Imagine a rod protruding out of a holder. We put a dot on its end so we can see it rotate. We apply the right amount of force on the free end to make it deflect, rebound and return to center in exactly one second. We rotate the rod at exactly 60 RPM, or one revolution per second, so that one back and forth cycle equals one revolution. The dot ends up in the same spot in the back and forth cycle every second. Now let's extend that rod out further from the holder and apply same amount of force on the free end. Because it is now more flexible, it deflects further and takes longer to return. It now takes two seconds to complete one back and forth cycle. To make that one cycle equal to one revolution we now have to slow the rotational speed down to one revolution per TWO seconds or 30 RPM.
Replace that dot with a tooth or multiple teeth and you can see the challenge presented by milling tools. So why would be given the SAME speed recommendation for all the endmills shown at the bottom. How can that be?
Fortunately, there are ways of measuring the tool point frequency and calculating stable speeds specific to the length of tool and number of teeth.
Theoretically, yes. However, using one my least favorite phrases, “in the real world” it doesn’t. There are obvious reasons for this. There will be runout from the endmill to toolholder connection. It might be very little or a lot. Imbalance may increase runout at speed. There is also the dynamic behavior of the tool during the cut. Milling is unique in that the cutting forces are acting on the side of the free end of a tool, a cantilever beam, at its weakest point. These forces are discontinuous, coming with each tooth impact. If the forces are small the endmill will cut close its diameter. If the forces are greater, the displacement of the cutter causes it to cut a larger slot. New tool path strategies avoid full slots, but the static (runout) and dynamic (vibration) behaviors still remain, perhaps resulting in inaccurate parts.
It is possible to measure the tool point and predict stable speeds that will minimize displacement, increasing surface accuracy.
Folks that come from the 3D printing world seem frustrated by CNC machining, especially milling. They want to be able to take a 3D CAD model and machine it…automatically….with one button…like a 3D printer. Modern CAM software is amazing. You can take that part model, enter some tool and stock information and generate a geometrically ideal tool path. But, physics can play havoc with geometry. The tools, fixture, workpiece and the machine itself are not rigid bodies. They all move and vibrate and that creates limitations.
The top image shows the stability lobe diagram in red that displays the physical limits of the tool and toolholder in a machine. Below the red line you are stable, above the line instability and chatter. Also on that chart is the torque and power curve displaying the limits of the machine’s spindle in pink. In the pink you are overloading the spindle. The lower left image shows how we embed the torque and power information into our software and, on the right, how it appears on our Dashboard. If you tap-test and respect the limits shown on the tool’s Dashboard, your geometrically perfect part program will now be physically possible to produce the part (that's lot’s of p’s).
Sharing information about high performance milling technologies, the result of 30 years of research.