Getting rough with inserts

Surface texturing puts inserts in their place.

Surface texturing puts inserts in their place.

Rough sport a spiral pattern that machined into their This textured surface helps inserts securely in the pocket cutting operations.

Ceramic inserts, by their very nature, are hard to keep around. They run fast and hot and refuse to stay in one place for long. But now, they have no choice thanks to a textured pattern found on new Rough Stuff inserts. Even though it looks more decorative than functional, the highly engineered texture can withstand more pull-out force than dimpled inserts.

Hot-pressed silicon-nitride inserts were the first to have the precision spiral texture machined into their surfaces. When any top clamp tightens down on a Rough Stuff insert, the clamp actually becomes embedded in the surface for an incredibly tight hold.

"The concept is almost too simple," says Dave Rydbom, national accounts manager with Greenleaf Corp., Saegertown, Pa., "but it will take productivity to a new level." Strewn across his desk are a bunch of ceramic inserts that look pretty much the same until Rydbom points out one crucial difference. Most of the inserts have small holes where a lock pin would go through. Two, however, have the spiral design instead of a hole.

This little design is a big deal because ceramic inserts have long had a reputation for being hard to hold in the pocket. These hard, finely ground tools must be run at high speeds and temperatures to be effective, but such conditions cause the inserts to slip in the cut.

When ceramic inserts were first introduced, they were held in place by a top clamp and a mechanical chip-breaker and, eventually, by just clamp. In both cases, the pull-out (coefficient of friction the gripping force) can overcome the gripping at high speeds, causing some movement of these "flat-top" inserts.

Next came pin-lock holes, which hold inserts in place fairly well, says Rydbom, but also weaken the cross-section of the insert. "With ceramics, you want to keep as much of the solid integrity of the ceramic as you can," he comments. To deal with this problem, the cutting tool industry devised recessed inserts.

These recessed tools have been a hot commodity in recent years, especially in the automotive sector. Companies making these tools use a cold-pressed silicon-nitride material and, rather than a through-hole, place a "dimple," or pocket, in the insert. A top clamp that fits into the dimple impression tightens the insert down.

"We feel that the dimple presents some problems," comments Rydbom, "First, although you don't penetrate the insert, you do get a thinner web in its middle. This is not as fragile as a hole, but it could be a weak point."

Secondly, he points out, every company making a recessed system has its own dimple pattern and matching clamp. This makes it critical that the clamp fit precisely into the dimple to avoid point contact, where only one part of the clamp touches the insert. This condition also happens when the clamp doesn't match the impression exactly, or the clamp it-self doesn't have a flat edge. "If the clamping surface isn't consistent, the insert can rock in the pocket," remarks Rydbom.

Finally, shops using recessed tools are often locked into one supplier for both the insert and top clamp. "Even though Greenleaf's hot-pressed silicon nitride, GSN, has a much higher speed capability than cold-pressed ones, which lets you run machines faster at the same feed-rates, the clamping device was always a stumbling block to companies using our tools," Rydbom says.

Greenleaf found that once a shop tooled a machine with a recessed clamping device, it was hard to convince it to change — even if hotpressed tools could do a better job. "Really, the productivity that a hot-pressed silicon nitride can offer versus a cold-pressed was the reason we went to the lengths we did," explains Rydbom.

The Rough Stuff textured surface is the result of six months of intensive product development by Greenleaf. Six months may not sound long, but in that time, the company engineered and conducted tests on 39 different patterns to find the one with the optimal gripping texture to hold the insert in place.

Finding the best pattern, however, wasn't the only challenge Greenleaf faced. It also had to determine the best way to machine the intricate design into different grades of hard ceramic. Just how Green-leaf machines the surface is proprietary, but Rydbom does say that his company uses a special CNC machining center to hold a ±0.003-in. texturing tolerance.

Currently, Greenleaf offers Rough Stuff in GSN (silicon nitride) and WG-300 (whiskered ceramic). "We have plans to expand into other grades," Rydbom says, "but because this is a machined surface, different issues come up with each ceramic grade. We have to address each one, because we want to provide a consistent clamping surface all the time."

How Rough Stuff works
A ceramic tool is hard, about 92 Ra or better, while the clamp that holds it runs only in the 40 Rc range. Rough Stuff takes advantage of this difference. "When you torque the clamp down onto the Rough Stuff surface, you actually imbed the clamp into the surface," says Rydbom. Rather than point or line contact, this provides multiple surface contact for a vise-like grip. "Literally," states Green-leaf's James Meehan, "the clamp becomes part of the insert. For the insert to move, the clamp has to be sheared off."

"It's not a gimmick," claims Rydbom. "The insert has the same strength and integrity as one without a hole or a dimple in it. It's almost fail-safe. If you're torquing it down properly — we recommend a minimum of 80 in.-lb of torque — and the insert is seated correctly, you get an impression-type grip that stabilizes the insert in the cutting application."

The impressions left in the clamp do not present problems as an insert is indexed. Says Rydbom, "The precision-machined surface simply makes new impressions in the clamp."

Shops using the new inserts are enthusiastic, comments Meehan. "Rough Stuff has great operator appeal. They can feel the clamp imbedding into the insert, and it gives them a sense of security that it will hold the insert properly in the pocket."

Another attractive aspect, says Rydbom, is that the inserts work with any top clamp. This lets shops reduce inventory costs and gives them another source for their ceramic cutting tools.

Among the companies already using the inserts are those suppliers making brake parts, rotors, and drums. "At the onset, we focused on cast iron machining — brake rotors and that type of industry — where you have problems with microvibrations through the cutting tool," says Rydbom. These microvibrations can work their way through an insert to the clamping surface, causing microchipping in the insert, premature wear, and insert movement that can lead to size and finish problems. The Rough Stuff's multicontact clamping surface, though, can minimize the effects of microvibration on the tool.

Greenleaf also believes the inserts can help out the high-strength alloy finishing industry. "When you're doing a final cut on a jet-engine turbine, for instance, it's critical to hold a ceramic tool in place," says Rydbom. For this reason, many of these manufacturers use a ceramic tool for the rough cut and then a carbide insert with a pin-lock hole for the finish cut. With Rough Stuff, a shop can use the same insert and clamping device for both roughing and finishing, saving time and reaping the benefits associated with ceramic tooling throughout the machining process.

Rough Stuff at work

So far, field testing of Greenleaf's Rough Stuff inserts has shown significant benefits as compared to cold-pressed silicon-nitride ceramic inserts with dimple impressions.

In one test, a straddle cut on a cast iron brake rotor, Greenleaf ran both its insert and a competitor's at 2,870 sfm, feeding 0.0165 ipr. The depth of cut was 0.060 to 0.100 in.

The results were positive: The Greenleaf insert produced 120 parts, versus the competitor's 50.

Another field demonstration, also a cast iron brake rotor, compared Rough Stuff versus yet another company's dimpled ceramic insert. This time, the competitor's insert was run at 2,300 sfm and fed in 0.0197 ipr on a rough pass and 2,600 sfm and 0.0138 ipr on a finish pass. The customer required a 3.0-µ surface finish, but the competitor's insert only achieved 3.5 µ. Greenleaf's insert was run at 3,300 sfm and 0.0197 ipr on the rough pass and 4,000 sfm and 0.01 ipr on the finish pass. The resulting finish was 1.5 µ.

TAGS: Features
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