I recently moved my shop from an industrial space to my home garage and in order to fit my Bridgeport Boss II R2E4 mill into the garage I had to take the head off. With the head off, I thought it would be a great time to upgrade the spindle motor. I converted the mill to LinuxCNC 3 years ago, see build thread here, and have always been dissatisfied with the factory spindle motor. I've been toying with the idea of replacing the 3 phase, 1.5HP factory motor with a servo motor for a while, and I finally pulled the trigger on a Chinese servo last week. However, I think I may have purchased an underpowered servo motor as a replacement.

There are a few reasons why I wanted a servo motor for the spindle:
  • Automatically set spindle speed in G-code. I can't tell you the amount of times I've done a tool change and broken a tool because I forgot to adjust the spindle speed for the new tool.
  • Absolute positioning of the spindle. For probing I would like to account for tip misalignment by rotating the probe for each probing event. I would also like to make a small, three to four, tool rack on the side of the table for tool changes.
  • Get rid of the vari-drive unit. It's loud and takes forever to change speeds. I'm lazy and anything I can do to streamline a process will make me less likely to take shortcuts.
  • Lower total mill height. I have lowish ceilings in my home garage and can't fit the factory motor/vari-drive assembly on top of the mill unless I cut into the ceiling and re-do some of the rafters.
  • Increase the mill's power. The factory motor is only 1.5HP and I've bogged the cutter quite a few times with HSM tool paths in steel.


I purchased a 130ST-M10025-BZ servo motor and drive combo on E-bay from China last week. The motor is rated for 2500RPM max speed, 2.6kw (3.48HP) and 10Nm. I initially thought this would be fine, but I forgot to account for the gearing necessary to get the max desired spindle speed of 6500RPM, which requires a ratio of 2.6:1. The factory Bridgeport motor is 1.5HP and 1425RPM.

With the vari-drive and back gear of the original setup, torque at the tool is decreased when spinning faster than the base motor speed and increased when spinning slower, correct? With the servo motor, the torque at the tool will always be the same, regardless of spindle speed, right? With the 2.6:1 reduction in power, the new servo motor has an effective 1.33HP at the tool. This means that the new motor will have the same equivalent power as the factory motor spinning the tool at 1,900RPM. Considering I do most of my milling above this speed that seems fine, but they also sell a 3.8kw (5.1HP) motor. Should I try and return this motor and upgrade to the more powerful one?