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SB 10L single tumbler gear box renovation.

Gard

Aluminum
Joined
Mar 18, 2016
I have been working away on rebuilding this war production board badged south bend 10 inch lathe for a couple years at this point. I have gotten excellent advise in several previous posts. Now I am working on the gear box, then on to the apron.

On the left side of the gear box (facing the lathe), the cone and clutch shafts currently ride in replaceable steel bushings. There is 0.017" of vertical movement of the cone shaft in the bushing and 0.006" on the clutch shaft. This seems excessive. My plan here is to turn down the shaft ends enough to clean them up and make new bronze bushings. These are held in place with set screws.

There is also wear on the right side, here the shafts ride directly in the cast iron of the gear box and have 0.018" and 0.011" of vertical play. My thought here is to clean up the bores with an adjustable reamer, probably end up about 0.890" diameter then turn down the shafts and make custom press fit bushings. The question is how thin can the wall of the bushing reasonably be? I don't want to remove more from the shaft than I need to. Common bushings are about 0.062 thick. Could I make and install one for example 0.04" thick? Less? Another possibility would be to set up the gearbox on the mill and bore larger holes for the bushings and then remove less from the shafts. Thoughts?
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The above photo is from the bottom/back of the gearbox so the left side on the lathe is the right side on the photo. Below is more of a closeup of the bearing surface in the gearbox.
KIMG1838.jpg
 
You are planning to do what i have completed on a 10K 2 lever box a number of years ago.

Several notes:

On the left side of the gear box (facing the lathe), the cone and clutch shafts currently ride in replaceable steel bushings. There is 0.017" of vertical movement of the cone shaft in the bushing and 0.006" on the clutch shaft. This seems excessive. My plan here is to turn down the shaft ends enough to clean them up and make new bronze bushings. These are held in place with set screws.

seems a good plan, exactly what i did for some of the shaft ends.

There is also wear on the right side, here the shafts ride directly in the cast iron of the gear box and have 0.018" and 0.011" of vertical play. My thought here is to clean up the bores with an adjustable reamer, probably end up about 0.890" diameter then turn down the shafts and make custom press fit bushings.

I disagree with this.
Set the box up on end, perhaps in between some right angle irons, indicate it true, find the actual intended shaft centers, and bore.

I don't know if anyone had prints that show such, bit it can be extrapolated from measuring/indicating on un-worn areas of existing bores, and on the other end of the box, where diameters might be different, but centers would be the same.

If you ream the existing distorted hole, the new hole will be displaced off-center from where it should be, practically by the same amount as the wear.
It might even cock a little bit. You will not only make the problem worse, you will be fixing it in stone.

IOW, do this as you described in this sentence:
Another possibility would be to set up the gearbox on the mill and bore larger holes for the bushings and then remove less from the shafts. Thoughts?

The question is how thin can the wall of the bushing reasonably be? I don't want to remove more from the shaft than I need to. Common bushings are about 0.062 thick. Could I make and install one for example 0.04" thick?

Recognizing that you might not have a separate lathe while you are working on the gearbox; still, i hope you will be using actual bushing bonze, say 660 CDA/932 alum bronze for soft shafts, and not trying to get away with using oilite, which compresses. In that case you can make the bushing any thickness you want. I probably would not go under .050" for 660/932. Some of the tougher bearing bronzes such as 954 could go thinner, but will tear up unhardened shafts.

Nice to catch it before the gears start popping away &/or jamming/breaking teeth.
Good Luck!

smt
 
I have purchased some oversize bearings from asbbearings.com. From their website, "These products are manufactured from continuous cast bronze material that conforms to specifications ASTMB 505-C932, SAE 660 and CuSn7ZnPb." It sounds like this is the same material you are suggesting so good news there. I do have another lathe to use, after this 10L is finished I will be able to use it to make parts for its big brother, a 14 1/2 inch from about the same vintage.
I am not sure if reaming the hole and fitting a tight bushing will make it worse than it is now but you make and excellent point, the best way to fix this is to get the hole centers back to where they should be.
I have a clausing milling machine which should be big enough for the gearbox. The steel bushings on the left side are a tight fit in the gearbox so likely in the original location, I have not got my head around the best way to transfer this to the opposite side in the right place. I will see if I have some angle plates I can mount it to and measure all the bore centers relative to the surface where the box mounts to the lathe. Then flip it over and see if alignment is the same. I can either just use the next larger size endmill or set up the boring head.
 
I found an angle plate that has the outside surfaces nicely machined but the inside is rough and uneven enough I was not sure how well clamping bolts would work. I used a carbide face mill but did not have a smaller diameter end mill long enough to get into the corners. I did have some large MT2 drill bits. So I cut one down to a reasonable length and sharpened it by hand to look like an endmill. I drilled and taped a hole in the tang for the 3/8 draw bar. It worked fairly well, I needed to resharpen it a couple of times which I expected machining raw cast iron. I was pretty happy with how this worked and I may repurpose some of the other MT2 drill bits I have. The MT2 shank makes the end mill significantly stiffer than a similar length endmill with a 1/2" shank (maximum collet size for my mill). The drill bits were NOS, a little rusty and included for free with other stuff I was buying years ago, I do not have a drill press with enough HP to make good use of them.
KIMG1842.JPGKIMG1843.jpgKIMG1847.jpg

Here is the gearbox mounted to the mill table. I first mounted it with the left side up and measured the locations of the unworn bushing OD on the holes using an edge finder with the DRO to measure the + and - side of each bushing in X and U as well as the surface where the gearbox mounts to the lathe bed. (photo)
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Then I flipped the gear box over and repeated the measurements. One of the holes is off by 0.016" which is more than it is worn by so I think my fixtureing and measurement needs some improvement to be more repeatable when I flip it. I was explaining all this to my wife and mentioned that what I really need is a coaxial indicator, she kind of rolled her eyes, asked what the heck is that, then googled it on her phone and started reading reviews and specs. She said I should not buy anything made in china and that the Blake indicator had the best reviews, and might as well just buy a new one. So that is on its way, I am pretty excited as I have never used one. I do love that woman!

In the meantime I decided I should be able to calculate the correct shaft spacing from the gears. A little reading and it seems pretty simple, The cone to clutch spacing came out to exactly what I measured. The pinion to cone is more complicated because the tumbler is between them and the angles change depending on which gear is selected. Working on a way to measure that.
 
10L Single tumbler gearbox dimensions
By counting the teeth on each gear in the gearbox I calculated pitch diameters and thus shaft spacing. Most are 14 pitch, the lead screw end is 16 pitch. The cone to pinion spacing is a little more complicated because the tumbler gear is between them and the angle changes with each feed speed
. I measured the slope of a plate temporarily attached to the shift lever and then measured the angle between the tumbler and pinion centers and the same plate. From this I calculated the distance between the pinion and cone shaft with a little Trig. This was repeated for each gear location while lifting up slightly on the handle. It agrees well with what is measured on the good side of the gearbox.
KIMG1854.jpgKIMG1865 (1).jpg
Following is a table showing the center to center dimension of each gear shaft
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Following is the coordinate list for the holes measured relative to the machined surface where the gear box attached to the lathe bed along with a plot of the locations measured with the DRO on the mill. This data is my best guess based on a worn gearbox but perhaps it will be of some use to someone someday.
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I was unable to develop a repeatable enough way to mount the gearbox to the mill to measure consistent coordinates on each side of the gearbox relative to the mounting surface and when the gearbox was removed and replaced. For example, each time I removed or flipped the gearbox the relatively unworn pinion shaft coordinates changed by up to 0.010" relative to my angle block. I eventually gave up on trying to transfer dimensions from the unworn side to the worn side. I was worried about using the new coaxial indicator on the holes with slots for felts. So I made brass bushings that pressed into the holes and had a round hole in the middle. This resulted in a lot of confusing data where nothing seemed to line up perfectly between the left and right side coordinates.

Finally I realized the center of the original 0.875" shaft must be someplace inside the existing oversize hole. I made a 0.875 OD bushing, pushed it to one side of the hole, measured the center and repeated while pushing it in other directions. Next there was just a little trial and error to find locations that fit within the existing holes and agreed with the calculated gear pitch diameters.

With the locations found I bored out both holes to 1.000" for the new bushing. Almost seemed anticlimatic.
KIMG1872.jpg
When I ream the bushings to final ID size (based on the cleaned up shafts) the adjustable reamer will be supported and aligned using a temporary bushing in the opposite bearing so all holes will be in line.

My plan is to ream the brass to about 0.0015" larger diameter than the cleaned up shafts? Oil supply holes are drilled thru the brass and I plan to install felt plugs between the Gits oil cups and brass. I think I will cut some oil channels with a dremel inside the bore and skip the felt that was in groves along the shafts. I am also curious about how worn would a bushing get before you would consider replacing it?
The lead screw is about 0.004" smaller than the bore, I am leaning towards leaving this one alone, assuming it will use more oil but will otherwise be fine for the amount I will use it.
 
The tumbler gear shaft was sloppy and clearly not original and I was not sure what it was supposed to look like so I redesigned it to work with a needle bearing. I cleaned up the bore in the handle and made a new shaft that press fits the bore. I got a new R8 needle bearing 1/2x1/2x11/16. I thought it would be best if it rode on a hardened and ground surface but could not find a suitable inner race. I used a 3/8 ID x 1/2 long drill bushing from McMaster. The bushing OD is 0.5015 OD so I reamed out the bore of the tumbler gear to 11/16+0.0015". The new parts.
KIMG1881.jpg
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Needle bearings generally conform to their housings and it is still a snug press fit into the gear. The bearings can be bought with seals so they are "lubricated for life". It seems to work smoothly and can be replaced without removing the gearbox if it ever gets loose. The new nut locks the drill bushing in place on the shaft and has a 0.003" gap to the spinning gear. Here is a cross section sketch.
KIMG1896.JPG
If I do this again on a single tumbler gearbox I think I will counter bore the hole in the handle to fit a washer between the drill bushing and handle so when the nut is tightened it will clamp the handle between the washer and head of the shaft. This will make assembly and disassembly easier and not rely on a press fit for a stable shaft. Proposed sketch.
KIMG1909 sm.jpg
 








 
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