Higher Quality, Reduced Defects Via Remanufactured Grinders

This off-highway vehicle manufacturer's grinders were difficult and time consuming to set up, and holding tight tolerances had increasingly become a 'hit or miss' proposition.

Case Study From: 5/15/2000 Modern Machine Shop

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Piston body

“The piston body,” Mr. Petersen says, “has to be ground to very precise tolerances. The piston in these wobble plate hydraulic pumps must withstand up to 8,000 psi, so the clearance between the piston and the bore has to be extremely accurate.”.

Two remanufactured grinders

“One of the things you’ve got to keep in mind,” Mr. Petersen says, “is that while the two remanufactured grinders don’t completely finish the parts—they’re used primarily in a progressive roughing application—they are absolutely critical and a key to the success of the operation. If they don’t hold tolerance on diameter or roundness, the finishing operation will not be able to correct it.”

Dave Petersen, product improvement coordinator for SauerSunstrand (Freeport, Illinois), a manufacturer of axial piston pumps and motors for off-highway vehicles—backhoes, tractors and road building equipment—found himself at a crossroads. His manufacturing process—three vintage grinders circa the early 1970s—just wasn’t doing the job. The grinders were difficult and time consuming to set up, and holding tight tolerances had increasingly become a “hit or miss” proposition.

“The piston body,” Mr. Petersen says, “has to be ground to very precise tolerances. The piston in these wobble plate hydraulic pumps must withstand up to 8,000 psi, so the clearance between the piston and the bore has to be extremely accurate. If the piston body is undersize, oil leaks between the piston and the bore can develop; if it’s oversize, the piston can seize up in the bore, neither of which is acceptable and can lead to failures in the field.”

One of the Mr. Petersen’s alternative choices was an obvious one: Replace the old grinders with new ones. But Mr. Petersen wanted more than just assured close tolerance grinding. In addition, he wanted inprocess gaging systems that would do 100 percent part inspection with feedback that would ensure process control and verification—in real time.

After some thorough research, Mr. Petersen recognized that even if he bought new grinders he’d still have to identify a third party to provide the process control, part gaging and inspection he wanted. “We wanted a turnkey supplier,” he says. “We didn’t want to buy a grinder from one source and the gaging equipment from another. In those instances, when something doesn’t go right, you’ve got the grinder manufacturer pointing at the gaging equipment and the gaging supplier pointing at the grinder, and nothing but frustration results.”

Enter AirTronics (Elgin, Illinois), manufacturer of gaging and control systems, as well as compensation and other systems for centerless grinders. AirTronics has built a global reputation for quality gaging systems for 35 years.

While the AirTronics name was well known in gaging and control systems, the company expanded to include the remanufacturing of centerless grinders, primarily Cincinnati machines, for much the same reason that Mr. Petersen wanted a “turnkey” supplier. The AirTronics strategy was to adapt its gaging systems to an AirTronics remanufactured grinder and, thus, offer a complete turnkey package.

Mr. Petersen and Jack Yoder, senior manufacturing engineer for SauerSunstrand, met with AirTronics engineers to tailor a grinding system specifically for their piston grinding needs. Two remanufactured Cincinnati 34020 Twin Grip centerless grinders, adapted with AirTronics precision Inchworm compensators arranged for tandem operation, were the result. They are also equipped with AirTronics gaging and control systems.

“We chose AirTronics not only because they were a onestop, turnkey supplier,” says Mr. Peterson, “but because they assured us that their grinders would meet or exceed new machine specifications and tolerances and wrote right into their performance proposal that their processes would produce parts to our tolerances with 100 percent part inspection.”

According to Mr. Petersen, the two remanufactured Cincinnatis are linked in tandem and then further linked to a third lapping machine for final finishing operations. “One of the things you’ve got to keep in mind,” Mr. Petersen says, “is that while the two remanufactured grinders don’t completely finish the parts—they’re used primarily in a progressive roughing application—they are absolutely critical and a key to the success of the operation. If they don’t hold tolerance on diameter or roundness, the finishing operation will not be able to correct it.”

The gaging systems, which consist of a computerized XL2001 Qualifier, Redliner control and Inchworm compensators, constantly monitor part size variations and can make corrections in increments down to 0.000010 inch to the regulating wheel slides through precise forward and reverse stepping motion of the compensator. When a size trend is detected, corrections are made before tolerance limits are exceeded. The compensator utilizes a backlash free magnetostrictive drive in which an armature, made of a special alloy, exhibits the property of increased length when surrounded by an electromagnet field. The compensator harnesses magnetostriction, along with a unique clamping action, to provide forward or reverse linear stepping motion. Stepping distance is adjusted by varying the strength of the electromagnetic field with specially designed electronic controls. Thus, extremely small, highly controlled motion can be obtained without backlash.

“Up to 0.008 inch of stock is removed in the first grinding operation,” Mr. Petersen says. “Each exiting part is monitored so that wheel wear can be automatically compensated, maintaining a tolerance of ±0.0003 inch.”
In the second operation, approximately 0.001 inch additional stock is removed from the pistons. “The parts exiting the second grinder pass through the XL2001 gaging and control system,” says Mr. Petersen.

In essence, Mr. Petersen explains, the XL2001 gaging system is a carbide contact gage. As the part passes through the gage, size variations are monitored at a rate of 1,000 to 4,000 readings per second. These readings are then entered into proprietary algorithms in the XL2001 control. The resultant readings are then displayed as a size. Based on the size, parts are segregated into good parts or rejects. These part sizes are then used to automatically calculate a trend.

Customerselectable control limits can then be entered into the system. In the event a limit is exceeded—perhaps due to a wheel breakdown or a drastic size change—not only does the system automatically compensate for the change, but the part is actually kicked out of the mainstream of parts via a pneumatically or hydraulically powered paddle that shuttles out of tolerance parts in one direction, intolerance parts in another.

Mr. Petersen points out that also important is that the XL2001 calculates, displays and, via a parallel printer, prints statistical data as well as histograms, X-, R- and Sigma charts, and through an RS232C interface can transport data to Sunstrand’s SPC system for process verification for use in part shipment.

“This closedloop grinding process cost us about 20 percent less than buying new equipment,” Mr. Petersen says. “These grinders produce parts to our specified tolerance or better. Our roundness tolerance, for example, is better than was quoted by AirTronics.” MMS

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