Author: Tronserve admin
Tuesday 3rd August 2021 08:51 AM
Optimizing Machining and Workholding for Metal Additive Manufacturing
Additive Manufacturing (AM) has made a splash as the technology for producing highly customized products in almost any material you can think of. Thanks to design flexibility, 3D printed metal parts can be produced for every application from aerospace to medical.
However, AM’s unique designs come with a hitch.
Freshly printed metal parts require extra machining steps to remove structure supports, and the rough surface finish and unreliable dimensional accuracy of some additive processes can make precision features such as mating surfaces and tapped holes impossible. Because complexity is free with additive manufacturing and parts need not conform to conventional orthogonal shapes, customized parts may also come in shapes that make workholding especially challenging.
This article will review how manufacturers today can overcome some of these challenges and optimize their AM processes.
Why Metal for Additive Manufacturing?
AM is an ideal process for metal as it allows metal parts to be built without the need for conventional tooling, ignoring many geometric limitations, and allowing for part consolidation for more efficient designs. This makes several metal alloys ideal materials for aerospace, automotive and medical applications as well as improves productivity in injection molding and expands possibilities in creative industries such as architecture.
Metals that are particularly suited to welding and casting are ideal for 3D printing. Conversely, metals that are difficult or costly to cut and machine are also ideal for AM. Some of the most popular metals used include titanium, aluminum, stainless and tool steels, Inconel, copper and tungsten.
Titanium is highly popular in aerospace and medical applications for its lighter weight. With internal lattice structures, implants can be designed to accommodate bone ingrowth. Steel, on the other hand, can be used to print strong parts which are able to withstand extreme environments, such as combustion or high holding pressures. These and other metals can be printed from powder or melted wire.
Metal AM can be performed with several techniques including Selective Laser Sintering (SLS) for powder beds, Laser Metal Deposition (LMD) for either powder or wire, and Wire Directed Energy Deposition (WDED).
SLS is popular for creating smaller parts with more complicated designs using a “build-up” technique, which fuses metal powder using a CO2 laser along an X and Y axis, over a descending tray in a sealed, inert gas environment. A roller evens out the bed as the tray descends before the next pass of the laser.
WDED uses lasers to melt wire fed through a nozzle, usually in a sealed gas enclosure. This method achieves higher deposition rates compared to SLS, but can suffer from lower resolution. WDED is popular in applications requiring simpler parts and where conventional manufacturing techniques are slow or expensive, like in aerospace and automotive.
With such techniques, AM can provide significant cost and time savings for difficult to manufacture parts, while also allowing for more complex, smaller batch parts. AM has its fair share of weaknesses, however.
Extra Machining Steps Often Necessary for Metal 3D Printing
Selective Laser Sintering with metal powder for Additive Manufacturing. (Image courtesy TRUMPF.)
To put it bluntly, 3D printing metal parts is often not the solution for a manufacturer looking to produce parts at a high, mass-production volume. AM is ideal for smaller batch productions that emphasize customization and unique design over sheer quantity. Unfortunately, even the great degree of design flexibility causes its own issues.
Printed metal parts don’t come out of the machine with a smooth surface, and are often made in such unique shapes as to require support structures to help keep the part upright and stable during the printing process. As a result, extra machining steps are necessary in order to remove support structures and perform surface finish procedures.
Removing supports may require manual processes such as using a bandsaw, and deburring may be necessary for certain parts, especially for applications which may not require machining, says Mark Kirby, additive manufacturing business manager at Renishaw Canada.
“The only time we don’t do machining is, ironically, on some medical parts like cranial plates which will be polished,” says Kirby. “We’ll have a lot of manual work to do to make them smooth.”
If a part requires holes, it is often better to drill them afterward than print them with the holes, to prevent rough surfaces in these negative spaces which can often come out at improper measurements.
While certain non-critical hole features Kirby suggests are acceptable to print with good datum points, anything large in size or especially precise is likely more trouble than it’s worth.
If a printed hole is out of position, it could demand a time-consuming manual fix, Kirby explained. “I might have to helically interpolate it and put it on a center where it needs to be and so on. If I just drilled it, it would have been in the right position.”
Furthermore, printed holes could be inconveniently placed on the part and hard to reach for machine tools during the finishing process.
Parts with unconventional shapes cause additional challenges when it comes to workholding. During the finishing process, a part may need to be held at odd angles, requiring the manufacturer to have a good understanding of where the work piece will relate to the cutting tool and how it will interface with the table of the machining center.
Clamps and vices are usually the first go-to workholding solution, but such options are ineffective for 3D printed parts with complex geometries.
For more complex and delicate parts, soft jaws are a good solution as they can be contoured to more accurately fit the part. These soft jaws could be 3D printed themselves. Kirby finds these solutions effective, but smaller printed parts, such as those for medical applications, are not typically designed with load tolerances able to withstand machining.
Keeping these limitations in mind is essential when considering machining techniques and proper workholding solutions.
Machining Operations for Surface Finishing 3D Printed Metal Parts
Depending on the additive process, a fully printed metal 3D part may first need to be cut away from its base or backing plate. Certain materials are very tough and abrasive, making manual bandsaw operations dangerous, wasteful and expensive. Wire EDM, or Electronic Discharge Machining, is a good option in these cases.
EDM can function as a single pass operation for part removing and surface finishing with vastly superior repeatability and precision, says Brian Pfluger, EDM product line manager at Makino. Options like Wire EDM greatly reduce waste versus manual bandsaw operations and are much less likely to damage your part.
“With Wire EDM, you’re going to hold that consistency probably within ten microns of material left on the part for final grinding, milling or completion of the part,” says Pfluger. “The parts, when cut off, are far more consistent and you can eliminate scrap.”
EDM is also a superior solution versus conventional machining and surface finishing options, capable of producing oblique holes, odd-shaped features, and finishing a surface along a part with variable thickness.
“Every Wire EDM machine has what’s called adaptive power control,” Pfluger explains. “Variable thickness is very taxing because of that dynamic shift in material thickness. The machine generator must be able to see it, sense it and react to it. If it doesn’t, it’s going to cause a wire break.”
Typically, granular material like powder is filtered out, but larger pockets of powder can disrupt the system beyond what adaptive power control can compensate for and cause a wire break, which will slow the machine down and disrupt the process. These pockets accumulate in hollow spaces designed into 3D parts.
When a flushing nozzle passes over such a pocket, the fluid stream loses pressure, becoming less effective, dropping in pressure and creating turbulence. Floating particles can then damage the EDM wire. Larger diameter wires, or using different types of wires, can withstand such interference for longer periods, but many wire EDM machines are also capable of automatically rethreading themselves.
Sinker EDM is an alternative that specializes in smaller features requiring higher accuracy and finer surface finish. Sinker EDM is ideal for producing difficult-to-machine sharp inside corner radii or a blind hole, for example.
If Wire or Sinker EDM do not seem to be the ideal solution, hybrid machines include both additive as well as machining capabilities, explains John Zaya, product manager of workholding at Big Kaiser.
“Theoretically, what you can do is have a part that is 3D printed onto a platter and it builds up the 3D printed parts and then the machine would turn into or switch into machining mode. It would then take an actual cutting tool, a subtractive cutting tool, and do final machining operations to that work piece.”
In hybrid machines, the part stays securely located in the same fixture during both operations, eliminating the headache of locating the part for accurate machining.
Workholding Solutions for Metal Additive Manufacturing
Workholding is one of the most important elements to the AM process, Zaya argues, and must be considered at every step of the production process. Ensuring a part is manufacturable must include size, shape and material considerations as they relate to workholding solutions for whatever machine the part is going into.
“With a little initial consultation with a manufacturing engineer, you can sidestep a lot of problems that would occur down the line,” Zaya says.
With some preplanning, one can determine whether to palletize parts onto a fixturing plate or consider special mounting geometry in the design of the workpiece to attach a clamping knob to the part after it has come off the printer, Zaya continued.
One way to work with metal 3D printed parts is to have a 5-axis machine and either clamp on at least 1/8th of an inch of material, or use a dovetail, or put some clamping pins directly in the work piece, says Michael Gaunce, group manager for stationary workholding at Schunk. “You can basically machine the whole thing complete, all in one set up, and then just flip it over and clean up the other side.”
In contexts where such forward solutions just won’t work, such as due to highly complex geometry or sensitive designs, soft jaws are the key. However, this method requires attention to the contours of the part and the radius of the soft jaw.
Gaunce recommends making the radius of the soft jaw marginally smaller than that of the part to ensure a proper grip, clamping on two points. This requires an understanding of how well your 3D print will come out, Zaya adds.
“When you put your work piece into the soft jaw and the surface finish of the work piece is very rough, then it's going to locate at the ‘high points’ on that feature,” says Zaya. “You may not be getting full complete contact and it may only register against certain areas of it.”
The design of a part can make workholding simpler and more reliable when pre-planning holding features. Locating bores and fastening features can be pre-printed into the workpiece or into the casting, and hold down tabs or recessed areas for clamps or bolts can be printed into the work piece for larger parts, Zaya explained. “Pre-planning the design of a work piece to include clamping locations or tooling locations is very beneficial to prepping the work piece for subsequent operations.”
The worst-case scenario is when the part is particularly delicate and fragile, and no vice or clamp can do the job safely. Alternatively, creative teams can come up with especially unique solutions for thin or small work pieces with adhesives.
Hip surgery locator device nicknamed “Cobra” by machinists. (Image courtesy Renishaw.)
Renishaw and the University of Waterloo recently collaborated to design a device used to align the human hip joint during a surgery. Because of the part’s unique, generatively-designed shape, the team nicknamed the part “The Cobra.” On the part, four pins needed to be machined in order to attach retroreflectors for the medical procedure. It was important not to distort the part during the machining process.
To fix the part to its baseplate, a series of pads were 3D printed and screwed to the plate. A special glue was applied to the pads to fix the part in place.
“The glue is activated by ultraviolet light and holds the part in an undistorted state,” Kirby explained. “When I put a part down in a pile of glue and cure it, I know approximately where the part is, within about a millimeter. We have to use our best fit probing after that.”
With exposure to UV light, the glue takes only 90 seconds to cure, efficiently sticking the titanium part in place.
Metal additive manufacturing is a tool in the toolbox for manufacturers looking to solve unique challenges in part design and manufacturing, but finding ways to properly hold and secure those parts for final finishing, machining and post-processing may require some engineering and ingenuity.