Fast and Flexible Workholding
Hold it right to machine it right.
By Robert B. Aronson
Workholding systems have to accommodate the newer demands for high-speed, lower-volume operation. Here's a look at how some manufacturers are meeting this challenge.
There are two aspects to the need for high speed. First is high-speed production. Costly multifunction multiaxis machines have to keep operating to be cost effective, so the ability to rapidly position a workpiece is critical.
High speed in the sense of high-speed machining often means greater loads on the part during the machining processes. Workholders have to be able to maintain precise part positions under the increased pressure without damaging the part.
Robots have become a common addition to workholding systems.
Tennessee Rand (Chattanooga, TN) specializes in fixtures for parts that are welded by robots. Automotive Suppliers such as Johnson Controls, Lear Corp. and Magna are the major customers. Customers outside the automotive industry include Harley Davidson and Yamaha. The company designs and produces fixtures which chiefly use pneumatic clamping. The parts are manually fitted into the pneumatically actuated fixtures and the robots MIG weld or spot weld the components together. Typical production parts are steel motorcycle frames and fuel tanks, automotive seat frames, and instrument panels. Positioning accuracy is typically ±1 mm.
| Pull-Down Chucks Used for Milling
In this application, a twin-pallet VMC with six chucks per pallet is used to make military missile parts. Formerly, these parts were produced in a single-pallet machine using standard vises. But there were problems:
Being round parts they could not be held well in a standard vise. When form jaws were tried they caused part distortion and out-of-round problems, so many parts were rejected.
The vises had no active pull-down, which caused considerable vibration. This reduced tool life and caused poor surface finish. In addition, the lack of an active pull-down led to parallelism problems on some work surfaces.
The large chip volume had to be periodically cleaned out, causing unwanted downtime.
Because the vises operated manually, time was lost opening and closing them.
These problems were resolved when the manufacturer switched to the Autoblok Model TS pull-down chucks from SMW. These chucks provide several advantages:
- The jaws pivot to seat on the part allowing six-point contact on each workpiece. This eliminated the roundness problems.
- An active pull-down seats the part on a hard-stop locator. This solved the problem of vibration and resulted in increased tool life. It also eliminated the parallelism issue.
- Because the chucks are completely sealed daily, maintenance and lubrication are not needed, and neither fluids nor chips can enter the chuck body. While it is necessary to blow the chips off the jaws and locating surfaces, there is no downtime for cleaning.
- An air cylinder operates the chucks simultaneously, eliminating the problem of operating each chuck independently, and considerably shortening the load/unload time.
The SMW Autoblok TS pull-down chucks can be applied both as turning chucks or static fixtures for a machining center.
One-off suppliers. A group that works on specific problems, more of a design and build concept instead of supplying an off-the-shelf system, is the workholding division of Emuge Corp. (Northborough, MA)
"Our specialty is fixtures that require the highest accuracy and repeatability, and our market encompasses everything from low-volume job shops to high-volume automotive production environments," explains the company's Dave Jones. "We are a niche operation specializing in problem workpieces and operations, often replacing original equipment that simply does not meet the requirements of the customer. Our philosophy is to be close to our customer from early on in the quotation process right through delivery and beyond. This relationship helps us design a device which is not only specific to the workpiece and operations being performed, but additional considerations may be uncovered that prove integral to the solution for a problem workpiece.
"Our systems are suited for these problem-issue environments and workpieces, such as holding very thin-wall parts where aggressive operations are being performed. For example, our System SG, which is based on a series of short tapers, spreads the clamping forces over the largest possible area of the workpiece. This allows us to use as much of the available clamping area as possible, resulting in a very rigid, accurate and repeatable clamping solution for some of these more difficult workpieces. Typical applications for our System SG include hobbing, shaping, shaving, turning, grinding and inspection.
"For the best possible solutions, we encourage customers to do two things. First, supply us the complete operation specifications. Second, if there is any deviation from these specifications, it is important that all parties be informed. In one case a customer neglected to inform us of special clearance problems that arose, in another, specifications on a grinding wheel design were changed. Luckily, our devices allowed us to adapt to these changes, and minimize any added cost and delay to the project.
"System accuracy is hard to comment on because the answer depends on many factors that are out of the clamping supplier's control, including operating conditions and tooling. Additional variables include the accuracy of the locating and clamping surfaces on the workpiece itself. Cleanliness is very important. A simple build up of debris in the clamping area of a workpiece may effect how a workpiece is held, and ultimately the results of the operation being performed may be compromised.
"Other considerations are the projected volume that will take place on the workholding device. In environments with a high volume it is always best to use a mechanical workholding solution as opposed to say a hydraulic solution. Certainly some workpieces, specifically workpieces with a challenging bore-to-length ratio may mandate the use of a hydraulic system. Our approach in higher volume production is to use our System SP design. It is our most accurate system and can compete directly with a hydraulic solution. is also used in areas that require added accuracy such as tool holding, grinding or workpiece inspection to name a few.
"As for trends, we see a move toward higher cutting speeds, and consequently, higher cutting forces. This certainly relates directly to the adage that time is money, and that everyone wants to get the most out of their newer faster machine tools. This time-is-money adage really became apparent a few years back in our cutting tool lines, and now we are seeing the same in the areas that our precision workholding line runs in."
Established fixture elements, such as strap clamps, are experiencing a rather flat demand," says Greg Arnold, Jergens Inc. (Cleveland). "This is because these are mature, basic products, easy-to-make products, and so can be easily produced overseas. At the same time, the move to lean, high-speed production has stimulated a demand for more complex fixturing systems.
"Because of market demands, we have become more of a solution provider instead of a component supplier. More of our effort is in helping a customer solve specific problems.
"We are now looking at products more related to lean manufacturing. For example, we are expanding our line of precision dual-station vises to meet the need for automated fixturing. Advanced clamping is also a focus for us.
"Our established Ball-Lock mounting system is being incorporated into fixture plates and subplates for quick changeover.
| Magnetic "Legos" Replace Conventional Fixtures
Magnetic hold-downs for iron and steel parts have been used for decades. But researchers at the GM Research and Development Center have put magnets to a unique use in a fixturing system that they expect will save millions of dollar in their machining operations for aluminum engines and transmissions.
The unit looks like a conventional tombstone fixture. However, within the tombstone block are the key elements: electro-permanent magnets that can exert multi levels of magnetic force and don't require constant electric input.
Initially, the patented "agile fixture" will be used with HMCs. In a fully developed system, the tool magazine will carry a number of fixture elements such as clamps, locators, and supports. In addition, the CNC programs of the machining center will carry commands for the positioning of these fixture elements on the tombstone to form workholding fixtures for various parts, designs, or machining operations.
In operation, the machine-tool spindle, with special pickup tools or adapters attached, positions the fixture elements on the tombstone in a method like "Magnetic Legos." At that time, only the lower-level magnetic force from the magnets is functioning. This force is just strong enough to hold the fixture elements in place. When all the fixture elements are positioned, a pulse of electric current is sent to the magnets. This strongly boosts the magnetic holding force, and the fixture elements are then held rigidly to the tombstone.
GM wanted a truly agile fixture. The researchers noted that in some cases a product had to be designed under the constraint of a common locating scheme of fixturing to facilitate manufacturing agility. That is, the design of fixture configurations or workholding positions determines the product specification. GM believes that the magnetic fixture is a true agile design in that it does not place restrictions on the part design so it can be made as the designer wants it, and be held in the fixture in the most balanced and secure manner.
According to GM R&D Project Manager Pulak Bandyopadhyay, PhD, "With our fully developed system, it will take no more than 10 minutes for a fixture changeover."
Yhu-Tin Lin, PhD and lead project engineer, says GM's research results indicate that the magnetic fixture is as good in accuracy and rigidity as a conventional solid-metal, hydraulically actuated fixture. With the help of modern machine tool capabilities, the agile fixture offers an accuracy less than 10 µm, a pull resistance of 12,000 lb (53 kN) and slide resistance of 3000 lb (13.3 kN) max. Early test results indicate that this fixture is accurate and stiff enough to provide greater accuracy than conventional fixtures.
GM began working on this concept four years ago chiefly because flexible and common manufacturing systems will help the company maximize its capabilities. With dynamic market demands and shorter product life cycles, the cost of permanent rigid fixtures for such runs becomes prohibitive.
The current prototype, which has a magnetic tombstone measuring 650 by 1000 mm, has completed validation testing. It will be introduced in a low-volume GM Powertrain production application in the first quarter of 2005.
Although designed initially to hold parts for 4, 6, and 8-cylinder engines for low-volume production, the agile fixture could be applicable to any medium or even high-volume parts, says Bandyopadhyay.
The agile fixture is a follow on to the company's successful C-Flex design with which GM was able to greatly improve the flexibility of its body assembly operations. With this system, multiple body elements can be welded using the same set of programmable C-Flex tools and robots.
"One of our new designs is a three-sided vertical twin vise which allows greater access to a part. It provides greater clearance and accessibility to the workpiece than a four-sided design because there is a wider access angle."
Special customer needs are encouraging many manufacturers of volume workholding products to move into the specialty area. "Although we are best known for our standard Kitagawa chuck line, we have extensive experience offering custom workholding solutions using niche, specialty products," explains Spencer Hastert, President, NorthTech Workholding/Kitagawa, (Schaumburg, IL). "This is to answer needs for any high-volume applications that our standard 4 - 24" [102 - 610-mm] chucks can't fill. It's a growing part of our business because of the demand for more customized workholding solutions to achieve even higher quality and meet stricter productivity requirements.
"The Kitagawa ML and DL chucks from NorthTech Workholding/Kitagawa are two new examples of our engineered solutions to meet the demands of this trend. Both of these new power chucks feature unusual functionality."
The ML series has extra-long-jaw stroke that is useful when gripping unusually shaped workpieces. It's available in three, two, or single-jaw versions. The single-jaw version incorporates a custom-designed fixed locator or jig for location (i.e. for workpiece nesting). The two-jaw version is useful for square, rectangular, or unusually shaped workpieces. The three-jaw version is essential when gripping workpieces with flanges that must be "swallowed"--requiring the jaws to open far enough to clear the larger diameter flange and grip smaller diameter datums.
Some workpieces require highly accurate axial, Z-axis location for length control. Traditionally, this requires a chuck with axial or a combination of axial/radial jaw movement. In some cases, this axial/radial jaw movement can interfere with the workpiece and complicate manual as well as automated workpiece loading.
The new DL Series directly addresses this workholding application problem with a chuck that actuates and clamps the workpiece using traditional radial actuation. Then, in the second part of the actuation, as the chuck develops gripping force it performs a final axial, Z-axis movement in which the part is pulled back to a fixed stop for length control with near-zero repeatability.
This two-step actuation ensures accurate length control and axial repeatability while avoiding the possible jaw interference issues seen with traditional "pull back" solutions. The DL series power chuck is available in 6 and 8" (152 and 203-mm) sizes.
More turnkey contracts are causing some workholding system manufacturers to establish partnerships to cover all customer needs. "With the advent of more turnkey operations, we have found it advantageous to partner with companies that make a variety of toolholding elements to offer complete workholding systems, instead of only our traditional workholding elements," explains Gerard Vacio, product manager, workholding systems, BIGKaiser Precision Tooling Inc. (Elk Grove Village, IL).
"Another trend we see is a move to handling larger parts, particularly in the aerospace industry where modular elements are replacing assembled sections.
"At the same time, lower production volume means less need for stacks of waiting pallets. There is more emphasis on setup reduction. Customers want a reliable quick-change clamping system."
In the newer designs from Kosmek USA Ltd. (Bridgeview, IL), a company that makes hydraulically actuated workholding systems, the trend is to run at lower operating pressures. "The range is 500 to 1500 psi with 1000 psi average," explains company president Chris Turcich. "The reason is that this pressure drop reduces overall system cost because the user can tap into the machine tool's hydraulic system. Therefore, fewer parts are needed for the workholding system since fewer pumps and accumulators are needed. This also reduces system maintenance.
"Another trend we are seeing, particularly in the Japanese market, is more enclosed hydraulic systems as opposed to external plumbing. The internal system is more costly, but cleaner and less subject to damage.
"Most of our equipment is for dedicated fixtures. One of the challenges is to be sure the pressure on the part is right. That is, strong enough to hold the part during machining, but without damaging it. We use a series of elements--throttling valves, sequencing valves--to modify the pressure.
"Among our newer items is a quick pallet-change system in which we clamp a pallet to a machine base with a tolerance of ±118 µin. (3 µm). It allows much more rapid change of fixtures with great precision. With the system, the pallet is aligned on a series of preset pins, then clamped, all in a matter of seconds. There is no need for manual adjustments," Turcich concludes.
The toggle clamp has been around since the 1930s, maybe longer, and is a key element on many fixtures. But this well-known fixture element has undergone a number of changes. De-Sta-Co Industries (Madison Heights, MI) has made a number of modifications to this venerable design.
According to Tom Stimac, product manager, the company recently offered a new 2000 series clamp which at first glance looks like any other clamp, but offers some new features. "First, the linkages have been reproportioned to eliminate pinch points in critical areas," he says. "The same linkage redesign has also increased the mechanical advantage so that the holding power is increased two or three times. Some elements of the 2000 series clamps are smaller, lighter weight, and lower cost.
"In addition, earlier designs had a small pull-back motion once seated. This has been eliminated in the new design, which is a big advantage in high-precision work."
Another advance is an enclosed pneumatic clamp. These clamps are often used in harsh welding or machining environments where weld spatter and chips can erode the clamp's performance. The new design has the action enclosed, ensuring much longer life.
Because of the need for reduced setup time, the company offers pneumatically actuated clamps to replace the manual version. The clamps can be set up to close simultaneously or in sequence. For greater automation, sensors can be added and the entire cycle computer-controlled.
Although the bulk of the company's products are off-the-shelf, about 20% are specialty work. Customers often have unique part-shape or load requirements that require some modification. These changes are normally taken care of in a short time, explains Stimac.
This article was first published in the September 2004 edition of Manufacturing Engineering magazine.