What do violin strings and cutting tools have in common?
I know this is a weird question not many machinists ask during their lives, but I assure you the answer is relevant to the topic at hand.
Aluminum is one of the easiest materials to machine. Its physical properties allow machinists to attack it with heavy cuts at high speeds, unlike other metals that respond better to lighter cut depths. However, these strong cutting forces combined with high speed inevitably produce chatter.
Chatter, contrary to what some believe, is not the result of hitting a piece too violently. Chatter is a self-excited vibration just like the one produced when a violin bow passes over a string. It happens when the tool comes in contact with a surface that has already been machined. Each pass produces a waviness in the material´s surface. When the cutting edges move in perfect synchrony with the material´s waviness, they vibrate in harmony with each other causing the tool to deflect or jump away from the surface.
Curiously, aluminum has a particular speed sweet spot that allows tools to enter 180° degrees of harmonic dephasing, effectively silencing all chatter. This speed is located at around 15.000 revolutions per minute. Lower or higher cutting speed settings do not guarantee this level of stability, but many factors can move this sweet spot a few thousand revolutions up or down.
At these high speeds, you won’t be seeing any of the typical curled chips that are so common in aluminum milling. Instead, chip load must be optimized so the tool does not waste output energy creating beautiful aluminum curls. Instead of cutting the chip, the tool must hit the material at higher speeds so the chips break loose through shearing before deforming.
This last factor makes tool geometry extremely important. For this job, a steep helix is recommended. If you use a large angle value for cutting aluminum at high speeds, you inevitably increase pull forces and displace the harmonic sweet spot. Go for 35° angle helix tools to prevent this.
Flute count is also relevant when cutting aluminum. The tool needs to hit the material fast enough to match aluminum´s natural frequency, which is around 500 to 800 hertz or strikes per second. There is no chance of matching this frequency when spinning a two-flute tool at 20.000.
On the other hand, 4 flutes or more do not provide enough room for chip evacuation, causing the aluminum to adhere between the tool flutes.
When milling aluminum, three is the magic number. It allows your spindle to match the material´s natural frequency while leaving enough room for chips to freely escape.
Finally, we must consider tool material. These high speeds require rigid tools that can withstand high speeds and temperatures without losing sharpness. Carbide end mills for aluminum provide the hardness and resistance required to keep your spindle running for longer periods of time. Their rigidness also grants increased accuracy when running at high speeds. Solid carbide end mills can be coated in Zirconium Nitride (ZrN) to increase corrosion and wear resistance.
If your operation requires high aluminum removal rates, you better be armed with the best end mills for aluminum. Fortunately for us, companies like Online Carbide offer a wealth of carbide tools specially designed for high-performance and high-speed cutting. Find the right coating, geometry, and flute count for your specific needs at direct manufacturer prices. Visit their website or call them at 630.238.1424, and one of their representatives will help you find the right end mills for aluminum.