Courtesy of Plastic Design
A micromold machined at Plastic Design is positioned on a Hermann Schmidt magnetic chuck mounted on a unit of System 3R’s Nano reference system.
Shop takes ‘systems’ approach to holding microparts.
Accurately machining and measuring a part requires it to be located and clamped with precision that matches or exceeds specified final tolerances. Generally, a shop can combine good machine tools, vises and tooling to get a good result. But when working with micron-level tolerances, good is not good enough. Beyond assembling high-accuracy workholding components, it is crucial to control how they relate to each other as an integrated system, and assure they are applied in a repeatable, systematic manner.
Zero-point palletizing, or referencing systems, can maximize precision in machining operations. In these systems, workholding pallets feature a centrally located drawbar or stud drawn into a machine-mounted chuck to provide consistent and reliable positioning.
“In the case of developing or validating a process with small tools or small features, at some stage you have to remove the part and take it to a dedicated metrology device to inspect your results,” said John Bradford, micromachining R&D team leader for Makino Inc., Mason, Ohio. “You then put it back in the machine, make adjustments to your process and continue machining. You must be able to remove, check and replace the part without repeating the whole setup process.”
Plastic Design Corp. (PDC), Scottsdale, Ariz., deals with those challenges every day. The company manufactures molds and uses them to produce small medical devices, microfluidic circuits and in vitro lab ware.
“We are cutting true microcavities,” said PDC President Mark Kinder. “Typically, we are doing one- and two-cavity tools; a luxury for us is a four-cavity mold. In the case of microfluidic devices, he said, “the X-Y of the cavity may be pretty big—3 "×5 "—but we are cutting features in the 30µm to 50µm range.”
Machining features as small as 10µm, he said, “is no big deal.” However, there are many variables that affect the shop’s ability to consistently generate those tolerances. For example, all rotary chip-removal systems feature some degree of spindle whip. “When you bring the machine up to speed, the centerline shifts subtly,” Kinder said. “It’s a centripetal phenomenon. The better the spindle, the less there is, but it occurs. We’ve mapped all of our machines. We can predict it, but it is never an exact thing. It’s subtle—tenths (10-thousandths of an inch) or microns.”
In consideration of that and other variables, Kinder regularly checks part dimensions while machining. The parts are taken from a vertical machining center or EDM to a Nikon NEXIV VMR-3020 optical/laser 3-D coordinate measuring machine, inspected and returned for re- machining, if necessary. The actual steps followed are dictated by a shop’s familiarity with, and confidence in, a specific operation, as well as the tolerances of the part being machined.
“If we have a high level of confidence, we will make the cut and check the part just for verification,” Kinder said. If the part is out of specification, it’s either scrapped or, if possible, refixtured for further machining.
On the other hand, if tolerances are especially tight, Kinder said he takes a different approach. The initial CAM program is written to cut the part to slightly larger-than-final dimensions. Then the part is inspected, the program is adjusted and the part is refixtured for machining to final dimensions.
For that reprogramming and remachin-ing to be accurate, the part must be positioned identically for measurement and machining. About 2 years ago, PDC was having trouble repositioning work after removing it for inspection. “When you get down to splitting tenths, measuring the part is difficult enough,” Kinder said. “But getting it back into a location where we could take a 0.00004 " (1µm) cut is where we were really struggling. We were spending 2 hours on a good day, and 4 to 5 hours, on average, getting the block trammed back into the machine.”
A palletizing system might eliminate much of the time spent repositioning the molds, Kinder observed, but standard systems didn’t provide accuracy high enough for his needs. Then he saw a demonstration of the Nano referencing chuck system from System 3R USA Inc., Elk Grove Village, Ill. “Repeatability was basically unmeasurable,” Kinder said.
Tweaked and Finessed
Jack Sebzda Jr., System 3R Northeast regional manager, said the Nano system is a “tweaked and finessed version” of System 3R’s standard Macro chuck system. “We take standard chucks out of the production line and basically balance and blueprint them, sort of like they do to an automobile engine to improve its performance,” Sebzda said. “The chuck is hand-lapped and hand-measured; everything is done with the goal of making it as accurate as possible.”
For example, he said, instead of being simply rough milled and tumbled, the locking surfaces of the pallet drawbar are ground so those dimensions are consistent drawbar to drawbar. “It’s overkill for what you might normally expect, but we want the same exact pull force to be exerted on the pallets every time,” Sebzda explained. The result is repeatability better than 1µm, he said.
Courtesy of System 3R
The Nano reference system’s chuck components are ground and lapped to maximize accuracy and produce repeatability better than 1µm, according to the company.
Kinder enlisted his workholding supplier, Hermann Schmidt Co., South Windsor, Conn., to integrate Schmidt’s workpiece-holding chucks with the System 3R palletizing system.
PDC required precision vises and different styles of magnetic chucks for its machining center and EDM, said Peter Schmidt, president of Hermann Schmidt. The company provided 6 "×6 " magnetic chucks ground to better than 0.00005 " square. When mounting the magnetic chucks to the System 3R pallet chucks, Schmidt said, “We indicated the rail around the outside of the magnetic chuck (against which the part rests) from the centerline of the System 3R reference chuck, so that in multiple chucks the work offset is within 0.00003 " square and parallel from the center location.”
A Step Higher
Palletizing is a long-established technology and most suppliers will guarantee their products to repeat within 2µm. However, while 2µm repeatability is good enough for most applications, “Mark Kinder’s world is a whole other step higher,” Schmidt said. “Mark wanted to take a pallet out of one machine, put it in another machine and repeat to 1µm or better. We are talking about going from 0.00008 " repeatability in one machine down to 0.00004 " repeatability machine to machine.
Hermann Schmidt takes the prefinished chuck and bolts it to the referencing system. When two objects are bolted, they are stressed, so at some point in the process one of the surfaces is lightly machined to qualify it. For example, the flat plate on the back of a magnetic chuck will be requalified via grinding and lapping.
PDC just had to mount the chucks and tram them in. “It has become our standard workholding system,” Kinder said. The pallets are installed on PDC’s VMC, sinker EDM and CMM. Some hold magnetic chucks, some grinding vises and some System 3R electrode holders. After initial machine setup, no further setup is required.
Such precision is not inexpensive, Kinder noted. “We put as much money into that system as we would a machine tool,” he said. “Originally, when we priced the system out, I had an ROI of 3 years on it, based on the shortened setup time after measurement.” However, because of the system’s positive effect on constraint management at PDC (see sidebar on page 50), the payback was 9 months.
Peter Schmidt stressed that this kind of precision workholding system is not a set-and-forget proposition. “It requires a systematic approach not only in how we build it, but in how they use it,” he said. “If they don’t use it the same way every time, if they change that procedure, they are not going to hold tolerance.”
Jack Sebzda agreed. “That last 10-millionths is an expensive and difficult thing to get at,” he said. “Maintenance, cleanliness and consistency are critical. You can put the best chuck in the world in a shop and you’ll fail miserably if the machines are not maintained properly, or the operators don’t handle things with care.”
Some variables, inconsequential in macro applications, become significant in micromanufacturing. For example, System 3R specifies that even the air pressure used to actuate the chucks be tightly controlled. “If we want the reaction of this chuck to be identical every time, the procedure for opening and closing the chuck must be as accurate as everything else,” Sebzda said. While System 3R recommends air pressure of 5 to 7 bar (about 70 to 100 psi) to operate its standard chucks, air pressure for the Nano should always be 6 bar.
Courtesy of Plastic Design
Pallets in the System 3R Nano referencing system can be removed and replaced with repeatability at levels less than 0.5µm (500nm), according to the company.
One challenge is that advances in machine tool accuracy may limit the usefulness of some palletizing systems, according to Makino’s Bradford. He cited machines such as Makino’s Hyper 2J VMC that features 0.000000020 " (0.5nm) scale feedback with guaranteed positioning accuracy of ±0.3µm (±0.000012 ") and repeatability accuracy of ±0.2µm (±0.000008 ").
In actual applications, he noted, the machine has provided positioning accuracy and reliability on the level of ±30nm (0.030µm). When machining parts or features that take advantage of those machine capabilities, removing the part from the machine for measurement may not be an option since part tolerance might only be a few microns.
As the machines are introduced with repeatability in the 70nm to 80nm range, more accurate workholding systems are a must, he added. “If your tooling only gives you repeatability of 700nm to 800nm, you are losing the benefit of the machine’s accuracy and stability.” In those cases, he said, machine tools will offer increasingly sophisticated on-board measuring systems that permit inspection without removing the part.
However, according to Sebzda, advanced palletizing systems are already in the same tiny ballpark as the machines Bradford described. System 3R’s Nano referencing system can repeat at levels under 0.5µm (500nm), but the problem has been how to prove it, he said. “By placing optical sensors on both the pallet and reference surface of the chuck, we are able to monitor and confirm system performance.” Testing of the system has taken place when optical grinding, he said, including work with the Fraunhofer USA Applied Research Institute.
The Whole Package
To remain competitive, shops must continually seek and apply new technology. Regarding workholding, Schmidt said there are manufacturers who struggle because they are unaware of new equipment and integration services that enable high precision. To those who struggle, Schmidt said, “There are people who understand what you are talking about and your problems.”
Sebzda said all machining elements must be included in the upgrade. “I can put the most expensive tires possible on a Yugo, and it’s still a Yugo,” he said. “The whole package has to be there. Sometimes that package has to be chosen from a variety of suppliers. It all has to be researched and constantly reviewed because there is always a better way to do it.
“What is coming down the pike must be understood, accepted and embraced,” Sebzda continued. “Unless you move with the changes, you are going to be behind the times. We are coming to the point where our industry is doing the elite work; the no-brainer mold work, the blow molds, cheap toys and things like that are all gone. What’s left are the upper-level, tip-of-the-pyramid processes that only a few can truly achieve.” CTE
About the Author: Bill Kennedy, based in Latrobe, Pa., is contributing editor for CTE. He has an extensive background as a technical writer. Contact him at (724) 537-6182 or by e-mail at firstname.lastname@example.org.
Plastic Design Corp. operates under the theory of constraints, also known as constraint management and “debottlenecking,” according to company President Mark Kinder.
“We look at the aggregate efficiency of the shop, not the efficiency of a given machine tool,” he said. “We will yank a setup if there is another job that we need to get through that piece of equipment to keep the overall workload in the shop moving.”
Most shops focus on reducing downtime for individual machines. Kinder counters that “if you throw away micro profit centers and efficiencies based on single operations and look at overall plant efficiencies, you can really achieve a lot more with limited resources.”
High-precision, palletized workholding helps PDC make decisions on shopwide workflow. Kinder described a situation where the shop followed a run of EDM electrodes on its V-22 Makino VMC with a hard-milling job. A programming error on the sinker EDM produced features that were too shallow. “We didn’t scrap the block, but all the electrodes were consumed,” he said. “We looked at the shop schedule and decided that the block being burned was more critical than a job we were hard milling.”
The decision was made to take the hard-mill job off the VMC, switch the machine back to mill electrodes, remake the electrodes and finish EDMing the part that day. Prior to implementing an integrated System 3R/Hermann Schmidt palletization system, such a change would require hours, Kinder said.
Kinder authorized the change back to electrode machining and went to lunch. Upon his return he found operators working on the hard-milling job. When he asked why the electrodes weren’t being remade, he was told they were already done. “With the pallet system, the operators could interrupt the hard-mill program, pop that pallet off and drop in the electrode pallet. The electrode program was stored in the machine and, fortunately, we hadn’t pulled the endmills we were using for the EDM work.”
The electrode pallet had a vacuum box for dust removal, and the VMC was configured to handle either flood and mist coolant or vacuum dust collection. “They were set up and running electrodes in about 10 minutes,” Kinder recalled.
Hermann Schmidt Co.
Plastic Design Corp.
System 3R USA Inc.
Related Glossary Terms
Way of displaying real-world objects in a natural way by showing depth, height and width. This system uses the X, Y and Z axes.
Cone-shaped pins that support a workpiece by one or two ends during machining. The centers fit into holes drilled in the workpiece ends. Centers that turn with the workpiece are called “live” centers; those that do not are called “dead” centers.
Workholding device that affixes to a mill, lathe or drill-press spindle. It holds a tool or workpiece by one end, allowing it to be rotated. May also be fitted to the machine table to hold a workpiece. Two or more adjustable jaws actually hold the tool or part. May be actuated manually, pneumatically, hydraulically or electrically. See collet.
- computer-aided manufacturing ( CAM)
computer-aided manufacturing ( CAM)
Use of computers to control machining and manufacturing processes.
Fluid that reduces temperature buildup at the tool/workpiece interface during machining. Normally takes the form of a liquid such as soluble or chemical mixtures (semisynthetic, synthetic) but can be pressurized air or other gas. Because of water’s ability to absorb great quantities of heat, it is widely used as a coolant and vehicle for various cutting compounds, with the water-to-compound ratio varying with the machining task. See cutting fluid; semisynthetic cutting fluid; soluble-oil cutting fluid; synthetic cutting fluid.
- electrical-discharge machining ( EDM)
electrical-discharge machining ( EDM)
Process that vaporizes conductive materials by controlled application of pulsed electrical current that flows between a workpiece and electrode (tool) in a dielectric fluid. Permits machining shapes to tight accuracies without the internal stresses conventional machining often generates. Useful in diemaking.
- flat ( screw flat)
flat ( screw flat)
Flat surface machined into the shank of a cutting tool for enhanced holding of the tool.
- gang cutting ( milling)
gang cutting ( milling)
Machining with several cutters mounted on a single arbor, generally for simultaneous cutting.
Machining operation in which material is removed from the workpiece by a powered abrasive wheel, stone, belt, paste, sheet, compound, slurry, etc. Takes various forms: surface grinding (creates flat and/or squared surfaces); cylindrical grinding (for external cylindrical and tapered shapes, fillets, undercuts, etc.); centerless grinding; chamfering; thread and form grinding; tool and cutter grinding; offhand grinding; lapping and polishing (grinding with extremely fine grits to create ultrasmooth surfaces); honing; and disc grinding.
Finishing operation in which a loose, fine-grain abrasive in a liquid medium abrades material. Extremely accurate process that corrects minor shape imperfections, refines surface finishes and produces a close fit between mating surfaces.
- machining center
CNC machine tool capable of drilling, reaming, tapping, milling and boring. Normally comes with an automatic toolchanger. See automatic toolchanger.
- magnetic chuck
Workholding device used on surface grinders and milling machines for holding ferrous parts with large, flat sides. Holding power may be provided by permanent magnets or by an electromagnetic system. See chuck.
Science of measurement; the principles on which precision machining, quality control and inspection are based. See precision machining, measurement.
Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.
- milling machine ( mill)
milling machine ( mill)
Runs endmills and arbor-mounted milling cutters. Features include a head with a spindle that drives the cutters; a column, knee and table that provide motion in the three Cartesian axes; and a base that supports the components and houses the cutting-fluid pump and reservoir. The work is mounted on the table and fed into the rotating cutter or endmill to accomplish the milling steps; vertical milling machines also feed endmills into the work by means of a spindle-mounted quill. Models range from small manual machines to big bed-type and duplex mills. All take one of three basic forms: vertical, horizontal or convertible horizontal/vertical. Vertical machines may be knee-type (the table is mounted on a knee that can be elevated) or bed-type (the table is securely supported and only moves horizontally). In general, horizontal machines are bigger and more powerful, while vertical machines are lighter but more versatile and easier to set up and operate.
Strip or block of precision-ground stock used to elevate a workpiece, while keeping it parallel to the worktable, to prevent cutter/table contact.
- payload ( workload)
payload ( workload)
Maximum load that the robot can handle safely.
- sawing machine ( saw)
sawing machine ( saw)
Machine designed to use a serrated-tooth blade to cut metal or other material. Comes in a wide variety of styles but takes one of four basic forms: hacksaw (a simple, rugged machine that uses a reciprocating motion to part metal or other material); cold or circular saw (powers a circular blade that cuts structural materials); bandsaw (runs an endless band; the two basic types are cutoff and contour band machines, which cut intricate contours and shapes); and abrasive cutoff saw (similar in appearance to the cold saw, but uses an abrasive disc that rotates at high speeds rather than a blade with serrated teeth).
Minimum and maximum amount a workpiece dimension is allowed to vary from a set standard and still be acceptable.