Get a Handle on Your Chips

Chip thickness, curving, breaking, and evacuation are all important to the overall machining process

Selecting the proper chip breaker requires
examination of chip
formation in relation to feed rate, spindle speed, and depth of cut. Photo courtesy of Kyocera Precision Tools.

Selecting the proper chip breaker requires examination of chip formation in relation to feed rate, spindle speed, and depth of cut. Photo courtesy of Kyocera Precision Tools.

Multiple factors affect chip breaking, including chip thickness, chip curving, and the material’s properties. But why is it important to control chip formation and evacuation? What goes wrong if control is lost?

“If chip breaking is not controlled, it really slows down the operation because you’ve got these uncontrolled chips getting caught in chip conveyors. They also use up much more volume in the chip bin, so you end up having to clean out the chip bin much more often. Long chips also can catch around the tool or the workpiece, affecting the surface finish,” explained Kurt Ludeking, product manager for Walter USA.

If you want to get a handle on chip breaking when turning, observe the colour, size, and shape of the chips being produced.

“Chip colour and chip size can tell you a lot about the application. Judging by the chip colour, you can determine if the heat is going into the chip or the workpiece,” said Paul Rice, applications engineer for Kyocera Precision Tools. “When looking at chip formation, you can determine the proper chip breaker selection in relation to feed rate, spindle speed, and depth of cut.”

The shape of the chip also tells you how well the chipbreaker is working.

Colour Matters

As for the specific colours in question, it’s important to note the difference.

“A dark blue chip could mean that your surface feet per minute (SFM) speed is too high, and a silver chip could mean that the SFM speed is too low,” said Michael Hunter, business development manager for bore machining and reaming products at Komet of America. “For most materials, I try to get a straw-coloured chip.”

The same basic rules apply to both turning and milling applications.

Select a geometry that matches the material and machining conditions.
If that fails, select different, more aggressive geometry. Photo courtesy
of Walter USA.

Select a geometry that matches the material and machining conditions. If that fails, select different, more aggressive geometry. Photo courtesy of Walter USA.

Thickness Matters

Chip thickness, meanwhile, has a huge impact on the chip-breaking process.

“[You aim for a] sweet spot. A too-thick chip gets hard to break because it’s too strong and you end up getting long, uncontrolled chips. Conversely, if it’s too thin, the chip is ductile [the ability of materials to be stretched or bent without breaking] and you also have a hard time breaking it. That’s where the geometry of the insert really comes in to expand the range of feed rate, depth of cut, and speed parameters that give you decent chip breaking,” said Ludeking.

“Chip thickness plays a major role in chip breaking and shape,” agreed Rice. “If the chip is too thin, a ribbon effect may take place, resulting in an uncontrollable chip that will decrease tool life and create poor surface finish because the chip will likely wrap around and/or drag across the workpiece.”

It’s important to match the feed rate to the chipbreaker’s design.

Curved or “spiral” chips are created during turning, drilling, and boring. But curved chips are created when milling too.

“It is the nature of metal cutting operations. The depth of cut is generally going to be higher in milling, but the chips still will be curved. The degree of chip curl is dependent on the chipbreaker geometry you are using,” said Ludeking.

So with that in mind, how do you produce the best chips?

“First try to adjust the parameters, especially feed rate and depth of cut. Also, try to get a geometry that matches the material and machining conditions. If that fails, you need to go to a different, more aggressive geometry,” said Ludeking.

Material Matters

Another chip handling rule of thumb: Ductile materials often are more challenging than hard, brittle materials such as cast iron.

“If the materials are higher on the Rockwell C scale, like 28 HRC and higher, the chips will break easier. Stainless steels and other steels that are less than 28 HRC are harder to break, and it’s harder to control the chips,” said Hunter.

“Generally we don’t have chip control issues with cast irons. Stainless steels, high-temperature alloys, and most nonferrous materials are going to create chip control issues because they’re ductile materials,” added Ludeking. “You need to have the right combination of cutting parameters and inserts to control those chips. And the more nickel or cobalt that’s in the alloy, the gummier (and more ductile) it becomes. That makes it more difficult to break the chip.”

In addition to making chip breaking a tougher challenge, ductile materials can produce long, stringy chips.

"Long, stringy chips are the worst form of chips. These types of chips can scar the machined finish and require secondary operations to remove them,” said Rice.

Chips of the long, stringy variety also are difficult to handle and dangerous to the operator.

“They tend to wrap around the tool and workpiece and ruin surface finishes, causing all kinds of havoc,” said Ludeking.

Eventually these long, stringy chips can lead to the annoyance known as birdnesting.

“Birdnesting is perhaps the worst type of chip formation. When this occurs, no chip control is present. This scenario usually presents itself when turning ductile or other low-carbon materials,” said Rice.

Coolant Matters

Coolant, particularly the high-pressure variety, also plays a big role in the chip handling process.

“High-pressure coolant will improve your ability to control chips,” explained Ludeking. “It works in two ways: One, it cools the chip. [When cooled] the chip is going to be more brittle and have a tendency to break easier. Second, if high-pressure coolant is aimed correctly so that it is hitting the cutting edge right underneath the chip, it creates pressure on the chip, forcing it into a tighter form that will get it to break more quickly.”

“For light depths of cut, 500 to 1,000 lbs. of pressure can assist in breaking the chip, especially if the coolant exit hole in the tool body directs the coolant right at the tip of the insert,” said Hunter.

Depth of cut is another major factor in the chip control process.

“With lighter depths of cut you have less material, so you get thinner, lighter chips. These can be more difficult to control, so you should use a more aggressive chipbreaker. For example, finishing operations tend to be at a low depth of cut and relatively light feed rates, so they get nice surface finish. Finishing inserts are designed with this in mind. Typically, the chipbreaker features are close to the nose radius and the rest of the cutting edge to break the thin chips typical in finishing,” said Ludeking.

“It’s more difficult to break chips or control [chips with] light depths of cuts because they bend easier. Regardless of the depth of cut, however, if you match the chipbreaker design to the depth of cut that you have, you most likely will have success in controlling the chip,” said Hunter.

There are some new developments coming in chipbreaker design too.

“[Companies are] continuing to work on breakers for high-alloy materials, like high-temperature alloys and stainless steel, which are becoming more prevalent. As productivity demands continue to increase, having good chip control that can handle higher parameters like higher feed rates and higher speeds is going to be very important. I think everyone’s working on that,” said Ludeking.

Experts also ponder one of the key aims of the chip handling process: producing the “perfect” chip.

“Many factors affect chip formation. Chipbreaker selection, material, coolant application, setup, feed rate, spindle speed, and depth of cut all contribute to chip formation. The best chips should look like small 6s and 9s that fall away from the workpiece. All factors mentioned must be considered to achieve acceptable chip control,” said Rice.

Contributing writer Nate Hendley can be reached at nhendley@sympatico.ca.

Komet of America, 847-923-8400, www.komet.com

Kyocera Precision Tools, 800-823-7284, www.kyoceraprecisiontools.com

Walter USA, 800-945-5554, www.walter-tools.com