Details so tiny they can't be machined any other way.
Because micromachined parts involve tiny machine movements, glass scales on all axes are essential for microEDM motion accuracy. (Photos courtesy of Makino.)
Using discharge dressing, shops create electrodes (top) for microEDMing micron-sized holes. (Photos courtesy of Agie.)
Working with electrode wire four to five times smaller than a human hair, holding parts barely visible with the naked eye, and accurately producing holes as tiny as 0.0005 in. in diameter is nothing new in micromachining medical parts with EDM. What makes a part micro is having details so minute that they can't be ground, milled, waterjet cut, laser cut, or EDMed with standard-sized diameter (0.008 in.) wire.
Medical shops use both wire and sinker EDMs for micromachining. However, cautions Brian Pagano, prototype EDM department manager at Ron Witherspoon Inc. (RWI) in Campbell, Calif., for tiny parts, the process takes special equipment. "Any shop can burn a 0.010-in.-diameter hole, for instance, but diameters smaller than 0.004 in. can't be done competitively/consistently on conventional EDMs." For such miniscule jobs, shops must carefully consider workholding and machine design; wire and small-hole electrodes; and generators and simulation software.
Obviously, holding tiny medical parts for microEDMing is difficult. EDM OEM Charmilles, Lincolnshire, Ill., recommends using tooling systems with high repeatability that let shops set up parts off-line (perform measuring and aligning) prior to loading them onto an EDM. Such systems also allow for checking parts prior to cut-off passes, so shops run initial passes, remove parts and holders, measure with CMMs, and adjust for precise final skim cuts.
Charmilles recommends that if possible, shops design micro parts so they are held with two tabs 0.008 to 0.010-in. thick, instead of just one. Using two tabs prevents small parts from moving out of position.
While considered somewhat unorthodox, some shops further secure tiny parts using chewing gum. Prior to cutting part tabs, gum is stuck to the part's top and bottom. It gets into the wire gap to hold the parts in place after tabs are cut and prevents parts from being washed away by flushing pressure.
For microEDMing applications, dead-nuts machine accuracy is paramount because small parts require minute movements on a stable base. According to Charmilles, the average EDM is not ready for these demanding accuracies.
The company says an ideal machine moves workpieces in relation to the machine's fixed column. For instance, tables move while wire guides remain stationary. The company also says machines should have V-guides ground flat into their castings and submerged worktanks.
In addition, Charmilles suggests, if possible, the EDM features a separately cooled worktable. That's because cooled worktables protect workpieces and tables from shop-air exposure that negatively affects part position when worktanks are drained.
Also, cooling entire machines — columns and castings — along with housing them in air-conditioned rooms further prevents part inaccuracies caused by thermal growth and shrinkage. Or, fully enclosed machines are another option. Within their covers, such EDMs typically maintain working temperatures to within 1° for submicron work.
For motion accuracy, glass scales on all axes are essential for accurately measuring wire position. Fortunately, microEDMing doesn't involve moving a lot of large part masses, so there's little overshooting or high speeds to contend with.
Down to the wire
Typically, the smallest wire used in microEDMing is 0.0008 in. in diameter, while common sizes are 0.001 and 0.002 in. The challenge is creating start holes for such tiny wires.
Often, shops cut from outside the workpiece and then into the part's shape to eliminate using a start hole, or they burn a large start hole outside of the part details to be machined. But if these options aren't possible, they must discharge dress a small-hole electrode on a sinker EDM.
For this job, RWI's VA10A sinker EDM from Mitsubishi EDM/Laser, Wood Dale, Ill., features a special arm attachment with a guide positioned at the machine spindle's centerline. The shop mounts a piece of tungsten wire in a chuck and threads the wire through the arm's guide. With this short piece of tungsten sticking out past the guide, RWI powers the spindle, which spins the electrode wire true/concentric. It then positions the spinning electrode wire against a piece of carbide mounted in a vise and discharge dresses the appropriate diameter for burning holes as small as 0.0005 to 0.0007 in.
Start holes not only have to be small enough to prevent shorting, they also must be accurately positioned. This is why R&D shop Engineering Techniques Inc. in San Jose, Calif., reports it burns start holes on its Toolmaker III sinker EDM from Sodick Inc. of Schaumburg, Ill. The machine accurately positions start holes to within 0.0001 in.
Once start holes are burnt in, the wire routes through a machine's tensioning system and into its automatic wire threader. Unlike with conventional EDMs, these two systems must handle the tiny wire diameters for microEDMing.
According to Charmilles, an EDM's CNC should measure wire brake, the system that regulates wire tension. Conventional EDMs run 1 or 2 kg of wire tension, while with microEDMing wire these tensions are 50 to 100 gm — light pressures that require fine-tuned wire-transportation systems.
"MicroEDMing wire is so fragile-and small, you can barely feel it between your fingers," says Pagano of RWI. "The wire's fragility means it must be tungsten. Brass doesn't have the required strength at such small diameters."
RWI has a Vertex wire EDM from Agie Ltd. of Lincolnshire, Ill., that threads wire diameters down to 20 µ in 20 sec with its guides spread as much as 2.500 in. apart. The system threads wire via a precision waterjet stream, and sensors located at critical points monitor the threading operation. If the wire fails to reach any of these sensors along the way, the system triggers a re-try.
In automatic wire threading, the method used to cut wire must leave its ends burr-free. This is why some systems stretch wire and separate it with a heating process that tapers the ends for easy threading. For added assistance, threading systems may incorporate suction in the machine's lower head to suck wire through workpiece start holes and into lower guides.
But what if start holes are crooked? For this, microEDMs should include routines that work the wire out of shorting situations in start holes. So even if the wire is touching the side of a start hole, the machine cuts small spiral patterns, for instance, to free it both at the top and bottom of the hole.
Generating and simulating micro
For micromachining, EDM generators must have the capability to reduce spark energy for small wire diameters, but simultaneously produce enough energy for fast cutting. Because medical applications often require extremely fine surfaces finishes and hand-polishing extremely small parts is usually impossible, it's imperative that generators produce microsurface finishes — basically polished surfaces right off the machine.
Medical and biomedical work is a mainstay at Engineering Techniques, and customers often tell company President Marsh Syverson that surface finishes need to be "as good as he can get them." And good is actually excellent, with surface finishes near 0.07 Ra with its Agie Vertex EDM. The trick is gaging such fine finishes on part details measuring 0.010 X 0.010 in., or, in some instances, on parts smaller than a grain of salt, says Syverson.
The shop accomplishes this microscopically, using both digital and optical microscopes with powers up to 3,0000 X. But most of its medical parts are easily gaged within a 200 to 500 X range.
In addition to a generator built for micromachining, microEDMs need good simulation software within their controls. This software should not only simulate programmed wire paths, but also all necessary offsets.
When programming and doing dry runs for miniscule medical parts, it is critical that shops clearly see the tiny offsets involved so as not to accidentally cut off a whole detail from a workpiece. A clear, precise cut-path view in this situation also indicates whether to eliminate a pass, change the program, or switch to a smaller wire diameter to avoid scrapping the part.
Along with the required micromachining accuracies, today's twin-wire EDMs make such wire diameter switching easy and fast and provide job flexibility. As opposed to singlewire machines dedicated to small parts, twin-wire systems let users do both micro and standard-sized work on one machine.