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The State of the Art of Milling is Really Quite Thrilling

 

Advances in tooling, workholding and software deliver benefits in terms of speed, efficiency, quality and profitability


Troy Stashi
Product Specialist Milling
Sandvik Coromant
Fair Lawn, NJ

For anyone who owns, operates, or manages a machining center, these are exciting times. Never before has there been such an awesome assortment of high-quality cutting tools. Newfangled toolpath algorithms are removing metal like there’s no tomorrow. Toolholders grip tighter and more accurately, vises and fixtures can be swapped out in the blink of an eye. If your shop isn’t taking advantage of all the industry has to offer, the one thing you can be sure of cutting is profit margin. Here are some new tooling developments together with a few best practices to think about while you watch the spindles go round.


The Right Tool for the Job

It seems simple. Hole drilling operations are best done with a drill, face milling should always be performed with a face mill. But many shops see great success using center cutting end mills to drill holes, albeit shallow ones. Why bother with a face mill when a shoulder mill is handy? And what’s wrong with using that old standby, the two-flute, 1/2″ (12.7-mm) solid carbide end mill, to cut everything within reach?The right tool being applied to the right job: Shown during a finishing operation, this tool is specificially designed for the automotive industry, with eight cutting edges. The same multiedge inserts are used as working inserts and wipers inserts, mounted in the same position.

Using a less than perfect tool for any given operation might be an expedient choice, but most would agree that making a hole with a center cutting end mill is nowhere near as efficient as drilling one with an actual drill bit. Some of these bad habits evolved from the days of manual tool changers and 200 ipm (5080 mm/min) rapid traverse rates, when swapping out tools was a painfully slow process, but today there’s absolutely no reason to skimp on cutting efficiency to save a second or two on a tool change. And if you’re worried about the time needed to touch off a few extra cutters, thinking it’s easier to use what’s already in the carousel, then you’re not taking advantage of offline presetters and quick-change tooling. Shame on you.

Some of this “get ’er done” mentality extends to carbide as well. A low-volume job shop might get by with a general-purpose PVD-coated grade to cut Inconel one day and aluminum the next, but this less-than-optimal approach invariably leads to longer cycle times and poor tool life. For example, improvements in carbide molding and grinding techniques are producing unorthodox insert shapes with a dozen or more cutting edges, promising to greatly reduce tooling costs.

Coating chambers are getting smarter, as are their owners, providing us with such innovations as unidirectional CVD to increase wear resistance, and edge-line secure PVD coatings that resist flaking and chipping. TiC and TiN have evolved to multiphase TiAlN, AlTiN, ZrN, TiAlCN, MT-TiCN, Al203, and a host of application specific grades able to tame even the toughest of materials.

Carbide itself has improved as well. Grain sizes have become progressively smaller, going far beyond the now blasé micrograin carbides introduced decades ago. Cobalt rich, submicron substrates are now common, as are dual property “gradient” substrates that pair cubic carbonitrides for hot hardness at the cutting edge with tungsten carbide binders beneath, offering a best of both worlds solution to cracks and chip hammering.

The message here is that cutting processes in any shop should be carefully analyzed. The right tool, the right geometry, the right grade and chipbreaker—perhaps perfection is out of reach, and in some cases unnecessary, but there’s no reason to settle for a solution that’s not at least darned good. With most cutting tool manufacturers boasting dozens of grades and literally hundreds of insert shapes, styles, and sizes, huddling with a knowledgeable application engineer for some fine-tuning can lead to big payoffs downstream.

Don’t Discount Solid Carbide

In a large percentage of machined parts, ripping away metal as fast as possible is the surest way of making a profit. Maintaining accuracy and surface finish, reducing setup time and improving tool life—these are all important factors to successful machining, but these may come to naught on larger workpieces, where roughing operations comprise the lion’s share of metal removal. Insert cutters are often a cost-effective strategy for heavy milling, but don’t discount solid-carbide end mills.

Some shops settle for technology that predates NC machine tools. High speed steel (HSS) roughing end mills—a.k.a. corn cob cutters—are super tough, relatively inexpensive, and care little for chip loads or recutting. Just bury them deep and go. Unfortunately, these low-cost cutters may end up being the most expensive tools in the shop in terms of lost productivity.Optimized security and high metal removal rates are two key factors when milling titanium and other extremely hard metals.

Solid-carbide roughers are admittedly expensive. A 1″diameter tool can easily cost $600 or more, five to six times that of HSS. And while shop owners and procurement folks might shudder at the price tag, solid carbide offers the ultimate in metal removal performance. Many are equipped with differential pitch flute configurations to reduce harmonics and vibration under heavy loads. Special edge prep, chip-breaker geometries, and optimized flute designs allow for full width slotting capabilities up to 2× depth.

But just because these cutters can plow metal like their HSS counterparts, it doesn’t mean they should. Trochoidal and slice milling techniques are proven productivity winners, and their low radial engagement, high-feed cutting principles are just as applicable here as they are with indexable tools. Nor are solid-carbide tools limited to hogging operations. A number of cutting tool manufacturers now offer high-helix end mill designs with six or more flutes per tool. These make short work of finishing and superfinishing operations, and are also effective in constant cutter engagement, high-feed milling (HFM) applications.

Keep it Quiet in There

With the increased popularity and use of five-axis machining, tools have to reach farther than ever before to clear fixtures, clamps, and vises. Moldmaking operations require cutters that can dive deep into cavities, often while machining hardened steel at feed rates and spindle speeds that would have been laughable 20 years ago. Multiaxis aerospace parts have complex shapes and are made of robust materials such as titanium and Inconel, metals notorious for high cutting pressures, premature tool wear, and chatter. Built up edge on the cutting tool, material inclusions in castings and forgings, and interrupted cuts such as a cross hole in a bore also introduce vibration into the machining process, an unpleasant condition that can quickly escalate into scrap parts and total tool failure.

The best way to avoid these noisy situations is with a rigid setup. Stub tools up as much as possible, employ carbide tool shanks and make sure workholding offers no chance of part movement or flex. When this isn’t enough, however, a more proactive solution is called for. Several tooling manufacturers offer dampened toolholders and boring bars, which typically utilize a heavy mass mounted on rubber elements within the tool body, and surrounded by a viscous fluid. By utilizing the largest body possible relative to the cutting tool itself, and employing sound programming techniques to minimize loads—climb milling with consistent depths of cut, for example, and rolling into the workpiece—depth-to-diameter ratios of 8× are possible, twice that of nondampened tools.

CoroMill 419 uses a 19° lead angle to reduce forces on the machine, enabling the application of higher feed rates.One final consideration is the toolholder itself. Back in the day, Weldon shanks and ER collets were the preferred—and often only—way to hang on to cutting tools. And while those tried-and-true toolholding strategies still have their place, they’ve largely been usurped by stronger, faster, and more accurate ways to grip cutting tools. Much of this technology has been driven by the increase in spindle speeds on machining centers today—with ranges of 20,000 rpm and higher the norm, the need for well-balanced toolholders with minimal runout is critical.

Shrink fit is one of the favored contenders in this arena. Heat up the gripping end of a toolholder, slide in a carbide tool, and within seconds an interference fit of 0.001–0.002″ (0.025–0.050 mm) is created. Depending on the tool diameter, this can easily generate upwards of 6000 pounds in gripping force and tool runout of 0.0002″ (0.005 mm) or better, making it suitable for the majority of roughing applications as well as very accurate finishing of almost any material imaginable. The downside to shrink-fit toolholding is one of investment: the equipment needed to induction heat the tools starts at around $15,000, and perhaps twice that for production units. There’s also some investment in time, as it takes perhaps 6–10 seconds to heat the tool, and slightly more than that to cool it down.

Hydraulic toolholders are another robust way to grip tools. Offering equivalent accuracy and gripping force as shrink fit, hydraulic holders are simple to use, require no special equipment, and can be purchased with internal antipullout pins for heavy roughing. This is especially important when machining titanium and other materials that tend to “grab” end mills and gradually draw them out of the holder while in the cut. Some attempt to counter this phenomenon has been made using Weldon flat end mills, but A) Weldon holders are inherently unbalanced, making them unsuitable for high-rpm milling, and B) the holding screw in Weldon flat holders has been known to work itself loose during heavy cuts, leading to catastrophic results.
Using helical interpolation and linear ramping to create holes from a solid workpiece requires a strong insert face geometry.
Indexable inserts are prone to movement as well. Microshifting is often difficult to detect, but it causes unpredictable process control and negative impact on tool life and part quality. Here, too, cutting tool manufacturers continue developing creative ways to lock tools in one place and keep them there. Look for designs that incorporate locking rails on the sides or bottom of the insert. Shimmed toolholders—though slightly more expensive—generally offer greater insert life and lower costs in the long run. And don’t skimp on maintenance. Routine cleaning of insert pockets, replacement of screws, shims, and clamps, and judicious use of lubricant on threaded parts are important ways to extend indexable cutter life and improve metal removal. And when tool pockets wear, replace the tool or send it back to the manufacturer for reconditioning. Keeping tools past their prime is a recipe for failure.

Ditch the Deburring

It’s unavoidable: machining creates burrs. Many of us remember the days when freshly machined parts would leave the milling department only to spend days or weeks in deburring. There, a group of meticulous workers would sit at microscopes, scraping away burrs, picking at hole intersections, sanding and buffing workpiece edges until smooth. Maybe your shop still employs those patient people—if so, it might be time to rethink their positions. A variety of indexable chamfer cutters and exchangeable head milling systems are available, offering cost-effective and flexible ways to produce burr-free parts without human intervention.

For example, dedicated chamfering tools should be used to break sharp edges while still in the machine, or consistently prepare parts for downstream welding operations. Threaded hole chamfering is easily accomplished with specialty cutters able to reach the front and back side of the part. Today’s CAM systems can generate the toolpaths necessary for rotary deburring tools and ballnose cutters to interpolate difficult hole intersections, and remove burrs from complex sculpted surfaces. And five-axis machining centers can tilt and rotate workpieces at whatever angle is needed to make even square-nose end and shoulder mills into effective chamfer tools.

For those old-schoolers who argue CNC spindle time should be spent machining parts, leaving the scraping work to humans, think again. Every minute a human spends touching a part is a minute of lost profit. In-process machine deburring is fast, predictable, and easily accommodates whatever part complexities present themselves. If your shop is still scraping by hand, it’s time to toss out that blade and start using your CNC equipment to its full potential. Scrapers are best used on frosty windshields.

 

Get with the Program

Times change. That tired adage is applicable to most machining operations, but especially so with cutter toolpaths. In yesterday’s programming world, milling cutters used wide radial depths of cut and shallow axial engagement to remove metal, like scraping your finger across the icing on a birthday cake. The result was high heat in the cut, heavy spindle loads, and cutters that wore out quickly, leaving much of the tool length unused.

There’s a better way. Designed originally for cutting heat-resistant super alloys and for hardened steels, today’s CAM developers employ a variety of catchy and proprietary names to describe toolpaths the complete opposite of yesterday’s “hog it and hope” approach. Most employ a phenomena known as chip thinning, which relies on shallow cutter engagement—often no more than 10—15% of the tool width—to produce a C-shaped chip that starts out thick but tapers as the cutter exits the workpiece; pressed flat, the chip profile resembles an icicle.

Since each flute is engaged far more briefly than with conventional cutting, there’s less heat generated and tool life improves dramatically. Rubbing and tool pressure is likewise reduced. This results in surprisingly high feed rates, often 10 times that of yesterday’s outdated methods. The best part of this approach is that greater axial depths of cut are possible; often the cutter’s entire flute length can be engaged, making the most of solid carbide end mills and indexable shell cutters.

There’s more. Constant cutter engagement maintains consistent tool pressure through the toolpath, eliminating the bumps in the road that can easily break a cutting tool. Slicing techniques clean out pocket corners using bite-sized pieces, avoiding broken tools and the general unpleasantness that comes when tools get buried between intersecting walls. Smoother cuts, less wear and tear on machine tools, predictable processes and more parts out the door—these are just a few of the benefits that come with modern metal removal techniques. If you’re still programming in the ’90s, you should take a hard look at a new CAM system. Your bottom line will thank you.

This article was first published in the May 2016 edition of Manufacturing Engineering magazine. Read “The State of the Art of Milling is Really Quite Thrilling” as a PDF.


Published Date : 5/1/2016

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