| Drilling microholes requires expert approach
“Drilling a 0.003″ hole is no small task. Virtual Industries Inc., a manufacturer of vacuum systems, discovered this when it needed vacuum tweezer tips capable of handling parts as small as 100μm. Based on experience with larger tips, the Colorado Springs, Colo.-based company knew local machine shops would no-bid such a small tip, so it turned to its subsidiary, Prime Axis Manufacturing LLC, also in Colorado Springs. In the following, the author explains, step by step, how Prime Axis solved the problem.”—Ed.
To produce vacuum tweezer tips, we not only needed 0.003″-dia. carbide microdrills, which only a limited number of toolmakers produce, but also ones with a 0.040″ flute length. The toolmaker we were buying from, however, stopped offering drills that size as a standard item and required a minimum order of 100 drills to produce them as specials. That meant spending about $7,000 on drills at the time, which we didn’t want to do.
Fortunately, Harvey Tool Co. LLC, Rowley, Mass., began offering 0.003″-dia. drill bits with the appropriate flute length as an off-the-shelf item, so we used those. The drills are uncoated.
 Miniature Drills From Harvey Tool Material selection
We considered several different materials for the tip—brass, Torlon plastic and electrostatic-dissipative (ESD) Delrin plastic—and evaluated their characteristics to select the one that would satisfy Virtual Industries’ customer base.
We determined the fibers used in the fabrication of Torlon tended to redirect the 0.003″-dia. drill bit, causing tool breakage, which eliminated that choice. Brass is easy to machine but might physically damage the delicate parts a tweezer tip handles, so it was also eliminated.
The ESD Delrin seemed like the logical choice. It has a surface resistivity of 108 to 1010 ohms and bleeds off any buildup of electrostatic charge. That is a concern when handling microparts, because it only takes a few electron volts to cause a part to stick to the tip and not release. In addition, an electrostatic charge can damage electrical and microcircuitry components. Delrin is also soft, so it will not physically damage parts.
Although we knew the material’s characteristics were appropriate, we experienced a problem with tool breakage because the center of the ESD Delrin bar stock—where the hole is drilled—has a lot of porosity as a result of how it’s produced. Every few parts the drill would hit a small pore, which would cause the tool to break. After spending time adjusting the feeds and still breaking several hundred dollars worth of drills, I located a supplier for Delrin with much lower porosity and overcame the problem.
Process selection
Several ways exist to manufacture these small tips, which have a ±0.0005″ tolerance. One method is to turn the larger features, which include a 0.060″ OD that tapers at about 20° along a 1⁄16″ length to an 0.008″ OD at the tip, on a CNC lathe and drill the 0.003″ hole at the tip and a connecting 0.020″ hole at the back of the part as secondary operations on a CNC mill. The smaller hole can’t be drilled on the lathe because it has a maximum spindle speed of 5,000 rpm, which isn’t fast enough to achieve a suitable surface footage for the tiny tool. Although this method worked, it requires a part to be set up and run on two separate machines. This drove up the manufacturing cost.
The second method is to use a CNC Swiss-style machine with optional live tooling. This arrangement is advantageous because the part turns at 7,500 rpm while the drill also turns at 7,500 rpm in the opposite direction. Counter rotating the part against the drill bit produces an equivalent drill speed of 15,000 rpm.
Spinning the drill bit helps eliminate the drill drifting off center because the faster something is spun, the more likely it is to stay centered. The high speed also prevents burr formation when drilling a 0.003″ hole into plastic. In addition, the higher spindle speed reduces breakage of the $55 drills.
Feeding at the appropriate rate also eliminates burr formation. If the feed is too high, the drill pushes the material too aggressively and causes burrs to form. A high feed also generates thick chips, which can clog the flutes and cause a drill to snap. On the other hand, too low a feed rate causes the drill to rub rather than cut the workpiece. A feed of 0.0005 ipr proved to be a happy medium. Pecking is required to control chip length and prevent flute loading. The pecking depth is 0.004″ for about half the 0.049″-deep hole. The tool is then retracted every 0.003″ for the remainder of the hole.
Other considerations
When drilling a 0.003″ hole, applying fluid to cool the part and aid chip removal must be balanced against distorting the true position of the drill bit. At the higher spindle speeds, it is critical to keep the tool cool. Instead of coolant, however, we use a cool-air unit to maintain the required temperature and help remove chips so the bit does not melt its way through the part.
Chip control is also critical so the flutes do not become clogged with plastic chips and tear at the ID, making it larger than the drill bit. We keep the cool-air unit about 2″ to 3″ from the tool/workpiece interface to prevent the high-pressure air from breaking the drill.
Typically, the supplier provides extruded plastic rods ground to a ±0.0005″ tolerance. Grinding the OD to this tolerance ensures the process is repeatable. To prevent having to adjust the Swiss-style machine’s guide bushing—which supports the material during machining—we create two groups of bars: those up to 0.5005″ and ones as small as 0.4995″. Having two groups of bars at opposite ends of the tolerance range prevents having to adjust the guide bushing after machining each rod, which lessens the chance of making inaccurate parts or galling them. If the guide bushing is adjusted too loose, we cannot make accurate parts, and if the guide bushing is too tight, the material can gall and get stuck in the bushing.
We produce about 700 parts in each run. We manufacture a complete part in roughly 50 seconds, including turning, profiling and drilling. µ
About the author: Patrick Lemos is part owner of Prime Axis Manufacturing LLC, Colorado Springs, Colo., and has more than 20 years of experience manufacturing small to medium-size parts. Telephone: (719) 572-0577. Web: www.primeaxismfg.com. |
