The Art and Why of Trochoidal Milling!
Form solid to deep cavity in nine minutes! Now that's Productivity!
The video above is not only is a good example of how one would want to use a Trochoidal Milling tool path it also illustrates the reason why and when you would use a Trochoidal Tool path. I have searched the web high and low to find a good definition of Trochoidal Milling, but alas I have not been able to find one the hits all the key elements. There are several tool path options, circular, pill or football shaped, "D" shaped and more. Most CAM software now come with tool path choices. But there is more to Trochoidal Milling than just the tool path.
A Trochoidal Milling tool path is one that utilizes a light radial width of cut usually in the range of (7 to 15%) of the tools diameter and a long length of of cut usually greater than two-times the diameter. The light radial width of cut reduces radial forces and when coupled with a 45 degree or greater helix or cutting edge angle allows for the greater axial length of cut and good chip evacuation. In addition to reduced radial forces, the light radial engagement of the tool thins the chip.
This chip thinning action requires a significant increase in feedrate in order to create a chip thickness large enough to carry the heat away for the shear zone. This increased feed rate equals high metal removal rates and increased productivity. Another positive result of the light radial engagement and high feed rate is a reduction of heat transfer into the component being machined. This results in less thermal expansion of the part and more accurate size and tolerance control.
And why using coolant isn't always the best way to get rid of chips!
As you view the video you may ask yourself why there is no coolant. In reality coolant in milling operations in many but not all cases is a bad thing more than a good thing. What happens when you pour hot water onto a cold windshield? You buy a new windshield. Why? Because the rapid thermal transition from hot to cold causes the windshield to crack. The same thing happens in milling.
The cutting edge is 1200-1400 degrees Fahrenheit when in the cut. When you use coolant the extreme and rapid temperature changes is called thermal cycling. The thermal cycling from hot to cold causes the tool to rapidly expand and contract which results in what we call thermal cracks. As the cracks grow and propagate through the tool you end up with catastrophic failure. So how do you evacuate the chips? Use compressed air.
If you must use coolant, you will need to use a cutting tool material with a higher Cobalt content. The higher Cobalt/Marshmallow allows the tool to manage the rapid temperature changes and expansion and contraction without cracking. Be careful though, tools with higher Cobalt content typically must be run at slower surface speeds.
IMPORTANT NOTE: There are many cases where coolant must be used; when machining superalloys or refractory metals, aluminum, some brasses and bronze materials. In cast iron it's not required; however, it is sometimes used to keep the dust down. Use judgement; just remember coolant in milling requires cutting tool materials with higher cobalt content which requires slowing cutting speeds.