I originally wanted to submit a full report on my balancing work but as Mike is also working on this I decided to share the first part concerning the motor and motor pulley.
After removing the belt and just running the motor I could still detect a significant vibration. I continued by removing the motor pulley but the vibration was still present.
Looking up inside the “dome” at the top of the motor I could see something white, wobbling a little as the motor turned. Removing the dome revealed a plastic fan. I removed the fan and supported it axially on a light, horizontal tube as an initial balance test. This fan is clamped to the motor with a single screw and although it has a cutout this is presumably not intended to compensate for the weight of the screw, washers and nut. The first photograph shows an additional screw that I added (shown on the right side) as a counterweight. My original idea was to be able to first select an appropriate screw length and then adjust the position of the screw as a fine adjustment but it turned out that the fine adjustment wasn’t necessary. However, trial and error didn’t get me close enough, so inspired by the work of G. G.Kamysz (see
CNC Milling Spindle @ DIESELRC.COM) I set up a small loudspeaker as a sensor and a reflective opto detector as an indexer for the position reference as shown by the second photo. The motor was removed from the mill and placed horizontally on rubber strips in order to allow it to vibrate freely.
THIS IS A DANGEROUS CONFIGURATION AND IS NOT RECOMMENDED!
The rotating pulley and fan were fully exposed while rotating at high speed
. I was also very careful to switch the motor on and off only at the minimum speed setting because I was particularly concerned that fast deceleration caused by the brake could throw the motor off the table! Please don’t get hurt – either find a better, safer way to do this, or just forget it!
The third photo shows an example of what I saw on the oscilloscope. A peak in the waveform indicated the position of the excessive weight relative to the index pulse. Using this technique I was able to balance the fan end of the motor quite easily.
To balance the pulley end I drilled and tapped 3 pairs of M5 holes, 10mm deep at 120° separation in a pulley (I had ordered an additional set of pulleys because I needed an operational mill to machine the pulleys). The idea was to add grub screws as necessary in order to achieve the balance. My initial attempts with allen grub screws wasn’t very effective because these screws are essentially hollow and weren’t heavy enough. I therefore made some heavier grub screws by cutting the end off a couple of standard screws and making a small slot in one end. A combination of both types of screws was sufficient.
The end result was quite pleasing. With the motor and pulley re-mounted in the mill they now ran with very little vibration over the entire speed range.
While an oscilloscope may not be in everyone’s toolbox it might still be worth trying to balance the fan statically on a light plastic or thin walled alu tube.
In Part 2 (in a couple of days) I’ll describe what I found on the spindle end, and of course the all important results.
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