Chatter in Milling
You are sitting in an auditorium listening to a beautiful song from a renowned singer. You are totally immersed in the song and suddenly, the song is shattered by a loud screeching noise from the PA system. You are rudely awakened from your delightful experience. I believe all of us have experienced this scenario.
The high-pitched screech from the PA system is known as audio feedback. This happens due to the presence of a sound loop between the audio output and the audio input. The screech says that there is something undesirable about the arrangement which is not suitable for the setting.
And “chatter’ says the same thing when it happens during machining, something undesirable is there.
Chatter is a resonant vibration (also called as resonant chatter), which feeds on itself. Resonant vibration can be defined as a forced vibration in which the frequency of the disturbing force is very close to the natural frequency of the system and the amplitude of vibration becomes very large. The resulting response vibration is amplified and can be huge. Chatter can be quite violent and will create a distinctive loud noise. It is very bad for the tool life, spoils the accuracy and surface finish of the component. It is also bad for the machine spindle.
Chatter can also be described as regenerative vibration; it can feed on itself. Vibration at the cutting edge leads to a wave in the workpiece and constant vibration creates a series of waves. When the second pass is made over this surface, which is wavy, the forces on the cutting edge will vary according to the peaks and valleys of the waves. Chip thickness also varies. This will intensify the vibration which in turn makes more waves of the same frequency on the surface. And this repeats during every pass.
Chatter can be either related to the cutting edge or to the workpiece. In the first case, the tool and machine vibrate and is transmitted to the workpiece. Whereas in the second case generally, this happens while machining thin walls.
To control workpiece chattering, first and foremost is to use the sharpest cutting edge possible for the material to be machined. Uncoated cutting edges can help since the cutting edges will be very sharp, but not a very good proposition considering the cutting parameters. Tool overhang is also to be considered, use the shortest tool initially and longer tools step by step till the required depth is achieved (if possible). To increase the rigidity of the walls some easily removable filling material can be used to fill the cavity, but this is not always possible.
Apart from the above, the machining strategy is the most important. Using the alternative side machining method will help to control chatter to a great extent. Keep the maximum possible stock on the walls and machine the alternative sides. For example, the first DOC on the first side of the wall, the same DOC on the second side of the wall, the second DOC on the first side than on the second side and so on.
When dealing with chatter related to the cutting edge, the matter becomes more complicated than related to workpiece. Since chatter is a regenerative vibration, what can be the best way to attack? Answer is simple, prevent vibrations, which is not simple.
First, let us discuss why does the tool tends to vibrate. Imagine applying force on one of the prongs of a tuning fork. It will deflect. Once you release the pressure it springs back and vibrates at its resonance frequency. The same is the case with the tool. When the cutting-edge digs into the material cutting forces act against it and once it comes out of the cut, the forces are released, it springs back and vibrates at its resonance frequency. This is called as tool deflection. Tool deflection is a natural phenomenon, but when it exceeds the limit, vibrations occur which can lead to chatter.
While addressing chatter, reduction of the cutting forces is the prime objective. Most of the precautions areas the same as in workpiece chatter. Using the most positive cutting-edge geometry is the first step. This depends on the material to be cut. For example, if you are machining hardened steel you need a strong geometry, a positive geometry intended for aluminum will be disastrous. So, select a comparatively positive geometry recommended for hardened steels. Reducing the number of cutting edges will naturally help to reduce the cutting forces and always use a differential pitch cutter. Use the maximum diameter with the shortest overhang.
But, is it only the cutting edge responsible for chatter? The cutting edge is the part of the tool which is held in an adaptor. So, adaptors also play a major role in controlling chatter. Well balanced adaptors (G2.5 class) with a good runout (3 microns at L/D 3) will the best choice. Vibration dampened adaptors also help to eliminate vibrations and thereby chatter.
One more major aspect cannot be forgotten, the machine. Since chatter is a resonant vibration that is excited by the cutting edges, sometimes a certain RPM will influence the workpiece at a frequency that will maximize chatter. These spindle speeds should be identified and avoided. Some experienced operators know that at some certain spindle speeds, there can be chatter and they avoid those speeds. They know the “sweet spots” or the “Stable Milling Speeds” of their machine by sheer experience.
If we approach chatter phenomenon scientifically it is possible to identify stable milling speeds of the machine using the “Stability Lobe Diagram” by Tobias. The diagram is plotted by a series of intersected borderlines of stability. A rough representation of the diagram is given below.
The curved lines divide the diagram into three areas: unconditionally stable, conditionally stable, and unconditionally unstable. Area below the line is unconditionally stable, which is independent of the chatter frequency of the RPM. Whereas the area above the line is unconditionally unstable, chatter will occur in this area where the chatter frequency of the RPM will influence. In the conditionally stable area, points are stable when they are below the lobes, and unstable above the lobes.
If the stability lobe diagram cannot be provided by the machine manufacturer, there are two ways of doing it. Manually, by listening to chatter and plotting the points, which is a laborious process. As mentioned earlier an experienced operator will know these points and will avoid certain spindle speeds. Another way is by investing or getting the services for analytically determining the lobes. There are various providers and equipment capable of doing chatter analysis.
Chatter is a phenomenon that will give undesired results. But it is not unavoidable. Precautions mentioned in this article can be helpful to address this phenomenon.
About Author:
Sashi Menon, Director of Gratias Solutions, providing consultancy services in various aspects of manufacturing. With a career spanning over 35 years, he has experience in Metal Cutting, Promotion of New Metal Cutting Techniques, Product Launches, Conducting Seminars, Educating Customers & Internal Personnel, Sales Management, and Business Development. He has worked as General Manager (Product Management & Application Support) for Seco Tools India and Head (Technical) for Hoffmann India. A high performing executive with a proven track record of accomplishments and has led teams of highly professional technical experts.
