Smoothing the Path to Large-scale Additive Manufacturing
For the past year, engineers at Local Motors (localmotors.com) have been producing its autonomous electric minibus at the company’s Knoxville, TN, micro-factory. They’re getting better at it. In fact, they anticipate that by the end of the year they’ll have learned to make the vehicle about 20 times faster than they were able to when they started.
The vehicle, named the Olli, has a lightweight plastic/carbon composite chassis. In July 2018 the company installed a Thermwood (thermwood.com) large scale additive manufacturing (LSAM) system, a hybrid machine that can print a chassis and then machine critical surfaces to spec, all within its 40-foot length. A full 90 percent of the vehicle is now fabricated through 3D printing.
The process starts with pellets made of thermoplastic imbued with carbon fibers. The pellets are driven by a large screw into a 1.6-inch melt core; melted and then extruded through a print head to build up the workpiece layer by layer.
Where 3D polymer printing is usually done by extruding a small print bead onto a hot table in a heated chamber, so the printed part is kept hot until printing is complete, the LSAM uses a large bead and, prints at room temperature in a “continuous cooling” process: The beads are large enough, with enough heat energy, to completely fuse with the previous layer. The system depends on understanding at what point the printed bead is cool enough to support the next layer, but still warm enough to fuse completely with it.
There are three print heads that can be used; they vary in size to control how much material is being extruded at a time. The largest of them can put down a 0.8-inch bead, allowing printing at a rate of up to 500 lb. per hour.
At such speeds, it’s hard to maintain a constant layer thickness, so as the extruding print head moves along its path, a servo-controlled heated roller follows and presses the extrusion down to a consistent height. The entire printing system is carried on an automated gantry.
The resulting chassis is near-net shape: A five-axis CNC mill, located on a second gantry, is then used to trim it to its desired size and provide the desired surface finish.
The biggest challenges to getting production up to speed were not on the relatively modern additive side but rather on the old-school material removal side, explains Local Motors advanced manufacturing engineer Tim Novikov. For one thing, the carbon fiber in the material is abrasive, resulting in a short tool life. Secondly, the chassis walls are relatively thin in places, creating extreme chatter during the milling operation. None of this was helped by the long tool lengths needed to reach inside the vehicle’s wheel wells and other deep features.
“The milling cutter we started with created so much vibration that the chassis seemed like it was going to self-destruct,” Novikov says. “We had to reduce the feed rate to 5% just to get a good part.” As a result, the time it took to complete printing and finishing a chassis was a full 80 hours.
“A big part of that was due to the HSK-F machine spindle, which limited us to fairly small cutters and ball-nose end mills,” he explains. “These might be fine for light surfacing work and prototype parts, but we need to machine large surfaces and do so in a production-oriented manner.”
To address the chatter issue, they used a CoroMill 390 milling cutter made of titanium and optimized for rigidity and low mass. They coupled it with a Silent Tool milling adapter, a tool holder with a built-in vibration dampener. The result was the elimination of chatter when machining those thin walls at higher speeds.
They also introduced a more rigid spindle connection in the form of a Coromant Capto quick-change modular interface. This increased tool stability enough that the team was able to use larger cutting tools, which greatly reduced the number of passes needed, explains Sandvik Coromant engineer Matt Brazelton.
“And since we could now use an indexable face mill rather than solid carbide end mills, we loaded them up with PCD [polycrystalline diamond] inserts,” Brazelton says. “The result is significantly increased tool life, higher feeds and speeds, and a huge reduction in perishable tooling costs.”
And, according to Novikov, the time needed to complete each chassis on the LSAM is down from 80 to just five hours.
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