July 2012 / Volume 64 / Issue 7|
Avoiding holding penalties
By Susan Woods, Contributing Editor
| When selecting toolholders for endmills, a sidelock holder or an ER collet may be the best choice, with alternatives offering a new twist.
While every type of toolholder has an advantage in a specific application, some shops just need something that will work most of the time across the board. And for endmilling, a high-quality sidelock endmill holder or ER collet chuck can be the answer. While high-performance toolholders are available, they are not always necessary or affordable.
“With endmilling, you are pushing on the side of the cutting tool. So that means the interface, where the cutter is held by the toolholder, no matter what type, becomes one of the weaker points of the system,” said Alan T. Miller, engineering manager, product manager-BIG for BIG Kaiser Precision Tooling Inc., Hoffman Estates, Ill.
Courtesy of BIG Kaiser
Endmill holders secure the cutting tool with a setscrew. When tightened, the screw engages the Weldon flat on the tool shank. Because that setscrew pushes the tool to one side as it is tightened, endmill holders are not concentric, which increases runout and unbalance. However, they do provide incredible holding force because the screw acts as a physical stop, preventing endmill slippage and pullout when heavy roughing.Concentric Collets
ER collet chucks consist of a collet holder body, a collet that is inserted into the body and a nut that tightens a screw over the collet. The collet forms around the round tool shank and exerts a strong clamping force when the nut is tightened.
Collet chucks hold the cutting tool concentrically, which results in increased tool life, higher feed rates and finer workpiece finishes compared to endmill holders. “Because of the concentricity, the cut is spread out between all the cutting edges of the tool evenly,” Miller said. “With the sidelock holder, because it pushes off from center, one edge of the endmill will be doing more work, and once that edge is worn, the entire tool is worn.”
Collet chucks do not provide as much holding force as endmill holders because there is no physical stop. Collet chucks collapse around the tool shank and use friction to hold it.
“Also, when you are milling, the collet sits a little bit higher up out of the chuck so with that side load you are putting a lot of pressure on the collet and the nut rather than the body of the chuck,” said Brendt Holden, president of Haimer USA LLC, Villa Park, Ill. “You tend to get some deflection.”
Because of their concentricity and balance, collet chucks are effective for spindle speeds up to 30,000 rpm. An endmill holder run at high speeds, because of the high runout and unbalance, will not provide as much tool life and there is a risk of cutting edge breakage.
The main advantage of using a concentric holder like a collet chuck is that, compared to a sidelock holder, it has less runout, which significantly improves tool performance and tool life, especially on tools ½ " in diameter and smaller, according to Brent Godfrey, product and application specialist for toolmaker Sandvik Coromant Co., Fair Lawn, N.J. “With a small-diameter endmill, just a few microns of runout up inside the endmill holder bore can cause excessive runout at the tool tip.”
How much runout is typical? “For high-end collet chucks, about 0.0001 " at 4 times diameter,” Miller said. “Middle-of-the-road collet chucks may go up to 0.0003 " to 0.0005 ". For sidelock holders, best case is around 0.0005 " but values of 0.001 " and up are more common.”
David McHenry, product engineer for Rego-Fix Tool Corp., Indianapolis, disagrees that collet chucks should be primarily used for cutting tools ½ " in diameter and smaller. “People think that with an ER 32 collet chuck, for example, a ½ " tool is all they can endmill with. That’s not correct. It’s because they are using the wrong collet, wrong holder or not setting it up correctly,” he said. “You can definitely endmill with the maximum size diameter the collet will hold. So for an ER 32, which goes up to ¾ ", you can endmill with that.”Pullout Problem
Endmill holders are commonly used where roughing cutting forces might cause the tool to slip or pull out of a less rigid toolholder.
“The advantage of the sidelock holder is that if you are performing heavy roughing, that setscrew prevents the cutting tool from pulling out from the bore, which is something that can happen, especially [when machining] heat-resistant superalloys and titanium,” Godfrey said.
“If you are using an endmill holder with a certain helix angle on the cutting edge—I’m referring to a 50° helix angle and up—the material engages that helix and creates a negative axial force that tries to pull the tool out of the holder,” he added. “The higher that angle, the more that negative axial force is created and the more possibility of pullout.”
Courtesy of Sandvik Coromant
Godfrey noted negative axial force occurs when machining superalloys and titanium primarily because of their strength and toughness, and partly because of their elasticity. “We’ve seen a lot of the aerospace shops standardize on some kind of sidelock holder solution for all their roughing operations because they experience this tool pullout,” he said.
When using a high-helix endmill, aluminum also pulls on the tool because of its gumminess.
Under heavy cutting forces, collet chucks tend to let the tool slip down or pull out. “Collet chucks have good clamping force, but that force just prevents the tool from spinning around inside the bore when it is cutting,” Godfrey said. “The clamping force alone isn’t always enough to prevent the tool from pulling out.”
BIG Kaiser’s Miller noted that as the cutting tool starts to pull out and the collet chuck allows it to slide, the tool may lengthen. “If you are trying to do any critical features or control any depths, the tool is now longer than it should be and you’re going to cut material you don’t want to.”A Different Answer
Shops are always looking for something better than the endmill holder, but because of its ability to overcome potential pullout, they are forced to keep using it, particularly when heavy roughing.
“We were challenged by an aerospace customer that said they want to take very aggressive roughing cuts but are tired of having to use the sidelock holder because it compromises tool life and balance and the surface finish is not as good,” Holden said.
Courtesy of Haimer
So Haimer developed the Safe-Lock holding technology and has integrated it into its Power Collet Chuck.
“We have partnered with cutting tool manufacturers and they are now marketing their tools with what we call the Haimer groove,” he continued. “Our goal is to eventually replace the Weldon flat with the Safe-Lock groove as a standard.”
With Safe-Lock, the user inserts the cutting tool with its spiral-shaped grooves into the chuck. The tool positively interfaces with pins in the rear of the collet, preventing the tool from twisting or being pulled out of the chuck.
In addition, the design allows the collet to sit deeper in the chuck, preventing the inherent issue of deflection under side load applications that is typical with an ER collet chuck.Proper Use is Key
Everyone interviewed for this article agreed one common problem is overtightening a collet chuck. Overtightening distorts the collet and diminishes holding strength and accuracy.
“As you tighten the nut, which comes down and makes contact with the faces of the collet, it creates friction,” Rego-Fix’s McHenry said. “If you overtighten, that friction wants to twist the top of the collet. Instead of more clamping force, it reduces it because with the overtightening, you are creating gaps in the contact path and introducing more runout.”
The proper way to tighten a collet chuck is to follow vendor torque recommendations, which vary. “For an ER 32 collet chuck, it can be anywhere from 50 ft.-lbs. to 75 ft.-lbs.,” Miller said.
Overtightening can be avoided for the most part by using a torque wrench. “When operators clamp the nut on the collet, they try to get it really tight,” Sandvik Coromant’s Godfrey said. “They put a standard wrench on it and hit it with a hammer, or they put a big bar on the wrench and just tighten it as tight as they can. And the thread that is on the chuck eventually ends up cracking. Torque wrenches are often recommended but are typically considered a spare part, adding cost to the tool, so a lot of shops just don’t buy them.”
Another common problem is not cleaning the collet chuck. The chuck, collet and nut must be thoroughly cleaned before assembly to maintain accuracy. Any dust or dirt around the collet bore affects runout and clamping force.
Also, the collet is designed to wear and be replaced, which requires keeping track of collet use. “They only become aware of it if their tool life suddenly decreases or runout increases,” Miller said. “But careful inspection and cleaning of the collet, as well as regular replacement, is suggested.”Lack of Training
McHenry believes part of the problem is people are not properly trained on how to use a collet chuck. This includes knowing what a backup screw does to an assembly.
“In most collet chucks, you have a screw located behind the collet—the backup screw, which people use incorrectly,” he said. “They use it to set the tool length. So they put the cutting tool in, drive that screw up and have the tool hitting the top of that screw while tightening the nut. As the collet tries to pull down, the tool can’t move with it because it is sitting on top of a screw, so all your tightening force goes into friction. You end up having less clamping force.”
Courtesy of Rego-Fix
Another common mistake is shops grinding flats on round tool shanks. Carbide endmills are not usually supplied with a flat for endmill holders, so “a lot of shops grind their own flats, which is not good at all,” Haimer’s Holden said. “They do it unevenly and then the setscrew is not holding right, which can lead to breakage. They take a $300 cutter and then use a little hand grinder to put a flat on it.”
It looks like those flats are going to be needed for a while, though, as there is still a place in the industry for endmill holders. “Unlike in Europe, where they do a lot of high-speed machining and can run collet chucks very easily,” McHenry said, “in the United States, we still like to take an endmill holder, bury it as deep as we can to take the material out, and then come back in a second operation and clean it up.”
However, from a cutting tool manufacturer’s standpoint, “we always recommend keeping up with the evolving technology,” Sandvik Coromant’s Godfrey said. “Every year, there are new holding products that come out that create a lot better performance and holding power and are worth the cost sometimes if you are having trouble with ER collet chucks or sidelock holders.” CTE
Courtesy of Sandvik Coromant
Exploring the milling chuck option
Courtesy of BIG Kaiser
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