September 7, 2012
Ball-bearing die-set guide components are a critical part of the stamping operation. Properly selected, lubricated, and positioned, they improve tool operation and promote longer component life. If they aren’t properly maintained, excessive press downtime may result.
Die-set guide components must, as their name suggests, guide the tool to the bottom of the press stroke. Properly selected, lubricated, and positioned, they improve tool operation, promote longer component life, and contribute to the production of quality parts. If these components aren’t properly maintained, press technicians will be replacing worn components continually and, as a result, cause excessive downtime.
These days few companies can afford such downtime, and this has made smart use of die-set guiding more important than ever.
Stamping dies use either plain or ball bearing die-set guide components. This article will focus on the ball bearing variety. Components consist of a ball bearing cage that wraps around the outside diameter (OD) central guidepost, which descends into the inside diameter (ID) of a bushing. Traditionally, the guidepost and bushing mount onto plates with precision-bored holes. The guidepost mounts to the upper punch holder, the bushing to the lower die shoe (although surface-mounted guides are arranged differently; see The Basics Behind Surface-mount Assemblies sidebar).
Technicians must take the die’s operating conditions into account to select the proper guidepost, bushing, and cage. Factors such as press speed, shut height, stroke length, workpiece requirements, and operating environment play roles. All these factors and more help determine just how long each guidepost, bushing, and bearing cage should be.
Ball bearing guide components work in a preloaded condition. Preload is a rolling press fit, or negative clearance. When the ball cage bearings are between the guidepost OD and bushing ID, the cage OD is slightly larger than the bushing ID. The amount of preload varies with the manufacturer, ranging anywhere from 0.0001 to 0.0021 inch. Sometimes the amount of preload increases slightly with the guidepost diameter. Also, most die component-makers position cage bearings at a slight angle to ensure they have different areas of contact as they travel vertically along the guidepost OD and bushing ID.
Components can be fully preloaded throughout the stroke; preload-relieved, or partially relieved, at the top of the stroke; or be totally disengaged, with the bearing cage fully disengaged from the bushing at the top of the stroke (see Figure 1).
The fully preloaded condition works well for high-speed, short-stroke applications. In this arrangement, the cage bearings contact the guidepost OD and the bushing ID for the entire stroke. This eliminates pinch points.
The preload-relieved condition also works well with high-speed applications, especially those using medium- or long-stroke dies, but it may not be suitable when the press strokes are more than 120 times a minute. At the top of the stroke, one part of the bearing cage is in contact with the guidepost OD, while another part contacts the bushing ID. Along with eliminating pinch points, the preload-relieved condition makes it less likely that foreign matter will get into the assembly and damage components. Personnel also can remove the punch holder or die holder without removing the die.
The total disengagement condition works well with long press strokes, but the condition isn’t recommended for applications higher than 150 strokes per minute (SPM). The arrangement also introduces a pinch point in the assembly.
Cage creep occurs when the ball bearing cage deviates from its normal end-of-stroke position. This seems to be related not to individual guide components, but instead to the overall die design. Guide components experiencing cage creep sometimes can be removed, installed in another tool, and not experience cage creep. Left alone, though, cage creep can accelerate wear of the guide components.
Cage creep can cause the ball bearing cage to move up the guidepost slightly, but it’s far more common for the cage to creep downward. It is most common when guide components are fully preloaded. When they are partially relieved or fully disengaged at the top of the press stroke, the ball bearing cage can reset itself to its starting position with every stroke.
If technicians believe the ball bearing cage is creeping out of its proper position during the press stroke, but the creep is not visually evident, they can check for some common indicators. First, they can inspect the cage’s set screw dog point—that is, the extended tip at the end of the screw. A damaged dog point on the set screw almost always occurs because of cage creep. The ball bearing cage creeps out of position to the point where the cage set screw is hitting the end of the guidepost slot during the preloaded stroke, which should never happen during designed operating conditions. With the set screw at the end of the guidepost slot, the cage stops traveling (it should be traveling about half the stroke distance), and the preloaded ball bearings no longer can roll as they were designed to do. This means that in the preloaded condition, the bearings skid, steel on steel. As the cage attempts to travel, it sends excessive force to the dog point on the set screw.
Technicians also should check the color of the cage and its bearings. A blue color can indicate cage creep. The blue comes from excessive heat generated by the skidding (not rolling) ball bearings in the preloaded condition during the press stroke.
To correct for cage creep, press technicians have several options. They can increase the diameter of the components and move up to the next-largest-diameter guidepost assembly. They can reduce the preload by modifying the guidepost OD or the bushing ID. Or, they can modify the length of some of the components to create a preload-relieved or fully disengaged operating condition. All these options may work, but there are no guarantees they will. Moreover, many stamping facilities may not have the capabilities to make these adjustments.
The only guaranteed fix for cage creep is to incorporate a positive stop, or bumper, to prevent the ball bearing cage from moving out of position (see Figure 2). Cage creep typically occurs in very small increments per press stroke, so the stop need only bump the cage back slightly to its proper position at the bottom of the stroke. The technician first must determine the correct cage position at the end of the press stroke. He then places a bumper or stop inside the bushing. The positive stop’s ID must be sized appropriately to allow the guideposts to pass through but also stop the cage travel at a determined point. It is best to use urethane that has some give to it, so it will not deform the aluminum ball bearing cage.
For the rare case of upward cage movement, a positive stop can go on the guidepost to prevent any travel upward beyond its designed position at the end of the stroke. Also, if technicians detect upward cage movement, they should confirm that the bushing is properly vented at the bottom. If it isn’t, air pressure can build up excessively as the cage descends into the bushing, pushing the cage up slightly and out of position.
Although rare, cage creep still can occur when die guide components are partially relieved or fully disengaged at the top of the press stroke. This condition has been documented with some servo presses. The press may cycle, accelerate, and change direction very quickly—so fast, in fact, that inertia can cause a ball bearing cage, not preloaded, to travel up and out of position. This places the cage at the incorrect starting position for the next press stroke. To correct this, press personnel may need to adjust the stroke speed or induce some type of drag to the ball bearing cage.
Tracking describes the visible vertical lines on the guidepost OD or bushing ID (see Figure 3). Lines that do not indent the working surfaces are common and should not be of concern. But if the tracking indents the working surface significantly, personnel should take action. Because components operate with negative clearance, the location and straightness of the bored mounting hole are critical to obtain proper guidance. If tracking appears only on one side of the guidepost or bushing, technicians should investigate the bored-hole location for the guidepost or bushing mounting, the bore straightness, and guidepost parallelism.
Side loading can cause excessive tracking, particularly if more tracking lines appear on one side of a four-guidepost die set, with guidepost assemblies in each corner. If workpieces put extreme side loads on a tool, ball bearing guide components may not be the best option, because those excessive loads are being placed on individual bearings, which cause them to dig tracking marks into the guidepost OD or bushing ID during the preloaded condition. Similar workload forces also may produce teardrop indentations on the guidepost OD or bushing ID.
If tracking is a concern, technicians should confirm that the ID dimension of the installed bushing is to specification. Straight-sleeve ball bearing bushings should be installed with a minimal amount of interference; an interference fit should be no more than 0.0005 in. More interference than this results in a bushing ID that’s too small, which in turn causes more preload than the components were designed to handle. This ultimately leads to premature component wear.
To eliminate the concern over tracking, an operation can use demountable-style bushings rather than straight-sleeve bushings. Demountable bushings do not rely on an interference fit for installation. Instead, they are installed with a wring fit, and then held in place with toe clamps on the bushing flange. This produces better holding force and also makes them easier to remove and replace if necessary.
All guide components, as well as wear components in general, require proper lubrication. Guide components may work without lubrication, but this severely reduces their working life.
For ball bearing guide components, grease is not recommended. Because there are negative working clearances between components, grease tends to hold contaminants, which will work themselves into the rolling press fit of the assembly. Grease may also inhibit the bearings from rolling as required. This will generate heat, potentially cause flat spots on the bearings, and contribute to excessive component wear.
Instead of grease, personnel should use a refined mineral oil (viscosity of 290/340 SSU at 100 degrees F) that contains extreme-pressure additives and rust inhibitors. If stamped parts will have at least incidental contact with food, an application may call for a food-grade lubricant. These are labeled “NSF Category H1.”
Some operations use lube pins, which are guideposts that have internal cross-drilled lubrication holes. These usually incorporate a 0.125-in. NPT hole for a fitting at one end of the guidepost. This fitting connects to a system that automatically lubricates guide components at specified intervals.
Properly selecting guiding components can help prolong component life and improve tool operation and parts quality. As always, selecting tooling components early in the design phase has significant cost advantages.
Technicians should investigate and correct any unexpected problems, whether it entails cage creep, excessive tracking, inappropriate lubrication, or anything else. Otherwise, a stamper must frequently replace die components, manage excessive press downtime, and deal with part quality problems. That, of course, is a costly alternative.
In a traditional arrangement, with the guidepost attached to the upper punch holder, the die-set guide assembly relies on gravity. As the ram lifts from bottom dead center, gravity forces the ball bearing cage down to the proper starting point on the guidepost for the beginning of the stroke.
An alternative arrangement—the surface-mount ball bearing assembly—reverses the guidepost position, mounting the holder to the lower die shoe. A spring holds the ball bearing cage in the proper position for the start of the press stroke. This arrangement can be less expensive, because it doesn’t require precision-bored holes in the base plates.
This does, however, give die designers less flexibility. They can specify guidepost diameter and length, but unlike the traditional setup, the bushing and ball bearing cage length are fixed based on the guidepost diameter. Another limiting factor is the required amount of inside die height when the die is closed. Instead of using bored holes in the base plate, the cast bushing holder and guidepost holder mount to the plate surface with bolts or dowel pins. Without enough inside die height, surface-mounted guide assemblies just won’t physically fit in the die space.
The designer can work around this, though. For certain low-inside-die-height conditions, the cast bushing holder can be flipped over so that the casting is placed in a clearance hole in the upper punch holder. Another less common option is to invert the guidepost holder casting and install it in a clearance hole in the lower die shoe. To accomplish this, a technician must remove the guidepost from the holder and reinstall it in the opposite end.
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