# Broken tooth



## Flatfendershop (May 4, 2012)

Hello, 
A few months ago I broke a tooth on the 80 tooth (banjo?) gear on my SB 10K (it lives on the left side, large gear under the end cover).  I've been reluctant to run it, so haven't done much with the lathe.  I finally found one on epay last night.  Yikes, it was $1 per tooth!  Hope to have it back in service soon.  My question is, how critical is one tooth on an 80 tooth gear?  Can the machine be run even though there is a tooth missing?  I guess something got in the gears when it was running, I recall hearing a small pop when the machine was cutting one night.  It has covers on the end, but something may have flipped over there.

What else may cause such a thing?  There is not a lot of swarf or other crap in the area, just grease.  I think the back lash is set correctly, but I don't really have any specs on what the spacing should be.  Just my own mechanical experience - you know . . . not too tight, not too loose, ahhhh!

Input?

Thanks, Flat.


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## bvd1940 (May 4, 2012)

Why didn't you just repair the broken tooth??  You can fix a broken tooth on a gear with very little tools and material, just time & patience.


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## 8ntsane (May 4, 2012)

Bill has a good point. You probably could have repaired gear with out the exspense of a relacement for one tooth. But on the flip side. Try to avoid using grease on your gear train. Grease is a chip magnet. I use gear oil in the gear cover. I dont like grease on gears at all, the chips tend to stick to the grease and cause problems. Though many people tend to use grease because they run quieter, Ill stick to using oil.


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## Flatfendershop (May 4, 2012)

Repair it?  I'm all ears.  Is this a JB Weld job or do I build up the missing tooth with my MIG welder and grind it back into shape?  Are there temper or other metallurgy issues I need to consider?  Are these gears cast or forged?  Looks like cast to me, in which case I have my doubts. FWIW, I have at my disposal the lathe, milling attachment, drill press, welders, press, grinders, dremel tool, files, pocket knife, and a big hammer if needed.


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## bvd1940 (May 4, 2012)

Flatfendershop said:


> Repair it?  I'm all ears.  Is this a JB Weld job or do I build up the missing tooth with my MIG welder and grind it back into shape?  Are there temper or other metallurgy issues I need to consider?  Are these gears cast or forged?  Looks like cast to me, in which case I have my doubts. FWIW, I have at my disposal the lathe, milling attachment, drill press, welders, press, grinders, dremel tool, files, pocket knife, and a big hammer if needed.


The gear is probably cast iron and it would take a few minutes to aply a blob of brass int the broken spot, face off each side level, either grind a bit to fit a good tooth & mill it to shape or you could use a lathe jerry rigging a holder  OR do the shaping by hand with a file (brass files real easy) 
If you want to go further you can drill the base of the broken tooth and put a pin in it and then a blob of brass as stated earlier.
I have seen where they made a new tooth and dove tailed the base of the tooth into the hub with a tight fit & set screw.
And the list goes on & on I have seen metal filled devcon putty used and you could not even see the repair.
Just my 2 cents worth.
By the way did you check with Boston gear, etc for a stock cut gear. Oh yes you could cut your own gear real easy by using a single point gear cutting bit with an indexer which your old gear would index it just fine.


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## Flatfendershop (May 5, 2012)

Gentlemen, 
You have inspired me, tomorrow's Saturday, I'll see what I can do.

Mayhem, you're 4000 word summary is great, thanks for posting the pics.  

My entire tooth is gone, so I think putting in a few pins will be the way to go.  I don't have an oven, but I can pre-heat with the torch to bring the temp up slowly.  Since the gear on my lathe just drives the lead screw when the lever is engaged, there won't typically be much pressure on it.  I'm sure a brass filler will last for a long time.

Flat.


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## martik777 (May 5, 2012)

Mayhem said:


> I had to repair several broken/missing teeth on some of the gears on my lathe. I pre-heated the gear in an oven i have in the shop for powder coating and then again after the repair to allow it to cool down slowly.
> 
> Some of the teeth were just badly chipped, so I just cleaned up the surface with a die grinder and built up the deficit with a brazing rod. The ones where the entire top or most of the tooth was missing took a little more work. On one I welded a rod across the top and on another I drilled and tapped some holes and then screwed in some small diameter rod and then brazed it all up.
> 
> ...



As I have no welding equipment, I repaired 7 missing teeth on my bull gear with custom made screws and loctite.

http://s1182.photobucket.com/albums/x445/martik777/Bull gear repair/

 Would it be possible to braze the screws together with a propane or mapp gas torch?  Not through to the cast iron gear, just the space between the screws?


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## Hawkeye (May 5, 2012)

Martik,

My concern with trying to braze the screws together is that the cast is so massive, by comparison, that it would suck all the heat away as fast as a MAPP flame could supply it. 

Good photos and vids. There are so many approaches to what seems like a disaster. Nice fix.


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## 8ntsane (May 5, 2012)

Flat

You may want to use the pining method, and braze it up, and profile the tooth what ever way you can. I have a buddy that knocked a few teeth off a gear some yrs back. He showed up at my place asking me to put a blob of weld on the spots with the damage with my mig welder.

I did advise against doing it this way, but he said he isnt going through the trouble of pining it, and brazing. He wanted a quick fix that he could slap the gear back on , ahd get the rest of his work done. After welding the gear with the mig, he asked me for a grinder that had a cut off wheel mounted. He proceeded to hand grind the teeth to shape, and when he got them real close that way, he went home to finish them with a hand file.

That was the worst method Ive seen to repair the broken teeth, but he still has that repaired gear in his lathe, and claims it hasnt givin any trouble since the repair. Now he has the equipment he could make his own, and says if, and when it lets go again, maybe he will. Or maybe he will be back at my shop looking to mig weld it again.

Go figure
I didnt think that method would make it through one job, let alone several yrs. :biggrin::whistle:


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## Flatfendershop (May 5, 2012)

Update: Broken tooth fixed!  

Thanks for the ideas that everyone contributed.  

I had great plans for documenting the process, but it turned out that most of my pictures sucked, so you just get a few.  

I wound up deciding to MIG to tooth because I didn't want to mess with any temper that the nearby teeth might have.  Keep in mind, I know just enough about characteristics of metals to know that I don't know squat.  I cleaned the area with carburetor cleaner and brushed it with a flame to burn out any old oil or grease.  Then I stacked two small weld beads into the area, they just about filled the space between the two adjacent teeth.

Using a Dremel tool with a small fiberglass cut-off wheel, I carved out the new tooth, constantly checking the fit with the gear that meshes with this one.  In the end, I had trouble telling which tooth I fixed.  It worked out great! 

Reinstalled the gear and put the lathe back in service.

I don't know how to get my captions under the pictures - ? - oh well!  The next project is the chipped tooth that the pick is pointing to.
Flat.


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## Flatfendershop (May 5, 2012)

Thanks for the thoughts Paul, I guess we'll see how it lasts.  I know typically using a MIG on cast does not produce the best strength, I'm banking on the relatively low pressure that this gear has. We'll see!


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## martik777 (May 6, 2012)

Heck, if I am seeing the photo right, it was just a chipped tooth, probably would have worked for years as is.  Nevertheless, you did a great job. Congrats!


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## ScrapMetal (May 6, 2012)

martik777 said:


> Heck, if I am seeing the photo right, it was just a chipped tooth, probably would have worked for years as is.  Nevertheless, you did a great job. Congrats!



Look closer at the top - middle pic.  Tooth is pretty much gone.

-Ron


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## 4GSR (May 6, 2012)

The missing tooth on the 80 tooth gear will probably hang up with a tooth on a much smaller gear like the 18 tooth mating gear.  Probably won't run for very long until it hangs up with the mating gear teeth.
I have a back gear on my 9" SBL that has a tooth missing on the smaller gear.  Been running like that for 50+ years!  Yet to be fixed.  Dad would not let me fix the tooth when I was younger, neither would he.  I now have bought a replacement back gear that has all of it's teeth.  Haven't installed it, will do it one of these days.


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## martik777 (May 7, 2012)

My 9A bull gear had 7 teeth missing, still worked with a little noise, didn't try it under heavy loads though.  Even after I repaired it I didn't notice a significant difference.


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