Tool failure while roughing scaly castings remains one of the chief headaches facing fabricators of 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,” says Tom Noble, tangential milling manager at Ingersoll Cutting Tools. “One instant the insert is ‘cutting air;’ the next it’s burrowed deep enough into a high spot to snap off.”

He added that today’s best practice for roughing uneven cast surfaces is tangential milling (TM), which presents the insert’s strongest cross section to the brunt of the cutting forces. “As a result, you can dramatically increase throughput, yet count on inserts lasting consistently longer.”

Cases in Point 

Tangential 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 a big G&L mill to eliminate the need for a second machine.

Recent experience at several big fabrication shops proves the point.
Off-road equipment maker Terex doubled throughput of huge tubular transmission boxes and increased edge life by 5:1 with the switch to TM (fig 1). They also eliminated cutting fluids and their inevitable oil-mist cloud. “Debottlenecking our existing machines – reliably – boosts our capacity without big ticket investments,” says Jim Rice, operations manager at Terex’s Ciudad Acuna, Mexico plant. “That’s exactly what tangential milling has done for our operation.” Annualized savings are estimated at $50,000 on that one part alone (annual volume 144 pc).

Hol-Mac Corporation eliminated tool wrecks and spindle blowouts while doubling throughput on a variety of steel castings used in mining and earthmoving equipment (fig 2). Despite the increase in feed rate to 36 IPM from 16, catastrophic tool ruptures – a chronic problem before -- disappeared altogether. On the very first part that the company retooled with an Ingersoll S-Max tangential cutter, cycle time was reduced by 44 minutes per part, saving $125,000 annually. As a result, Hol-Mac 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 steel casting, and ended up with a throughput gain as well. The part is a plate shaped roughly like a pizza box, which goes into the cab mountings on off-road equipment. With a conventional radial cutter, edges lasted six pc. With a tangential V-Max mill, edge life doubled to 12 pc -- even at 25% higher feed rates. Moreover, since the tangential inserts were double sided, insert life actually quadrupled.

Edge Life Predictably Longer

Typical skin milling operation on big steel castings at Hol-Mac. The company has standardized on tangential milling with 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 all three cases, the TM inserts failed solely due to gradual, predictable wear, never from catastrophic rupture. Risk of such serious losses as tool wrecks, ruined high value-added parts, safety hazards, spindle damage or motor burnout is gone. “At the very least, rupture of one edge renders the entire insert useless, regardless of how many good edges are left,” explains Mr. Noble.

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 (fig 3). “Orientation of the insert in a heavy roughing cut contributes at least as much to longer edge life as a wear resistant substrate.”

Lessons from a Matchstick

“Picture a matchstick clamped to the edge of a table with a little bit of the end protruding above the surface” says Mr. Noble. “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 which generates lower cutting forces.

Here is a closer look at the TM operations at Terex, Hol-Mac and Winegar.

Debottlenecking at Terex

The Terex parts are transmission boxes, essentially huge tubes big enough to stand in -- 9 or 12 ft dia, 8 to 10 ft long with walls 4 ½ to 8 in. thick. The bottleneck operation, run on a 40 HP G&L horizontal mill, is to rough-mill about .700" of Rc32 cast alloy steel off both ends. Previously, using a 6.00 inch milling cutter with conventionally mounted inserts, it limped along, taking 3 ½ hours per part, wrecking inserts midway through the cut, and occasionally fogging the work area with oil mist. Now with an 8.00 inch diameter S-Max tangential mill, Terex runs the operation at double the feed rate as before -- 30 IPM vs. 15 at the same 0.100 inch DOC -- and without cutting fluid. Cycle time is now 1 ¼ 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 Mr. Rice.

At one point during testing the new cutter, they doubled the DOC 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 over to tangential milling for all heavy roughing jobs.

No More Blown Spindles at Hol-Mac

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

“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, .100" (4 mm) depth of cut (DOC). For finishing, only the DOC is reduced, to .010".

More recently, Hol-Mac switched to TM for a deep-reach plunge milling operation on a big clevis. The setup uses an S-MAX tangential face mill on a 10 inch 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 also quicker because the processes are more consistent.

Doubling Feed Rate “Conservative” at Winegar

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 tangential milling cutter with a 30 degree 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 routinely index every twelve 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 Mr. Noble. “You don’t replace the engine: you simply get better performance out of the existing machine.”

Ingersoll pioneered the tangential milling concept back in the ‘60s. Today the line has 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 also available on request.

“We’re especially excited at TM’s success and potential in smaller parts and on low HP machines,” says Mr. 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.”
 

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