Here is a link to a post I did on increasing the RPM on the LMS3900 SX2
$30 RPM Improvement for LMS3900
And info I based my change on from poopypants here on the forum.
poopy_pants is on a distinguished road
Re: Drive Ratio
I replaced the bearings right after I got the mill anyway since I heard they were cheap. As it turns out they appeared to be similar to the ones I got from VXB on Amazon with lower quality shields.
Here's what I bought:
6007ZZE Nachi Bearing 35x62x14 <-- Rated for 12,000 RPM with metal shields
Original Bearing - 6007RS rated for 6,800 RPM with rubber shields
6206ZZE Nachi Bearing 30x62x16 <-- Rated for 11,000 RPM with metal shields
Original Bearing 6206RS rated for 7,500 RPM with rubber shields
As you can see, the original bearings should be fine for 1:1 but I was planning to later switch to a high rpm and more powerful spindle motor so I was future proofing. I would probably just run it with the stock bearings and replace if they fail. The motor and mount tend to get pretty hot after running full speed for over an hour straight but the spindle head doesn't appear to be too warm so I think the bearings are fine.
Hope it helps.