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Chinese mini lathe spindle bearing upgrade

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  • Chinese mini lathe spindle bearing upgrade

    Just have to share this with you guys... I've been tuning my Chinese mini lathe, which happens to be a Chester Conquest, but I just haven't been able to stop it chattering on surprisingly light cuts. Finally, I decided to change out the plain ball bearings in the head stock for angular contact bearings, bought from ARC Euro Trade, and at the same time swapping the plastic gears in the head stock for metal ones, just in case of an accident; I'd rather something more accessible breaks.

    Along the way, I discovered that my lathe is *not* suitably configured to accept angular contact bearings without modification. The original bearings are plain ball bearings that do not require a preload - and the head stock in my lathe does not make provision for any preload. Since the old and the new bearings have the exact same dimensional specifications, after disassembly, I pressed the old bearings back into the head stock until they bottomed out in their seats, and measured the distance between their inner races. This turned out to be 1.6mm less than the total height of the stack of gears and spacers that go in between them.

    So, I milled down the plastic spacers that go next to the bearings on the inside, until the height of the internal stack was slightly less than the measured space between the bearings, and then reassembled my head stock with the new bearings. This worked out perfectly; I was able to adjust bearing preload without compressing the internal stack at all. I ended up at about 2 inch-pounds of torque required to break my main spindle loose (in neutral, of course), which seems to me to be about right.

    Speaking of adjusting preload: the C nuts on my lathe were laughably inadequate, being significantly non-axial between their threads and their seating surfaces. After a lot of searching, and the purchase of what turned out to be even worse, completely unuseable, C-nuts from China, I finally managed to find a good source: the nuts happen to be compatible with the nuts that do the exact same job on several different models of BMW stub axles, including on their top 3 series models. If it's good enough for them, it's good enough for me! The BMW part number is 33411133785, and you only need one - it's deep, and will lock sufficiently well on its own. (Needs a 36mm 12-point socket.)

    How's it working? Well, I've only done a quick first test so far, but: before this change, a 20 thou cut at standard automatic carriage feed was scarily marginal - in aluminum. After the change, I can do a 20 thou cut in steel, and it's all pretty little curly chips, no sign of chatter, and a very nice finish. I haven't dared push further just yet.
    Last edited by tih; 12-11-2020, 05:33 PM.

  • #2
    That seems like good work!

    If you would like to make the adjustment more non-critical, you could source some Belleville conical spring washers from McMaster-Carr. They give a range of adjustment at essentially constant tension, so you can select the preload by choosing what washer you use, and how many are used together. They add-up if all oriented the same direction.

    That will let the adjustment be more constant as temperatures change, since a certain amount of expansion or contraction will not change the preload.

    When I bought a new spindle bearing from Logan, for my model 200, I was somehow shipped a C3 clearance double-row bearing (it was supposed to be zero clearance/preloaded internally). I rigged up a system to preload it about 90 lb with Belleville washers, and it has been very good ever since. It was literally unusable with that C3 bearing in it and no preload.
    Last edited by J Tiers; 12-11-2020, 05:14 PM.
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    • #3
      Good job, always nice when you make something much nicer.


      • #4
        Tih, looks like you just joined up. So first off Welcome Aboard!

        Your changes to the mini lathe sound like you've re-discovered the steps to modifications that have already been done by others. Have you searched You Tube for "mini lathe modifications"? Or in this case "mini lathe bearings"?

        There's a pretty healthy number of videos about all manner of upgrades to the basic 7x series mini lathe which would likely interest you. Some of them go quite far. Others are fairly simple and lower in cost to do yet greatly improve the mini's performance. I'm pretty sure you'll find that it's a few viewing hours well spent.
        Chilliwack BC, Canada


        • #5
          Thanks for the advice, folks!

          Those Belleville washers look like a really smart hack - I think I'll have to order a few, and see if I can make them fit the mini lathe (and, for that matter, the mini mill; I'm changing bearings on that next).

          I have been searching Youtube, and spent hours watching videos. What I haven't seen, though, is any mention of the lack of room inside the head stock for the stack of gears and spacers on the standard mini lathe. It's well covered how tapered roller bearings are thicker, and need adjusting for, but on my lathe, I discovered that there simply wasn't room to have even the original bearings pressed all the way into their seats in the head casting. This may explain the (by now quite old) thread I saw on this site, where someone was unable to properly preload angular ball bearings like the ones I mounted; he may have been trying in vain to compress the gear stack inside. Until the bearings seat in the head, there won't be any preload happening.

          I was quite surprised when I aligned the lathe afterwards: I was prepared to spend time shimming the head stock to get the thing running true, but it turned out to be just about perfect elevation-wise, with just a touch of bed twist. What surprised me was how easy it was to dial out the latter with the tightening of the bolts holding the feet of the lathe to my work bench. This'll now be a periodic check and adjustment, because the bench is wooden, so I can't expect it to stay completely fixed.

          Anyway, I'm really happy with how my lathe behaves now, and will do the mill next. It's a Sieg SX2, and I've already rebuilt it with ARC Euro Trade's heftier, non-adjustable, column and deeper saddle, a tachometer, and a Chinese glass scale DRO, and have it trammed well within the limits that can be expected, so the bearings will be the last step, I think. (Here's a picture of it:
          Last edited by tih; 12-24-2020, 10:30 AM.