Shops that machine prototypes take aim at RP turnaround times
In the television series Star Trek: The Next Generation, weapons engineers aboard the USS Enterprise didn’t need to search the galaxy for rapid-prototyping (RP) shops to model their phaser designs. All they had to do was program the ship’s holodeck with whatever death-ray design an engineer dreamed up in the shower, test the device on a few hostile Romulans and go into production.
Blast away, Capt. Picard.
The manufacturing process is a little more challenging on Earth. Whether you’ve come up with a new phaser design or a radically new paper cup dispenser, product development is frequently an iterative, learn-as-you-go process of design, prototype, test, which is often followed by redesign, reprototype, retest.
Luckily, product designers today have a range of prototyping options to choose from that speed the process.
Since the introduction of stereolithography (SLA) in the late 1980s, RP technology has grown to include fused deposition modeling (FDM), inkjet printing and selective laser sintering (SLS). RP processes share some basic similarities. All are additive, meaning parts are built from the bottom up by adding material, one layer at a time—like making a wedding cake. The process used determines what the “cake” is made of—nylon, ABS (a thermoplastic) or polyurethane, to name a few.
There is virtually no waste with additive processes, and machine programming is nonexistent. Simply slice the 3-D CAD model into paper-thin layers and start extruding, sintering, lasing or binding each one. With few exceptions, part geometry and complexity are unlimited with additive RP. If you can dream a part’s design, chances are at least one RP process can turn it into reality.
Back to basics
With all these additive processes to choose from, whatever happened, you might ask, to the local machine shop that cuts prototypes from metal? Nothing—if your vendor is Reed Prototype + Model, Austin, Texas. According to RPM Projects Manager Brian Stobaugh, machined prototypes are alive and well.
RPM specializes in quick delivery of one-offs and low-volume work with complex part geometries and 3-D surfaces.
“We typically quote 2 to 3 days for delivery, depending on shop load and part specifics,” said Stobaugh. “From a tolerance and surface-finish perspective, machined prototypes are as close as you’re going to get to the final product. This means you can do final fit checks and regulatory testing, or just get an actual part into your salesman’s hands, before going to production.”
Parts with tolerances in tenths of a thousandth of an inch, steep draft angles, and fine details and surface finishes are beyond the capabilities of most RP processes. In spite of these limitations, Stobaugh sees additive prototyping methods as complementing machining.
Machining allows a shop to produce prototypes with finer features than is possible with RP. Image courtesy Reed Prototype + Model.
“We don’t compete with additive technologies,” he said. “Machining is just a different portion of the prototyping process—one that comes after RP but before full-blown production.”
Brad Cleveland, president and CEO of Proto Labs Inc., Maple Plain, Minn., agreed with Stobaugh. “We don’t view additive processes as competition for CNC-machined prototypes,” said Cleveland. “Traditional RP complements what we do here.”
One of Proto Labs’ offerings is the Firstcut CNC machining service, which boldly goes where no shop has gone before. “We believe we are the world’s fastest quick-turn CNC machine shop. We don’t employ machinists, and we don’t have people sitting at CAM stations.”
A machine shop without machinists?
Cleveland explained that Firstcut has completely automated the job of CNC programming and setup. “We’ve developed software to analyze a customer’s 3-D CAD model and automatically generate the toolpaths needed to machine that part. We’ve also developed a proprietary fixturing mechanism. We can go from CAD model to quote within minutes, complete with a graphical representation of the finished part and an analysis of what can’t be reached with the machine tool.”
Rapid prototyping allows almost any design to be produced within hours. Shown is the San Francisco skyline fabricated by a stereolithography machine. Image courtesy Solid Concepts.
If the customer likes what he sees, explained Cleveland, the toolpath is sent to a machining cell at one of three worldwide machining locations. Every machine at Proto Labs is programmed to know the toolset it has available and has been equipped with the company’s proprietary fixturing technology. A machine operator loads the correct material blank, hits “cycle start” and the part is machined. “We can go from customer order to a part out the door in the same day. It’s about the same speed as additive processes.”
What’s the catch? Cleveland admitted the Firstcut process is limited in terms of part size, complexity and material. Customers can choose from one of several dozen materials, including ABS, nylon, Delrin, PEEK and ULTEM (thermoplastics), aluminum, brass and, in some cases, stainless steel and magnesium. Parts must fit within a 10" × 7" × 3.75"
envelope, and be machinable on a 3-axis machine fitted with standard cutting tools. Features deeper than 2" may not be possible, and parts measuring under one-quarter-inch square or with tolerances tighter than ±-0.005" would likely be no-quoted.
Said Cleveland: “We have hundreds of 3-axis mills that can machine from six orthogonal sides of a part, but that’s it. We currently don’t have 5-axis capability. So if you want a hole drilled at a 45° angle, we won’t do that. We show our customers the limitations up front, and if they can’t live within our capabilities, they’ll need to take that part somewhere else.
“Even so, we can solve around 80 percent of the geometries that we receive today” he added. “We continue to add machines and improve our software, but until we’re ready, we’ll say ‘no’ to work that doesn’t fit our process.”
Covering the bases
Designers who’ve been no-quoted by Proto Labs might take their work to a shop like Solid Concepts Inc., Valencia, Calif. It offers quick-turnaround CNC machining and a number of RP services.
“It’s important to have the right tool for the job,” said Chuck Alexander, the company’s product manager for additive manufacturing. “We cover all the bases, with additive RP being about 40 percent of what we do.”
Sounds like a coin toss. What determines when Solid Concepts makes chips?
“There are a number of different drivers for that,” Alexander said. “Quantity, time, cost, cosmetics, functionality—all those things factor into what we recommend.” He explained that machined parts offer unmatched accuracy and surface finish, out of virtually any material. But, depending on the part geometry, they are often the most expensive to make in prototype quantities.
Solid Concepts fabricated our cover subject, Rapid Man, on a 3-D inkjet printer from Objet. Image courtesy Solid Concepts.
And while RP can deliver pretty much anything part designers can dream up, it has limitations. “For example,” said Alexander, “the only way to get good cosmetic appearance on additive-manufactured parts is to add finishing labor. This drives up cost. You typically avoid this step with machined parts.”
Although designers have a wider selection of workpiece materials to choose from than ever before, each material is largely tied to certain RP processes, which isn’t the case with machining. For instance, stereolithography can meet part tolerances of a few thousandths of an inch and produce very fine details, but it uses epoxy-based thermoset resins. “SLA offers the highest value for prototyping, meaning the biggest parts at the lowest cost,” said Alexander, “but you’re limited to epoxy-type materials.”
Selective laser sintering offers designers a choice of several engineering thermoplastics, including nylon and PEEK, but because the process builds parts from a bed of plastic powder, tolerances and fine details can be problematic. Also, the material is granular, so surfaces are somewhat rough, like a casting.
“A big phase-change [occurs] going from powder to solid, which leads to high shrinkage,” said Alexander. “But the density of the finished product can reach 100 percent, making SLS parts very strong.”
Fused deposition modeling is another process that makes parts from production-grade engineering plastics. With FDM, plastic—such as ABS, polycarbonate or ULTEM—is extruded through a nozzle. “It’s like a hot-glue gun,” said Alexander. “FDM is a slower process than either SLA or SLS, and it has the lowest resolution, with layers 0.010" thick (compared to 0.004" or thinner for SLA). But because there’s less shrinkage, it also has the highest accuracy.”
The right stuff
Does this mean that additive RP can only be performed with a handful of materials, that tolerances are loose and finishes are poor? No.
Enter PolyJet, the newest kid on the RP block. Looking like a Xerox on steroids, PolyJet 3-D printers, manufactured by the Israeli company Objet Ltd., support a wide range of plastics and rubbers. They can put down layers as thin as 0.0006" and boast resolutions exceeding 600 dpi. What’s more, prototypes from a PolyJet printer can be made from multiple materials in the same build. Want to fabricate a model airplane with clear windows, black propellers, a silver fuselage and rubber wheels? It’s possible with a PolyJet.
Schmit Prototypes Inc., Menomonie, Wis., owns a PolyJet 3-D printer, the Eden 260V. Said Schmit’s president, Steve Upton, “3-D printing is starting to compete with the machining side. Compared to SLA and some of the other RP processes, the Objet gives a much tighter build. It’s more repeatable and can be used with a wide range of materials.”
FineLine Prototyping used the direct-metal-laser-sintering process to produce this part from 17-4PH stainless steel. Image courtesy FineLine Prototyping.
Upton said that the PolyJet can be printing within 30 minutes of an order coming into the shop. “Set it, print it, ship it. With machining, you have to program the job, fixture it and [sometimes] there are multiple operations. Nine times out of 10, printing is faster than machining.”
Despite this, Upton noted one big advantage to machining: “You get parts out of the material you want.”
One RP process that expands the list of materials designers can choose from is direct metal laser sintering (DMLS), said Rob Connelly, president of FineLine Prototyping Inc. The Raleigh, N.C., shop operates 20 SLA machines and a handful of SLS machines that it uses for DMLS applications.
“You don’t have to make material sacrifices with DMLS,” he said. “With this technology, you can do additive rapid prototyping using 316L, 17-4, tool steel, cobalt chrome—even titanium—and the parts come out every bit as strong as those made from forged bar. In some cases even better, with nice grain structures and close to 100 percent material density. The tolerances and layer thicknesses are just as good as SLA.
“Regardless of the [prototyping] process used,” continued Connelly, “it’s important to understand that RP and machining complement one another. CNC-machined parts will have far better accuracy and surface finish than RPed ones, and in those instances where the parts are not terribly complex, machining is faster and cheaper than RP methods.
“Where RP-based methods shine is in difficult geometries—thin walls, undercut areas you can’t reach with a cutter, and where the engineer has dreamed up something you can’t machine, period,” he continued. “There are some cases where CNC is great and others where it just won’t work. The same can be said for additive processes. There is no ‘one size fits all’ when it comes to prototyping.” µ
FineLine Prototyping Inc.
Proto Labs Inc.
Reed Prototype + Model
Solid Concepts Inc.