What excites bending & torsion modes in cutting tool?

[mechie23]

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.

 

[BrianE22]

There are radial loads on the tool bit during the cutting process. Each time a tooth engages there is an radial impact happening.

 

[mechie23]

Thanks Brian. I agree with that. But my doubt is why does the frequency varies. I mean for some revolutions in succession only bending mode is dominant. And then for some revolutions only torsional mode and for some both are dominant. It varies without any logic. I am not fully understanding this pattern.

 

[MikeHalloran]

You're working with a recording made during a particular machining sequence, right?
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

 

[BrianE22]

How are you measuring the vibration of the tool while it is cutting? I've seen dynos that are used under the material being cut. I can't think of a way to measure the cutting tool vibration while it is cutting.

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?

 

[mechie23]

Hi, thank you.

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.