Holemaking operations deploy a variety of rotating cutting methods, including drills, reamers, milling cutters, trepanning tools, and boring tools, but some applications demand different approaches. Two holemaking alternatives include the Thriller tool from Emuge Corp. (Northborough, MA), which enables hole drilling, thread milling, chamfering, and spot facing or counterboring all with a single tool....

While thriller technology has been available for several years, Emuge licensed the original design and has incorporated improvements into its BGF Thrillers, which are designed to both drill and thread holes by using helical interpolation. The solid micrograin carbide-substrate thrillers feature refined profile correctness in the tool's cutting geometry and axial, through-the-tool coolant capabilities, resulting in significantly longer tool life and optimal chip removal while decreasing cycle times.

At BorgWarner's (Chicago) 1,000,000 ft2 (93,000 m2) transfer case plant in Muncie, IN, the Emuge thriller tools have shown to be extremely durable since being installed on BorgWarner's four-wheel-drive transfer case production lines in June. The BorgWarner plant uses the thrillers in transfer cases produced for General Motors Corp. (Detroit). The automotive supplier also makes similar equipment for Ford Motor Corp. (Dearborn) and other automotive OEMs at that facility.

"We chose to use this tool on the interface between the customer's transmission and the transfer case," says Jim Sells, manufacturing engineering coordinator for BorgWarner's casting group. "There are always threaded tapped holes, 10 X 1.5-mm tapped holes into aluminum or magnesium, and the depth/diameter ratio is usually quite high--21/2 to 3 times the diameter is normal for this particular tapped hole."

BorgWarner machines the transfer case holes using thrillers and high-speed MC25 HMCs from Heller Machine Tools (Troy, MI). In the current application, BorgWarner used the thrillers to machine the threaded holes in the 4WD transfer cases used in GM's Tahoe, Yukon, Escalade, and new Hummer H2 models.

Prior to selecting Emuge's thrillers, the BorgWarner plant used self-reversing tapping heads to produce the threaded holes required for transfer case production, but problems with thread quality and the tapping equipment's high maintenance costs prompted Sells to try the thrillers. Installed about eight months ago during pre-production on the GM transfer cases, the thrillers went straight into production, and have been running without replacement since then in an application that Sells says calls for deeper holes than the typical depth/diam ratio of about 11/2 to 2 times the diam of the holes.

"The thread at that depth, and the location of that hole, made it difficult, because you're always drilling into a cored hole that's part of the die-casting," Sells notes, "and core pins that are that small and that deep usually have a tendency to bend or break. You're always fighting the cored hole. When you use a conventional drill and then tap, it's a tough operation to put the tool on location, because the tools always want to follow the core pin."

With 69 HMCs at the Muncie plant, which uses four different Heller HMC models, Sells says the company switched to the thrillers for the new GM production based on the number of tools the Emuge tooling claimed to eliminate. In the GM workcell, six Heller HMCs drill and thread 12 holes per operation, with six holes machined on two transfer cases.

Besides eliminating tool changes, the thrillers improved thread quality, although Sells says he doesn't yet have an accurate figure on overall productivity gains. "I don't know that we've had enough time to figure that out because overall, the cycle time is a little longer," Sells concedes. "It takes a little longer to process each hole. In some cases, we've had to make double passes with this thread milling portion, and it's always slower to interpolate than it is to just screw in, screw out. But the overall quality level of the part has gone through the roof, and I haven't had any failures yet. These tools have been in three of the six machines in the GM cell that used this tool, and they've been in them since we launched it. So I don't know where the end's at yet.

"It puts the hole in the proper location because it's a very short, rigid tool, so location problems, which have always been a problem in the past, are almost eliminated," Sells adds. "This is like running a boring bar, boring the holes and putting them on location, instead of drilling them and letting them float around wherever they need to go.

"Also, with a tap, when the thread size started getting small, you simply replaced the tool. There's nothing you can do about it. It's worn, and it won't cut the proper size anymore. With this tool, you can run an offset, a tool-comp offset in the NC program, and make it bigger, so you can run the tool longer. If your hole starts getting small, you can make it bigger. That was a feature you couldn't do with a fixed-size tap."

With the thriller, an Emuge-supplied program enables users to easily run the tool-compensation offsets. "It's not a very complicated program, but when you look at the NC toolpath, this tool has to go through some pretty strange gyrations," Sells observes. "It drills, backs up, moves sideways, goes around a helical interpolation, goes back to the middle, and then it comes out, so it makes quite a few moves while it's inside that hole."

Using helical interpolation, the Emuge thrillers produce holes that are not like a normal drilled-and-tapped hole, Sells says, but instead have an undercut to the hole. "Normally, on a drilled-and-tapped hole, you'll just drill a straight hole, deeper than the tap, then that hole size represents the minor diameter of the thread," he notes. "Then you run the tap into it, so you've got the threaded portion of it, and there's a little pilot at the end, where the drill had been before.

"On this one, the actual thread OD is a lot smaller than the tap would be, and on the very tip of it, there's a portion like a spade drill. It drills down to depth, and this spade drill's very short--the diameter of that spade drill equals the minor diameter of the threaded hole. But now remember the threads are back here behind it at the shank of the tool and they're small in diameter, so they're not cutting. You back the tool up and move over sideways, and now the spade that's down at the bottom of the tool starts cutting an undercut ring at the bottom of the hole as you're interpolating and generating the thread."

The main reason BorgWarner chose the thrillers was that GM wanted a transfer case including threaded holes with fasteners in them, while other automakers had blind tapped holes in the cases that received a bolt during final assembly, Sells notes.

"To begin with, the reason to use the thriller was purely that it promised to eliminate some tool changes, which is always a big deal if you have 69 machining centers. If you have even half of those machines running this type of tool and you can eliminate one tool change, you've got a major productivity increase," Sells recalls. "Plus you have maintenance and wear and tear, durable tool costs that go down, down, down, just by eliminating one tool, and this had the promise of eliminating a couple of tools, and some very high-priced, high-maintenance tools, to begin with. It just sounded like a 'too-good-to-be-true' thing. We actually started the program with a process using both taps and thrillers. The Emuge tool was just a test, when we bought the machine tools, and then it worked out so well we just took it into production.

"This tool cost a lot more than a conventional tap, so it had to run longer and produce more holes," Sells says, noting his group normally would replace a tap about every day. "And so far it has done so, and I don't know when it's going to quit."

Sells points out that the eight months during which BorgWarner has used the thrillers includes prototype and pre-production testing. "It's only been used in full production since about late June. We've used it to build all the prototype parts, all the initial quantities, the pre-production, and now it's doing all the production parts. So the tool has been through some pretty rigorous programs, and it's singing like a bird."