Finish line: Drilling Performance

Finishing hardened workpieces on multitask machines that provide grinding and hard turning and/or hard milling capabilities can prove beneficial.

Multitask machines are typically thought of as providing milling and turning operations, but applications are increasing for machines that perform grinding and turning and/or milling to finish workpieces hardened up to about 65 HRC in one setup.

View a video courtesy of United Grinding Technologies. A Studer brand S242 machine, which combines hard milling and grinding, is shown hard milling a toolholder without coolant, then grinding the holder’s taper with coolant to achieve the required surface finish.

The trend is being driven by the need to have the highest quality part at the lowest cost per component, according to Gary Hulihan, who recently retired from EMAG LLC USA. The Farmington Hills, Mich., machine tool builder offers an array of equipment for “hard finishing,” such as the VTC 315 DS for vertical turning and grinding of demanding shaft-type components. “Hard finishing to complete a part merits that type of machine because it eliminates handling between separate machines, increasing accuracy,” he said.

Jeff Reinert, president and CEO of INDEX Corp., Noblesville, Ind., agreed that finishing a part in one operation increases part accuracy by never releasing the part before it’s completed. “It maintains part alignment and the orientation between part features,” he said.

Courtesy of INDEX

The C200 CNC production turning machine from INDEX can be configured to provide grinding. (Additional information on this machine is available in the online version of this article at ctemag.com.)

Courtesy of Bohle Machine Tools

The work area on a Buderus Schleiftechnik multitask machine from Bohle Machine Tools with an ID grinding wheel on the left and a turning tool on the right.

INDEX has been building its V product line of vertical lathes for turning, milling and grinding in one machine and recently introduced the C200, which can be configured for hard finishing. “In the C machines, we may reposition the part from the main spindle to the counterspindle, so there would be minimal possibility of misalignment and the orientation is never changed because the spindles are in sync with each other,” Reinert said. “The part is never released from the main spindle until the counterspindle has clamped on it.”

For example, when a part is turned and ground in one clamping, the TIR between the machined diameters is only about 2μm, noted Wolfgang Henkel, vice president of Bohle Machine Tools Inc., Plymouth, Mich., which distributes the German-made multitask machines from Buderus Schleiftechnik. “Each reclamping can generate a TIR error of 10μm,” he said.

Hard finishing for the roughing operation can also reduce cycle time. That’s the case, Henkel pointed out, when the stock to be removed is greater than the 0.2mm to 0.3mm (0.008 ” to 0.012 “) typical for removal via grinding, and then only about 0.05mm (0.002 “) remains for grinding after hard turning.

Hard turning is faster than grinding but it is not able to impart as fine a surface finish as grinding, and turning is unable to remove material in increments as small as grinding can. Grinding can remove millionths of an inch of material, according to Guenter Schad, technical manager at INDEX. “If you’re hard turning, you need to allow for sufficient depth of cut, and this depends on tool geometry,” he said. “This is typically 0.002 ” or more to permit the insert to break the part surface.”

“That comes with the territory when dealing with hardened materials—you get one chance to do hard turning,” concurred Tyler Economan, proposal engineer at INDEX, who noted that with hard turning and grinding in the same operation, the end user could leave as little as 0.02mm (0.0008 “) of material for grinding, thereby reducing grinding time.

Hard milling might also rough a feature prior to grinding, and it could be used to machine features not accessible to a conventional grinding wheel. Reinert noted that an end user, for example, can stop the main spindle on the C200 and move a second grinding wheel in and out using the Y-axis capability of the machine’s lower tool turret to grind a flat across the part. “You can’t do that on a regular grinder,” he said.

Gone with Grinding?

Hard turning and hard milling to tolerance specifications have long been considered alternatives to grinding, but the applications where those processes could effectively substitute for grinding were fewer than they are today, according to Jim Endsley, product specialist for machining centers at Okuma America Corp., Charlotte, N.C., which doesn’t build special multitask machines for hard finishing. He noted that 5 to 10 years ago, about 5 percent of hard milling applications could achieve single-digit microfinishes comparable to grinding without tool marks or lead lines. “Today, it’s as high as 60 or 70 percent of the time, and it’s mainly through tooling and technique” Endsley said, adding that developments in cutting tools and machine tools are now advancing together, which benefits hard milling, rather than leapfrogging each other. “You could never get that happy balance.”

To determine which machine processes a customer needs to achieve the required part specifications, United Grinding Technologies Inc. uses a matrix, explained Hans Ueltschi, vice president of the Miamisburg, Ohio, machine tool builder. “It depends on the quality requirement,” he said, “and there is a band where we can go either way: grinding or turning.”

Courtesy of United Grinding Technologies

The interior of the Schaudt CombiGrind S242 machine is shown equipped with a grinding wheel and the tool magazine.

United Grinding’s Schaudt brand offers the CombiGrind for grinding IDs and ODs, hard turning, milling, brushing and gaging in one setup. The vertical version is a chucker and the horizontal version is more for shaft-type work. According to the company, Combi- Grind machines are for mass production of components and subassemblies in the automotive and supply industries. That would include a parts manufacturer supplying a family of parts to different OEMs that have varying standards. “Fundamentally, there’s no difference in the part [family] other than maybe size and shape,” Ueltschi said. He noted that a job shop would probably opt for separate machines.

In addition, the company’s Mägerle MGC surface profile grinder is capable of milling and drilling.

Ueltschi outlined several examples where it makes sense to hard turn and grind on one machine. More toolholders are processed that way because they have many features suitable for hard turning, but the tapered area that interfaces with the machine tool spindle must be ground. A multitask machine allows the customer to changeover with minimal setup time because he doesn’t necessarily have to reprofile grinding wheels, where one set of wheels is sufficient to grind different toolholder tapers and different toolholder sizes.

Components with interrupted surfaces can be another suitable application. “Interrupted surfaces have a negative impact on turning tooling and they usually determine whether a part can be hard turned or not,” Ueltschi said.

In addition, long, highly cylindrical, small-diameter bores often require finish grinding to impart the needed surface finish after turning because it’s difficult to avoid generating a taper when grinding them completely, according to Ueltschi.

Conversely, as end users continue to process more exotic alloys that are increasingly difficult to machine, hard turning is eliminated as an option when a material can only be effectively ground, Ueltschi noted.

Courtesy of Usach Technologies

Hard milling (top) a precision three-lobe tapered polygon removed 1⁄8 ” or more of material per lobe before grinding took off the remaining 0.008 ” per side.

Martin Nobs, engineering manager for Usach Technologies Inc., an Elgin, Ill., grinding machine tool builder, recalled an application where an end user needed to produce a precision three-lobe tapered polygon from a cylinder that justified the combination of hard milling and grinding in one operation. Hard milling removed 1⁄8 ” or more of material per lobe and grinding took off the remaining 0.008 ” per side. In addition, the part required nonround ID grinding. The processing of the male part, using OD hard milling and OD grinding, and the female part, using ID hard milling and ID grinding, is achievable in one machine with a multiple-spindle turret.

According to Nobs, the finish grinding accuracy requirement was the determining factor with a form accuracy tolerance of less than 0.0001 “. Achieving that accuracy for a nonround and tapered part on two machines would have been challenging because the end user would have needed to determine exactly where the zero position was in the rotary axis when reorienting the part on the second machine. “If you machine it in the same setup, you don’t need to worry about it because you know where it is,” Nobs said. “Multitasking made sense in this application, simplifying the overall process by eliminating part reorientation.”

Selecting the Process

On the other hand, Bohle Machine Tools’ Henkel noted that multitasking isn’t always the best route to finish hardened parts. It’s not appropriate when only a single grinding pass is needed to remove a small amount of stock. Another example is if the required cutting speed of 150 to 200 m/min. for hard turning cannot be achieved; then only grinding is appropriate because significantly less rpm is required.

Excessive chip generation can also make multitask hard finishing impractical. If, for example, hard turning produces a large quantity of long chips that might get caught in the part and damage the grinding wheel if not manually removed, then multitasking doesn’t make sense, according to Nobs. “After all, a grinding machine is not a turning machine and vice versa,” he said. “It’s a combination that has to make sense. You don’t want to do it just for the sake of doing it.”

The small chips, or swarf, grinding produces and the grit from the grinding wheel can also cause problems on a multitasking machine, according to Okuma’s Endsley. He noted that, traditionally, a grinder is sealed more completely than a multitasking lathe or machining center, where the swarf and grit can penetrate way covers and wiper systems and cause long-term damage to the machine.

For customers interested in performing grinding, Endsley noted that Okuma can help seal the machine with a graphite electrode machining package because graphite accesses machine locations similar to grinding swarf and grit. “We can seal a machine much better today than even 5 years ago,” he said.

Courtesy of INDEX

Taper grinding is one of the grinding operations end users can perform on the INDEX C200 CNC production turning machine.

Schad countered that grinding swarf isn’t much of an issue in this application because only a small amount of stock is ground. Economan added that fine coolant filtration using paper media to remove particles as small as 5μm ensures that all of the coolant returning to the machine contains only a minute amount of particulate matter. Magnetic chip removal is another option. “The ferrous grinding dust that sinks to the bottom of the coolant in the machine can be removed with a magnetic chip conveyor if that works for a customer’s particular process,” Economan said.

In addition, some multitask machines for hard finishing are initially intended for grinding and sealed accordingly. “As we come from the grinding field, these machines are more designed and engineered as a primary grinding machine where we’re adding turning or milling—where other people are probably coming more from the machining center and turning field and adding grinding capabilities,” said United Grinding’s Ueltschi.