A Clean Break: Turning Performance

Parting, or cutting, off is a delicate, sometimes dangerous operation. When separating a part from bar stock, the spinning part can fly off and damage the cutting tool or machine. If the blade is misaligned, the part may not separate cleanly or not separate at all, and if the proper tools are not applied, chatter can ruin the surface finish or even break the insert.

However, with proper setup, tooling and fluid application, the cutoff operation is manageable in virtually any machine shop.

Image courtesy Horn USA.

Machine Matters

An ideal situation for parting off, also known as a “pick-off,” is when the subspindle clamps on the finished end of the part while the main spindle clamps on the bar stock, which keeps both sides of the cut-off operation rigid, reducing risk of the part damaging anything, according to Cooper Ferguson, applications engineer at machine tool builder Okuma America Corp., Charlotte, N.C. However, many machines don’t have a subspindle, so parts catchers are a good alternative. Beyond that, one machine characteristic is important above all others.

“Rigidity is paramount in all areas,” Ferguson said. “If any part of the setup is not rigid, you’ll have chatter, which could scrap your part, throw things out of balance, or potentially damage your tool or machine if the part is ejected. And, of course, you want to make sure you ensure good chip evacuation and don’t exceed the maximum rpm in order to maintain tool life.”

However, machine shops do not exist in a perfect world.

“There are things we, as a tooling company, can control and things we can’t,” said Eric Carbone, applications specialist at Horn USA Inc., Franklin, Tenn. “One of the things we can’t control is the type of machine the customer uses. Ideally, you want the machine tool to be very rigid, but, ultimately, we’re dealing with what they have.” Whether it’s a high-end piece of equipment or an outdated model influences the recommendations a toolmaker provides.

Iscar’s PentaIQGrip parting inserts have five cutting edges and a geometry designed to handle virtually all workpiece materials. Image courtesy Iscar Metals.

The machine itself must be accounted for, but what is done with the machine is more important. According to Clay East, GRIP systems product manager for toolmaker Iscar Metals Inc., Arlington, Texas, tool alignment is the single most important consideration, regardless of the tool or machine.

“The first thing I do when I look at an application is mount the tool and make sure it’s in the correct position,” he said. “Even if you’ve got the best tool in the market, it won’t compensate for misalignment; it won’t compensate for the cutting edge not being where it needs to be.”

Keeping the tool on center is paramount, but the methods for doing so differ depending on the diameter of the bar stock. It’s not a simple set-it-and-forget-it rule.

“For cutting off, you should always use the shortest possible tool length,” said Horn’s Carbone. “If you’re cutting a 4 “-dia. part and your tool is 0.0050 ” off center, that might not be as big of a deal as when the part gets down to 0.5000 “. That difference becomes proportionally much greater.”

With 4 “-dia. (101.6mm) stock, the insert blade must be at least 2 ” long, which creates a lot of potential for the blade to vibrate and flex. “Don’t go any longer than you have to,” Carbone said.

Another consideration for rigidity is insert width. Selecting the widest insert possible without sacrificing material cost will allow for the most support under the insert, creating the best rigidity.

While blade flex is less of an issue with smaller bar stock, it is an important consideration that is often overlooked by machine shops despite being widely recommended by some toolmakers and machine tool builders, according to Carbone. “A lot of manufacturers recommend running small-diameter parts, typically anything under ½ “, slightly above centerline, and most people don’t do that. The theory is you set the insert 0.001 ” to 0.002 ” above centerline, so as you approach zero on your size, the material will start to climb the part. By the end of it, you will be back on centerline.”

Need for Speed

Once the tool is properly centered, it’s time to consider speed. Iscar’s East noted there is a right and a wrong way to measure that speed. Whenever possible, Iscar recommends using surface footage instead of spindle speed.

“Constant surface footage means the contact of the material to the carbide insert is consistent,” he explained. “If you program something for the same surface footage all the way through the cut, it’s going to stay there, and the carbide won’t know if it’s a 4 ” OD or a 1 ” OD, because the material that slides across the carbide will be at the same speed throughout the entire cut. On the flipside, if you program with constant rpm, the closer you get to center, the slower the material will be moving across the cut, which negatively impacts the ability of the insert to plasticize the material.”

However, constant surface footage brings its own risks, noted David Essex, turning product manager for toolmaker Tungaloy America Inc., Arlington Heights, Ill. As the tool goes in toward the center of the spindle, the rpm must increase to maintain a constant surface speed.

“Potentially, the part could fly off and damage the machine tool,” Essex said. “On the other hand, keeping a constant rpm could put more pressure on the cutting tool, shortening insert life and possibly leading to breakage.

“The best practice when you part to center to reduce tool pressure on the cutting edge is to program the job at the suggested feed rate for the particular tool you’re using and feed it down to the diameter that is equal to the width of the insert,” Essex continued. “At that point, you want to reduce your feed rate by 50 to 75 percent. Backing down on that feed rate will minimize pip size, minimize vibration and extend tool life tremendously.”

Additional machine features are available to assist the operation. For instance, Okuma offers spindle synchronization to align the jaws or part features from one side to the other, air blow to clear chips and other debris, and torque skip, which utilizes feedback sensors to indicate when the jaws touch the part and how much pressure is being applied.