What excites bending & torsion modes in cutting tool?
What excites bending & torsion modes in cutting tool?
(OP)
Hi,
I am analysing the vibration signal acquired during a milling operation in frequency domain. Some times I see first bending mode of tool (of cantilever) in dominant peaks, sometimes torsion mode, sometimes both at similar magnitude.
Considering that a milling process involves cutting with a rotating tool, with cutting edges on its periphery, excitation of torsion mode is fully understandable. But what might be causing the bending mode to excite?
Can someone offer their opinion on this?
Thank you.
I am analysing the vibration signal acquired during a milling operation in frequency domain. Some times I see first bending mode of tool (of cantilever) in dominant peaks, sometimes torsion mode, sometimes both at similar magnitude.
Considering that a milling process involves cutting with a rotating tool, with cutting edges on its periphery, excitation of torsion mode is fully understandable. But what might be causing the bending mode to excite?
Can someone offer their opinion on this?
Thank you.





RE: What excites bending & torsion modes in cutting tool?
RE: What excites bending & torsion modes in cutting tool?
RE: What excites bending & torsion modes in cutting tool?
Do you have a video to go with it, or a listing of the G codes?
Either might bring understanding.
My speculation is that pure torsion mode dominates when the cutter is plunging into the workpiece, like a twist drill making a hole. I.e., in addition to spindle rotation, the tool attached to the spindle is advancing in the Z axis, but the X and Y axes are stationary.
Whereas, during a normal 'milling' cut, on a 3 axis CNC mill or anything kinematically similar, the workpiece is being fed laterally while the spindle is rotating. The Z axis is fixed, but the X and/or Y axes are translating continuously. This operation uses the sharp flutes on the side of end mill to remove material, and it stresses the milling cutter in bending.
You will have a better understanding of the modes and such if you can get access to a manually controlled mill and break a few cutters.
Mike Halloran
Pembroke Pines, FL, USA
RE: What excites bending & torsion modes in cutting tool?
There may be a cross coupling between the modes that modulate each other. There's also the frequency of the forcing function that will affect the vibration of each mode. Is the cut continuous or are you changing directions or amount of stock being removed or feed rate, etc. Is it an end mill milling a slot or is it side milling? What are the natural frequencies and the spindle RPM and feed rate and number of flutes on the mill?
RE: What excites bending & torsion modes in cutting tool?
I am doing peripheral (end) milling on a hollow cylinder. So it is essentially G2 code.
I mounted an accelerometer on the cylinder and measuring vibration and taking FFT later.
So ideally we would expect a bending mode as Mike has told. I also expected the same. But we have to remember that the end milling by process itself has torque involved in it - tangential forces. And also bending - due to radial forces.
So thinking what could be the dominating factor at different instances during a cut so that I am seeing torsional at one time and bending at other.