Cutting Tools: Good Vibrations
- June 6, 2017
Advanced cutting tools eliminate vibration during machining while improving part surface finishes
Once chatter starts, it’s enough to send even the hardest of hearing among us running for the ear plugs. The machine enclosure rattles, the floor vibrates, the high pitched squeal can be heard even in the parking lot. Cranking the feedrate and spindle overrides in either direction does little but change the frequency of the howling, and unless you’re lucky, the part’s probably headed for the scrap bin.
Fortunately, there’s no reason to live with chatter. Cutting tool manufacturers have addressed your high pitched pain and imminent hearing loss with bold new cutter designs that stop chatter in its tracks. One of these is Dormer Pramet. Product specialist Dan Cormier says the company’s been offering variable pitch, variable angle solid carbide end mills in Europe for a number of years, but will soon release these to North America in both inch and metric sizes.
“With conventional end mills, the flutes strike the workpiece at regular intervals,” Cormier says. “This tends to create a harmonic effect, which manifests itself as chatter and vibration. But by varying the flute pitch and helix angle slightly on alternating flutes, these harmonics are immediately dampened.”
The difference between each flute is small. Spacing and helix angles are offset by only a few degrees in either direction, depending on the manufacturer, yet this is often enough to break up the metronome-like tempo (albeit one with a very fast rhythm) of carbide striking metal.
But there’s far more to curing chatter than cutting tools. Cormier says a properly balanced toolholder is every bit as important to eliminating machining vibration, as are toolpaths, the type of machine, and the material being cut. “Chatter is especially problematic at lower rpms, which is where you typically find super alloys and stainless steels. Variable helix end mills often help in this application, but you still need to select the correct spindle speed to avoid any harmonics. Companies like Blue Swarf and Manufacturing Automation Laboratories here in Canada offer tools that help determine the ‘sweet spot’ for a given machining application.”
Not so fast
As speeds increase, the rules change. Get much beyond 20,000 rpm and variable pitch end mills are generally not used. This is due to the extreme toolholder assembly balance requirements that come with high speed machining, something impossible to achieve when the cutting tool itself is inherently “not balanceable.”
Also, the machine tools used for high rpm milling, which often have ceramic spindle bearings, are less forgiving of vibration. “You can’t wrap a neat little bow around variable pitch end mills and say they work for everything,” Cormier warns. “You need to look at the entire process. But in the case of high speed milling, they generally cannot be used, because any imbalance might cause the spindle to shake itself to death.”
Dan Doiron, milling products manager at Emuge Corp., says the type of toolholder used is also important. Many of the company’s Top-Cut VAR and other lines of variable helix end mills are offered with a Weldon shank, a feature that, according to Doiron, presents a no-slip way to grip end mills.
If you’re shaking your head over the thought of Weldon style end mills, which many dismiss as old school and out-of-balance, not so fast. Doiron points to Emuge’s FPC toolholding system, an all mechanical collet style chuck with an optional pin lock collet system that engages the Weldon flat, thus balancing the assembly and preventing any movement during heavy cutting. “It’s a robust toolholding solution with guaranteed protection against pullout,” he says.
Regardless of the type of toolholder used, Doiron and Cormier both recommend trochoidal toolpaths. “Bury the tool in the Z axis, and use a small step over and constant cutter engagement,” says Doiron. “That’s what most end mills are designed for nowadays. They last longer that way, with predictable wear, and when used on one of today’s faster, more accurate machine tools, it makes for a good combination all around.”
Watch the path
Trochoidal toolpaths stand to benefit the most from new variable flute cutting tool technology. “Get much below 20 per cent radial depth of cut and you’re thinning the chip so much that two things happen, both of which are bad,” adds Cormier. “One is that you can’t transfer enough heat into the chip, because it’s not thick enough. Secondly, the tool’s not biting like it should, so it chatters. This is especially true with a 90° cutter, where the forces tend to push the tool away from the workpiece wall, and is another example of where a variable pitch end mill really helps.”
Even in situations where chatter isn’t evident, Cormier notes that surface finish may be improved through the use of a variable geometry end mill. That’s because the tiny cusps and waves on the part surface are reduced when the flute and helix spacing are unequal, sort of like mowing the lawn with irregular step over amounts. Of course, achieving good surface finish and maximum cutter life requires that tools are setup properly (zero runout), held rigidly, and properly maintained: this is why everyone interviewed for this article agrees that variable pitch end mills (and perhaps all high performance cutting tools) should in most cases be sent back to the manufacturer for re-sharpening.
Paul Jakala, director of product development at LMT Onsrud LP, prefers to call it dual helix. “A true variable helix end mill is one where the helix changes within the flute from the front of the tool to the back,” he says. “We’ve done that several times for aerospace customers with very demanding titanium applications, but most companies—us included—prefer to keep the helix angle constant.”
Whatever you call it, alternating the flute pitch and helix angle not only reduces chatter, it also extends tool life. “Dual helix end mills like our Max-Q line do a better job of protecting the cutting edge and stabilizing the tool in the cut,” Jakala says. “This eliminates chipping at the corners of the tool, which is usually the first thing to fail.”
That assumes users pay attention to the cutting tool manufacturer’s feed and speed recommendations. Jakala says end milling isn’t a “plug and play” situation, and that going with what you know may result in having cutting parameters too conservative for high performance tooling. That’s why he welcomes those customers willing to reprogram their machines according to his advice, allowing him to show off what his tools are capable of.
Keep it home
IMCO Carbide Tool Inc. is another supplier eager to show off its wares. President and owner Perry Osburn says the most successful shops are the ones that pay close attention to their tooling selection, leveraging whatever technology is available to improve their metal cutting operations. “When I was a sales rep, I’d run into shops that purchased their cutting tools based solely on cost, instead of what those tools can do for them,” he says. “A lot of those companies have since gone out of business.”
Like the others in this article, Perry says efficient machining is a combination of many things, with high performance cutting tools just one piece of the puzzle. Successful shops match their tooling selection to the capabilities of the machine tool, the rigidity of the workholding, the CAM system, the materials and shapes being cut, as well as the part volumes being produced.
“It’s a really a combination of all those things, together with good tool geometry and coatings that makes the process effective,” says Perry. “However you go about it, it’s important for shops of all sizes to embrace high efficiency machining techniques. We as a supplier of tooling don’t want to see anyone’s business going offshore. We want to see North American companies step up and adapt these techniques, and therefore become able to compete one-on-one with anybody in the world. That’s the message we as tooling suppliers need to convey to our customers in the machining industry.” SMT