Tool And Die Shop Doubles Throughput On Steel Tubing Grooving

With their conventional high-speed steel milling cutters, this shop experienced a series of problems producing critical axial grooves in steel tubings.

Case Study From: 10/1/1997 Modern Machine Shop

With their conventional high-speed steel milling cutters, Kline Tool and Die experienced a series of problems producing critical axial grooves in steel tubings. The cutters were ganged up in two sets of four to perform the milling operation. The grooves, cut into the edges of the tubing slit lengthwise, form mating parts of a tongue and groove joint when the halves are fit back together. The workpiece forms a soil core sampler used in geological drilling. The mating halves must come apart so field workers can withdraw the sample core. So, accuracy over the entire 28-inch length of cut is critical.

Cycle time, tool life, and cut repeatability were poor. When the Phillipsburg, Pennsylvania shop first started making the workpiece, their customer specified a soft Rc 6 steel. Gradually, hardness specifications increased, as the customer sought a more durable product. Over time, they went to a heat-treated Rc 21 steel, and eventually to Rc 30. The harder, heat-treated steel demanded higher cutting forces, which pushed the original milling cutters to their limit. Kline had to switch out all four cutters after one pass, compelling them to stock three spare sets. Tool resharpening alone was costing $5,000 per year. Furthermore, they couldn't improve on speeds and feeds without compromising precision even further and overloading the machining center.

That's when president Mike Kline looked into a tooling switch. After considering several alternatives, he selected the CoroMill 331 indexable side and face milling cutters from Sandvik Coromant (Fair Lawn, New Jersey) with high-positive rake cutting. As a result, Kline boosted throughput 2:1, reduced cutter inventory by half and has a straighter, better-finished end-product. They also got payback on their retooling investment in only six months.

Kline is a tool and die shop serving the drilling industry for 17 years. They make parts for drill rigs, oil and gas wells, and blast-hole rigs.

Let's examine how they benefited from the tooling modernization.

To make a soil core sampler, Kline starts with deliberately out-of-round 4130 annealed, heat-treated Rc 30 steel tubing 2½ inches in diameter and 30 inches long. The tube is cut in half lengthwise. Both halves are fixtured and all four edges are milled simultaneously so that when they are mated, the finished tube shape is exactly round, with a two-inch diameter. ACME threads are machined onto each end.

Previously, to cut the axial groove on one half of the tubing, Kline ganged two sets of high-speed steel milling cutters on a single spindle, 50-taper Cincinnati 20VC. The other half of the tubing got an axial tongue matched to the groove. The groove's internal dimensions are ¼-inch deep, 28 inches long, ¼-inch wide.

Edges of the high speed steel cutters wore out within six to seven minutes of cutting the 30 Rc steel. To maintain dynamic balance, all four cutters had to be changed out together and shipped off-site for resharpening. In addition to the stoppages for cutter sharpening, there were also stoppages for breakage. These were serious because breakage of one cutter entailed the replacement of all four cutters, to ensure there was no overload on any single cutter in the set.

To compound matters, Kline experienced tool axial runout causing grooves that weren't always straight. This made it difficult to match the two machined halves.

Maximum speed was 50-70 rpm, feed was 5-6 ipm, and cutting speed was 80-100 sfpm. Cycle time was 24 minutes per part.

Now, Kline is ganging up three sets of two CoroMill 331 facemill cutters on the same Cincinnati 20VC and achieving approximately five times higher machine speeds, feeds and cutting speeds — 270 rpm, 30 ipm, 400 sfpm, respectively. Instead of having to resharpen the tools after one pass, they merely index to another edge after two passes. With the CoroMill 331 inserts in grade GC4030, they get four edges from each insert.

"Cycle time is half of what it used to be," says Mr. Kline. "We now produce one part in 12 minutes. And the parts are 25-30 percent straighter than before." Total savings from capacity gains, longer tool life, lower tool inventory and lower machining cost per part are approximately $80,000 per year.

Groove straightness is a result of the CoroMill 331's design. Kline is able to precision set the tool for accuracy. Repeatability is high due to the precise lock-in feature, a spring loaded cassette ensures smooth adjustment, and precision serrations provide firm mounting and security. A locking wedge screw facilitates quick and safe clamping of the cassette.

Kline also invested in a Varilock shank mounting system for added stability and security. In fact, Mr. Kline attributes their ability to maintain a straight, dimensionally consistent groove to the CoroMill/Varilock combination. "The secret to getting minimal axial runout lies in the stability of the cutting edge," he says. "We're getting that stability with the CoroMill 331, as well as repeatability." A locating pin limits setting ranges to provide maximum security, precision and safety within the total setting range.

In addition to other tongue and groove jobs, Kline uses grade GC4030 inserts for light (WL) and general purpose medium to heavy (WM) milling of low alloy, HB 180 steels. The Waveline insert geometry and top surface features high positive rake to reduce cutting forces and divert hot chips away from the insert body. Cutting forces are directed to the best supported portion of the insert for further control. The insert's aluminum oxide coatings are resistant to sticky materials and BUE formation, which are common with heat-treated steels, such as the Rc 30 steel used for Kline's core samplers. These inserts are effective whether milling wet or dry.

There were a few pleasant surprises with the changeover. One of them was the higher than expected increase in feed rate. "Quite frankly, I would have been delighted with a 3:1 improvement to 18 ipm feed rate," says Mr. Kline. "We're at 30 ipm right now."

Kline got the 5:1 feed improvement as a result of a recommendation from Sandvik to adjust the chip thickness. "Keith Stroup told us to get an optimal chip thickness for the 30 ipm," states Mr. Kline. "We increased our setting by at least 400 percent, and it worked like a charm," he adds.

Kline has been standardizing on Sandvik tooling for milling and other operations since 1993. They have purchased the CoroMill 200 system to create blind pockets for gear housing. The tooling, which replaced a mill drill, cut cycle time 8:1. They have also implemented the Q-Cut system for parting and grooving.

Plans for the future are to expand utilization of the CoroMill 331 system to a family of core sampler parts and also generate ACME threads with the same tool. "Right now we're only limited by our fixturing system," explains Mr. Kline. "Once we redesign our holding fixture, I believe we can increase speeds and feeds another 30 percent." MMS

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