Published 1/15/2002

Moving Up To CNC Swiss Screw Machines

Three new CNC Swiss machines in 18 months have provided real growth for this Illinois shop.

Edited by: PM Editorial Team

Until about 2 years ago, Precision-Tek's production equipment consisted primarily of 2-dozen Swiss automatic screw machines, which are great for relatively uncomplicated, turned parts with large length-to-diameter ratios. However, according to Mr. Pflum, most of that kind of work has gone overseas. Increasingly, the shop found itself quoting more sophisticated parts that required cross-drilling, tapping, milling and similar work, which the shop had to handle as secondary machining operations.

"We weren't competitive," Mr. Pflum recalls. "The prices that we quoted included the additional setups, machining operations and the extra part handling needed to produce the parts. We weren't getting the work. When we'd follow up with calls to the customers, they would tell us that our quotes were 20 percent to 30 percent higher than those from other shops."

Some jobs were so complex that the shop declined to quote them. However, Mr. Pflum kept a file of those jobs, and as the file grew, it was becoming increasingly obvious that he lacked the equipment needed to compete for what appeared to be, in most cases, very profitable jobs.

Specifically, Precision-Tek needed multi-axis, CNC Swiss screw machines capable of producing parts complete in a single setup in order to obviate those expensive secondary operations. That meant that the shop would have to make two technology jumps at once: First, it would need to go from cam-driven Swiss machines to CNC Swiss machines; and second it would need to go from what are essentially two-axis Swiss machines to five- and six-axis machines.

Because cost was a consideration, however, the shop initially looked at a two-axis CNC Swiss machine. Unquestionably, the CNC machine offered important advantages over the shop's cam-driven counterparts. For example, the shop's Swiss automatics typically take 2 to 3 days to set up: The cams must be designed and machined and the tooling must be prepared and installed. By contrast, the CNC Swiss machine can be set up in hours, which makes it much more practical for prototype work and small-lot production. Also, where the tooling must frequently be ground to size for use on the Swiss automatic, tooling for the CNC Swiss machine is often insert-type tooling that can be quickly installed and more easily maintained.

"The more basic CNC Swiss machine would enable us to provide our customers with prototyping capabilities, faster part deliveries and, for some jobs, more reasonable pricing," Mr. Pflum is quick to agree. "Important as those advantages are, however, the more basic machine could not produce complex parts complete in one setup. Therefore, even though the multi-axis, multi-process CNC Swiss machine was considerably more expensive, we decided to buy it to get that very important capability."

Help From Some Friends

"Our customers helped us make the decision," Mr. Pflum adds. "They convinced me that we needed multi-axis CNC equipment to be competitive on the more sophisticated parts. And while there were no promises, they hinted that if we had such equipment they would be able to place more work with us."

Precision-Tek reviewed the multi-axis CNC Swiss machines on the market and decided that the Model SA-16R CNC Swiss-type automatic lathe from Star CNC Machine Tool Corp. (Roslyn Heights, New York) provided the combination of bar size capacity and backworking capabilities best suited to the shop's existing and potential job mix. The five-axis (X, Y, Z, A and B) machine has a 2.2-kW, 12,000-rpm main spindle with a synchronous, revolving guide bushing. The tooling at the main spindle includes a gang tool post that accommodates up to six turning tools and three rotary tools for cross-drilling, milling and tapping operations.

An 8,000-rpm subspindle with an AC spindle motor is located directly opposite the main spindle. A four-spindle attachment, with four tools facing fore and four aft, swivels (A axis) into position between the main spindle and subspindle to permit separate or simultaneous ID work such as drilling, boring and tapping at the main spindle and/or subspindle.

The CNC Swiss machine not only permits complex parts to be completely machined in one setup, but it also permits many operations to be performed simultaneously instead of sequentially, substantially reducing the total time required to machine the part. For example, while the OD of the bar in the main spindle is being turned by one or more tools on the gang tool post, the four-spindle attachment described above can be drilling and tapping the front of the part. Or while the cross-drilling unit is drilling and tapping the (stopped) bar in the main spindle, the four-spindle attachment can be drilling and tapping the back end of the part in the subspindle. Synchronous control of the main and subspindle rotation and longitudinal movement permits the bar to be supported "between centers," for machining, which, among other things, eliminates cut off burrs.

Precision-Tek became more competitive on short-run work as soon as the CNC Swiss machine came on stream. "Today, customers don't want to place an order for 50,000 or 100,000 pieces for that new product," Mr. Pflum explains. "They want to see 250 prototypes first and they don't want to pay a tooling charge.

"With cam machines you need to make a set of cams, so you incur costs up front," he stresses. "If there's a design change, you may have to recut the cams. It takes a long time to set up the cam automatic: Each tool must be centered on the machine. Days and sometimes weeks can go by before the customer gets production parts.

"By contrast, we can set up the CNC Swiss machine to run a job a lot faster," he continues. "There are no cams to cut, and the tooling is easier to change. We have a smaller up-front investment in tooling and setup costs, and if the customer makes a design change in the part, we can usually incorporate it with a minor change in the program."

Buying The Second Machine

Precision-Tek's first CNC Swiss lathe was operating at capacity within a year of its purchase. Therefore, Mr. Pflum decided to invest in another Star machine, this time a Model SR-20R, six-axis, CNC Swiss-type lathe. "We wanted a CNC that could run larger diameters," he explains. "Also, the newer model has more power and more capabilities, which enables us to run jobs a little faster than we could on the first machine."

Probably the most important difference is that the newer machine has more tooling capacity, including full C-axis and separate and independent frontworking and backworking units that enable front- and back-end operations to be performed independently. Front- and back-end operations can be performed without any waiting or interruption, making for the shortest possible cycle times.

Where the first machine has an end-working attachment used for both front and back end-working, the SR-20R provides separate and independent units for front- and back-end machining. A single gang tool post, which holds up to six turning tools, four end-working tools and four cross-working tools (photo on page 33), handles front-end operations at the main spindle. The gang tool post also accepts optional attachments for off-center face drilling (such an attachment can be seen below the cross-drilling tools in the photo) as well as for milling, slotting and other operations. The subspindle is served by a separate tool block that holds up to four stationary (or rotating) tools.

The larger tooling capacity of the second machine enables Mr. Pflum to balance the machining more evenly between the main spindle and subspindle. "At the vary least, we are able to perform all of the backworking operations without increasing the cycle time for the part," he stresses. "It's like getting all of those operations for free."

Precision-Tek's sales picked up significantly as a result of having invested in the CNC Swiss machines. Three months later—during a period when most shops were experiencing slowdowns and order cancellations—the firm ordered its third CNC Swiss machine, another SA-16R.

Mr. Pflum estimates that by producing parts complete in one setup on his CNC Swiss machines, he is saving between 5 percent and 35 percent of the cost of machining those parts the conventional way with secondary operations. The lower costs permit him to provide more competitive quotes, which are bringing more work into the shop.

The parts produced on the CNC machines are not only less expensive but also better in a number of ways. "We get better repeatability on the CNC machines than on the cam machines," Mr. Pflum reports. "It takes a skilled operator to hold five tenths (0.0005) on a Swiss automatic all day long, but the accuracy and repeatability of the CNC Swiss machine enable it to routinely hold such tolerances.

"Surface finishes are better, too," he continues. "On a Swiss automatic, once you've cut your cams, you have to live with the resultant surface finish—or recut your cams. You can retrofit the machine with variable cams that allow you to make adjustments for surface finish, but they're expensive. On our CNC Swiss machines, we can vary the speeds and feeds at will to achieve the finish that the job requires."

Faster Turnarounds

Precision-Tek has improved its turnaround time on jobs, for both prototype work and production runs. The Swiss automatics typically take 2 to 3 days to set up and get into production. By contrast, the CNC machines can be changed over to a new job in one day or less, assuming that the program has been prepared and the tooling for the job is on hand. Ability to set up fast for the next job boosts the shop's flexibility: Orders for a handful of prototypes can be accommodated quickly and easily, and short-run jobs become economically feasible—an important consideration for customers interested in establishing just-in-time relationships with vendors.

Because the parts are completely machined in one setup on the CNC Swiss machines, they are uniform. "The more parts are handled by shop personnel, the greater the opportunity for error," Mr. Pflum notes. "When we were doing secondary machining of parts, there was always the danger that a machine operator would load a part in the machine backwards or drop a part and nick it, or somehow miss an operation. Any number of things could go wrong that might result in a reject. With the CNC Swiss machines, we've virtually eliminated the opportunity for human error and have greatly improved consistency from part to part."

Lights Out

Each of Precision-Tek's three CNC Swiss machines is equipped with an LNS Hydrobar bar loader that can automatically load 20 to 40 bars—depending on their diameter—in succession without operator intervention, making it possible for the CNC machines to operate for extended periods without stopping for loading. (By contrast, each of the shop's Swiss automatics must be loaded by hand by the operator, one bar at a time.)

Precision-Tek normally operates one shift, but certain, carefully qualified jobs are set up to run unattended on the CNC Swiss machines after hours. The shop usually will not consider running a job unattended unless it is brass, mild steel or some other free-machining material. First, it will run the job for 2 or 3 days to monitor tool wear, and even watch the way the chips pile up around the tooling to make sure they do not prevent the coolant from getting to the cutting zone. Once the shop starts running the job unattended, an operator must inspect the lights-out parts first thing in the morning to make sure that they are within specifications.

For jobs that run unattended where tool wear is a concern, the shop sets up the CNC machine with a second, identical tool. After the first tool has run a thousand or so pieces and can no longer be offset to compensate for wear, the control switches to the backup tool, allowing the machine to continue making parts.

Cams Still Have A Future

Mr. Pflum's enthusiasm for his CNC Swiss lathes has not lessened his regard for Swiss automatics. After nearly 30 years of working with the automatics, he feels there is no more efficient way to produce relatively simple parts with large length-to-diameter ratios in large quantities. In fact, he believes he has the best of both worlds: the Swiss automatics for less complex parts in large quantities; and the CNC Swiss machines for complex parts in prototype and short-run quantities.

Far from competing with the Swiss automatics, the CNC Swiss machines are used at Precision-Tek to make the cam machines even more efficient for the jobs they handle best. Mr. Pflum explains: "We automatically schedule any large-quantity job for our many Swiss automatics," he begins. "The customer knows up front how the job is going to be run; in fact, he knows that he will pay a one-time tooling cost for the job to run on the automatics.

"Before we tool up our Swiss automatics, however, we run the job on one of our CNC Swiss machines," he continues. "Using the flexibility of the CNC machine, we experiment with various feeds and speeds until we find the best combinations for the various cuts. Only after we have optimized the cuts do we specify the tools and cut the cams to run the job on the Swiss automatics. When the job calls for a million pieces or a half-million pieces, taking a second or two out of the part can make a big difference. Also, we avoid the guesswork by starting out with proven speeds and feeds. Production starts are smoother, and we avoid the cost and delay of having to recut cams. Using the CNC Swiss machines in this manner more than repays the effort."

Business Up 30 Percent

Mr. Pflum reports that the three CNC Swiss lathes, purchased over an 18-month period, have increased Precision-Tek's sales by about 30 percent—not bad for a recession. He has nearly outgrown his 8,000-square-foot facility and expects to expand to 12,000 square feet. He'll equip the addition by rebuilding some worn Swiss automatics—that's how he started some 8 years ago—and he plans to buy one or two more CNC machines. Does he plan to buy more Swiss automatics? "Swiss automatics will continue to be important to us for the foreseeable future," Mr. Pflum notes. "However, any new machines that we add will definitely be CNC.

"Parts produced on the Swiss automatics are less complex, less challenging and, at 10 cents to 20 cents per part, less profitable," he continues. "Parts produced complete in one setup on the CNC machines tend to be more complex and, at 40 cents to 90 cents per part, more profitable. The CNC machines have helped raise Precision-Tek to a new level. They played an important part in helping us achieve our ISO 9000 certification, and I feel confident they will continue to help us grow, particularly as a source for complex parts with demanding tolerances and surface finishes."