Published 12/15/2002

Holding The Center

Using a collet-type workholding system, a high-volume shop improves accuracy when machining round parts.

Workholding

Bill Dundas, Editor

Fig. 2—Studies at the University of Illinois at Urbana-Champaign demonstrate that the damping effect of a workholding fixture reaches its maximum value at a relatively low level of clamping force (left). As additional clamping force is applied, the damping value drops (left) as stiffness increases (right).

To hold round parts for accurate milling on horizontal machining centers, American Turned Products has attached eight hydraulically actuated, collet-type fixture modules to each side of a double-sided tombstone.

Fig. 1—A workholding setup with six contact points arranged as shown is the most efficient method for flat, rigid workpieces.

The same airbag part is also milled on vertical machining centers that use a nine-module setup.

Collet workholding modules can be arranged in various configurations according to the specific application.

In the constant drive for greater metalworking efficiency, two factors have important effects on today's workholding applications. On one hand, as the dimensions of many types of parts have been continually downsized by designers, the demand for high-density fixturing has grown. On the other hand, continual reductions in the levels of human intervention are fundamental to the pursuit of fully-automated processes, as well as being essential for competing in a global economy. This compels U.S. machine shops to reduce the number of manual setups required for various metalworking operations. Efficiency also demands that these setups be completed faster and with less operator fatigue.

A high-density workholding system is usually evaluated first in terms of the numbers and sizes of parts that it can accept in a single setup. In addition, crucial performance factors such as resistance to deflection and avoidance of chatter during machining are taken into consideration. When switching from a conventional workholding fixture to an alternative system, however, improving the positioning accuracy of parts is not usually included among the anticipated benefits.

But better accuracy is exactly the purpose that American Turned Products (Fairview, Pennsylvania) intended to achieve when it installed a new, multi-part workholding system recently. Manufactured by Lexair, Inc., this system comprises modular clusters of hydraulically actuated collet-type closers that may be arranged in a variety of configurations. The company's Fairview plant is currently using this system to hold cylindrical parts that it manufactures for automotive airbag systems.

Geared For High Volume

American Turned Products is a privately owned company that operates two production facilities in the vicinity of Erie, Pennsylvania, and one facility in El Paso, Texas. The firm employs approximately 100 people at its 50,000-square-foot headquarters plant in Fairview. This high-volume plant operates 24 hours per day, 7 days per week.

Recognizing the importance of maintaining a highly skilled workforce, the company was one of the first Pennsylvania manufacturers to develop an extensive training program based on standards developed by the National Institute of Metalworking Skills (NIMS). Working in conjunction with a regional skills center, the company provides various levels of training to enable its employees to attain certification of their skills.

Because it specializes in large-scale production for some prominent industrial customers, American Turned Products certainly doesn't fit the definition of a job shop. Approximately 70 percent of its work is performed for automotive customers, while the balance of the company's business includes producing parts for manufacturers of off-highway equipment, hydraulic valves and pumps.

As its name indicates, the company produces many types of smaller, turned parts. Thus, the Fairview plant's equipment includes Hydromat rotary transfer machines, CNC bar and chucking machines and CNC multi-spindle lathes. But the plant also produces larger parts and performs a substantial amount of milling work. For example, although the airbag component is a round part, it's produced entirely by milling rather than turning. At present, the Fairview plant is using Daewoo horizontal machining centers as well as Fadal and Chiron vertical machining centers to produce these parts.

The Round Part Dilemma

Accurate positioning of round workpieces requires a fundamentally different approach to workholding than that employed for rectangular or polygonal shapes. Because the points where a round workpiece contacts its fixture are quite small, precisely positioning the centerline of the part can be quite difficult. This is true because roundness variations or surface imperfections at the points of contact can skew the centerline location sufficiently to create tolerance conflicts.

Because the plant's new workholding system incorporates collets that distribute holding force around the entire part, positioning is more precise than a conventional type of locating fixture. This is especially important because these airbag components are machined from 9310 extruded steel. In this regard, the Fairview plant's manufacturing engineer, Art Sider, says, "Prior to getting the new workholding system, we used V-shaped fixtures that located off the diameter of the part. With this setup, any variance in the part's extruded diameter would affect the true centerline of the part. With the collet system, we are able to machine each part on center."

To manufacture the airbag components, two setups are required to mill each part on opposite sides with the plant's horizontal machining centers. For this purpose, Lexair provided a workholding setup that attaches the collet modules to double-sided, pivoting tombstone fixtures. This enables the shop to expedite its production flow by rapidly moving parts between successive machining operations.

Mr. Sider has also been impressed by the efficiencies in setups made possible by this new system. "We've experienced about a 50 percent reduction in setup time compared to our previous, manual fixturing method," he says. Designed to hold parts securely when pressure is present in the system, the double-acting collet closers can be quickly released by using a pallet disconnect handle. This eliminates much of the physical strain associated with repeated hand-tightening of conventional workholding fixtures. The system also represents an important advantage in the Fairview plant's high-production environment.

This system may be operated via hydraulic or pneumatic power, depending on the required level of holding force. Although American Turned Products uses a system with a maximum collet capacity of 1.625 inches in diameter, the manufacturer can provide its standard collet closers in capacities ranging from 0.25 inch to 3.5 inches. Step-chuck closers are also available that increase the diameter capacity to as much as 6 inches.

Alternate Applications

Although the company has dedicated the collet system to its machining operations for the airbag parts, Mr. Sider believes that his shop will find additional applications in the future. In particular, he anticipates that this system will be useful in a variety of secondary operations for other types of round parts. At present, however, this system enables American Turned Products to add value to one of its most important parts by providing higher quality for the customer. At the same time that quality improves, the company also removes a significant amount of labor from its overall process. In the modern world of metalworking, these are the types of gains that represent a winning combination.

Workholding Basics

As part of its training program for Kennametal University, Kennametal Inc., instructs students in the fundamentals of workholding setups. The following is a summary of some key principles prepared for this course of study by Kennametal engineers Ruy Frota and T.J. Long:

Three main types of workholding fixtures are used for metalworking operations. General Purpose Fixtures incorporate standard and reusable components. These fixtures are suitable for short run production and prototypes. Modular Fixtures incorporate standard components, but they also have the flexibility to be used for production of varied batches of parts, such as those that are typical of the aerospace industry. Dedicated Fixtures are more complex devices that are designed to hold specific workpieces in mass production applications such as automotive parts manufacturing. In general, dedicated fixtures are more costly to design and fabricate than modular fixtures, and they also are more difficult to assemble and disassemble.

Workholding fixtures typically incorporate various components such as adjustable angle plates, parallels and blocks to restrain various sizes and shapes of workpieces. These components typically attach to sub-plates, base-plates or tombstones via a grid alignment of mounting holes that allows restraint of various parts. In addition, fixtures may incorporate components such as locating pins, V-type locators, round locators, edge locators and rest pads.

Clamps are used to hold parts against the various locating features of the fixture. These clamps may be mechanical, strap- or side-type, lever-type, toggle-type, pneumatic or hydraulic. The proper clamping configuration and sequence for a given workpiece depends on many factors including workpiece stiffness and inherent stresses, number and position of locators and the applied clamping forces. In general, however, clamps and locators should be placed as far apart as practical. Clamps should also be positioned directly over locators or other supports to avoid part distortion under clamping force.

Although increasing the number of contact elements in a particular workholding fixture will reduce the reaction forces during machining, fixtures should not necessarily incorporate redundant locators. This practice can lead to positioning errors due to imperfections on the surface of the workpiece. A good analogy for this circumstance is the well-known fact that a table with three legs will be more stable resting on an irregular surface than a stool with four legs. Thus, one of the most common and efficient workholding setups for flat, rigid workpieces is the 3-2-1 method shown in Figure 1.

To achieve optimum workholding setups, it's necessary to apply sufficient clamping force to withstand the maximum cutting forces without significantly deforming the workpiece. Because applying higher clamping forces does not necessarily provide greater stability, it's important to strike a proper balance between stiffness and damping values. As shown in studies conducted by the University of Illinois at Urbana-Champaign (Figure 2), these two properties are inversely related to the level of clamping force. Thus, if clamping forces are not consistent from one part to another, variations in the damping value can produce differing levels of vibration that might affect surface finish or perhaps generate chatter.

For these reasons, a powered clamping system may provide the features necessary to avoid introducing variations into a particular workholding setup. For example, powered clamps may be actuated in a specific sequence or equipped with measuring devices to accurately indicate the levels of clamping force. These features enable operators to make workholding setups repeatable.