Cut the Chatter

Reducing end mill chatter enables higher speeds and feeds, better surface finish, longer tool life

Variable-pitch end mills

Variable-pitch end mills fight chatter by varying the pitch and breaking up the harmonics that occur during cutting.

Manufacturers need to push the envelope to compete. Machines are running 24/7 and often are at their limits.

Delivering high volumes in the shortest amount of time is the new normal in today’s machining facilities. In addition, the sectors driving these shops, such as the automotive and aerospace industries, are now requiring strong, light, and increasingly complex parts. The use of hightemperature alloys, aluminum, and hardened steel is on the rise, but these materials also are more difficult to machine.

End mills are popular for both roughing and finishing and come in many types, shapes, and sizes. They typically are used for horizontal cutting and therefore cut laterally. However, these tools also can be used to cut axially because their flutes extend to, and often stick out from, the end face, which enables plunging.

Machinists generally have four goals in mind when selecting an end mill. They are:

  1. Long tool life. Quality, solid carbide end mills extend tool life, resulting in fewer stoppages to replace the tools.
  2. High throughput. Machinists want to run the tool at high speeds and feeds, which results in increased production rates.
  3. Smooth cutting. An end mill’s geometry must stabilize the tool, especially at deeper DOCs, while still creating the required surface finish.
  4. Time savings. Tools that can perform both rough and finish cutting save time for the machinist. Tools that can be changed out quickly also are preferred.

In a milling operation, end mill choice is a critical variable. Machinists demand cutting tools that have high Q rates, which can increase throughput while still being costeffective and producing good-quality parts.

A key factor working against these desired results is chatter.

“Chatter occurs because of the instability between the workpiece and the tool,” said Tom Hagan, milling product manager for Iscar Tools Canada. “The harmonics produced by the machine during cutting create vibrations, especially when the tool meets the workpiece. These vibrations can become worse if the cutting conditions are set wrong, if coolant is used improperly, or if there is a problem with the workpiece setup.”

Addressing the Chatter Problem

Tool chatter has an obvious adverse effect on surface finish. As the tool vibrates, it creates unwanted roughness and marks on the side wall and bottom of a cut. Additionally, tool life shortens, cycle time increases, and therefore total cost for the operation increases. If chatter is reduced or eliminated, speeds and feeds can be increased, and surface finishes will improve.

To reduce chatter, machinists typically slow down the cutting process and reduce the chip load per tooth. These changes to the cutting conditions will reduce chatter, but slowing down the cutting process also reduces productivity.

“Chatter also can be caused by improper toolholding (for example, if the cutter is sticking out of the holder or the overhang is too long) and fixturing, or by a machine tool that is not rigid. These possible problem areas should be checked and eliminated before looking at the tool,” said Hagan.

Reducing Vibration

When chatter occurs, it in all likelihood will remain until the problem causing the vibration is corrected.

Typically, machinists reduce chatter by reducing cutting forces. They do this in three ways:

  1. Changing to an end mill with fewer flutes or a variable pitch.
  2. Reducing the feed rate, thereby decreasing the chip load per tooth.
  3. Reducing DOC (axially and radially).

“A tool with fewer flutes has less contact with the workpiece. This means the tool isn’t working as hard, so chatter is reduced,” said Hagan. “Variable-pitch end mills also can be used. These tools have been available for some time to help fight chatter by varying the pitch to help break up the harmonics that are occurring during cutting.”

Iscar Tools recently released the ChatterFree ECH, a variable-pitch tool with additional different helix flute angles (35 degrees and 37 degrees).

“This design helps break up machine harmonics and reduces cutting forces. Machine tools with less power also can use variable-pitch tools because the cutting forces are reduced,” said Hagan.

Setting the correct cutting conditions also can reduce chatter.

“There is a ‘happy’ range that tools should work in,” said Hagan. “We can give you a guideline—a range of working conditions that should work best—but then the machinist needs to zero in on what works best for his individual setup.”

These steps will reduce chatter, but slowing down the cutting process is not widely seen as a good course of action. Slowing down the cutting conditions may reduce chatter, but it also may reduce productivity to a point that is unwanted.

Better first steps are to improve rigidity and stability. This can be done in a few different ways. They are:

  1. Dialing in the correct speeds and feeds.
  2. Using a large end mill with a large core diameter.
  3. Using the shortest tool setup possible.
  4. Employing balanced toolholders and staying away from collets and hydraulic chucks.
  5. Fixturing the workpiece as securely as possible.
  6. Using programming techniques such as trochoidal and circular interpolation.

“Always be aware of how your fixturing acts,” advised Hagan. “Make sure you have a solid fixture because if you don’t, that by itself will be enough to produce vibration. This means the chatter will be worse because the tool is now contacting an unstable workpiece.”

Chatter in Corners

Chatter is very common when machining corners. As the end mill enters the corner, the percentage of engagement increases the number of teeth in the cut. This drastically increases the cutting forces, causing chatter.

“You see more chatter in corners because there is a little more load on the tool in these locations,” said Hagan.

To reduce chatter when machining corners, consider using circular interpolation to produce a bigger corner radius than indicated by the part print. Then remove the remaining stock with a smaller end mill, also using circular interpolation.

“The trochoidal or ‘peel milling’ technique can also help reduce chatter,” he said. “In this technique you are taking small depths of cut at high spindle speeds and the load on the tool is much lower. Because chatter can be caused by having a chip that is too thick, this technique can eliminate that load.”

Peel milling is a cut with a large axial DOC—often the cutter’s maximum—but it uses a small radial engagement (usually less than 10 percent of the tool’s diameter). This reduces cutting forces and, at the same time, enables machinists to increase both feed rate and speed.

Reduced vibration equals reduced chatter. A good way to start, according to Hagan, is by choosing a tool with a variable- pitch configuration, which has patterns that eliminate harmonic vibration, known to be the main source for chatter.

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