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After boogering up my spindle I was forced to replace it. While I was at it I decided to replace the cheap bearings with high quality Japanese bearings. The procedures you see for mini mill bearing replacements will work, but the methods used put unneeded stress on the bearings and require some specific tools. I decided to do it an easier and better way.
The mill uses deep groove radial bearings. They're not ideal for a mill spindle, but they'll work adequately. Since I have a MT3 spindle I use two 6206 bearings, and you can get them either sealed or shielded. Shielded actually have a tiny gap between the shield and the inner race, so they don't protect as well, but they also produce less heat and have a higher top speed. Sealed protect the bearing better, but they'll produce noticeably more heat and, once they're installed and preloaded, their top speed will be about the same as the top speed of a belt driven mini mill.
Most Japanese bearings, even if they're listed as ABEC-1 will actually be ABEC-3. Really, either is good enough for the tolerances of the mini mill. A lot of replacement 6206 bearings you see are C3 tolerance. The C3 indicates they have greater clearance inside the bearings. On the mini mill they should be ok to use since the bearings are preloaded. You can also use CN (normal internal clearance) or C2 (reduced internal clearance). I ended up with SKF sealed C2 bearings since that's what was readily available to me.
Getting the bearings out wasn't hard. As I hammered the spindle out using a rubber mallet it pushed the bottom bearing out. The top bearing was removed using a piece of pipe and a mallet. The bearing bores were thoroughly cleaned and lightly oiled.
First I needed to install the new bearings in the mill head. To do so I removed the mill head (you'll need to remove the motor to do so) and put it in the oven at 220* F for 1.5 hours. The previous evening I put the bearings in the freezer. After using my IR thermometer to make sure the mill head was up to temperature, I took it out of the oven and pushed the two bearings into place. You have to be quickly because the heat iron will quickly heat up and expand the bearings.
If you can't do it, or couldn't get the bearing all the way in before getting stuck, you'll need to press it the rest of the way in. The key is you can only press the bearing on the outer race. The way I do this is take the old bearing, remove the shield from one side, and grind the inner race down. Now you can use the old bearing to push the new bearing in without applying pressure to the inner race. I then use a 1/2" threaded rod, a couple nuts, and several thick washers to push it into place.
Installing the spindle is a similar procedure. I put the spindle in the freezer overnight and the mill head, with the bearings in it, went back in the oven at 200* F. I put a very light mineral oil on the spindle to help lube it during assembly. Once the mill head was up to temperature I quickly removed it and seated the spindle in place. I had to tap it once or twice with a rubber mallet when it jammed during insertion. However, I never had to hit it very hard so it didn't damage the bearings.
Once everything was together I reassemble the mill, head, and motor. I then tightened the reverse thread nut on top of the spindle down tight to remove the play from the bearings, backed it off slightly, and tightened down the set screw.
Once back together I ran in the bearings using the SKF recommended procedure, basically running the mill in increments of 500 rpm until the bearing housing temperature stabilized.
UPDATE: Well, the new spindle's bore was ground at a slight angle, so I replaced the spindle for a second time. This time using angular contact bearings: http://benchtopmachineshop.blogspot.com/2015/05/mill-angular-contact-bearings.html.