I also had one of these and was very happy with it… until it died after about a year. When I opened it it was completely packed with dust that had broken some of the copper wire inside.
The spindle has six holes at the bottom. In the product picture you can see five of them. Two of those have screws in them from inside that hold the spindle together. The other four are open and let all the dust in. So I would recommend putting small screws in them to stop the dust from creeping in or close them some other way. (Although I have to say that I have never tested it. The holes might provide ventilation to cool the spindle in the inside but I kind of doubt it. But I won’t pay for it if this breaks your spindle. So test this at your own risk. :-))
And a word of caution: The metal housing of the power supply is not grounded. Do yourself a favor and connect a ground wire to it. And before connecting the power supply to your Arduino to switch it on automatically or control the speed measure the voltage difference between spindle negative and Arduino ground. Mine was 80V which would have fried my electronics.
This might read a little bad, but I was happy with the spindle. It was even precise enough to mill PCBs with 0,2mm traces.