Shop Solutions: Precision Control for Micromachining Parts
The trend toward micromilling parts instead of risking the high cost of a micro-mold for prototype parts is growing, according to Andy Phillip, president of Microlution Inc., a Chicago builder of micromachine tools.
The parts produced by these micromilling machines are no larger than a 2 1/2" (63.5-mm) cube and are used in a variety of industries, including aerospace and biomedical. Microlution's micromilling machines were developed based on graduate research conducted at the University of Illinois by the company's founders. The result was the company's micro-meso-scale machine tool (mMT) technology that addresses the requirements of microscale manufacturing with machine tools that are much smaller and easier to operate than conventional machines.
The Microlution 310-S three-axis micro-milling machine is capable of spindle speeds to 200,000 rpm, 2µm positioning accuracy, 0.02µm resolution, and a maximum acceleration of 5 g. The 310-S machines rely on Geo Brick automation controllers from Delta Tau Data Systems Inc. (Chatsworth, CA) with an embedded turbo PMAC2 CPU to provide precise positioning for high-speed machining of features measured in microns.
Micromachining parts with contoured surfaces and/or very small features generally necessitates many small moves, requiring a high servo-loop update rate. In addition, factors such as the ability to interface with ultra-highresolution encoders at speed, feed forward, and resonance had to be considered.
The Delta Tau Geo Brick was used because it provides a controller and amplifier in a compact package with the ability to deliver precise positioning. Although very compact, the package size does not sacrifice performance. The fully scaleable Geo Brick automation controller utilizes a 48-bit processor, ensuring higher fidelity processing than 16 or 32-bit processors.
"The Geo Brick delivered the combination of precision, open architecture, and computational power to machine small, high-accuracy parts that we require," says Phillip. Capabilities of the Geo Brick controller critical to the success of micromachining small parts include its ability to process ultra-high-resolution encoder counts, open-source architecture, high servo-update rate, and smooth trajectory-generation engine. Not all controllers can interface to ultra-high resolution (count) encoders, nor is that ability necessary for all applications. However, machining features on micro parts typically requires numerous small moves to be made very quickly. For precise positioning of the cutting tool, an ultra-high-resolution encoder is necessary. Not only must the controller interface with the encoder, but it must do so at a high-speed rate.
"We talk about overshoot in the range of a few counts and each count is 20 nm, which is a significant distance for the types of miniature parts we make." Phillip explains: "That's why the ability to interface at speed with an ultra-high-resolution encoder is so crucial. When using small cutters to machine microscale features, there are many very small moves involved. Without the controller's ability to direct each move, no matter how small, it would be impossible to create precisely machined micro-scale parts."
In combination with a high servo-update rate, high fidelity (long word length) improves accuracy. Adjusting the feed-forward parameter to its maximum value reduces servo lag or following error, i.e. the difference between the commanded position and the actual position at any moment in time. Additionally, the Geo Brick features a very smooth trajectory engine that generates fine incremental motion and when combined with continuous velocity and acceleration delivers smooth motion.
For cutting micro part surfaces and curves that require high-accuracy dynamic contouring, the control must provide very high-accuracy motion. This enables the cutter to follow the contours precisely as designed. With the Geo Brick, the servo cycle rate, segmentation time, and look ahead can be set to achieve the highest servo-update rate possible. This increases the dynamic bandwidth of the stage.
"When profile cutting a part, even if the feed rate is only 10 m/min, making a move that is 100µm long occurs in the blink of an eye. The control and stage must be extremely responsive," Phillip says.
The 310-S micro-milling machine features a ball and vee kinematic-type mount system for both spindle and workpiece. The kinematic mount has six independent single-point constraints, e.g. the number of degrees of freedom, and the number of physical constraints is six. This mounting configuration allows high repeatability as a workpiece pallet or spindle is removed and replaced.
The system allows removal of the workpiece in the middle of a machining operation for inspection. For example, the operator can take the workpiece to a vision metrology system, such as an electron microscope, and measure progress, then put it back on the machine without sacrificing submicron repeatability.
The cutting operation can continue without re-registering the parts. Similarly, the spindle can be removed and replaced with a laser metrology head for on-machine measurement. This allows inspection of parts without the time it takes to remove them from the machine and transport them to a vision inspection system and back to the machine.
Shop Cuts Time-To-Part 33%
Rotating Equipment Repair Inc. (RER; Sussex, WI) has taken a major step forward in its technology with the use of a new CNC cycle-controlled lathe for machining piping, pump shaft, and other long, heavy workpieces for its customers. RER has realized a 33% overall reduction in time-to-part, with some jobs involving a 50% reduction, based on the tracking done by the company.
Owned by Kurt Weis, RER performs heavy repair and rebuild operations on pumps, primarily for the power generation industry. In addition to in-house machining, welding, final assembly, and diametrical/runout testing procedures, the company also operates a portable "machine shop on wheels" that can travel onsite for breakdown, repair, and rebuild operations at the company's power generation customers. While machining shafts is the company's primary focus, RER also repairs or rebuilds threaded nuts, sleeves, bearings, balance pieces, and forgings for its customers.
RER acquired an E70 CNC lathe from Weiler North American Corp. (Mt. Pleasant, SC) through the local Weiler dealer, Weller Machinery (Pewaukee, WI), which also supplied training to RER.
The E70 CNC lathe is equipped with a dedicated graphical programming interface, designed by Weiler and based on the open architecture of the Sinumerik 840D CNC from Siemens (Elk Grove Village, IL). According to Weiler, the machine has been designed so that simple parts can be made in the same way as on a manual machine, only better, while complicated parts can be made in the same way as on a CNC machine, only more easily. The operator doesn't need a CNC background or G-code knowledge, and can go from a drawing to cutting in far less time, as well as make adjustments on the fly, especially in the running speeds, without interrupting the basic program."
Bob Merriman, RER plant manager, says: "We installed the Weiler E70 with its 4.5-m bed about six months ago. Since that time, we've cut our per-part production by at least one-third and up to one-half on some jobs. The constant speed of the machine, the adjustable feed, the nonstop production rates, and especially the programming setup all contribute to an overall savings for us."
A typical job Merriman cited involved a part that previously took 45 hrs to run and is now being done in 28 hrs with superior quality. He says the straightness of the part, the tight tolerances held, to ±0.0005" (0.013 mm), and the finish quality, even on vertically heat-treated 416 stainless were very good. RER typically works on 3–5" (76.2–127-mm) diam, multi-stage, high-pressure boiler feed pump shafts that need to run at 3600–5000 rpm. As a turnkey operation, this job shop also repairs and rebuilds impellers, sleeves, and nuts for finished assemblies with tight interference fits, in the 0.001–0.003" (0.03–0.08-mm) range. On its largest job to date, a 15' (4.5-m) long pump shaft was machined, for use on a vertical pump.
The workflow on a job at RER, as it involves its Weiler lathe, begins when the pump is delivered, broken down, and analyzed for requirements. The engineering department prepares a CAD drawing for the shaft work. Material requirements are determined and the particular material is cut to length, usually from RER inventory, on the in-house bandsaw. The machine operator writes the program on his laptop and the material is loaded. The program is transferred to the machine and the cutting begins. One-off is the standard at RER, with a typical workpiece starting as a 600 lb (272-kg) billet and finishing as a 400 lb (181-kg) machined shaft.
A typical shaft rebuild done by RER involved a seven-stage water pump assembly, on which the shaft was machined on the Weiler E70 SNC lathe in 33% less time than previous methods. RER also machined the shaft nuts and impellers, cut keyways, and reassembled the unit for a 0.001–0.003" (0.03–0.08-mm) interference fit. The shaft is CA6NM, a modified 400 Series magnetic stainless with high corrosion resistance.
With the look-ahead feature of the Siemens CNC, the Weiler control can maintain a constant cutting speed, while adjusting for angle-to-radius changes especially on precise internal threading. Based on Siemens CNC architecture and its Wizard template, the Weiler-designed controller is provided to the customer with customized screens for particular machining functions, tool management, thread-cutting cycles, part profiling, and cut simulations. A drilling axis in the X or Z-axis can also be selected for machining bore holes and threads. To continuously improve the control, four software engineers at Weiler have Sinumerick 840D CNCs at their workstations to simulate all possible screen variations, based on the input from machinists worldwide.
Bob Merriman, RER plant manager, sums up the company's experience with the Weiler CNC lathe. "Anything you can do on a conventional engine lathe, you can do on this Weiler machine, only better and faster." RER is currently planning another facility for the Southeast to better serve RER customers in that region.
Milling Molds Right–The First Time
Proper end mill selection is critical for manufacturers of high-end, one-off custom molds. The luxury to "try and try again" might be available for some companies, but not for Mar-Lee Mold Co. Inc. (Leominster, MA).
For over thirty years, Mar-Lee Mold has focused on basic core and cavity mold work primarily for highend, one-off custom mold-making jobs. One-off jobs demand that Mar- Lee use the right end mill the first time so that molds can be machined in an automated fashion with minimal hand labor. To achieve this desired "zero stock machining" requires the correct tooling, machine tool, and software to produce cores and cavities without leaving extra stock. This eliminates the need for manual operations such as hand finishing, polishing, and fitting, while reducing cycle time and improving accuracy and surface finish.
"We have little room for trial and error, so it is critical that all key components of the CNC machining process are working together perfectly," explains Mark T. Lavoie, of Mar-Lee Mold.
Mar-Lee Mold, one of three Mar-Lee Companies in the Leominster area, was founded in 1972 and now employs 32 people at its 20,000 ft2 (1858 m2) facility. It isn't surprising that Mar-Lee chose Leominster as their home base. Mar-Lee Mold designs and manufactures complex, high-precision injection molds for the plastics industry, which historically has had a prominent presence in Leominster and was once dubbed "the Pioneer Plastics City" due to its thriving plastics industry.
Located nearby and employing about 48 people combined are Mar-Lee Packaging & Consumer Products, specializing in high-volume, quality products, and Mar-Lee Medical, which manufactures low-volume, complex, high-precision components and subassemblies for the medical device, single-use, and bio-absorbable markets.
About 60% of the molds Mar-Lee produces are used by their consumer and medical divisions to develop components and products, with the balance being sent to outside contract manufacturers.
In 1972, Mar-Lee began operations with a conventional EDM machine. At that time, a moldmaker that had an EDM machine was considered a pioneer of sorts. They followed up this acquisition by adding their first CNC milling machine in the early 1980s, and then fully embraced high-speed machining during its debut around 1998.
High-speed machining (HSM) presented opportunities and challenges for mold makers in the late 1990s. Since feeds and speeds were different for high-speed machining versus conventional CNC machining, different end mills were required. Also, Mar-Lee began its high-speed machining operations on graphite material, and then broadened its capabilities to include machining steel. Milling a harder material added to their challenges.
"We put our first Okuma highspeed machining center on the floor in 1998 with the promise of increased productivity and the potential to broaden our mold making capabilities," says Lavoie. "However, we needed end mills that would be effective for our high-speed applications."
John Gravelle, president of Mar-Lee Companies, adds, "For Mar-Lee to remain competitive in a shrinking mold making market, we needed the very best automation to efficiently and profitably produce a high-end, custom product. In order to continue manufacturing locally [in the US], we needed to create efficiencies wherever possible."
Around 1998, Mar-Lee invited several end mill manufacturers, including Emuge Corp. (West Boylston, MA) to demonstrate their solutions. "Right away, at first glance, we knew they had something different that we needed. We've never seen tools with geometry and coatings like theirs. It was clear they knew their stuff," says Lavoie.
"Emuge understood our requirements and had the right solutions. When I consider end mills, it is not just based on cost. Longevity and performance of the tools are more important. Ultimately, the proof is in the application—it has to be able to run," he says.
Much of the injection-mold work that Mar-Lee does involves short runs in hard material on high-speed machines. The company reports that Emuge end mills provide them with 30–40% more tool life than others they have tried, and a 50% reduction in cycle time.
Emuge end mills help Mar-Lee reduce labor costs by allowing one operator to tend to three automated work cells. Mar-Lee is able to conduct several hours of untended operation utilizing these efficient work cells. Scott Hastings, CNC machinist at Mar-Lee, says, "My machine runs all night using the Emuge end mill, many times working in hardness of RC48-56, and by morning the cutter is still good. We can't do that with other end mills because we would have to set up four tools and break up the tool program. Retooling wastes time and money."
Hastings continues, "When you need to take a few tenths off a mold, an Emuge end mill will do it to tolerance. It consistently removes those tenths without wear." In one example, Mar-Lee ran the tool to mill four cavities and the dimensions on the cavity were dead-on consistent. In another, using a six-flute Emuge end mill versus a traditional four-flute mill, produced a mold with a mirror-like finish because the end mill is removing material faster and more accurately.
Toolholding is also a critical element for successful milling. To that end, Hastings achieves further efficiencies by using the Emuge Shrink Master induction shrink workstation. Emuge reports that the shrink-fit technology provides the most accurate and rigid tool mounting available. Hasting agrees, and comments that Shrink-Master provides an extremely rigid setup that eliminates run-out and helps to prolong the life of the tool.
Les Regan, CNC machinist at Mar-Lee notes benefits from Emuge tools as well. "Emuge tools complement high-speed machining. They last longer and hold up better than other end mills we've tried for milling hard materials in the 48–50 RC range. Sometimes we even go as high as 54–56 RC. Other end mills would melt in cutting conditions like this."
Additionally, machinists at Mar-Lee report that Emuge end mills actually provide "twice the life and more" for which they were originally intended. At Mar-Lee, when a machinist is done using an Emuge end mill for a high-speed application, the tool is then reused for traditional machining applications like on a Bridgeport. "All the machinists want the Emuge end mills, even the used ones from our HSMs," said Regan.
This article was first published in the August 2007 edition of Manufacturing Engineering magazine.