Metals take a turn in new drills

When cordless drills first turned up on job sites about a decade ago, construction workers had a graphic way of showing their skepticism about the power of unplugged tools. "They would stop the chuck with their hands," says Christine Potter, a design engineer and assistant product manager for DeWalt Industrial Tool Company (Baltimore, MD). If someone tried that trick today, they could end up with a busted wrist. Potter notes that torque of professional cordless drills has tripled over the past ten years—to 450 in-lb for some of today's 18V models. And with battery capacity doubling over the same period to as much as 2.4 A-hr today, cordless drills don't often quit mid-job.

Power and run-time progress have without a doubt helped cordless drills gain acceptance with the tool-belt set. At the same time, however, these improvements have triggered materials selection challenges. Tom Bodine, a senior project engineer for DeWalt, points out that "more power and longer run times translate directly to increased thermal and mechanical loads." These loads have recently led DeWalt engineers to switch some drill components from engineering thermoplastics back into metal. "Where you run into trouble with plastics is that their strength decays rapidly with increasing temperature," says Bodine.

On its recent XRP line of conventional and hammer drills, DeWalt engineers made good use of two advanced metal technologies as a way to achieve a better performance than plastic components could—and do so while keeping costs under control. For the gear cases, the company adopted magnesium injection molding. The drills also feature the company's first all-metal power transmission, one whose gears come out of a state-of-the-art powder-metal process.

Magnesium gear case. Though plastic gear cases have gained a good reputation for resisting drop damage on the job site and promoting design flexibility, DeWalt engineers wanted something even tougher for the XRP drills. Potter says the design team rejected plastics because team members wanted a thin-walled case that didn't sacrifice support for the spindle—particularly when off-kilter users subject the drill to side loads. "Users perceive the stiffer case through reductions in bit run-out and chuck wobble," Potter reports.

And magnesium's improved strength and wear resistance comes in handy with the XRP hammer drills. As part of the mechanism that allows their axial translation, these drills have a fixed ratchet keyed into the gear case. Bodine points out that the ratchet and, indirectly, the case absorb all energy from the hammer as it beats 34,000 times per minute, often under bias loads exceeding 100 lbs.

Once plastics had been ruled out for the XRP drills, DeWalt engineers briefly considered diecast aluminum for these two-piece cases—a one-piece main housing plus a cap. But magnesium injection molding, or Thixomolding^®, had design and manufacturing advantages that gave it a clear edge in this application. "The difference in mechanical properties had little influence in the decision," says Bodine. "Either material could give us the strength we needed." But magnesium housings can do so at a much lower weight. Thanks to a density advantage and the ability of the Thixomolding process to produce thin walls—down to 0.080 inches in this case—the magnesium housings weigh about 45% less than a comparable diecast aluminum case, Bodine estimates. The design advantages don't stop there. Bodine notes that Thixomolding can turn out zero-draft parts and hold dimensional tolerances—particularly on feature size—better than aluminum diecasting.

Both these advantages really come into play on the bores that ultimately hold the drill's needle bearings. Not only do these bores have no draft, but they also require tight dimensional control to enable a tight press fit with the bearings. DeWalt's molder, Thixotech Inc. (Calgary, Alberta, Canada), has to hold the bore diameter to slightly better than ±0.001 inch. "That's pushing the envelope a bit for the Thixomolding process," admits Ron Bokkers, Thixotech's manager for the DeWalt job. Bodine agrees and explains that the tolerances normally cited for Thixomolding design guides—such as a ±0.001 in/in for a linear tolerance—normally apply to simpler parts than the gear case. "The published claims on tolerances at the time we started the job were based on thin, flat, uniform wall thickness parts whose measured features were formed from a common side," he says. "Our parts have complicated, three-dimensional geometry, which poses challenges for any casting or molding process."

While DeWalt and Thixotech have hit feature-size tolerances at least as good as published claims, they have experienced about twice the anticipated linear tolerances, Bodine reports. But he attributes this difference once again to the gear case's complicated geometry—and to the fact that DeWalt's feature position measurements extend across the tool's parting line. It's worth noting, too, that Thixomolding's tolerances may get even better as this relatively recent process matures. "The tolerances published in our design guide have been intentionally conservative," says Stephen LeBeau, vice president of Thixomat, the Ann Arbor, MI company that licenses Thixomolding technology. "But our molders are continually squeezing under the tolerances we cite."

Magnesium molding also shines from the standpoint of manufacturing costs. It saves money not just by enabling thinner walls than aluminum but also by eliminating the overflows often required with diecast parts. "These waste material," Bodine notes.

Thixomolding also lends itself to more productive tooling—both through higher cavitation or longer life. "In tools with side actions or lots of kiss-offs, the aluminum erodes the steel, and the flash just gets worse and worse," Bodine says, adding that comparatively low Thixomolding temperatures and the magnesium itself don't harm tooling as quickly as aluminum does. Finally, and most important from a cost standpoint, Thixomolding's dimensional tolerances and ability to mold-in holes eliminated secondary machining. "We have similar parts from another product in diecasting and they need secondary machining," Bodine says. These machining operations amount to 25-50% of the part cost, he adds. The only downside to magnesium—and something that slightly offsets the process's overall cost advantage—is that exposed, cosmetic magnesium parts need a corrosion-resistant paint job that aluminum parts don't require.

All-metal power transmission. The effects of higher power and better batteries have made themselves felt in power transmission too. Prior to the XRP line, DeWalt selectively used plastics in its power transmissions. A typical drill would employ three-stage planetary gear with thermoplastic ring gear and, sometimes, the planets. But today, with 18V drills capable of running for hours straight off multiple battery packs, continuous use temperatures have climbed "somewhere north of 200F," says Bodine. "The thermal loads are getting too high for plastics," he says.

For the XRP power transmission, DeWalt engineers decided to do away with plastics entirely: The three-stage planetary gear set and related components have all been created through a powder-metal process. "The whole industry is moving toward all-metal power transmissions," says Potter, "but we're the first."

Despite the well-recognized expertise in powder metal processing at DeWalt and its parent company, Black and Decker, thinwalled gears presented challenges on the manufacturing floor. Creating the powertrain parts, which range in density from 6.8 to 7.2 g/cm³, wasn't just a matter of bringing densities up—thereby ensuring good mechanical properties. "The trick was getting the powder to flow into the gear teeth and still compact to the right density," Bodine says.

To make these gears and some related components, DeWalt adopted proprietary tooling designs and used several types of ferrous powders from Hoeganaes (Cinnaminson, NJ). For critical parts of the drivetrain, the company used new AncormaxD powders, which have been created to boost green strength and densities. Bodine also noticed that these powders produce parts with drastic elongation improvements—as much as four times that of previous powders. This property gain helped DeWalt avoid a machining operation on the rotating ratchet used in the hammer drills. "Powder metal parts don't typically have great elongation," he says. So joining ratchet and spindle would normally require honing to get an exact fit. With the extra elongation in the new powders, "We don't have to hone," he continues. The savings are as much as 25% on that part. "In our business, even pennies add up," he says.

As for overall cost, all-metal gearing definitely wasn't the cheapest way to go. Bodine notes that plastic ring gears cost three or four times less than their metal replacements. Yet since some of the individual gears had already gone to metal, moving to an all-metal power transmission only added 10% to the total cost of the powertrain. And that's not bad, considering that plastics wouldn't have made the grade anyway.