Flexible Workholding For Flexible Cells

For maximizing throughput in high-mix, medium-batch processing--that is, keeping a tool in the cut--it's tough to beat the flexible manufacturing cell (FMC) for efficiency. Today's FMC is well designed to move pallets into and out of machine tools, load and unload tools, verify part programs and tooling match, alert the operator of problems, and a host of other metalworking functions. Automation levels and reliability are such that untended or lightly tended operation is becoming routine for many shops.

In application, however, efficient operation of a cell often depends on factors outside the responsibility of the cell builder. Fixturing, used to present the workpiece to the spindle, is one of these factors. It can be the defining element in successful cell operation.

The work that is put across a shop's machine tools makes each shop unique. Processing that work--particularly how it is fixtured and to some extent tooled--is what transforms a general-purpose flexible manufacturing cell or, for that matter, a stand-alone machine, into a shop-specific production tool.

Fixtures that allow quick and easy changeover yet provide rigid support and unerring positioning represent workholding goals any shop can achieve. This story looks at how one manufacturer, Henry Valve (Melrose Park, Illinois), successfully combined the attributes of a four machine, 16-pallet Fritz Werner FMC (Fritz Werner, Carol Stream, Illinois) with a flexible fixturing system designed and built by Royal Machine and Tool (Berlin, Connecticut) to realize fully the great production benefits of multi-machine manufacturing.

Knowing Your Needs

Henry Valve's primary objective for installing a cell is to maximize spindle utilization. Maximizing the time a tool is in the cut is how Henry Valve, and any other job shop, makes a living. To that end, decisions about which workpieces should go on the cell were necessary.

An overall evaluation of the shop's work was undertaken. "We looked at all the parts we currently manufacture and decided which ones were the best candidates for the cell," says Dave Palla, manufacturing manager.

Henry Valve made their processing decisions based primarily on production volumes, cycle-time, one-set potential and possibly workpiece size for multiple-part holding. Other shops might decide to more heavily weight other factors.

Defining overall workpiece parameters up front led Henry to a fixture strategy that involved two basic fixtures applied evenly over the cell's 16 pallets. Eight pallets were equipped with Royal's quad-vertical combination chucks, which are capable of holding up to 16 workpieces at four per side. On the remaining eight pallets, a smaller version of the combination chuck holds up to eight workpieces, two per side.

Need To Be Flexible

True to their name, Henry manufactures valves. They've been in business since 1914. While it may seem that there is much similarity among the workpieces--a valve is a valve--they in fact manufacture several hundred different sizes and designs. Currently, Henry Valve runs 120 different castings/forgings through their FMC, in average lot sizes of 100 to 200. More jobs are being transferred to the cell as programs, tooling and fixturing are created. At any one time, 16 different part numbers are possible in the cell, either in queue or being machined.

Their valves are used in the industrial refrigeration industry. More than 25 percent of Henry's business is export. Concern over global ozone depletion has resulted in more stringent regulation of atmospheric discharges. This has impacted Henry's manufacturing.

"Customers who use our valves are working with CFC and ammonia refrigerants," says Mr. Palla. "Some valves are being tested that allow a mere 0.1 ounces of leakage per year, from refrigeration systems."

That means tighter manufacturing tolerances for valve manufacturers. Critical to getting better parts, more consistently, is a fixturing system with the dual function of rigid, accurate blank location and quick changeover. For Henry Valve, another workholding variable is vagaries in the raw castings and forgings from which they machine many of their workpieces.

Holding different materials also plays a role in fixture consideration. Varying clamping pressures to accommodate brass, steel, bronze and cast iron is another fixture capability necessary for processing Henry's valves.

Some workpiece features also necessitate clamping consideration because of potential distortion from springback induced by overclamp. A bore may be round in the fixture, but it can distort when it's unclamped.

Process Considerations

There are many reasons for shops to consider flexible cell manufacturing. Throughput gains, reduced personnel requirements, single-set machining, improved part quality, potential lights-out operation and overall process efficiencies are some of them. For many shops in medium-batch production, cells make sense on many levels.

Many shops, however, find getting the FMC to a point where its full production potential is realized tends to be more blood, sweat and tears than plug and play. "While flexible manufacturing sounds rather inclusive, actually a shop needs to be rather specific about defining the operational role of an FMC," says Mr. Palla. Defining workpiece and production specifications is what makes a general-purpose cell become specific to a shop's production requirements.

Looking at the overall shop throughput, part of the workpiece evaluation process was a determination to make all parts complete. Obviously, this specification impacts workholder selection.

Many of the valve bodies produced at Henry require three-sided machining. They're arranged in a T-shape. They lend themselves to complete machining in a single clamping. "Obviously it's better to process a part in one handling," says Mr. Palla. "That's one of the reasons we invested in the cell--to reduce or eliminate multiple handlings."

For the 15 percent of workpieces run through the cell that require A/B operations, the fixture design must eliminate misalignment between relative features from set A to set B. Henry runs these parts half set A and half set B on a single fixture.

To accomplish positioning consistency from set A to set B, the sub-jaws that contact the workpiece are designed as a stepped-V. Each jaw, upper and lower, cradles the workpiece and automatically centers it using three-point contact. These insert jaws are made by Royal and are workpiece specific.

About The Fixtures

Two fixture sizes are used by Henry in their cell. Eight of each size fixture are mounted respectively on the cell's complement of 16 pallets. The primary difference between the fixtures is their height--the larger can accommodate up to 16 parts (four per face) while the smaller can hold up to eight (two per face).

Both fixtures are four-faced, dual-action chucks. A two-sided center jaw, on each face, is fixed. Upper and lower jaws are actuated together and close on the center jaw from both sides. Datum reference for the workpiece is determined by an off-set dimension measured from the fixed-center chuck jaw.

Upper and lower master jaws stay mounted on the fixture. Master center jaws stay mounted for multiple part setups but are removed for larger workpieces.

Key to flexible changeover without losing positioning accuracy is the use of insert jaws. These mount on the master jaws. Location is maintained from job to job because the master jaws stay with the fixture. Insert jaws are machined especially for the part.

Complete changeover of insert jaws takes about twenty minutes. To remove a set of these jaws from the fixture, six setscrews are removed and the jaws come free. Installation of the next set of insert jaws is the reverse of this process. Taking their alignment from the master jaws, the inserts do not require additional adjustment, once installed.

Planning For Variation

Ideally, all workpieces would have a dedicated fixture mounted on a dedicated machine tool. Every part blank would be dimensionally exact. Each fixture would be purpose-built for each part.

In reality, however, a vast majority of metalworking shops operate under a different set of production drivers. Most of the jobs that Henry Valve runs through their cell are castings or forgings--in batches of hundreds.

By their nature, these blank parts have variations. Some are oversized, undersized or parting-line shifted. The fixtures that deliver these parts to an automated cell must be able to compensate for these variations.

To accommodate variations in the castings and forgings used by Henry Valve, Royal designed a dual-float mechanism for the center insert jaws. If there is some variation in the raw stock, the compensating center jaw allows for it. Without such a variable jaw, grip on oversize or undersize castings or forgings might be inconsistent and therefore potentially dangerous.

Proper orientation of the workpiece is set by the upper or lower jaw, which does not pivot. "Variation is a reality with castings and forgings," says Mr. Palla. "We're constantly working with our foundries and forges to get better blanks--and it's working--but allowance for variation must be part of the shop workholding strategy."

Clamping Down

Henry Valve's FMC requires two operators per shift. A programmer/operator is responsible for keeping the work moving through the cell. A second person loads parts going in, and unloads, checks and deburrs parts coming off the cell.

The fixtures are clamped manually. But, there is not a frantic race to load the fixture. Unlike a single pallet machining center, loading work into the cell doesn't affect spindle uptime. Hydraulic clamp actuation was an option at one time, but it was rejected because there's time in the process to manually clamp the fixtures.

To make sure the fixture is not overtorqued, thus possibly distorting or damaging a fragile workpiece, a pneumatic torque wrench is used to close the fixture. A dial on the wrench is set to the recommended torque. It's a simple solution.

Fixture As Program?

Runoff on new jobs introduced into the cell is the responsibility of the cell's operator/programmer. For repeating jobs, programs are downloaded from a central computer. Tooling requirements, fixture jaw sets, and other production data are pulled from file by the programmer/operator. A request for raw material is sent to inventory at the same time.

Due dates drive the cell's production. "A two-week leadtime is normal for getting work through the cell," says Mr. Palla. "However, we can and sometimes must put a rush order through in less time." The cell is designed to handle these contingencies.

For a new job, after deciding to run it on the cell, a casting or forging blank is sent to Royal so insert jaws can be manufactured. Royal uses fixture setups identical to those at Henry to verify fit of the new part in the insert jaws.

A by-product of manufacturing the insert jaws is a CAD drawing. It shows the workpiece nested in the jaws with finish dimensions marked for the workpiece and the jaws.

From this CAD drawing the programmer/operator can program the machining center complete with part and fixture dimensions. Because the master jaws stay with the fixture, the programmer is concerned only with the dimensions from the new insert jaws and workpiece.

"Working from these drawings saves us programming time," says Mr. Palla. "It's a bonus for us because we can work from a single set of dimensions instead of combining separate drawings for the workpiece and the insert jaws."

Making It Work

"Early in the planning process is the best time to consider fixturing, whether the plan is to purchase a multi-machine cell or a stand-alone machining center," says Mr. Palla. "Fixturing decisions are appropriate even in the part design phase. If you can't hold it, you can't cut it."

In the case of Henry Valve, fixture considerations hinge on taking variability and complexity out of the manufacturing process. Reducing the potential for error during part loading and rigidly and accurately holding a workpiece are functions that any shop should expect their fixtures to bring to the process. MMS

About The Cell

For Henry Valve, a flexible manufacturing cell (FMC) was a natural progression. Like many medium-batch manufacturers, they had been working with cellular manufacturing, in concept, for several years.

Initially this meant simply grouping together machine tools that performed primary and secondary operations on families of parts. With this background, the leap into an automated four-machine cell was not too large for Henry Valve. Cell management techniques learned earlier transferred nicely to the Fritz Werner four-machine cell, purchased last year.

The cell they installed is composed of four horizontal machining centers. They are serviced by a rail-guided vehicle that picks pallets from a 16-pallet queue. Pallets are 500 mm square with a system capacity to handle 500 x 630 mm pallets if needed.

Tool capacity for each machining center is 120 pockets. Spindle horsepower is 35, and maximum spindle speed is 10,000 rpm. Cell control is Werner's SC1, a PC-based controller and monitor.

Installation of the cell was conducted in three phases. This was to help ease Henry's personnel into cell operation. First, two machining centers were set up and run as stand-alone units. This helped establish manning requirements.

The second and third machining centers were then installed, and training was conducted on them as well as on the soon-to-be-installed cell controller and rail-guided vehicle.

When phase three--installation of the pallet carrier and work queue--was complete, the cell was in production almost immediately. The entire process took about a year to finish.