Tackling the motor vehicle manufacturing challenge of turbos, electrics, and lightweighting
Tesla and the march to all-electric cars and trucks may get most of the press. But the reality is that most U.S. automakers need to tackle the twin challenges of building both new components unique to electric vehicles while also building internal combustion engines (ICEs) that are ever-more fuel efficient. As Kirk Stewart, vice president of sales for EMAG, Farmington Hills, Mich., explained, “even the projections out to 2040 call for 80 percent of the vehicles to have an internal combustion engine in one form or the other.” In other words, most U.S. cars will either be hybrids, having both an electric motor and an ICE, or just an ICE.
So, no matter what, motor vehicle makers need ICEs that are as efficient as they can be.
Supplying the Turbo Boost
One way to wring more power out of an ICE is to use a turbocharger and the market for turbochargers in North America is growing significantly, Stewart reported. He added that while North America will see the largest growth in turbocharger adoption (Europe and Asia already have high percentages of turbo use), “the supply chain in America is quite limited. There is a growing manufacturing base, more south of the border in Mexico than here in the U.S., but we see this changing as well. Even the OEMs are investigating doing their own turbochargers in-house, which is totally new.”
It’s a challenging component to make. The “cold side” has a stainless steel housing containing an impeller that’s generally made of aluminum. A shaft connects that impeller (often called a “wheel”) to a wheel in the hot side that’s typically made of Inconel. That wheel’s housing is stainless steel. There are variations in the sequencing of making and joining these components, but with operating speeds as high as 300,000 rpm and the need for a near-perfect mesh between the impeller blades and the housing, dimensional and balance tolerances are tight. EMAG is one of the few companies making machines that handle all the needed operations, with the option to integrate them into a complete turnkey production line.
Stewart said the housings can be turned, milled, or made with a combination of the two processes. “In the case of turning, you can invest in a one-time, custom fixture and turn the component with standard tooling, or alternatively, you can use a milling machine with an exotic tooling solution,” he explained.
EMAG brings an unusual machine construction to these applications. EMAG’s lathes hold the part vertically, with the chuck up top and, if needed, a tailstock at the bottom. This enables heavy stock removal, aids in chip removal, and simplifies part loading. The chuck acts as the pickup device for the parts, which can travel into the machine on a simple conveyor. Plus the machines have a cast granite machine bed. This greatly limits thermal variation and the growth and shrinkage that come with temperature changes. “It’s a big rock and likewise has a very low vibration signature,” said Stewart. “Some of the stainless steels used for these housings can be pretty nasty stuff, and a well-built machine reduces tooling cost.”
The cold side wheel usually starts as an aluminum slug, Stewart said, and the general form is turned and then moved to a five-axis mill to create the blades. The hot side wheel usually starts as a near-net Inconel casting, “and then you’re only machining the back face and creating the mating diameter for the shaft.”
Grinding Hits Tight Tolerances
The tolerances on that shaft and the outer blade geometries are tight indeed, and grinding is the preferred method of reaching them. According to Hans Ueltschi, vice president of sales for the cylindrical division of United Grinding North America, Miamisburg, Ohio, the shafts vary from about 4" (101.6 mm) long with a diameter of only about ¼" (6.35 mm) for cars, to about twice that for larger trucks. And in order to ensure the turbos run more smoothly and quietly, the diameter tolerances have been reduced from the 10 µm to the 4-5 µm range, Ueltschi said. Runout tolerances are in the same range or a little tighter, while the tolerance on the wheel profile is in the 20 µm range. He explained that holding a 4-5 µm tolerance typically requires a process capability index (Cpk) of 1.67 or even 2.0. That forces a tolerance band to about a 1.5 µm range. (Cpk of 2.0 is a six-sigma standard of quality.)
As Ueltschi put it, it’s one thing to grind a sturdy part to such tolerances. It’s harder still to grind such a flimsy part so well, especially since the required cycle times are short. He added that two or three different diameters are typically ground down most of the shaft length, with the most critical areas being those that go into the bearings. Also, the small area where the hot zone wheel is welded to the shaft is ground, and in some cases a threaded area in the back where the shaft screws into the cool zone wheel. “Some shafts have O-ring grooves, or grooves for seals, that need to be ground,” said Ueltschi. “So that’s another challenge. And sometimes you have a build-up from the friction weld of the wheel that needs to be removed.”
Grinding the impeller presents the challenge of an interrupted cut, which impacts grinding wheel wear. There is also a tendency for burrs to form on the trailing edge. What’s more, Ueltschi said, some customers also require the flexibility to run different types of turbos through the line, “so you need the ability to do a lot of changeovers.”
To tackle these challenges, United Grinding adds in-process gaging and a steadyrest to an already rock-solid Studer grinder. The exact model(s) and grinding wheel configurations would depend on the OEM and required assembly procedure, though Ueltschi said the most popular approach is to use a “do-it-all-in-one machine.”
Process know-how is key, of course, and Ueltschi revealed that “the machines repeat well enough for one gage to control one grinding wheel. So, if there is a multi-wheel machine, each gage is associated with each wheel, because the coordinate systems are independent.” The gage offsets the axis drives for every grind in a closed-loop system, compensating for any grinding wheel wear or change in the machine axis. “We’re talking about changes in the millionths,” said Ueltschi. “The gage R&Rs on these particular gages are about 0.1 to 0.2 µm in order to make sure you can achieve this accuracy.”
The steadyrest is retractable to enable rapid auto-loading, followed by engagement of the gages and steadyrest. It makes for a crowded little work area, Ueltschi observed. Studer typically uses a plunge or angle-plunge grind for both the shaft and the impeller wheel, the latter with a form wheel that matches the profile. Ueltschi explained that it’s also possible to use peel grinding for the impeller, in which the CNC generates the shape with a grinding wheel operating at about 140 m/s, versus standard speeds of 50 to 60 m/s. “It’s a quite flexible approach, versus the less flexible but more productive approach of plunge grinding,” he said.
Besides the steadyrest and in-process gaging, process stability also relies on the Studer machine’s mineral cast Granitan base, the use of variable-speed wheel drives that automatically adjust grinding wheel speed as it shrinks, and control of the coolant temperature and cleanliness.
Ueltschi added that many people don’t recognize the criticality of having extremely straight machine ways. “A rotary dressing system will reduce the wear on the diamond wheel. But you still have to dress the wheel straight so that the part is then straight and there is not any taper.” United Grinding also offers a deburring station within the automation portion of the cell to clean impeller wheels after grinding.
The final piece of the puzzle is balancing the shaft and impeller assembly, and as Stewart pointed out, “any variation in that process can yield very high scrap rates. High scrap rates for an Inconel part that has already been welded onto a precision machined shaft is throwing away a lot of money.” Traditional machining approaches introduce too much variability, he said. “If your drill tip or mill geometry is off even a little bit, the math doesn’t work. Second, if the component itself has just a little bit of waviness and it’s off where you want to remove some unbalance, that screws things up.” Machine variability is also a concern, he added.
EMAG’s approach is to use electro-chemical machining (ECM), which Stewart said yields a reliable “first-off correction” within 45 seconds, including part handling. “We don’t touch the part. And it doesn’t matter if the cathode is a little bit closer or farther away. It doesn’t matter if the component is a little more or less wavy. If we’re in the vicinity with ECM, we get predictable metal removal amounts based on the known voltage and time.”
Tighter ICE Tolerances
Another way to maximize the output of ICEs is to build them to tighter tolerances, and JTEKT Toyoda Americas Corp., Arlington Heights, Illinois, builds HMCs that deliver that. According to Dan Wietecha, Toyoda’s automotive proposal manager, the spindles in most Toyoda HMCs have a “dynamic variable hydraulic preload system. It allows us to run at lower speeds where required, but when we get into 12,000 to 15,000 rpm, we take solace in our machines having minimal tool runout because of our preload at the higher speeds, combined with ceramic bearings, which helps with the heat that’s generated.” Toyoda has also largely standardized on a direct drive B-axis (the rotary table) for its popular FH500J HMC. “It’s much more cost effective than adding rotary scales to a conventional B-axis. We inherently gain the index accuracy that we need in many cases, and, as a bonus, for higher production you get the high speed indexing. We can do 90o in half a second.”
Buster Barnes, Toyoda’s regional sales manager for the South, amplified this, saying that Toyoda packages the direct drive with encoder for “less than a traditional worm drive table with a rotary scale. So for less money, you get a faster table and the same accuracy, if not better, because there’s no gearing involved. It’s like a motor spindle for the table.”
Toyoda also equips all of its HMCs with a ball screw thermal stabilizer (BTS). “It’s a very precise sensor that’s monitoring the linear growth of the ball screw,” explained Wietecha, helping the machine achieve “tremendous accuracy” without the use of linear scales, which can be a maintenance headache. For example, Wietecha said that for cylinder boring, the tolerance for squareness relative to the crank bore is 50 µm. “So if you’re indexing from the right bank to the left bank, that indexing accuracy is extremely critical. We even see parallelism, if we’re doing two sides of a part in one fixture setup, in the 80- to 100-µm range.”
Toyoda holds these tolerances without glass scales. Barnes referenced a V-6 aluminum block line in which its machines maintain the head deck dowel hole location on a 300+ mm distance with a Cpk of 1.67. It has done so for over 13 years without linear scales, thanks to the BTS system.
There’s another feature of the Toyoda HMCs that helps maintain tight tolerances: a patented replaceable spindle cap that enables rapid repair in most situations. As Wietecha explained, the damage or wear that occurs in a spindle over years of use is usually limited to the toolholder interface. But that portion of the Toyoda spindle is an easily replaceable cap costing less than $6,000. “It’s about a 30- to 45-minute job and it repeats within 2 µm,” said Wietecha.
Barnes said it’s hugely popular with automotive users, who can’t afford inaccuracies or downtime. “If you damage the taper [in a traditional spindle], you have only two choices: pull the spindle out and put a new spindle assembly in for $30,000 to $60,000, or regrind the taper in the machine.” That means it is out of production while summoning someone to come in with a taper grinder and set it up on the machine and regrind the taper, test it, and adjust the drawbar. “It takes days if not weeks to get it done,” he said. The approach is fast, inexpensive, and its replaceable taper spindles are no less accurate than one-piece spindles to begin with. “And it’s not something new. We’ve been doing it for many, many years,” said Barnes.
Big Battery Housings
Battery trays have emerged as a key challenge in electric vehicle (EV) production, reported Wietecha. “They’re quite large aluminum parts, typically with a lot of smaller drill and tap holes along with some other features.” Given their size, the trays are a good fit for Toyoda’s SX-i HMC models, which have “oversized work zone diameters and increased strokes on the X and Y,” Wietecha said. “The FH630SX-i is a 60-mpm machine, while the FH800SX-i works at 54 mpm.” These are high-speed machines, even though they are typically earmarked for large, heavy milling operations. He noted that they are also offered with an optional 15,000-rpm spindle. The FX800SX-i has a maximum workpiece swing of 1,500-mm diameter and the same maximum height.
For its part, SW North America Inc., New Hudson, Mich., developed the BA W08-12, which unlike most SW machines is a single-spindle horizontal, though it shares SW’s usual “monoblock” design for minimal deflection. The machine has a collision circle 950 mm in diameter with a length of 1,710 mm and the X-axis travel is 700 mm longer than the two-spindle variant, nearly doubling the machining range.
Linear motors are another key ingredient, according to Reiner Fries, managing director of sales at SW’s German parent company. “Machining high-volume structural parts is equivalent to normal light metal machining,” said Fries. “Highly dynamic, fast machines that can cover long sections in the shortest possible time are clearly more suitable for this type of machining. The SW machine series with linear motors is especially suited for these market requirements.”
Fries also pointed out that the BA W08-12 has two independent rotary axes (the fourth axis), both of which can be fitted with a fifth-axis table. And the clamping fixtures can hold the parts being processed on the top or the bottom of the base plate. “Four different parts have to be processed for the battery housing. With the SW machine this can be done in one clamping with four-axis [machining],” said Fries.
Another advance aiding the production of these big housings is a punch tap that saves approximately 75 percent of the threading time. Originally developed by Emuge in cooperation with Audi, the tool is now available to other customers and Toyoda worked with FANUC to create the appropriate toolpath for their machines. Barnes described it as a helical path, instead of threading down each individual pitch.
EV Motor Components
Making vehicles as light as possible has been a virtual obsession for some time now, and it’s certainly crucial for EVs. That includes the powertrain, the core component of which is the rotor shaft, said Stewart. He explained that one common approach to creating a light rotor, especially one that needs solid features at the ends, is to start with two independent components, largely drill them out, and then weld them together, “like two coffee cups joined brim to brim.” Here again, beyond the green machining, EMAG offers solutions for the automated welding, induction hardening after welding, hard turning after the induction hardening, and the final grinding of the component after the hard turning. Stewart said the stator is a set of many sheet metal sections (rather than a conventional steel slug) that are eventually formed into one. “It’s not a really robust piece, but it still needs to have a close diametric tolerance after assembly,” he said.
Alternatives for processing the part include simple turning and grinding, he explained, but EMAG favors a variation of scroll-free turning. Rather than turn with an insert having a single nose radius, which builds up heat in the insert and increases pressure between the workpiece and the insert, scroll-free turning uses a “long blade set at a 45o angle, which is sweeping through the part simultaneously with the feed of the insert. This very long blade is incrementally being passed through, … incrementally cutting the complete diameter. So the point of contact along the length of the blade is rather instantaneous.”
He noted that this pushes all of the heat into the chip and results in much less load on the component. “So you have less variation, and likewise it gives you the ability to have a very good diametric control.” This results in a surface finish down to Rz 1-4 µm, Stewart reported, which is a critical requirement for these components. Also, scroll-free turning is up to 60 to 80 percent faster than traditional single-nose turning, he said.
What’s the downside to scroll-free turning? “The technology is relatively new,” said Stewart, though by that he meant roughly 10 years old. And “there are some software requirements to get the exact sweep. The complication is that when you engage the very front side of the blade you’re actually a little bit behind true dead center of the diameter of the part. So there is an interpolation that’s not exactly linear and some special software [needs to go] along with it.” The other challenge is the need to be careful in setting up the tool, because the surface roughness of the blade directly transfers to the surface of the part. “So if you’re not controlling that tool very well, then it won’t give the result you want on the component.”
Stewart added that EMAG also has shrink-fit technology. “We can now introduce the stator to the cell, bring that stator onto the rotor, do the finish machining of the assembly, and also provide equipment for the final balancing and pack out.”