March 2012 / Volume 64 / Issue 3|
Zeroing out Swiss
By Kip Hanson
Courtesy of Sandvik Coromant The right insert for Swiss-style machining of microparts is often one with a near-zero radius.
When turning 2mm-dia. and smaller parts on a Swiss-style machine, it only makes sense that the radius on the tool should be equally tiny. After all, you can’t cut a 0.5mm bone screw with a TNMG-431 insert—the tool’s nose radius is nearly as big as the part.
In addition, said Jim Gosselin, owner of Genevieve Swiss Industries Inc., Westfield, Mass., “some parts, such as in the RF (radio frequency)-connector industry, can’t have any radius at all. It messes up the component’s electrical properties.”
Without a radius, you’re probably thinking the tool will chip on the first workpiece. Think again. Microscale indexable-insert tools are designed to handle this sort of work. Gosselin explained: “We’ve gone to submicron-grade carbide on all our tools. That means the grain size is very small, so the tools are very tough—even with a dead-sharp radius. And the edges have to be very sharp, so everything is ground to size, and then we use a special process to remove the grinding burr, followed by a PVD coating for good wear resistance.”
Perhaps the most important geometry on a microtool is the wiper that’s ground on the tool tip. This wiper is a flat—perhaps 0.05 " wide—on the trailing edge, with a clearance angle of 1° or less. “Depending on the material and part diameter, you can actually feed one of these tools at a couple thousandths of an inch per revolution, even with a zero radius,” Gosselin said. Great, the tool might last, but won’t the part have a surface finish no better than a corncob? No. “The trailing edge does more than support the tool tip. It also wipes off the mountaintops, providing a smooth finish,” he added.
Courtesy of GenSwiss
“Everyone knows the usual solution to improving surface finish is to either increase the tool nose radius or reduce the feed rate,” Gosselin said. “Both of these are practical solutions in normal turning. However, for microturning, a larger nose radius is the wrong prescription for curing the poor surface finish ‘flu.’ The same is somewhat true for reducing the feed rate.”
The issue gets back to the cutting edge. For proper cutting to occur, the material must flow over the top of the cutting edge. If the cutting edge of the insert has a 0.0002 " edge prep, then the feed rate must be larger than 0.0002 ". “If not, then the tool does not have adequate clearance, so, in effect, you’re just pushing the material out of the way and not cutting it,” said Gosselin.
Although called “zero radius,” there has to be some radius, otherwise the tip would chip. Most microtool manufacturers claim anything from 0.0001 " to 0.0012 ". Just remember that the bigger the radius, the bigger the deflection, and because tolerances on microparts decrease in direct relation to the part’s size, part manufacturers might be targeting grinding-quality tolerances of +0.0002 "/-0.0000 " or tighter. The slightest amount of deflection on a tolerance this tight and you’ll have the QA department riding you like a circus pony.
Another important consideration when Swiss-style turning microparts is keeping the tool close to the guide bushing. “The guide bushing is everything on a Swiss,” Gosselin stressed. “It doesn’t matter how heavy the machine is, how fast it can go or what features the machine has; all this is meaningless unless you have a high-quality guide bushing and can get your tools right next to the bushing.”
Gosselin noted that experienced Swiss-style machinists don’t like to produce small parts on a big machine. But doesn’t a big machine provide greater flexibility with more tools and more power? Maybe, but the tools are closer to the guide bushing on a small machine. “On a 20mm machine, you’re 2mm to 3mm away from the guide bushing,” he said. “You’ll see more flex in the workpiece than the board used in a freestyle diving competition.”Small Details
Turning microparts on a Swiss-style machine is challenging, concurred John Dotday, small parts specialist at Sandvik Coromant Co., Fair Lawn, N.J. Aside from the need for high-quality, precision-ground, PVD-coated, zero-radius cutting tools, Dotday pointed out the importance of tool centerline. “Proper centerline is critical. Achieving it is up to the operator, and how carefully he touches off when setting tools. A lot of the parts being made these days are so small you need a microscope to see the details.”
Monitoring coolant pressure is also critical. “We’re seeing high-pressure coolant systems today up to 2,000 psi,” Dotday said. “Hitting a small part with that much pressure can distort it and introduce vibration, either of which can mean bad parts.”
Dotday recommends buying centerless-ground bar stock—the rounder and more consistent the better. “Any out-of-roundness in your bar stock gets transferred into the workpiece,” he said. “Ground stock costs more money, maybe 50 percent more, but your component quality goes up by about 90 percent.”
Successful Swiss-style turning also requires buying the highest-quality tooling, including cutting tools, toolholders, guide bushings and collets, according to Glen Crews, western regional sales manager for Swiss-style machine builder Marubeni Citizen-Cincom Inc., Allendale, N.J. “Don’t let your purchasing department go cheap on you,” he emphasized. “They might save a few bucks up front, but you’ll lose thousands later in rework costs or scrapped parts and broken inserts.”
When it comes to machine size, Crews noted that a small unit isn’t always necessary. “You can turn 1mm parts on a 20mm machine, but you have to configure the machine properly,” he said.
Again, the trick is to get the tools as close to the guide bushing as possible. This might mean modifying the toolholders or shimming the tool. “You have to be almost touching the guide bushing,” Crews said. “You might look at extended-nose guide bushings, especially if you’re going to be cross-working, as live tools are typically 8mm to 10mm further away than turning tools.”Not Up Everyone’s Alley
You can throw your Machinery’s Handbook out the window when you’re working with microparts. “There are no hard and fast rules,” Crews said. “With micromachining, you have to experiment and play around with things to get it right. Feed too slow and you’ll workharden (the workpiece), go too fast and you’ll have a rotten finish. Each company invents their own tricks.
Courtesy of Sandvik Coromant
“Working this small is painstaking and tedious,” Crews continued. “You don’t have a lot of room for error. For example, tool heights have to be absolutely perfect, so you need a good setup man. There’s definitely a set of skills that come into play here. That’s why there aren’t that many people who want to get into it.”
And you can pretty much resign yourself to shorter tool life when Swiss-style turning small parts. “Tool life won’t be as good as it is with larger parts,” Crews warned. “That small nose radius is prone to breakage. On a larger part, you might be able to accept a few thousandths wear—not so with microparts.”When Good Tools Go Bad
“Tools cut for two reasons,” said GenSwiss’ Gosselin. “They are harder than the material being cut, and there is clearance under the cutting edge.” The following are several reasons why commodity inserts don’t cut it for small parts.
■ Most inserts have a prepped cutting edge, sometimes called a K-land. The K-land is different than a nose radius. It is a hone applied to the cutting edge to smoothly round it. It strengthens the edge of an insert by removing sharp burrs, loose material deposits and weak inclusion areas that may have been introduced during sintering or grinding. However, when small-diameter turning, the K-land on a low-cost tool is too big. It causes the part to deflect or rubs the part rather than cutting it. This is because the chamfer, or radius, of the K-land is below the top cutting surface of the insert. Therefore, the tool has insufficient cutting clearance and won’t properly cut. This is usually evident by part breakage during cutting or surface scuff marks.
■ Usually, the coatings applied to general-purpose inserts are too thick for turning small parts. Even a properly prepared tool will not perform well if it is CVD-coated. CVD coatings can be 12µm to 18µm thick, and the result during the cutting process is similar to a tool with a K-land on a low-cost tool.
■ The tool nose radius is too large. The more surface contact a tool has with the material, the greater the applied force. As the material being cut is less rigid than the tool, it deflects away. The vibration and deflection can cause the material to climb on top of the tool.
Specialty inserts might cost more than standard inserts—depending on the manufacturer you could spend up to three times as much—but you can be sure you’re getting high-quality tools that deliver top performance and extend tool life in micromachining applications. Those advantages far outweigh the added investment. CTEAbout the Author: Kip Hanson is a manufacturing consultant and freelance writer. Contact him by phone at (520) 548-7328 or e-mail at firstname.lastname@example.org.
CUTTING TOOL ENGINEERING Magazine is protected under U.S. and international copyright laws. Before reproducing anything from this Web site, call the Copyright Clearance Center Inc. |
at (978) 750-8400.