Workholding for Cylindrical Grinding

As grinding technology continues to advance, the "black art" of cylindrical grinding is slowly being defined into a science. Advances in wheel technology, temperature control, and precision drive systems have been significant over the last couple of decades. With these advancements, however, the weak links in the chain become clearer.

One possible weak link is workholding. With the exception of craftsmanship and materials, the workholding disciplines of cylindrical grinding haven't changed much in the last 40 years. Following are descriptions of the four basic systems.

Workholding—Centerless

One of the most established and yet least understood methods of cylindrical workholding is in the centerless grinding process.

Primarily known for its use grinding stock into precise rounds, centerless grinding is accomplished by feeding stock between a fixed guide blade and rotating regulating roller and a rotating grinding wheel. The regulating roller controls the rotation of the part. The feed-through action is precipitated by slightly skewing the guide blade axis in relation to the regulating roller, causing the regulating roller to pull the part across the face of the grinding wheel. The workpiece is held against the guide blade and roller by gravity.

Traversing the wheel toward or away from the guide blade and regulating roller controls diameters precisely. Once the high spots are ground on a round, the diameter can be reduced precisely to the desired range.

The process is relatively simple to set up, and although the parts may need to be fed through the grinder multiple times to achieve the roundness required, it is very economical, especially as volumes increase.

A variation to the process can generate profiles in the part. Instead of feeding the workpiece through the grinder, the part is located against a stop and a specially dressed wheel is traversed into the part, crush-grinding the profile on the workpiece.

This process generally is used for high volumes, as the grinding wheels used are large and expensive, and custom dressing them is time-consuming. Additionally, to achieve the roundness that is generally required, the stock needs to be centerless-ground before crush-grinding.

Workholding—Between Centers

The most widely used workholding method for cylindrical grinding is done between centers. Between-centers grinding does not require a centerless ground workpiece. In fact, many agree that the only way to create a perfectly round part from raw stock, in one setup, is to grind between centers. Many tool designers start with this methodology in mind.

The reason for its popularity is the stability of the tooling. Once the tool centers are indicated to the grinding wheel and its axis, this method is the closest to hard tooling, in which the workpiece stays stationary in a clamp fixture. Although the workpiece is rotating, the centers remain in solid contact with the center points on the workpiece.

Centers are drilled in the end of the workpiece, and hardened, precision center points position the centerline of the part in space. The workpiece centers ride, surface to surface, on the stationary head- and tailstock centers, establishing one, unchanging centerline for the part.

There are limitations on length and diameters, of course, as smaller diameters on longer parts can create deflection, making diametrical accuracy, roundness, and finish more difficult to obtain. However, accessories, such as a steady rest, can be used to minimize deflection in these cases. Also, as diameters get smaller, the possibility of using center points on the ends of the part decreases.

Care needs to be taken that the center drills match the taper on the machine centers.

Also, an ongoing effort needs to be made to ensure that the centers, on the workpiece and the grinder, are meticulously cleaned and smooth when loading each part. The slightest bit of contaminant or swarf in the coolant can change the centerline of the part and generate a taper. The effects are even more pronounced if the centers are contaminated during a regrind, causing runout and ovality.

Profiles can be ground using a CNC profile grinder, or custom wheels can be dressed to crush-grind parts much faster.

There are many tools and shapes, however, that cannot be created between centers. Complex end features, for instance, eliminate the possibility of drilling centers to suspend the workpiece. While it may be possible to turn most of the part using centers, and then cutting off and prepping the end in a secondary operation, this method is more costly and introduces the possibility of runout or other inaccuracies that are inherent with secondary setups.

Collets and Chucks

Collets and chucks are used for grinding workpieces that are either too small for center points or require one end to be featured or profiled.

Chuck and collet systems are dynamic workholding systems. Unlike centerless and between-centers setups, where the setup is stationary, the workpiece moves with the chuck or the collet. This means in addition to any inaccuracies they suffer, the quality, wear, and balance of the drive system is critical to maintaining the overall stability of workpiece holding.

In the case of the chuck and collet, blanks have to be preground using centerless grinding.

The more accurate of the two methods uses a four-jaw chuck. Each workpiece then can be indicated to precisely align the centerline of the part with the centerline of the chuck and spindle assembly. An indicator can be slid into position along the length of the blank, and any initial runout can be tuned out using the adjustments in the individual jaws of the chuck.

The time it takes to indicate and adjust in each part, however, can be excessive.

Collet systems are much faster to load and unload. They consist of a tapered barrel and an expandable collet insert that collapses on the workpiece diametrically, as the insert is pulled into the tapered barrel.

Setup and indicating the collet system are critical. Indicating and aligning the collet's tapered barrel to the centerline of the work head are essential. Once that is complete, the operator must consider the accuracy of the collet system itself.

Ultraprecision collets claim an accuracy of 4 microns when they are new. Some even claim 2 microns. For some jobs, 2 microns may be permissible, and the compromise in accuracy is accepted to save the time it takes to load the machine.

Two microns (almost 0.00008 in.), however, may be unacceptable in cases in which manufacturing machine tools will be used to create very precise features. This is especially true for smaller tools. Any inaccuracy in the collet moves the centerline of the shank of the tool, resulting in the ground features adopting a taper, at best, but more likely runout on all ground or machined features.

Add to this the possibility of swarf contamination, or any wear or compliance in the grinding wheel spindle or the collet/chuck bearing assemblies, and even the most precise software and drive system could be completely compromised.

While each part can be tweaked using the infamous "brass hammer" method, there is no precise, controllable way to indicate a part when using a collet system.

Some tool grinder manufacturers that have recognized the weaknesses of collet systems in demanding dimensional applications have begun offering stationary V-block fixtures that ride under the part, outside the collet, in an effort to correct collet inaccuracies. They effectively deflect the part toward theoretical center, but add to the setup process, and do not account for significant blank size variations.

Workholding—Perimetric

The most recent advancement to come along is perimetric workholding. It is similar to that in centerless grinding, but with some significant differences. Like collet and chuck systems, perimetric workholding systems require centerless ground blanks.

Like in centerless grinding, perimetric workholding employs a regulating roller to drive the workpiece and a guide blade, but the perimetric method precisely locates and qualifies the guide blade and roller to the grinding wheel, and employs a tension roller that ensures solid contact of the workpiece to both. In addition, the grinding wheel moves along an additional axis, relative to the workpiece. In traditional centerless grinding, only one axis is available.

Stops can be located at either end of the part, and a slight skew of the guide blade or tension roller can bias the workpiece against the stop during the grind.

The basic method has been around since the early 1960s, but the term perimetric was only recently coined by Tru Tech Systems Inc., a grinder manufacturer in Mount Clemens, Mich.

"We discovered there was a broad misunderstanding of how the system functioned," said Tru Tech Systems Vice President of Operations Steve Smarsh Jr. "There's a substantial difference between this workholding system and that of centerless grinding setups, and we sensed a need to delineate the two to avoid confusion. We've highly refined the method over the last 20 years, but some still confuse it with centerless grinding. People would like to say they understand, but they really didn't grasp why the setup is so accurate."

Using the grinding wheel and its traverse axis to dress the regulating roller and guide blade in position eliminates all setup misalignments, according to Smarsh. The workpiece axis runs true to the grinding wheel every time. The system even negates most of the effects of contamination.

Although efforts always should be made to keep the setup clean during grinding, contamination is less of a concern in a perimetric system. This is because perimetric workholding is a dynamic system.

In a static holding system, such as a chuck and collet system in which the workpiece is clamped, any contaminants trapped between the jaw/collet and the workpiece will skew the part during the entire grinding process, causing either taper or runout, relative to the shank.

In a perimetric system, however, the workpiece is dynamically moving against the guide components, and all are continually flushed with coolant to keep them free of contamination.

In the event that a foreign particle enters the system, however, the hardened work blade is running tangential to the grinding wheel, so any momentary contamination between it and the workpiece will not affect dimensional accuracy.

The porous surface of the regulating roller largely negates the workpiece movement that could be caused if a contaminant should pass beneath the workpiece before it is flushed away by the coolant flow.

"It's very simple, flexible, and quick to set up," Smarsh said. "Over the years we've added software and hardware technology to interpolate profiles with standard wheels, compensate for wheel wear, blank size variations, and temperature swings, but the real secret is in the perimetric workholding system."

The perimetric system has some length and diameter limitations, and it is not necessarily suitable for heavy grinding operations such as fluting. The dimensional results that it yields are impressive, though, allowing submicron specifications and strict control of parallelism, tapers, back-tapers, as well as complex shapes and configurations.

Perhaps one of the most significant features of this method is its repeatability during part loading and reloading. Because the part is rolling precisely on the shank of the workpiece, it can be ground, removed, inspected, and then reloaded without indicating. Runout is virtually impossible to introduce when loading a part, making the method ideal for regrinding used tools.

Though the workholding method depends largely on the size and type of the part being ground, the number of parts, and the accuracy demands of the design, workholding is the underpinning of the cylindrical grinding process.

Understanding how workholding methods work, how they locate the part, and how much labor is involved to use them is key to producing parts within the required tolerance and finish and at the cost necessary to be competitive.