The Case for Grinding Perfection

QUICKPOINT grinding machines from JUNKER allow end users to streamline production and increase productivity.

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In The French company ZF Bouthéon has a long history as a manufacturer of truck transmissions. The company has been producing complete drive assemblies since 1970, when it was still Berliet. In 1998 it was taken over by ZF Friedrichshafen and now supplies big-name truck manufacturers all over the world, including MAN, DAF, and the Volvo Group. To boost its productivity, the company relies on a QUICKPOINT 5000 from Erwin Junker Maschinenfabrik GmbH.

The project specifications: grind the OD, as well as the inner taper on drive shafts, in one clamping setup with the highest possible process stability and including automation. The project manager at ZF Bouthéon explained the initial situation: "Before, we’d been grinding the outer diameter with CBN on one machine and the inner taper with corundum on another machine."

The decision to use JUNKER did not come out of the blue. The company had already acquired a QUICKPOINT 5000/40 S in 2001 and is more than satisfied with its flawless operation. To ZF, one important criterion for investment was the high rate of availability for which JUNKER machines are known and prized in the industry. This led the company to decide on a QUICKPOINT 5000/60S. This machine is now operating at the plant and is changed over once a day. The machine’s extreme flexibility and fast changeover times (max. 60 minutes) allow it to grind several different types of drive shafts. Based on their excellent experience thus far, ZF once again decided on a QUICKPOINT.

This grinding method, developed by JUNKER, is particularly well established on the market for automotive component grinding. One of its advantages is the low grinding forces involved. The point-sized grinding contact area is achieved by using a four-millimeter wide, wear-resistant CBN or diamond grinding wheel. Inclining the grinding wheel axis toward the horizontal workpiece axis creates a clearance angle, reducing the contact line to a contact point. This reduces the grinding forces. QUICKPOINT can be used to grind any geometrical shape, such as shoulders, tapers, chamfers, or radii. QUICKPOINT OD grinding machines can be expanded for complete grinding by integrating modified JUNKER grinding modules with several grinding wheels. This way the machine can be enabled to perform such tasks as plunge grinding, thread grinding, shape grinding, and non-cylindrical grinding of components in just one clamping setup. The QUICKPOINT series is modularly constructed, which means there’s a version available for just about every imaginable application.

Different Operations
The 5000/60S version used by ZF Bouthéon is particularly well adapted for grinding large parts weighing up to 154.32 lbs. and measuring up to 47.24 inches in length. The wheelhead includes up to three grinding spindles and can be swiveled along the B axis, allowing for different operations. This means that a complex family of parts can be completely ground on just one machine. One added advantage is the automated part loading/unloading system, which Junker has already integrated in the design of the new machine. The project leader is happy with the improvement in quality. The quality values for roundness and surface quality are now at an unusually high level.

Operator Advantages
The JUNKER machines have a modular and nearly identical structure, which results in advantages for the operating staff. Training on the QUICKPOINT 5000/60 takes just five days. The operating platform—a solution developed in-house by JUNKER—is known as the Operator Panel, or EJ-OP for short. This menu features graphics and interactive programming operations, allowing for identical operating processes on the machines regardless of the controls used. This makes all components accessible for teleservice. JUNKER offers its own special brand of service: the JUNKER Service Center is available to its customers around the clock, 365 days a year. It has its own hotline and, whenever necessary, all the available data related to the condition of the machine can be analyzed online. Figure 1

Another feature that ZF Bouthéon holds highly is the long lifetime of the CBN grinding wheels, which are cooled with oil at high grinding speeds. The coolant medium is treated by the machine's own purification plant with extraction and filter systems. Safety has high priority for JUNKER. That’s why all JUNKER machines are equipped with a special safety concept: in the event of an explosion, the machine interior is automatically sealed off so that any potential fire is smothered before it can start. The project leader at ZF sums up the advantages. “This investment has saved us the purchase of three machines. With the QUICKPOINT high-speed method, everything now gets done in one clamping setup, which cuts the cycle time in half.”

CBN Technology
Roller bearings are used in a variety of industries: for example, in interfaces of dynamic and static generator components of wind energy systems, in winches for anchors and cordage in shipbuilding, or as electrically insulated bearings in electric motors.

New application areas are added constantly, leading to the development of more diverse variants of roller bearings. Nearly all applications require highly precise grinding in the bearing production process and tolerances are getting tighter for quality characteristics such as precision, longevity, and low friction. Bearing manufacturers now face new demands, and most of all they need machine concepts that will allow them to grind a wide range of parts quickly, reliably, and with minimal manpower.

Pinpoint Precision
The fact that JUNKER Nordrach has put together an attractive and extremely economical package in this area has been recently demonstrated by a successfully completed project. The specifications described the following task: to grind the OD profiles and faces of various bearing elements in the range of 1.57 to 8.66’’ in length and 1.97-6.96’’ in diameter. The rate of availability needed to be over 95 percent. In addition, the machine also had to grind radii on the plane surfaces of the faces and provide a reliable process for grinding asymmetrical barrel rollers.

This project was to replace an already existing process. Until now, the OD profile had to first be ground on Machine 1 using corundum plunge grinding wheels. Then, Machine 2 would grind the face using double disc grinding wheels. Today, a single QUICKPOINT 5000/50S uses two narrow CBN wheels to complete all the grinding, including the radii on the faces, in just one clamping setup. Compared to the previous method, the JUNKER grinding method gets far more accurate results on the face end profile. The advantage: the rollers run much more quietly and last much longer. To grind the OD profile, a B-axis with a hydrostatic bearing was installed for the first time on this model. The small wheel contact area significantly reduces grinding forces, resulting in a shorter warm-up cycle and higher part quality.

Automated Gauging
The wheelhead on a QUICKPOINT can accommodate up to three grinding wheels. For this special solution, though, only two wheels were used, and with good reason. Wheel 1 carries out rough grinding with stock removal capacities of up to 0.04’’, using the peel grinding method, while Wheel 2 takes care of the finish grinding. Installed on the third spindle is a newly developed piece of technology from JUNKER: A fully automated in-process gauging system that monitors the outside diameter and profile shape. This completely eliminates the need for corrections after grinding, and the machine produces only good parts. In addition, the machine features a length-positioning device and a diameter gauge, both mounted on the grinding table. The warm-up cycle, which is already brief thanks to minimal grinding contact, can also be constantly monitored. Figure 2

Other advantages of the QUICKPOINT include low tool costs thanks to long tool lives, low changeover costs since no carrier is required, and fast and accurate center-point changeovers thanks to the JUNKER three-point interface, as well as an operation interface adapted to fit the needs of the customer. Some examples of this are the graphic representation of interactive operations used for programming and the individual parameter input masks. Thanks to these masks it is no longer necessary to write a separate program for each type of bearing. Geometrical specifications for various roller bearings are stored on a “workpiece page” while information such as infeed speeds, spindle RPM, and stock removal is stored on a “technology page.”

When it comes to ranges of smaller sized parts, as an economical alternative to the QUICKPOINT 5000 JUNKER offers the compact QUICKPOINT 1000 with a clamping length of up to 3.94’’.

Maximum Flexibility
Despite all technical refinements, what really counts in the end is the result. For a barrel roller 4.72’’ in length and 2.76’’ in diameter, the final grinding time, according to the protocol, is under two minutes, including measuring. The high demands for dimensional accuracy are attained with roughness values of under 1µm on the OD profile and faces. Other process-related specifications such as roundness, profile deviation, axial run-out, diameter, and length tolerance are also met with low, single-digit micron results.

And all talk of roller bearings aside, it stands to mention that QUICKPOINT machines can actually grind any geometrical shape, such as shoulders, tapers, chamfers, or radii. Depending on the configuration and number of grinding wheels, these machines also handle such tasks as plunge grinding, thread grinding, shape grinding, and non-cylindrical grinding of components in just one clamping setup. The QUICKPOINT’s modular design allows for reliable, customized variations to suit nearly any possible customer need.