A tale of two materials

Difficult to cut doesn’t always mean high hardness

Today’s CNC machines enable high metal removal rates in aluminum, as long as manufacturers use the proper tooling and cutting data.

Milling can be performed in many, including up milling, ramping, and circular interpolation. The type of milling chosen is based on the part feature being created in that operation. For example, you would never choose the same type of milling motion or tool for face milling that you would for deep pocketing.

The biggest factor in the equation, however, is the material being cut. It doesn’t very often happen that the material falls in the Goldilocks zone of being “just right.” It can be too hard, too brittle, and even too soft.

While most machinists admit that hard materials, such as those 60 HRC and higher, are difficult to machine, other factors also make material hard to cut.

The largest proportion of material type that is viewed as soft is aluminum, a material that designers and engineers like for its strength-to-weight ratio. Aerospace, automotive, and general industry all love this material, but as any machinist will tell you, it’s gummy.

Milling aluminum

Aluminum is a soft material that is widely regarded as having good machinability properties. Modern machines enable high metal removal rates, as long as manufacturers use the latest cutting tools and proper cutting data.

“The cutting tools need to be able to resist both the cutting and centrifugal forces when milling. Also, to prevent built-up edge (BUE), inserts with a ground, sharp, very positive edge geometry, combined with a polished rake, should be used,” explained Sarang Garud, product manager, marketing for Walter USA.

These types of tools combine high performance, good chip evacuation, low cutting forces, and little vibration generation to create time savings when used on modern, powerful machines with high spindle speeds and fast feeds.

The main enemy when milling aluminum is BUE, a situation that can reduce tool life significantly.

“BUE happens when removed material adheres to the cutting edge on the rake face. It typically happens when chip fragments get trapped between the tool and the workpiece and get pressure welded to the tool’s cutting edge,” said Garud. “These fragments gather further workpiece material over a period of cutting time and then suddenly break loose, taking a chunk of cutting edge with them, leaving a rough, uneven cutting edge that is weak.”

BUE can be reduced or prevented by several methods.

The best tool for cutting graphite depends on the level of detail you are machining into the electrode.

  1. Increase cutting speed. Doing this creates less time for the adhesion process to occur, increases temperature in the cut for more thermal softening of the workpiece material, and typically helps increase throughput.
  2. Use a sharper cutting edge. A sharper edge with a more positive rake angle creates more shearing action during the cut with less cutting pressure as opposed to higher-pressure ploughing action created by a duller edge.
  3. Use a very smooth coating surface. Using a coating that creates higher lubricity reduces the possibility of the metal to adhering to the cutting edge.

Milling graphite

Another material that is deemed difficult to cut is graphite. If you are running an EDM process and cut your own graphite, you know its properties well.

The material structure is very important when it comes to machining the required part details. You want a very uniform structure throughout the block. Particle size is the next feature; the smaller the particle size, the finer the detail can be machined.

“The particle size also determines the flexural and compressive strengths. Graphite grades with low flexural strength or inconsistent structures will fracture when attempting to machine fine details or thin cross sections,” explained Sean Smith, sales manager for Machine Tool Systems.

Hardness of the graphite material plays a role in its machining too. Graphite materials with high hardness are generally more brittle, which can lead to chipping and premature tool wear.

Choosing a tool for machining graphite can be tricky, because the best type of cutting tool to use will depend on the detail you are machining. Graphite typically is milled to create an electrode for an EDM operation, so fine details often are required.

“As a general rule in [graphite] milling, diamond-coated end mills are used to extend the life of the tool because of the abrasiveness of the graphite,” said Smith. “Uncoated carbide end mills tend to cut sharper than diamond-coated tools and are also used when machining fine details.”

Standard surface grinders can be used with graphite, but the wheels must be kept sharp. The most common grinding wheel used is a green wheel, composed of 60-grit silicon carbide.

The milling machine also is often set up specifically for cutting graphite.

“The most common milling machines designed for machining graphite are set up for dry machining the material. These machines are fully enclosed with a dust evacuation system. They may also have added protection on the way covers and electrical components to protect these areas from the dust created during the machining process,” said Smith.

While dry machining is most common, some machines will run using coolant, such as the same dielectric fluid that is in the EDM die sinkers.

“Another design for dust control is to use a wet system to saturate the area around the cutter part with a liquid to prevent dust from entering into the surrounding atmosphere. The dust comes in contact with the fluid being used and is flushed away to a collection unit. While this can be an effective use of dust control, one must keep in mind the ability of the fluid to ‘wick,’ or be absorbed into the electrode material being machined,” said Smith.

If wicking occurs, the fluid must be removed before the electrode is placed in the EDM or it could contaminate the dielectric oil. In a wet-style dust collection system, a water-based solution should be used because this fluid does not wick into the graphite as easily as an oil-based solution.

Editor Joe Thompson can be reached at jthompson@canadianmetalworking.com.

Machine Tool Systems, www.machinetoolsystems.com

Walter USA, www.walter-tools.com

About the Author
Canadian Metalworking

Joe Thompson

Editor

416-1154 Warden Avenue

Toronto, M1R 0A1 Canada

905-315-8226

Joe Thompson has been covering the Canadian manufacturing sector for nearly two decades. He is responsible for the day-to-day editorial direction of the magazine, providing a uniquely Canadian look at the world of metal manufacturing.

An award-winning writer and graduate of the Sheridan College journalism program, he has published articles worldwide in a variety of industries, including manufacturing, pharmaceutical, medical, infrastructure, and entertainment.