SB 13 QC Gearbox Bearings - Advice Needed
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    Default SB 13 QC Gearbox Bearings - Advice Needed

    I'm refurbishing a South Bend 13" toolroom lathe. Currently, I'm working on the quick change gearbox. Some of the bronze bearings were sloppy with about 0.009 clearance, so I bought a bunch of generic bronze bearings from McMaster. Have made a threaded tool and removed all the old bearings. Just now I pressed in the first of the screw shaft bearings, but now the shaft doesn't fit. Specifically, I bought a bearing for a 1" shaft, and it barely slid over the shaft before I pressed it in. Now that it is in place I find that the shaft is about 0.998" and the bearing is about 0.997". Now I wonder, is it expected that those bearings need to be reamed after pressing in place? If so, do I ream them to 1.000" (providing about 0.002" of clearance), or do I need an oversized reamer? Is there another (less pricey) way to adjust the bearings to fit the shafts?

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    The id of any of this type will shrink when pressed in, even you make it yourself. There's different ways to solve the problem. Depends what tools or machines you got.

    Id prefer cutting bushing, before turning shaft down, but either will work.

    If the bushing is indeed bronze, it cuts real easy with sandpaper by hand. Start course, and finish with finer sand paper to make it look pretty.

    .001-.002" is good for oil clearance.

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    If the lathe is still running, you can make yourself a piloted reamer out of any old chunk of steel. If the bearing on the other side of the gearbox has been installed, make the pilot the diameter of the bore of the pressed in bearings and the rest of it the finish diameter that you want. Gash the bit with a grinding wheel where it goes from one diameter to another, then touch up with a stone. Then once one bearing is cut, that bearing serves as the guide (along with the pilot) to keep things aligned for the second bearing.

    Otherwise there's always a risk, however small, that the two bearings will get reamed ever so slightly off relative to each other. Bit of a fiddle fart to make the tool, but it will get you the result you want.

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    Way back in time when I was rebuilding lathes, the shafts in the gear box that ran in bushings were usually scored up and very worn. I would machine the journal just enough to clean up for the length of the bushing plus a little bit, then polish that OD. Then bronze bushings I usually bought them undersized on the ID, sometimes made them, and bore them out plus enough to allow for shrinkage and a couple of thousandths for clearance. Then installed them. I've had to go back and ream or run a brake cylinder hone to open up the bushings to give a bit more clearance. Ken

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    Many thanks for all the suggestions. Now I see that the bearing being undersized is not a problem but an opportunity to do a little reconditioning of the shaft surfaces as well. Fortunately I have a working Craftsman 12 already in my shop, so I can do that work. I decided to order some cheap adjustable hand reamers in 3/4 and 1" sizes. This will give me options on adjusting the shafts and bearings as needed. I am mindful of possible alignment issues but think I can insert the whole reamer and expand it a tiny bit at a time. I think that will keep the hole pretty concentric. If I run into a problem I can just buy another bearing since they are cheap. As long as I manage to avoid screwing up any of the shafts I should eventually get it working.

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    I ran into a bit of a snag in that the tumbler shaft on the tailstock end is a little worn and scored. Initially, I measured about 0.009" clearance between the bearing and shaft. After pressing in a new bearing and polishing the shaft just a little, I used an adjustable reamer to cut the hole to fit the shaft with clearance. That seemed fine until I tried to assemble the shaft with its parts. Then I found that while the whole length of the shaft is a nominal 1 inch, it can only be slid in from the tailstock end because the grooves for the tumbler gears don't extend all the way to that end. So, undersizing both the shaft journal and the bearing on that end doesn't work because it's impossible to assemble. The current shaft is about 1/2 thou under nominal, so to get the shaft in place I had to cut the bearing out to just about 0.9995". My new clearance is about 0.0045", so I have reduced my clearance by half. My question for the group is whether I should stop there, or is it worth making a new shaft? I'm thinking of using Stressproof 1144 ground and polished rod for the job. As the tolerance on that is +0.000, -.001, it should fit as received after replacing the bearings again. The shaft is pretty simple. All I would have to do is cut the two keyways on my milling machine and then turn the groove for the retaining pin with a form tool. I am not a very experienced machinist, but not a complete beginner either. I'm thinking that a 4-sided collet block with a machinist's jack on the far end would suffice for the grooves. I think I can grind my own form tool for the retaining groove as it doesn't seem that critical. What do you all think?

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    I'm more familiar with single tumbler, which I sorted out by boring and bronze sleeving the arm, and cut down shaft clean.

    Double tumbler is different.

    Food for thought though. Unless your a high speed and efficiency type on the lathe. Most speeds running through that box are NOT all to fast. Yea, it spins, but not like a car trans rolling 100mph down the high way. I'd prefer tighter, but I would not stop my overall build right now.

    Finish the lathe first. Run it and check noise or chatter. If its a problem, can sort it out then without totally ripping the machine apart.

    Even with it out, could probably use the same machine to help you in the repair. Just run dials by hand.


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