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The vibrations result in waves on the machined surface and also affect machine bed and spindle housing

Production Planning & Control Vibrations on CNC machines

Dec 1, 2015

Vibration problems during machining process generally result in noise, bad surface quality and sometimes tool breakage. The column talks on what causes these vibrations, and how they affect the part.

Machining vibrations, also called chatter, correspond to the relative movement between the workpiece and the cutting tool. As the spindle and axes motors rotate on a CNC machine, there are numerous parts vibrating at the frequencies at which these motors are rotating. These vibrations result in waves on the machined surface and also affect machine bed and spindle housing. The spindle is vibrating in its bearings, the bed is oscillating up and down on the floor to which it is bolted, the turret is making angular movements about its axis of rotation and the various axes are making linear movements.

Let us take an example of a small experiment, if one is trying to shake his right hand violently, as if shaking off water from it, one can observe the other parts of body shaking too –chest, shoulders, other arm and legs. They will all shake (vibrate) at the same frequency as the right hand. Because of the damping effect, the farther away a part is from the right hand, the less its amplitude, but its frequency will be the same. Damping is the effect of absorbing vibrations, and all the soft flesh and water (75% water) in the body is good for damping.

The same phenomenon works in a CNC machine. The most violent shaking happens close to the sources of vibration (spindle, axes, tool changer, etc) and their amplitude reduces as one goes away. Finally, the tool vibrates against the part, and this causes poor dimensional accuracy, poor surface finish and poor tool life.

On a well-designed machine, the quality of bearings and various mating parts reduces the amplitude of vibrations. The design of the various structures and the material used in them dampen vibrations. All this leads to reduced amplitude of vibrations between the tool & the workpiece.

In the market, we find same range of CNC machines but costing between X and 5X depending on the manufacturer. We wonder why there’s so much difference, and often say “Ah, those guys are just charging for their big brand name”. It’s not that. The price is higher because of better bearings, better material, better design, etc. This gives you closer tolerances, better surface finish, better tool & machine life.

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