How you handle workholding has everything to do with how productive your CNC vertical machining centers and turning centers will be.

By Ivan French
SMW Systems
Santa Fe Springs, California

ABC Co. is quoting a job against XYZ Co. Both companies have essentially the same costs. Both own or lease the same types of machine tools. Both pay their operators the same wages and benefits. And both have the same floor space, overhead costs and material costs.

Assuming that both companies truly know their costs (and many do not), the bids of the two companies should be the same or very close to the same for a given job. But they are not. ABC Co.'s price is significantly lower; it wins the bid competition and it makes money. How? Because ABC Co. gets more output from its machines, it is able to produce the job in less time and, thus, at less cost.

Old Habits Die Hard

XYZ Co. is led by a second-generation manager who was taught, perhaps by his father, to stick to the old basic operating principles, one of which is "when you need more output, buy another machine, and shop for the lowest possible price."

So when XYZ Co. goes out for bids on its new machine needs, the machinery dealer knows by XYZ's reputation that he must submit a bid on a stripped-down, "vanilla" machine if he wants the order. The dealer knows that today's machine accessories can dramatically increase the output of the new machine, but he dare not include them in his quote because the result will be a higher price, and he will lose the order.

Another hide-bound practice that inhibits XYZ's ability to profitably compete is its hesitance to embrace new processing methods and techniques that can dramatically reduce piece cost. Whereas ABC Co. is constantly experimenting with new ideas and methods that increase machine output and lower its costs, XYZ Co. is not inclined to try new ways that involve additional investment because it honestly believes that added investment translates into added cost.

Here's what ABC Co. knows about machine accessories and processing techniques that is consistently making it the winner in today's piece-cost war.

Increasing Your VMC Output

Fig. 1--A manual-operated pallet changer can dramatically reduce machine setup downtime by allowing load and setup operations to be performed on the off-line pallet while work is being machined on the pallet clamped on the machine table. Photo courtesy of Avibank Manufacturing, Inc. (Burbank, CA).

There are several ways to increase the output of vertical machining centers. Two that require a break with traditional thinking are the application of palletized workholding systems and the use of rotary indexer processing methods. Both can produce quantum leaps in machine output by making major cuts in machine setup downtime.

Figure 1 shows a manual-operated pallet switching system that ABC Co. employs on its VMCs. There are several such systems on the market; the one illustrated is called SetupSwitcher and is produced by SMW Systems. These systems break with the long-held practice of performing setups on the table of a machine.

XYZ Co. sets up a job on the machine table while the machine spindle sits idle, so machine output is zero during this time. By using the relatively low-cost pallet changer, however, ABC Co. sets up its jobs off-line—that is, on the pallet outside the machine. Meanwhile, the spindle is cutting workpieces mounted on the other pallet that is clamped in position on the machine table. When the machining cycle ends, pallets are easily switched in a minute.

Fig. 2--Pallet shuttle carts allow repeat workpiece setups to be stored on extra pallets, and brought into production at a moment's notice. The cart also serves as an away-from-the-machine setup table.

The pallet system also can be augmented with an adjustable-height pallet transport cart (Figure 2), and additional pallets. These additions allow repeat setups to be stored on the extra pallets. These setups are pre-aligned and thus do not require tramming in. The cart also serves as an away-from-the-machine setup table, which helps insure that a setup is always ready when the current run is complete.

Many people tend to associate pallet changers with long production runs. But because of the ability to allow job setup off the machine table, they are also very effective on short runs. In fact, the more short-run work they handle, the more money they save. They're truly an answer to JIT headaches. And they're very effective in shops that use mostly vises for workholding.

Some of the jobs ABC Co. produces involve medium and higher production volumes. This is typically accomplished by switching between two pallets, each fit with identical workholding arrangements so that they can hold the same workpiece. The big payback from the pallet system in this case is that it allows the workpiece loading and unloading operations, as well as chip removal, to take place on the off-line pallet while the machine is cutting parts on the other pallet. The only production time lost is the minute required to switch pallets when each machining cycle ends.

Another processing technique that boosts VMC output is the use of indexing fixtures and rotary tables that bring some of the advantages of the higher-priced horizontal machining center to the VMC. The primary benefit from this processing approach is that it can eliminate up to 75 percent of setup downtime for some workpieces by eliminating setups altogether.

Fig. 3--Indexers can be employed on vertical machining centers to expose four sides of cube type workpieces to the machine spindle in a single setup, thereby eliminating three conventional setups. Photo courtesy of MK Diamond (Torrance, CA).

Figure 3 shows a housing type workpiece clamped in a fixture that is designed to expose four sides of the workpiece to the machine spindle as the fixture is indexed in 90-degree increments. If this part were fixtured more conventionally, it would require four setups since only the top side of the workpiece would be exposed to the machine spindle per setup. With the ability to accurately index the workpiece, however, four setups are quite easily combined into one.

Fig. 4--This drawing shows how three or six sides of a cube type workpiece can be machined on an indexer-mounted tombstone fixture, eliminating up to four conventional setups.

Figure 4 shows how up to six sides of a cube type workpiece can be machined on a tombstone fixture mounted on an indexer. Note that mounting the workpiece on a four-sided fixture exposes three sides of the workpiece to the machine spindle. But by reloading the workpiece as shown at the right of the drawing, all six sides are machined in two setups.

Maximizing Lathe Output

XYZ Co.'s penchant for buying equipment at the lowest possible price also promotes inefficient turning processes. For instance, they typically buy lathes with the cheapest tooling and the cheapest chuck available.

When changeover requires different turret-mounted tools to be switched, loosening and tightening screws and re-zeroing newly mounted tools can add 20 to 30 minutes to the total setup time. ABC Co., on the other hand, purchased a quick-change tooling system that allows tools to be changed in about two minutes, and these tools are pre-set off-line so that no additional time is required to re-establish the tool offsets through "touch-offs" or other time consuming techniques.

XYZ Co. also purchased a low-cost chuck that uses bolt-on jaws. Each time jaws are changed, it takes 10 or 15 minutes to remove bolts, switch jaws and tighten bolts. When soft jaws are remounted for repeat jobs or same-diameter applications, they must be rebored which, depending on the skill and dedication of the operator, can take up to another 30 minutes. When changing over from hard jaws to soft jaws, XYZ Co. may require up to an hour to accomplish the whole task.

ABC Co. bought its lathe with a chuck that cost about $10,000—three times the cost of XYZ's chuck—but it allows ABC to change from hard jaws to soft jaws in a minute, thereby saving a whole hour of setup downtime. Not all this wisdom, however, comes from ABC's management alone. Long ago ABC encouraged their machine dealer to recommend anything that would increase the efficiency of their machine and reduce piece cost.

Fig. 5--Quick changeover chucks can make significant contributions to lathe output by allowing hard jaw to soft jaw changeover in about one minute.

Figure 5 shows the type of quick changeover chuck purchased by ABC. This chuck offers two primary benefits: First, jaws can be repositioned, reversed or changed in less than a minute because the entire jaw is removed by one-quarter turn of the jaw release key. But perhaps even more important, his chuck eliminates the need to rebore previously mounted soft jaws. The accuracy of the jaws and jaw actuating mechanism allows remounted soft jaws to repeat as close as 0.0002-inch TIR. For the workpieces that require soft jaws to clamp barstock or pre-machined surfaces, this accuracy can pay huge dividends by virtually eliminating jaw change time as a significant factor in total setup cost.

Fig. 6--Short-bar feeding systems are gaining in popularity. Some systems can be automatically loaded to facilitate low-cost "lights out" production. Photo courtesy of Matthews Studio Equipment, Inc., (Burbank, CA).

Another example of new technology that improves lathe output is the short-bar bar feed systems that are available today (Figure 6). These compact designs generally take up less than 10 feet of space behind the machine and the barstock runs "dry" in a spindle liner tube, eliminating oil mess problems associated with hydrodynamic bar feed systems that process 12-foot bars and require up to 20 feet of space behind the machine.

The key to the effectiveness of short-bar bar feeds is that they process bars up to 3.5 inch in diameter that have a maximum length of 5 feet. These bars require no end preparation; they rarely require straightening; and they generally are easily handled by one person without the assistance of a power hoist. Bar size changeover on the unit takes less than 5 minutes, including spindle liner change. Some shops that specialize in short-run work make 12 or more changeovers per shift. And because the bar is supported in the spindle liner—and the spindle, liner, and bar all rotate together—there is no limitation on machine turning speed.

Another benefit driving the demand for these systems is that they can be loaded automatically. The bar table or magazine can store enough bars to allow the lathe to run unattended for up to 24 hours. This feature not only frees the machine operator to run other machines or to perform inspection and other secondary operations such as deburring, it also facilitates "lights out" nighttime machining. Many companies load the system with bars and run it unattended on weekends. On Monday morning they are greeted by hundreds of inexpensive workpieces.

Maximizing Operator Output

One of the greatest problems faced by many companies today is the shortage of skilled machine operators. This problem has reached such huge proportions that a number of machine builders have established education centers to train CNC machine operators. This helps to facilitate machine sales, of course, but nonetheless illustrates how severe the skills shortage is for many shops. ABC's solution to the problem is doubling the output of the machines they already own, and to get twice the output from their operators as well.

Adios XYZ.

For more information from SMW Systems, call (562) 949-7991, visit their Web site at www.smwsystems.com, or select the Product Info icon at right.

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