Every CNC machine operator will have to deal with chatter from time to time. Milling, turning or drilling can all expose a part to this torment.
Sometimes it’s obvious what is causing the chatter. A slipping tool or inadequate work holding shouldn’t be too hard to spot. But what about if you are not sure what is causing it? Often machinists will go straight to reducing the spindle speed and feed rate. While this can reduce the problem it is not always the best solution. You might end up extending the machining time of the part when there was another way to solve it.
Read on to discover the different types, causes, and remedies for CNC machine chatter.
What is CNC machine chatter?
“Chatter” describes the unwanted vibrations experienced when machining a part. The vibrations are the tool and the workpiece moving periodically relative to each other.
These vibrations can be non-resonant, such as when using an unevenly worn tool. Non-resonant vibrations will typically be constant throughout the machining cycle and will often have a readily diagnosable mechanical cause.
Chatter caused by resonant vibrations is found when you’ve hit the right combination of tooling, work holding, machining strategy and machine set-up to produce a vibration at or close to your machine's natural frequency. This type of chatter may only be found at certain points in the tool path, such as in pocket corners.
Why is chatter a problem?
Chatter during a machining operation results in a sub-par surface finish and reduced tool life. Chatter marks will be visible in the machined surface, often showing as wave-like patterns or regular marks. Sustained chatter can cause uneven tool wear and even tool breakage.
CNC machine chatter is also often audible. An experienced machinist will be able to identify when chatter is a problem by its distinctive sound.
What contributes to CNC machine chatter and how to fix it?
Making certain adjustments to your CNC machine set-up can eliminate or reduce the causes of chatter. Start by taking actions that increase the rigidity of the system before playing with the feeds and speeds. A little bit of trial and error is typical in handling these kinds of problems. The best chatter reduction strategy may vary based on the particular machine, work holder, tooling, part, or machining strategy that you are using!
Tooling
A long thin tool will vibrate (and deflect) more readily than a short fat one. So choose a tool with the largest suitable diameter. Try to reduce the amount of the tool protruding from the tool holder and ensure it is held tightly with good run-out accuracy. Hydraulic expansion tool holders can be useful here. They generally have good run-out accuracy and the hydraulic action helps to dampen vibrations.
The right amount of tool pressure or chip load, consistently applied, is also important. Too little, too much, and too much variation can all result in chatter. On a CNC milling machine, too many flutes engaged in the cut can cause chatter. Fewer flutes and variable pitch end mills can be used to help to reduce resonant vibrations caused by chip load.
Work holding
Is there any chance that your part is moving or vibrating?
Check that sufficient pressure is being applied to the workpiece by the chuck, vice, vacuum table or other work holding device. Try to apply clamping pressure to the part as evenly as possible using the right size work holder for the job.
Avoid clamping just one end of a long thin piece of stock material (think about what happens when you twang a ruler on the edge of a table). If this is a likely problem consider using a larger work holding device or additional clamps on a mill. On a lathe consider using a tailstock or a steady rest.
Dampening part vibrations can be particularly challenging when working with thin-walled parts. One way of addressing this type of chatter challenge is to use a filling material like wax or plastic to increase part rigidity.
Machining strategy
With conventional toolpaths on a CNC milling machine, the degree of cutter engagement varies. This can result in too much force being applied to the cutter at certain points in the toolpath, resulting in chatter. Using a constant engagement toolpath or reducing the depth of cut can help to alleviate this.
The final thing to look to change is the spindle speed. If you believe that you have a problem with resonant vibrations try increasing and decreasing the spindle speed by just 5% at a time. Your set-up is likely to have many resonant frequencies and if you just halve the speed, you are very likely to find another one! Some CAM software packages have a feature that continuously varies the spindle speed.
Machine set-up
CNC machines should be sited on a solid concrete floor, free from jointing, cracks, or any other discontinuities. Loose, soft, elastic or broken flooring can exacerbate CNC machine chatter. Well installed anchoring or correctly adjusted feet are also basic requirements for reducing chatter.
CNC machine rigidity is the next most important issue. For the most part, this is set by design but no CNC machine can be 100% rigid. So if you are pushing the limits of what your set-up can achieve your machine may not be built well enough to dampen any resultant chatter.
Inadequate maintenance or repairs can lead to chatter and not achieving the stated machine accuracy. So if the chatter is a new issue this may be something to consider.
In summary here are some tips for avoiding CNC machine chatter:
- Ensure that every part of your CNC machining set-up is as rigid as possible
- Vary feed rates and spindle speeds by small amounts to find stable operating points
- Mitigate vibration by choosing specifically designed tools, tool holders and machining strategies
