View Full Version : Back Gear Repair - too ambitious for a newb?

08-12-2011, 10:17 AM
I picked up a South Bend 9C and discovered that the smaller gear on the back gear shaft is completely buggered. Before I spend $150 on a replacement, I was thinking about trying to turn down the broken gear (I'd probably grind it 99% down first) and replacing the gear on the shaft.

I do have a working Atlas 618 that I can do the work on, but this would be my first "real" project aside from just making random shavings for practice.

If you were going to attempt something like this, how would you mount the back gear? I have 3 and 4 jaw chucks but no backplate and dogs.

Any words of wisdom? I'm inclined to give it a try since all I really have to lose is time.

08-12-2011, 11:08 AM
I'd probably do it between centers, if the shaft has centers. Otherwise clamp it to a faceplate, on the large gear end with the small gear end sticking out. You could put the clamps on the side of the larger gear. I think the stub end of the shaft will fit in the faceplate hole. Use an indicator to be sure it's concentric.

If the stub end won't go into the faceplate hole far enough to permit clamping the larger gear to the faceplate, cut a disk of aluminum with a hole in the middle to shim it out.

08-12-2011, 11:18 AM

I haven't taken it apart yet, but I believe that the shaft is a hollow tube about 1-1/4" in diameter, so I don't think centers would work... Perhaps I should try to find something suitable to use as a mandrel.

08-12-2011, 11:20 AM
You have nothing to lose by trying to make a repair, the current gear is ruined as it is. You can always buy the replacement if your repair fails. As you say, time will be all you've lost; but there is gain even in that: experience.

When you put the shaft in the lathe be certain to indicate it so the machined area will be concentric with the original parts. As SGW mentioned doing the work between centers would be best, even if you have to make a mandrel or special centers to fit inside the hollow shaft.

All that taken into consideration you need to think thoroughly about the project and come up with a plan that has a good chance of working. If the current broken gear is cast as a part of the shaft how do you plan to retain a new gear? Can you get a stock gear from a supplier that matches the requirements you have; # of teeth, pitch, bore, width etc. If not the width and bore can be changed, not so for the # of teeth or pitch.

I am not familiar with the lathe you mention in particular, however the back gear takes quite a bit of load so retention is important. Can you use a woodruff key? I don't think a grub screw alone will make a lasting repair nor a professional one. If the shaft with the gear is hollow and rides on a bushed shaft a woodruff key may interfere with rotation. If you can use a key how do you plan to keep the gear from moving sideways? Perhaps a shoulder on one side and snap ring/circlip on the other? If a key can't be used perhaps silver brazing?

This should be an interesting project and will give a great feeling of reward if you successfully accomplish the mission. You'll also get some new experience and what works and what doesn't. Good luck and keep us posted with the outcome.

Bill McLeod
08-12-2011, 12:56 PM
do you have a picture of the damaged gear and by buggered do you mean teeth missing or ???

08-12-2011, 02:23 PM
do you have a picture of the damaged gear and by buggered do you mean teeth missing or ???

I can grab some pics, but yeah, about 3/4 of the teeth are broken off. The bull gear is missing a couple too, but those seem fairly common and quite a bit less expensive to replace.

Bill McLeod
08-12-2011, 02:41 PM
3/4 of the teeth missing is way too many for this method but I'll mention it for the gear that has 2 missing or damaged. This is currently working just fine on my 14x40. Clean where the tooth needs to go with a grinder then drill and tap in some small bolts in a row like #8's grind the heads off and build the whole thing up with brazing and file back to the shape of a tooth. Check the mesh and file as needed. The other gear will wear the brazing and it will get better with a bit of time. Yes it will wear out, mine was done about 3 or 4 years ago and it gets used pretty much everyday. Definitely falls into the category of Gorilla machining. If it does not work your no worse off

08-12-2011, 02:48 PM
I can grab some pics, but yeah, about 3/4 of the teeth are broken off. The bull gear is missing a couple too, but those seem fairly common and quite a bit less expensive to replace.

Sounds like it is just time to replace it.

Someone decided to reef on it to get the chuck off in back gear.

08-12-2011, 03:23 PM
If you can find a gear from a supplier that is the same width, diameter and pitch you can turn the old gear off the back shaft and then bore the new gear out to fit the shaft and use Dutch pins to lock it in place. I prefer set screws to using tapered pins for that.

If the gear it mates with is buggered up it will soon bugger up the new gear so you may have to replace both gears to make it a good repair.

08-12-2011, 03:50 PM
If you decide not to attempt it, contact Finegrain from the South Bend forum on PM. He has repaired gears for quite a few people there.


08-12-2011, 08:26 PM
I second the idea of getting in touch with Mike (finegrain) on the SB lathe yahoo group.
He makes a new steel small gear, and can either send you the gear so you can do it yourself, or he can easily turn down the old one and fit the gear for you.


08-12-2011, 10:40 PM
Thanks everyone.

I pinged Mike to see what the cost would be.... While I'm tempted to give this a shot, I think I might be better off starting off with something just a little less challenging as a first project. If the shaft were solid, I'd probably go for it, but there doesn't seem to be that much room for error.

08-12-2011, 11:54 PM
Oh yeah...the actual back gears are on a hollow tube that rotates on a shaft.

Not a problem: make some close-fitting shouldered plugs with centers that can fit into the ends of the tube. See if you can drill the center and turn the plug to diameter in the same setup, so they are as concentric as possible. Turn down the old gear until you're just beginning to cut the outside of the tubular section. Enlarge the center hole in the new gear to match the shaft o.d. where you turned down the gear.

As far as attaching the new gear to the shaft, I'd probably clean everything well, coat the turned-down section of the shaft where the gear is going and the hole in the new gear with Loctite 609, and slide them together. You will probably get one chance to line them up before the Loctite seizes them solid, so get the alignment technique down before you actually do it. There will such a large amount of surface area involved that I think the Loctite will be all you need to secure it.