subtractive manufacturing. The company has been machining parts for over 50 years and currently has over 650 machine tools that perform operations ranging from Swiss-type turning to five-axis milling. However, RMS has learned that this experience does not always apply to 3D printed parts. In fact, the pace and workflows are so different from the subtractive parts that the additive division needed a separate, dedicated machine shop with its own employees. âWe didn’t want additives to screw up what is already an industry-leading machine shop,â says Ryan Kircher, senior engineer in additive manufacturing.
Where conventionally manufactured parts require machining up to 90% of stock, add-on parts are already near-neat shapes that require basic operations such as drilling, tapping, and machined instrumentation slots. Cycle times are much faster. Depending on the number of features machined on a part, employees can machine between six and 20 parts per hour and change machines up to two or three times per day. In addition to the different workflows, the finished additive parts have to be treated differently as they are more expensive to scrap. The whole process is designed to produce zero scrap.
Two processes are better than one
The workshop’s five-axis machines, multi-spindle lathes and other advanced machine tools produce complete parts. Additive parts are different in that 3D printing alone is not enough. In fact, all parts additively manufactured by RMS require finish machining.
âWhen we looked at additive manufacturing, we saw it as a complementary technology, not a one-off technology,â says Olson. Kircher says he saw many pitfalls associated with treating AF as a stand-alone technology in his previous experience working with additively manufactured implants. âYou had to make compromises in the design to accommodate additive manufacturing or configure those horrible manual post-processing steps like deburring, removing the backing, and hand-tapping the threads that were almost printed on the right. By using both processes, RMS can manufacture implants efficiently and cost effectively without compromising the design.
The Additives Machine Shop is run by Aaron Glanz and consists of eight machinists on the first shift, seven on the second and two on weekends with the intention of adding more. The equipment in the additive machine shop is standardized, just like the rest of the company, for maximum flexibility. It has 12 FANUC Robodrills, as well as three CUT AM 500 horizontal EDMs, specially designed by GF Machining Solutions to remove 3D printed metal parts from their build plates. The additives division also has a Haas CMV dedicated to resurfacing build plates and a Haas lathe for machining AM specimens.
All of the parts on a build plate travel together through downstream operations as a batch. Each batch requires a separate setup in the machine shop, including first and last part inspections and full line clearance.
Subtractive workflows for additive parts
However, the process of machining additively manufactured parts begins before they reach this area of ââthe workshop. The same employees who are responsible for running the printers are also responsible for separating the parts and then resurfacing the build plates.
After being separated from the build plate, the parts are sent for hot isostatic pressing (HIP) and returned to RMS for machining. Between printing and HIP, the parts have had a lot of added value at this point, so machinists have to be very careful not to throw them out. âAaron has built his machining group around zero scrap – fixture, probing, standardized tooling, standardized programming – so that we don’t print a part to sacrifice for the setup of the machining operation,â Olson explains. Despite the relatively small amount of material removed from these parts during machining, getting it right is essential.
Based on his own experience, Glanz says embracing a zero scrap mindset can be a challenge after working in RMS’s subtractive divisions. âYou take machinists from the ground who are used to using wooden bars,â he says. âIf a machine tool operator makes a mistake, you can just go back to the warehouse. Operators must be taught that this is not possible with printed parts. Imagine the warehouse was three weeks away.
Machinists in the additive shop must also adapt to the increase in the number of setups. Orders for additional parts are broken down into production plate quantities. For example, an order of 1000 pieces might require 10 to 15 build plates. Each build plate typically contains between 75 and 100 parts, which travel as much during post-processing operations, including machining. However, this means that each build plate is its own configuration. âRather than setting up a job once and running it for 1,000 parts, Aaron configures that job 10 to 15 times to run the same 1,000 parts, because a lot of it all comes back to him,â says Olson. âAt a minimum, each batch should have a first part, one inspection, one last part inspection and full line clearance. “
All 3D printed implants have specially designed bone growth surfaces which are very complex and cannot be damaged or contaminated. Designing clamps for these parts is a major challenge.
One of the biggest challenges in machining additively manufactured parts is clamping. âMachining an additive part is complicated because it’s not a piece of bar where you have nice references and can extract it,â says Kircher. âYou have to figure out where that part is in the space and make sure that the features you are machining on the part are in the right place. This is not trivial when it is already 90% manufactured.
What further complicates the situation is the fact that all parts printed by RMS 3D are implants with features designed for bone growth that cannot be produced by conventional methods. âIt is the technology that our customers design in their implants that we cannot damage or contaminate,â notes Glanz. These surfaces can be so complex that they cannot even be touched with a piece of fabric. âWe have to work on fitting around all of this and make sure we don’t mark their surface technology,â he adds. For example, the company designed a clamp for one of its spinal implants around a hole in the middle of the room. The fixture captures this characteristic and keeps the parts stable without damaging the surface while additional stock is machined at the ends.
3D printed implants only require machining a small amount of material. However, scrap metal can be expensive as these parts have great added value by the time they arrive at the machine shop.
Ideally, RMS can provide information during the design phase to reduce these challenges. âWhen we can engage with our customers early on, we’ll actually design the part around the clamping and reference structures for post-machining,â Olson explains.
Despite the differences between machining additive parts and machining parts from stock, the team agrees that their experience in conventional manufacturing has been key to what they have accomplished with AM, given the challenges involved. to the machining of these parts. âI think it takes a very experienced subtractive manufacturing team to complete additive manufacturing and make it successful,â says Kircher.
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