# Mini Lathe Bearing Upgrade - Taper vs Angular: Taper WINS!



## ehamady (Oct 9, 2014)

Finally got tired of the mediocre finish and chatter I was getting on my 7x14 Big Dog Mini Lathe, even on aluminum and brass, and decided to try attacking the problem from a different angle - upgrading the bearings. I tried an endless parade of indexable tools, grinding HSS blanks, making everything as rigid as possible, different combinations of speed/feed rates, tool rake angles, etc.; all to no avail. This led me to the bearings as the possible culprits. I was initially scared to take this project on as I didn’t have a bearing press or any experience with bearings in general. It turns out there was nothing to be scared of!

Main Tools
1.  DIY bearing press made of ¾” threaded rod, nuts, washers, scrap plywood and 12” scrap PVC. Cost: $9

White bearing cover and first bearing pressed onto spindle by turning the top nut with a wrench. Didn't take much effort because the bearing was pressed on with even pressure:


2.  Harbor Freight bearing puller. Cost $34

3. (Optional) I used my router table to cut the step in the spacers so they won't rub the bearings. Many use a second lathe, but I don't have one. Also used the router to make better fitting bearing covers to keep swarf out. The stock covers were cheap plastic with large gaps around the edges and the paint was chipping, so I made new covers out of a durable poly cutting board that was the same thickness. I use poly cutting boards to make router templates because of their durability and they don't flex. Now I have better fitting, better quality covers. They're white and not painted, so no flaking.

With these tools I was able to press bearings on and off the spindle multiple times and fast without the inconvenience of having to go to a machine shop several times or buying an actual press for $200 that I’d hardly ever use. Not bad for $43.

Angular Contact Bearings
I tried angular contact bearings first because they were sealed, require less/little maintenance, and are lauded for their performance at high speeds. After methodically running them in, I noticed a significant amount of axial and radial play when lifting and pushing the spindle by hand even with a tight preload. I could also hear and feel the slight friction caused by the rubber seals. I wasn’t surprised when the spindle runout barely decreased and there wasn’t much improvement when facing, parting and chamfering, so I sold them on eBay the next day. They produced a better finish than the stock straight bearings, but I had higher hopes. On to taper roller bearings.

Taper Roller Bearings
Ordered a set of TRB’s from LittleMachineShop ($36), packed them 50% by my estimate with moly grease, methodically ran them in, and set a moderate preload. I couldn’t detect any axial or radial play by hand and couldn’t see the needle on the dial indicator move when checking runout. Things are looking very encouraging at this point. During the run in process, the headstock didn’t get warm until running 700 rpm for 20 minutes which I was very happy about. I think the key was not going overboard with the grease. Today I tried facing, turning, boring, chamfering, and parting aluminum, brass, and stainless steel – SUCCESS! Suddenly the HSS tools I ground sprang to life and cut like butter with the most beautiful chips. I used to think I didn’t grind the tools properly because they always left a rough finish no matter how hard I tried. There was a severe chatter problem when chamfering before, and now I get a long, smooth, curled chip. Now I get a great finish when facing in both directions, which owners of larger lathes probably take for granted, instead of only producing an average finish when moving the cutting tool toward the center. My mini lathe is finally cutting like lathes I drool over on YouTube!

I can remove and disassemble the headstock fast and easy now so there won’t be any problem setting up a maintenance schedule to check and re-lube the bearings. Maybe someday I’ll figure out how to make grease ports in the headstock. The schedule will also provide the golden opportunity to clean and check all components. The hardest part of this bearing upgrade was removing the long key from the spindle, but even that’s easy now that I figured out the technique without destroying the key in the process. I'm sure I did a more thorough job of reassembling the lathe than the Chinese factory which also helps the rigidity factor. I also replaced all of the rusted black socket head cap screws with stainless shcp's. The lathe just sounds and feels much more solid.

Cost of tools and bearings: $79.
Time: 2 days.
I highly recommend upgrading to TRB’s. They make an immediately noticeable positive difference and work wonders for my lathe.


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## tweinke (Oct 9, 2014)

Nice to hear your success story! I also have a Chinese made 3 in 1 that I bought used, and a few other items form that area. I have come to the conclusion that a lot of the goods from there come as assembled kits, some disassembly and tweaking required.   :thumbsup2:


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