Selecting The Right Workholding Solution

These tips help solve turning, grinding and milling workholding problems, particularly for thin wall parts.

Article From: 11/30/2007 Production Machining, ,

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Mechanical solutions for thin-wall parts

Mechanical solutions for thin-wall parts spread the clamping forces over the largest possible area of the workpiece for rigid, accurate and repeatable results.

Workholder for family of parts

Some workholding systems are designed for adaptability with families of parts. This example shows a basic machine connection (left) and an adaptable chuck (right).

Hydraulic workholder for thin wall parts

The hydraulically-actuated system holds thin-wall workpieces for face milling operations.

Mechanical workholder for grinding

This mechanical workholding system for grinding operations is designed for added longevity, accuracy and reliability.

With manufacturing operations using higher cutting speeds that place greater force on workholding equipment, choosing the correct system to hold on to the work is trickier than ever. Careful examination of the specific clamping application may lead to either a ready-made solution or a customized workholding answer.

One vendor, Emuge Corp. (West Boylston, Massachusetts), has a workholding division that provides tailored solutions to troublesome workpieces and manufacturing. This division specializes in fixtures that demand high accuracy and repeatability across a broad market, including low-volume job shops through high-volume automotive production environments.

Mr. Dave Jones, Emuge precision workholding manager, explains, “Our workholding group stays close to our customers to learn about their unique challenges and production environments. Doing so helps us avoid potential miscommunication and minimizes additional project costs.”

Mr. Jones and his team offer the following tips for obtaining a unique clamping solution for specific situations.

Difficult Workpiece Considerations

Most situations requiring a custom workholding solution involve a difficult workpiece and/or operating environment. For instance, one challenge is holding very thin-wall parts while performing operations with high transferable torque values. In these operations, the workpiece can have a tendency to deform. Emuge’s System SG (based on a series of short tapers) spreads the clamping forces over the largest possible area of the workpiece. The result is a very rigid, accurate and repeatable workholding solution for these types of difficult workpieces. This workholding system is commonly used for hobbing, shaping, shaving, turning, grinding and inspection applications.

Ensure Accuracy

Not only should the clamping system be precise, but several other related areas of the operation should also be reviewed to ensure the best possible results. The production environment and tooling must be suitable for the application. Just because it worked in the past doesn’t mean it is still effective. The accuracy of the locating and clamping surfaces on the workpiece should also be considered. Further, a clean workpiece yields the best end product. When debris builds up in the clamping area of a workpiece, it may affect how the piece is being held, and negatively affect the outcome of the entire operation.

The volume of work to be performed on the workholding devices should also be considered. In some industries a high volume can be 10,000 parts per month or less. In other industries, 50,000-plus parts per month would be high volume. According to Mr. Jones, the relationship of these quantities to workholding is important. “The workholding device must maintain its accuracy throughout the volume of workpieces required with as little maintenance as possible,” he says. “This is when you start to talk about longevity and reliability of the workholding device itself. Generally speaking, wherever possible, Emuge offers mechanical workholding solutions as opposed to hydraulic solutions, to provide that extra longevity, accuracy and reliability.”

There are, however, instances where the workpiece has a challenging bore-to-length ratio. An example would be a small-diameter bore with an overall part length, which is 2× the bore diameter. In these instances, a hydraulic system may be the correct alternative because of the workpiece geometry and perhaps the lack of spacing required for a mechanical solution. The trade-off in precision workholding with a hydraulic device, especially in the case of higher volume work, is that the hydraulic sleeve or clamping surface is, in most cases, integral to the device itself. This means that if the sleeve wears or develops a leak, then the majority of, if not the entire workholding device, needs to be replaced. In a mechanical system, however, the part-touching details would be considered easily replaceable in the event of failure.

While hydraulic arbors may be a natural choice for applications where extremely high accuracy such as 2-micron runout may be required, a mechanical solution could be more effective in cases where the volume is considered to be high and the desire to maintain mechanical reliability is a key factor. According to Mr. Jones, “The System SP is our most accurate system. This system is capable of a runout of 2 micron, similar to a hydraulic arbor, yet touting the reliability of a mechanical system. The system can also be used in other high-accuracy areas such as toolholding, grinding or workpiece inspection.”

Planning Ahead

The clamping solution of choice should be one that not only is effective now, but will suit the needs of the shop well into the future. When the vendor is designing the fixture, the shop should convey what its needs will be down the line as well. Perhaps the workpiece is one of many in a series or part family, which could mean that relatively minor changes would have to be made to the workholding device to accommodate the new workpieces.

Emuge’s workholding systems are designed to adapt to workpiece changes within a range. In some cases, a change in diameter would require only a new clamping sleeve or expanding bush that is designed to match the new clamping diameter of the new workpiece in the family. This adaptability within a range has advantages in quick change-over, where time is money, but can also reduce the number of basic assemblies required to manufacture a complete part family line.

Selecting the best workholding system for a given application ultimately leads to a better end product. By adhering to sound principles in choosing the workholding solution, and by maintaining a good and open relationship with the vendor, a shop can meet the challenges of today’s demanding production environments.

Establish Open Communications

The vendor should have an open ear beginning early on in the quotation process. It is critical that the vendor understands the requirements for the entire operation. Although not always immediately evident, some of this information may prove to be essential to the success of the workholding system.

The vendor must have complete operation specifics and then must remain informed of any changes to these specs, even if the modifications do not appear to be workholding related.

Mr. Jones explains further, “When we stay close to our customer and keep a steady flow of communication, we are usually kept informed without even asking. However, even when we are receiving timely updates, changes to the original specs may lead to some added cost or a delay in project delivery because the design or build process may be too far along.

“In most cases our designs allow us to react and adapt quickly to minimize the impact of unforeseen changes. There was a time when a customer didn’t tell us about a special clearance problem and another time when a customer changed the specs on a grinding wheel design. These unexpected variables really put the flexibility of the workholding devices to the test.”

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