Revving Up Tooling and Machining Strategies for Auto Parts
Article From: September 2019 Manufacturing Engineering, Ed Sinkora, Contributing Editor
Perhaps the most common challenge in automotive machining these days is aluminum. “Even chassis components are transitioning to aluminum due to the higher strength properties achieved by forging aluminum,” explained Jeff Gimino, PCD and CBN product specialist for Walter USA, Waukesha, Wis. That includes control arms, bearing carriers, and steering knuckles. Gimino added that the high-strength aluminum alloys used in these applications generally have more silicon and magnesium to increase the tensile strength, but that also makes them tougher to machine. Aluminum has also taken over in automotive engine blocks, according to Walter’s William Radtke, manager of application development and machine tool builders, “We saw a big transition from irons and steels to a short transition to CGI (compacted graphite iron). But now virtually all blocks are aluminum,” he said. “I believe only one automotive maker in the U.S. still has a CGI block.”
Machining the tougher aluminums generates a lot of heat, and heat causes tool wear. So, for heavy stock removal with minimal tool changes the preferred solution is polycrystalline diamond (PCD) tools, generally as PCD inserts brazed into a carbide tool body. Gimino added that brazing in multiple PCD inserts is the only practical way to cut multiple bores and other features with the same tool, doing more with less. Walter began making big investments in PCD tooling in 2004 with the acquisition of Werner Schmitt PKD-Werkzeug. Radtke said today’s tools will run for “months on end” in a typical aluminum engine block or head.
Iscar USA, Arlington, Texas, also makes PCD tools but has now added a new twist. Ron Crane, national product specialist for turning and threading, said Iscar worked with “a large aerospace manufacturer” to create a diamond-like coating that delivers “85 percent of the tool life of a true PCD brazed insert-type tool at a cost about 5 percent higher than carbide.” Iscar calls it “hard carbon” but does not publicize it or explain it in detail. Offered to big accounts in special cases, a typical application is automotive die-cast aluminum parts. Crane also explained that although adding a diamond coating to a carbide tool typically requires some degree of cobalt depletion in order for the diamond to grow, limiting the viable substrates, the hard carbon coating has no such limitation. It can be applied to any carbide substrate.
Taking a step down from PCD or diamond-like tools, there are also excellent carbide solutions for machining aluminum. For example, Iscar offers a variable pitch, variable helix, four flute solid round end mill for cutting aluminum called the ECA-H4-CF. “For 20 years the challenge has been evacuating the chips,” recounted Crane. “Three-flute tools have been the preferred solution for aluminum machining” and it was difficult to envision a four-flute design that could deliver the necessary volume for the chip removal without being too weak for the required machining speeds.
Crane said the ECA-H4-CF has a similar core diameter and flute chip gullet section size as the three-flute design and is chatter free at up to 33,000 rpm, and that extra flute delivers faster material removal. For turning, Iscar offers a new insert called ALUPTURN. That’s “alu” for the application (aluminum), “p” for the positive rake, and of course “turn” for the process. Crane explained that the industry standard has always been a ground sharp single-sided insert, while the “ALUPTURN looks like two CCGTs back to back. This means four cutting edges for approximately the same price as the two-cutting-edge CCGT insert without sacrificing any of the performance. That’s tough to do with an insert for aluminum because you need to be very aggressive with the chip breakers.” The ALUPTURN is designated as either a CNGG or CNGX form and is designed mainly for machining aluminum wheels.
Thin Walls and Other Challenges
Unsurprisingly, engineers are maximizing the use of aluminum in hybrid and electric vehicles, including parts that don’t exist in cars powered only by internal combustion engines. These include battery trays and stator housings, both of which are designed with thin walls that are difficult to machine without damaging the part. Gimino said most battery trays are die-cast and areas for mounting the battery to the tray as well as mounting the tray to the chassis of the vehicle must be machined. But Radtke explained that machining typically generates a force that pulls or pushes the part. So, Walter designed specialized tools that don’t exert enough force to damage the walls, creating tools specifically for side walls and different tools for machining the bottom. The added challenge in stator housings is maintaining a large-diameter straight bore down a long case.
Walter tweaks tool geometries to meet these challenges in part by using lasers to cut the entire form in the PCD. “We apply hones very easily to cutting edges, to either increase tool pressure, to help dampen the action, or make it sharper,” explained Radtke. Gimino added that a lasered edge is much better than a ground or eroded edge, “which translates into better surface quality on the workpiece.” Lasers also cut features like chipbreakers into the surface of PCD. Modifying the cutting edge geometry can also contribute to decreasing vibration (another key goal), sometimes in combination with variable helix angles and unequal flute spacing. Or as Radtke put it: “There is a wide range of tricks of the trade.”
The task of dealing with ever thinner aluminum parts doesn’t rest solely with the cutting tool suppliers, of course. Machine tool builders are rising to the challenge, as exemplified by Mason, Ohio based PCI-SCEMM, distributed in North America by Absolute Machine Tools, Lorain, Ohio. Max Paulet, key account manager, said that although automakers are using stronger materials, there is so much less material that parts are now more fragile and can crack or even break under the pressure of normal machining. Aside from such damage, the flexibility of the parts is also problematic. “On almost every aluminum powertrain application, there is a risk of bending or deforming the part,” said Paulet. Part of the solution, he explained, involves improved fixtures that can dampen vibrations, precisely adjust and sequence the clamping force, and communicate with the machine control about their position and status. This requires “more hydraulic, air, and electric supply lines to the fixture, and as a consequence an evolution of the internal design of the CNC,” added Paulet.
Paulet also observed that most high-volume automotive production is performed on high-speed machining centers, and the spindles are getting faster, enabling new machining modes that are less stressful on the parts. “Our standard HSK 63 spindle goes from 0 to 18,000 rpm,” explained Paulet.
“Part segments we used to machine at 9,000 rpm with 40 Nm of torque can be weaker in the new designs, requiring a higher speed with less torque to eliminate any damage or cutting process issues.” He added that for chassis components PCI is considering developments like a 24,000 rpm spindle operating at a lower feed rate to be as productive in material removal as before without damaging the lighter parts. PCI has also developed a range of “e-spindles” (i.e. connected spindles that communicate with the machine control) that are not only able to collect data through sensors (vibration and torque for example), but also instantly compensate for “any issue happening at the cutting tool/workpiece interface,” said Paulet. Depending on the feedback, the machine might automatically adjust the cutting conditions or possibly trigger a request for human intervention. The data can also be used for predictive maintenance. “This is how we overcome the cutting process difficulties that material changes bring to auto part manufacturing,” concluded Paulet.
From Jeff Moore’s perspective, the biggest challenge with aluminum is dealing with all the chips. Moore, a regional sales manager for EMAG LLC, Farmington Hills, Mich., said his company’s machines have one unusual, albeit natural, advantage: gravity. EMAG’s turning machines are vertical. The chuck is up top and, if needed, a tail stock is at the bottom. High pressure 1,000 psi coolant also helps wash everything down to the base and its large open area. In this configuration, the chuck also acts as the pickup device for the parts, which can travel into the machine on a simple conveyor. (Most EMAG installations are automated.)
This approach also enables heavy stock removal. “Because our tailstock is at the bottom and fixed I don’t have to worry about applying too much downward pressure or pulling the part out of the chuck, as you would on a normal horizontal machine with a traveling stock,” said Moore. “I can turn with a lot more velocity and depth toward the tailstock.” For example, using Sandvik’s Prime insert, the machine can remove 3-4 mm of aluminum in one pass going down, pushing all the material into the chip bead, and then stay in the cut for a finish pass going up, Moore said.
The disadvantages to this configuration? “It requires more spindle torque,” said Moore. “Without spindle power, it is going to stall,” he said. Keeping chips off the chuck is also critical, especially when picking up the next piece. EMAG uses air blasts to clean the chuck and coolant running through the spindle to wash off the datum structures, coupled with air checks of the gaps to ensure the chuck is ready for the next part. This is all necessary because tolerances are already in the ±50 μm range and getting tighter. In particular, Moore referred to the effort to make transmissions more compact and lighter, and combining a tiny three-speed transmission with a separate five-speed transmission in one vehicle. “The lighter they try to make these transmissions, the tighter the tolerances for everything.”
Carbon fiber reinforced plastics (CFRPs), so prevalent now in aircraft, are not yet common in autos, with the exception of exotic vehicles. And it’s expected that if automakers do turn to CFRPs, the parts won’t require some of the more demanding machining needed in aerospace (e.g. the thousands of tiny holes in acoustic panels).
However, one area where aerospace and autos overlap is in high-temperature alloys. In the case of autos, the main application is for turbochargers, which have become ubiquitous in order to wring extra horsepower out of smaller engines. And these are tight tolerance parts, adding to the difficulty. Radkte estimated that the overall average tool life for drilling, boring, milling, and tapping turbochargers is about 100 parts, which he again contrasted with aluminum in which PCD tools run for months.
“You run at surface speeds that are close to 25 percent that of steels and irons, maybe even lower,” said Walter’s Radtke. “That could be somewhere in the 130 to 180 sfm range in the high-temp alloys. That means carbide grades that are much tougher and heat resistant, because heat is needed to break the chips, especially in holemaking.” Crane of Iscar said he’s also seen high-temp alloys used in suspension components and Iscar offers solid-carbide end mills capable of full slot machining at 2xD in titanium.
Advances in Steel and More
For some automotive applications, there is no substitute for steel, and Moore pointed to new technology that makes common materials better. For example, he said EMAG’s patented simultaneous dual frequency (SDF) technology can be used to make softer steels like 5140 suitable for gears, and do so quickly. “Typically you’d start with a low-carbon steel, do the cutting and gear hobbing, send out a batch of parts to be carburized in a big furnace, and then bring them back for finish grinding,” Moore explained.
In EMAG’s new approach, users start with a common and easily machinable steel like 5140, perform the turning and hobbing operations normally, and then use SDF induction hardening right before hard turning and grinding, all in-house and in the same automated production line.
Besides the easier machining, this eliminates the hassle, cost, and delay of batching parts for outside carburization. Moore said SDF combines low and high frequencies at the same time, putting a lot of energy into the part quickly, so “cycle times are typically under a second and then quenching time is about seven seconds. Then we also temper in the machine. Tempering is the longest process, usually taking around 12 seconds.” And EMAG can also supply the grinding/hard turning machine, the gear hobbing machine (in partnership with either Koepfer or Richardon), and the linking automation.
These machines that perform hard turning and grinding hold a tolerance of around 10 µm, according to Moore. To hit even tighter tolerances or machine Inconel, Hastelloy or other super alloys with equal ease, EMAG also offers electrochemical machining (ECM) technology. As Moore put it, “ECM is like the opposite of electroplating. We’re breaking a molecular bond within the material and then using a salt solution with negative ions to draw out those atoms from the material, so it’s electrochemically machining the part.” The technique is excellent for things like turbochargers, polishing injectors and fuel pressure valves, and deburring tiny channels in parts.
It is also used to create an internal expansion gallery inside an injector. Returning to the turbocharger example, Moore pointed out that these parts must be balanced up to 20,000 rpm and it’s difficult to be sufficiently precise in using a drill to remove material to achieve this, usually resulting in a high rejection rate after secondary balancing. “With ECM,” he said, “I can 100 percent guarantee how much material I’ve removed based on the voltage and the amount of power consumed. I can be precise to within micrograms.” ECM also delivers fine finishes and produces no white layer, eliminating the need for any polishing.
EMAG also has an intriguing new approach to turning they call rollFEED that delivers both an excellent finish and extremely high material removal rates. The approach literally rolls a specially shaped insert across the part, with the point of contact moving along the edge, as the tool also moves along the X and Z axes (thus rollFEED requires a three-axis movement). Moore said the resulting chips are “like talcum powder. You can break them up in your fingers. That’s because the chip is very, very hot and we run zero coolant.” Even better, Moore said the technique is 80 percent faster than conventional turning and achieves surface quality comparable to scroll-free turning or grinding.
Phase Toughened Ceramics
Lest we forget the CGI applications that remain in automotive, let’s close with the productivity improvement achievable with the XSYTIN-1 phase toughed ceramic insert from Greenleaf Inc., Saegertown, Pa. Martin Dillaman, Greenleaf’s manager for applications engineering, reported on a case in which XSYTIN-1 delivered a cycle time of under five minutes milling CGI versus 20+ plus minutes with a comparable carbide insert. The carbide ran at 500 sfm (152.4 m/min) with a feed rate of 0.006″ (0.15 mm) per tooth and a DOC of 0.0060″ (0.15mm) while XSYTIN ran at 1,900 sfm (580 m/min) with a feed of 0.0085″ (0.22 mm) per tooth and the same DOC. A match-up in turning 30 HRC Class 30 gray iron also favored a WNMG-433 XSYTIN insert over the carbide equivalent running the same parameters (1,078 sfm (329 m/min), 0.0078″/rev feed (0.2 mm/ref), DOC of 0.050-0.250″ (1.27-6.35 mm). The XSYTIN insert cut 150 parts per edge versus 40 parts for the carbide.