Some time ago I buy a used (indeed abused) Sherline manual lathe planning convert it to cnc (a in-course project). The headstock spindle of this lathe had a worn feeling so new bearings were needed. Replacing bearings was easier than expected (no strong hits were required), and here is what work for me.
- Electric stove (something like this)
- Leather gloves
- 1 1/4″ soft metal rod
- A small part to protect spindle from damage when hammer is used.
- Oviously a pair of new 6004ZZ bearings. Mine were Japan made.
- Remove front cover and back nut.
- Set stove to medium level an heat headstock for a few minutes. Don’t now exact temperature, but you should not be able to touch without gloves. Too hot it’s not good of course.
- Wearing the gloves hit the spindle back while holding headstock by hand. Be careful as spindle and front bearing may jump away.
- Use the rod to hit rear bearing from inside.
- Place back front bearing in spindle housing, but in reverse position, an hit spindle back again.
- Wait headstock to cold (20 min or more).
At this point some cleaning and checking is required.
- Clean bearing housings and spindle, using sand paper and oil if required (I use 600 grit aluminum oxide paper). I found some nasty brown coating in the rear bearing, may be some kind of glue.
- Search for nicks in bearing housing borders, and file if required (those may difficult bearing fit).
- Check front bearing fit on spindle; you should be able to move it by hand (a little force may be required).
- Coat spindle, bearings and housings with oil.
This is pretty much the inverse process.
- Heat headstock as before.
- Mount front bearing on the headstock and insert spindle. Hit spindle nose while holding by hand, until complete fit. No too much force should be required. Remove assembly from headstock.
- Mount rear bearing; careful alignment is required. (I should hit a bit as the border had a nick and filing was not perfect).
- Put back spindle, hitting should not be required.
- Ensure bearing fit are ok and wait to cold.
Once the headstock is cold you can mount in the lathe and put back the nut to adjust preload (Sherline instructions here). To me this was a bit hard as the nut was a bit tight, so adjustment was difficult. I adjust preload so there was very low friction added. Measurements were:
- Nose runout: 0.008 mm
- Nose play: 0.003 mm (may be a bit tight but I can’t get any greater)
- Face runout: 0.006 mm
After checking old bearings, I realized that front bearing was severely worn, while rear bearing doesn’t feel bad.
I would like to try 7004 angular contact bearings, but haven’t found a provider (there’s only a ceramic version on ebay, costing around $100).
Now I’m pretty sure the nasty brown coating was bearing retaining compound. I suppose this is required when there’s some play between the assembled parts. Maybe I should have use this for spindle-bearing assembly.
I forgot to say it seems original and new bearings where normal class (or ABEC1). I guess that normal bearings will do the work, but with ABEC3 bearings costing US10 on ebay (as Andy points out), there’s no reason to spare.