What is MRR/VFA? How to distinguish and test MRR?

Unlike Ringing and other artifacts in 3D printing, MRR is usually shown as vertical ripples all over the vertical print surface. This is also why it is commonly known as Vertical Fine Artifacts (VFA). There are two types of "VFA": one with 0.6mm-0.8mm separation and the other one with 2mm separation The 2mm separation "VFA" is not MRR. It is caused by the polygon effect from the timing belt.

Both of the VFAs we talked about above are affected by speed. So printing a well-designed speed tower is an ideal way to test the situation.

How to solve MRR

1. The source of the problem: Motors

Motors are the root cause of MRR problems which is the source of vibration.So replacing the motors with low-vibration ones can effectively reduce MRR.

Why not use 0.9° stepper motors?

This is because, compared with 1.8° stepper motors with similar electrical parameters, the 0.9° stepper motor has a significantly lower torque, maximum speed, and performance. This means they are only suitable for 3D printers with low performance.

2. Adjust the belt to an ideal tension

Over tightening the belt can make MRR worse, However, untightening too much will result in the polygon effect which causes the 2mm VFA. So adjusting the belt to an ideal tension is necessary to improve the print quality.

3. Change the natural frequency of the motion system

You can achieve this by optimizing the design, changing the length, width, and structure of the timing belt. Or you can optimize the frame design. A stronger and harder frame has a much higher natural frequency, allowing a higher MRR free printing speed.

4. Avoid the resonance frequency

Using a much slower printing speed. This makes sure your printing speed is always below the MRR terrible speed ranges. Doing this may lose a lot of efficiencies but can reduce some MRR.

5. Extra tips: Driver settings

The driver settings are very important! 

If the current is too low, the motors will generate extra vibrations. When running with a current far less than the rated current, the smoothness and performance of the motor will be greatly reduced. Thus tuning the current settings of your drivers according to the recommendation from the manufacturer is necessary.

In addition, some advanced drives such as TMC need to be tuned carefully. Otherwise, the output current will be distorted and causing an artifact very similar to MRR.

Some attempts that proved ineffective

Motor damping mounts:

MRR originates from the output of the motors while damping mounts are an attempt to damp the transmission of vibrations from the motor body to the frame. This in principle cannot mitigate MRR.

Current damper:

The current damper is designed for old low end stepper drivers which have poor and distorted current waveforms. They are not for advanced drivers. If you found the current damper is effective for your advanced stepper drivers(such as TMC), This means your driver settings are not ideal this will result in a seriously distorted output waveform.

Keep “Optimize” the stepper driver settings:

The ideal motor driver setting makes the motor phase current to be an almost perfect sin wave over the print speed range. However, MRR is generated by the motor itself. Even if the driver output is a perfect sin wave, motors that are not specialized for 3D printing still produce MRR. So further tuning the settings of stepper drivers which is already tuned does not help reduce MRR and is a waste of time.

Closed-loop stepper motors:

Compare to servo motors, closed loop stepper motors will only correct when there is a losing step or about to lose steps. They do not optimize the motor vibration when the motor is running normally. According to our test results, closed loop stepper motors do not help to mitigate MRR.