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Finish Faster, Last Longer

Dec. 6, 2010
For machining rough-surface castings, tangential milling is a technique that will extend the useful performance of cutting inserts.
A tangential Ingersoll S-Max face mill buzzes through the ends of Terex’s big transmission boxes for large construction equipment. The switch to tangential milling doubled throughput, increased edge life by 5 to 1, improved finish and got rid of oil based cutting fluid. The gain freed enough machine time on big G&L mill to eliminate the need for second machine.

Tool failure while roughing “scaly” castings remains one of the chief headaches facing operations that perform finish-machining on large parts. But, the root cause isn’t the scale anymore, thanks to improved insert substrates. “Today the real villain is the uneven cast surfaces,” according to machining expert Tom Noble. “One instant the insert is ‘cutting air;’ the next it’s burrowed deep enough into a high spot to snap off.”

Noble — who is a product manager for Ingersoll Cutting Tools — explains that that today’s best practice for roughing uneven cast surfaces is tangential milling (TM), which is a technique that presents the insert’s strongest cross-section to the brunt of the cutting forces (see Fig. 1a and 1b). “As a result, you can dramatically increase throughput, yet count on inserts lasting consistently longer,” he says.

Recent experience at several important manufacturing centers proves this point. Off-road equipment OEM Terex Corp. doubled throughput of huge tubular transmission boxes and increased edge life by 5:1 with the switch to TM. The shop also eliminated cutting fluids and their inevitable oil-mist cloud. “Debottlenecking our existing machines, reliably, boosts our capacity without big ticket investments,” reports Jim Rice, operations manager at Terex’s Ciudad Acuna plant in Mexico. “That’s exactly what tangential milling has done for our operation.” Terex estimates annualized production-cost savings on that single part at $50,000.

Hol-Mac Corp. eliminated tool wrecks and spindle blowouts while doubling throughput on a variety of steel castings it uses to fabricate in mining and earthmoving equipment. Despite the increase in feed rate from16 to 36 IPM, catastrophic tool ruptures (previously, a chronic problem) disappeared altogether. On the very first part that Hol-Mac retooled with an Ingersoll S-Max tangential cutter, cycle time was reduced by 44 minutes per part, saving $125,000 annually. As a result, the operation has standardized on TM for roughing and finishing more than 20 of the mainstay castings that they run continuously.

Winegar Inc. got into TM simply to reduce tool-up costs for rough milling of a high-volume cast steel plate, and ended up with a throughput gain as well. The plate is a part of a cab mounting for off-road equipment, and it’s shaped roughly like a pizza box. With a conventional radial cutter, edges lasted six pc. Using a tangential Ingersoll V-Max mill, the edge life doubled, even at 25% higher feed rates. Moreover, because the tangential inserts are double-sided, insert life actually quadrupled.

Predictably longer
In all three cases, the TM inserts failed solely due to gradual, predictable wear, never from catastrophic rupture. Gone is all risk of such serious losses as tool wrecks, ruined high value-added parts, safety hazards, spindle damage, or motor burnout. “At the very least, rupture of one edge renders an entire insert useless, regardless of how many good edges are left,” Noble explains.

A typical skin milling operation on big steel castings at Hol-Mac Corp, Bay Springs, MS. The company has standardized on tangential milling with Ingersoll S-Max face mills for initial operations on most big castings. Despite the scale, sand inclusions and very irregular as-cast surfaces on the workpieces, throughput for skin milling has doubled as a result, with edge life rising by 4 to 1 and cutter wrecks a thing of the past.

In a tangential cutter, the flat insert orientation provides much more support and stability behind the cutting edge and a stronger seat pocket and cutter body. “Orientation of the insert in a heavy roughing cut contributes at least as much to longer edge life as a wear-resistant substrate,” he emphasizes

Noble continues: “Picture a matchstick clamped to the edge of a table with a little bit of the end protruding above the surface. Now take a horizontal swipe at it. That unsupported protruding part snaps right off because the cross-section facing the brunt of the forces is so small. But, if you lay the match down on the tabletop, and clamp it, you can hit it the same way and it won’t break. It survives because the force of the blow is absorbed by the entire length of the match.”

The V-Max pocket geometry also creates a positive presentation angle, reducing cutting forces on the insert. This results in a stronger cutting system that generates lower cutting forces.

The parts that Terex finishes are transmission boxes, essentially huge tubes big enough for a person to stand in, 9 or 12 ft in diameter, 8 to 10 ft long with 4.5 to 8 in. thick walls. The bottlenecked operation, run on a 40-HP horizontal mill, involves rough-milling about 0.700 in. of Rc32 cast alloy steel off from each end. Previously, using a 6.00-in. milling cutter with conventionally mounted inserts, it limped along, taking 3.5 hours per part, wrecking inserts midway through the cut, and occasionally fogging the work area with oil mist. Now, with an 8.00-in. diameter S-Max tangential mill, Terex runs the operation at double the feed rate as before — 30 IPM versus 15 IPM at the same 0.100-in. depth of cut — and without cutting fluid. Cycle time is now 1.25 hours, with absolutely no risk of insert failure. “Obviously the chip loads and cutting forces are reduced as well with the tangential system, or we couldn’t have gone with the larger cutter without stalling the machine,” says Rice.

At one point during their tests of the new cutter, the Terex machinists doubled the depth of cut and fed at 25 IPM. The spindle nearly stalled at this rate, but the tangential cutter was unscathed. Based on this success, Terex is switching to tangential milling for all heavy roughing jobs.

No more blown spindles
“During a recent skinning operation, one edge failure triggered a cascade that caused enough vibration to blow out the spindle bearings before the operator had a chance to shut down.”

So said John Scarbrough, Hol-Mac manufacturing engineering specialist. That’s what triggered the company’s transition to Ingersoll’s S-Max tangential milling cutters for all skinning operations. Conversion to TM not only eliminated catastrophic insert failures but also doubled throughput. Accordingly Hol-Mac standardized on the TM process for face milling more than 20 different large castings that the company regularly runs. Standard settings for roughing are 550 SFM, 36 IPM, 0.100 in. (4 mm) depth of cut (DOC). For finishing, only the DOC is reduced, to 0.010 in.

More recently, Hol-Mac switched to TM for a deep-reach plunge milling operation on a big clevis. The setup uses an Ingersoll S-MAX tangential face mill on a 10-in. extension. Previously, chatter fractured inserts even under very gentle cutting conditions because of the instability inherent in such a long reach. Now that operation runs five times faster, with edges lasting 12 times longer and never failing by rupture. In the first year since the transition, savings in machining time and tooling inventory are projected to exceed $1 million – at just one of the three Hol-Mac plants. Deliveries are quicker also because the processes are more consistent.

How Hol-Mac learned about TM is a story in itself. It began when owner Charles Holder noticed a magazine article on tangential milling as a solution for heavy roughing. Then, shortly thereafter, one of his bigger customers suggested TM as a way to improve deliveries, based on their own experience. So Holder called in an Ingersoll application specialist for an assessment and recommendations, and the rest is history.

These diagrams show how the main force vector aligns with strongest insert section in tangential milling cutter. 1a: conventional radial cutter; 1b: tangential milling cutter.

Doubling feed rate
With an 8-effective conventional face mill, Winegar was getting only six pieces per edge when milling the mounting plate in a two-minute cycle. Tool life and cost were their main concerns. Moving up to a 12-tooth Ingersoll tangential milling cutter with a 30° lead angle improved edge life to 12 pc and reduced cycle time by 30 seconds per part.

“We probably could run the inserts longer, but we routinely index every 12 parts as a precaution, given the vagaries in the incoming castings and condition of the machine we use,” says Winegar process engineer Jerry Engrav. The company runs the parts two-up on any of several mills, in varying conditions, depending on which is open at the time.

“I tell many people that putting a tangential cutter on a spindle for roughing a casting is like bolting a turbocharger onto a race car,” says Noble. “You don’t replace the engine: you simply get better performance out of the existing machine.”

In smaller sizes, TM cutters are much stronger because they retain much more metal after seat pockets are milled out. Left: one-inch 4-flute TM cutter body. Right: one-inch 4-flute conventional cutter body.

Ingersoll pioneered the tangential milling concept in the 1960s and has led its development ever since. Today its line if inserts has been expanded to include smaller diameters and improved insert geometries that allow more nonlinear toolpaths. Today, S-Max and V-Max tangential milling tools are available in assorted styles from one to twelve-inch diameters. Special configurations are available, too, on request.

“We’re especially excited at TM’s success and potential in smaller parts and on low HP machines,” according to Noble. “Inherent strength of the cutters coupled with freer machining geometries makes tangential machining the go-to solution for a much wider variety of rough milling work.”