Precision Grinding Often the Best Solution for Challenging Designs

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Parts that require fine surface finishes and extremely close tolerances are candidates for precision grinding, a process that relies heavily on operator skill as well as advanced technology.

"Awesome" is a word that fairly describes the capabilities of precision grinding shops. Those that combine the best of advanced technology and operator skill can consistently turn out parts requiring tolerances in the millionths and finishes as fine as 2 Ra (2 micro inches). Today, their specialty capabilities are increasingly in demand for parts such as gears, bearings, cylinders, and cams.

"Precision grinding is becoming increasingly important for the automotive, aerospace, medical-device, and electronics industries," said Anil Srivastava, manager of manufacturing technology at TechSolve, Inc. (Cincinnati), part of a team of industrial partners working with researchers at Purdue University to develop an "intelligent" system for improving precision grinding processes, according to innovations-report.com. "Grinding is often the final machining process for creating parts that require smooth surfaces and extremely fine tolerances."

While companies require precision, turnaround times are shortening. California Jig Grinding (Pico Rivera, Calif.), for example, turns around most jobs in 48 hours, and M&S Centerless Grinding, Inc. (Hatboro, Penn.) completes jobs in one week or less.

To maintain their competitive edge, many shops are deploying the latest advances in grinding machines and sophisticated abrasives. Wiscon Products, Inc. (Racine, Wis.), for example, specializes in complex form grinding between centers, using three new Junker 2-axis CNC universal angle head grinders. Wiscon also performs plunge centerless grinding with a new Tschudin CNC centerless grinder.

The new machines, having wheel speeds in the range of 50 meters per second, take advantage of new grades of ceramic wheels to attain higher feed rates, more metal removal, and better finishes, says Wiscon's owner, Rolfe Christensen. The machines also include automatic gauging for diameter, roundness, and like measurements. Wiscon gauges off-line as well.

Greater efficiency is also obtainable through a new generation of CNC grinding machines that give companies unprecedented flexibility for processing a wider variety of parts with shorter setup times. Absolute Grinding (Mentor, Ohio) realizes significant time savings with use of a Studer S31 CNC universal grinding machine from United Grinding Technologies (UGT, Miamisburg, Ohio). The firm's personnel can change over, within an hour, from an ID operation to an OD operation on a different part, according to Rob Murnyack, president of Absolute Grinding.

"We can't spend four hours on a setup," says Murnyack. "With other machines, I hear from people out in the field that they might spend four hours just writing the program," says Murnyack. "It takes us just five minutes to write these programs on the Studers."

Equipped with the Fanuc 16i CNC control system, the Studer S31 CNC universal grinding machine features a revolving wheel head that can be swiveled manually or automatically. It is particularly suited for grinding tooling and aerospace parts, especially parts that require close tolerances on angles.

Grinding Specialties

Many shops specialize in one grinding technique, such as jig grinding or centerless grinding, or carry expertise in a skill such as straightening long, narrow parts. Boston Centerless, Inc. (Woburn, Mass.), for example, developed skill in straightening shafts prior to grinding. Steven Tamasi, the firm's CEO, says, "We do a lot of long shafts, such as rollers for the printing industry, and have developed expertise in hand straightening so we can successfully grind them to the finished tolerance."

Grinding shops may also focus on particular part sizes. Deryl Craig, president of California Jig Grinding, says, "Most parts we grind weigh 10 lb or less and can be hand-carried." Most of Wiscon Products' parts are long and narrow, up to 2-1/4 inches by 10 inches long.

"We grind parts that others say cannot be ground," says John Shegda, co-owner, M&S Centerless Grinding. For example, M&S grinds a sesame-seed-size medical part with dumbbell-shaped ends that are 0.030 inch +/- 0.1. "We've kept pretty busy during the recent lean times in manufacturing by pulling tiny intricate jobs from all over the country," Shegda says.

Stema Punch & Die (Cambridge, Ontario, Canada) uses a Studer S32 CNC punch grinder from UGT to rough and finish punch blanks of very hard M2 tool steel. Most of the firm's punches, ranging from 0.125-inch to 3 inches in diameter and from 2 to 6 inches in length, require a tolerance of +/- 0.0005-inch on the OD. Some, however, require an OD tolerance of +/- 0.0003-inch. According to Stema's owner, Stefan Lorbach, the Studer S32 CNC punch grinder is capable of holding a tolerance of +/- 0.0001-inch. Besides allowing Stema to handle 1-inch ground profiles, the punch grinder enables the firm to "easily tackle 2.5-inch to 3-inch profiles for custom punches," says Lorbach.

As often as specializing, grinding shops may offer a full range of grinding techniques. Boston Centerless provides cylindrical grinding in addition to through-feed and in-feed centerless grinding. "This capability allows us to look at the most economical way to grind a part," says Tamasi. "For example, we might not need to cylindrical grind the whole part, but through-feed centerless grind instead, then finish with cylindrical grinding." Boston Centerless often sets up jobs in a cell, operating two grinding machines in tandem to grind a part's ID at the same time as the OD.

Applications

California Jig Grinding, reportedly the largest jig grinding company in the United States, has some 40 jig grinding machines (mostly Moore machines), 12 operators, and 2500 customers across the nation. The company won the contract from the U.S. Air Force as the sole source to recondition the critical front stabilizer on the B1-B bomber. The round, tapered part measures 4 ft x 3 ft x 4 inches thick. The company grinds off scars and galling acquired from wear, sprays on a protective carbide coating, and then grinds the part back to original size. "We developed special fixturing to hold the part so we can grind it properly," says Craig.

Using centerless grinding, Petersen Precision Engineering, LLC (Redwood City, Calif.) is reported to hold diameter tolerances of 0.000020-inch on parts for laboratory instruments. The company's applications also include OD grinding of hyperbolic taper forms for fluid analysis equipment, and the grinding of slots, measuring 3 inches deep and 1/2-inch wide, in Inconel 718 for parts used in aircraft engine mounts. According to Steve Neter, marketing manager, Petersen's recent installation of what he calls a "unique shuttle table creepfeed grinder" permits greater efficiency in grinding high-volume production parts.

"Creepfeed grinding is less stressful than conventional surface grinding," says Neter. "And, most importantly, [it] can be utilized to reduce the manufacturing cost of machined parts."

The company recently helped a customer that was having difficulty with the proper metering of pumped fluid from a precision pump-valve assembly. According to Neter, the customer's supplier had failed to achieve the proper angles and depths of slots that are ground on a center shaft. However, Neter says, once Petersen applied its tooled creepfeed approach to grinding the hardened parts, the company was able to achieve the correct angles and positioning of the slots, thus enabling the pump to function as intended.

Wiscon Products grinds a diameter on needle bearings for a clutch shaft to a tolerance of 0.00003 with a 16 micro–inch finish. The operator locates the part on a specially built mandrel and grinds the OD so that no runout occurs. When the bearings are mounted on the clutch shaft, their accurate dimensions provide quiet operation and long clutch–system life.

Boston Centerless grinds silver shafts that control the keys on flutes. "It's a very difficult part because straightness is critical," says Tamasi. "Any bend or kink and the key won't lie flat on the hole, altering the sound." The rods call on Boston Centerless' skill in straightening and grinding.

Operator Skill

Despite the automation and sophistication of today's grinding technology, operator skill and know–how remains a prime asset at many grinding shops. The high–volume world of double disc grinding "relies 80% on the operator, 20% on the machine," according to Fabricio Fregoso, quality assurance manager at Better Way Grinding, Inc., Santa Fe Springs, California.

"Often you're manually feeding the part, so [you] need to develop a sense, a feel of what is taking place," says Fregoso. "Thus, you can tell if you're working with the right abrasive; you can judge if you need to change out the grinding wheel, add more coolant. CNC doesn't sense whether you need to add coolant, or if you're burning parts, or if your abrasive is the wrong one."

Fregoso says that his shop emphasizes operator training. "You need the proper training to produce a proper part," he affirms. "Techniques here are passed from generation to generation. Two shops can operate the same double–disc grinding equipment, but the training makes the difference."

An engineer turns to double–disc grinding to grind two sides of a part—two parallel faces—simultaneously, with a high degree of flatness or parallelism. For example, a flat part that is bent needs straightening. Or the thickness of thousands of washers needs to be shaved from 0.150–inch to 0.145–inch. Or a burr needs to be removed from a long run of flat–shaped parts. Or brake rotors for racing motorcycles need near perfect flatness and parallelism to avoid brake surging at high speeds.

Double disc grinding has three modes: rotary, reciprocating, and feed–through disc grinding. The feed–through method produces as many as 10,000 parts an hour; runs can be in the millions. Parts, carried between upper and lower rails, are fed horizontally, track–like, between the spinning opposed disc abrasives.

Out–of–Spec Parts

Grinding shops are routinely challenged when receiving machined parts that deviate from spec, or have been knocked out of spec by heat treating. "Machine shops forget about effects like warpage from heat treating before handing parts to us, says Hank Matousek, president, Grind All, Inc., Cleveland, Ohio. "Now we have to find a way to hold the part after heat treating warps or bends it, or after the machinist's chucking distorts it."

Matousek says that parts often arrive other than as quoted by customers. For example, instead of parts in a range between 0.008–inch and 0.012–inch, the actual range comes closer to between 0.005–inch and 0.035–inch.

Although CNC is great for repetitive jobs, Matousek says that his operators must take great care in CNC grinding because the batch might contain oversize parts. "You can't start grinding close to the part, or you risk an oversize one blowing up the wheel," he cautions. "You take the first pass away from the part, then go in a little bit, making a few extra passes until you reach a safe zone."

Advice for Working with a Grinding Shop

After considering these pitfalls, how can a manufacturing engineer work most effectively with a grinding shop? Most grinders advise getting involved early on.

"When talking about fixturing for parts with complex run-out tolerances, early involvement is very important," says Boston Centerless' Tamasi. "Our operators can advise a manufacturer how to lay out a process prior to grinding, so you get a good part at the back end," he continues.

M&S Centerless Grinding's John Shegda advises: "We see a lot of prints with really tight tolerances that don't have to be there. Over–tolerancing adds unnecessary costs to the part at the grinding stage." Shegda suggests that engineers "open up their tolerances, decide what's necessary, and what isn't. Perhaps they can select an easier material to work with, or eliminate certain features of the part."

It's best to head off problems early before final grinding, because, Matousek says, "The customer has a lot at stake when he gives the part to us. He has invested in the material, the engineering, the machine time, the heat treating, and now needs the grinding job done right.

"This is not the time to look for how cheaply it can be done. Someone can grind to a shiny finish, but the part might not be round or straight. We're the insurance company, in that you get a good part from us after all you invested in it."

Intelligent Grinding Process Being Developed

Researchers at Purdue University are working with industry to develop an "intelligent" system that could save U.S. companies $1 billion annually in manufacturing costs by improving precision–grinding processes for parts production, according to innovations–report.com. The system will use artificial–intelligence software, which mimics how people think, in order to learn and adapt to changing conditions. TechSolve Inc., in Cincinnati, is leading the team of industrial partners in a three–year $6 million project funded through the National Institute of Standards and Technology's Advanced Technology Program.

"Precision grinding is currently an art that relies heavily on the experience and knowledge of employees who have been in the business for years," said Yung Shin, a professor of mechanical engineering who is leading the Purdue portion of the research. "The problem is that many factories don't have enough of these very experienced people, so a lot of grinding processes are run under suboptimal conditions. Our system strives to enable relatively inexperienced employees to operate grinding machinery with the same precision as these rare, highly experienced workers."

The intelligent system will use a wealth of data collected by various sensors, as a given part is being ground. It will apply advanced software, such as neural networks and genetic algorithms, to operate specialized CNC grinding machines that cost up to $1 million apiece. Computer numerical control machines are increasingly being equipped with sensors that provide information about the grinding process in real time. The sensors collect information about such details as forces exerted on bearings, speed, vibration, and temperatures during various parts of the process.

"A lot of machines are now coming out with these sensors," Shin said. "The question is, 'what do you do with all of that information?'

"We capture that information in the software to establish a database that will be used to set the machine to optimal operating conditions." Shin has demonstrated that his method works in small–scale applications, but he said it will be a challenge for the team to apply it on a large–scale industrial basis.

"It is high risk because we are going from the lab to full–scale industrial systems," he said. "That sort of endeavor is always difficult because the magnitude of complexity in industry is much greater than in the lab."

This article includes excerpts from a news release from Purdue News Service, posted on innovations–report.com, and additional reporting by Design-2-Part Magazine staff.

Source of information on intelligent grinding process: Purdue News Service: (765) 494-2096; purduenews@purdue.edu Writer: Emil Venere (765) 494-4709

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