# QC54 Disaster: Bull Gear Teeth



## KDLaun (Jan 5, 2018)

So, this happened....

I've been working on getting an Atlas 10" lathe back into running condition after it had suffered from a tip over and sat basically unused for over 20 years. I had just finished cleaning some gummy deposits out of the gear teeth in the headstock and back gears with a brass brush and thought I’d check the freedom of rotation with the back gear set engaged. I pulled the locking pin, engaged the gear then manually turned the headstock through a few rotations using the chuck. It moved easily with the exception of one slightly tight spot. It rotated through a little easier each time, so I returned to the original configuration. Then to my horror, I spotted a chipped tooth on what was before a basically perfect gear. And then...utter despair when teeth started dropping off one after another.

I've got a replacement part on the way and have been reviewing the spindle removal procedure on YouTube. I'm fairly confident I can get the replacement part in, but the larger question is: *Why did this happen in the first place? * I really don't want a repeat occurrence.

I'm hoping some of the members here might have some ideas or thoughts on the subject.


The bull gear is the only one out of the four that has any kind of damage (naturally).
I'll be looking at the engagement mechanism a little more closely today. Seems like it's a pretty tight mesh when engaged.
Is it possible that the Zamac material could have degraded? If so, there's concern for the whole machine.
I'm having a hard time believing that something could have been bent (vs broken) in the fall, but will also look into that.
I'll add more info here as I get it. Pretty discouraged right now, I may have to let this project rest awhile while I regroup.


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## RandyM (Jan 5, 2018)

My guess is that when the machine tipped over twenty years ago it cracked or stressed those teeth. When you started turning it just popped them off. I doubt it was anything you did. You may find other hidden damage with this machine as you move forward.


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## talvare (Jan 5, 2018)

I would suspect that something may have been in the teeth of what ever gears that bull gear meshes with. I would carefully inspect and clean all of the gears before re-assembly. Zamac is not a particularly strong material so I don't think it would take much of an obstruction to fracture those teeth. They may have already been fractured before you engaged the bull gear and rotated it. That tight spot that you mentioned would be my first point of inspection.
Hope you get it resolved without further damage.

Ted


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## Ulma Doctor (Jan 5, 2018)

i bought a TH42 a few years ago that was a basket case, it had the very same problem of missing teeth on the bull and pinion gears.
the cause of the damage in my case was the previous owner engaged the back gears WITHOUT disengaging the spindle lock
i went through the process of changing most of the old crusty zamak with newer zamak parts and restored full operation
then i traded it for a lathe that didn't work, that needed restoration .
that's how smart i am 

i may have a few leftover parts from the endeavor, even though i don't have the lathe anymore
if you in need or are interested, email me
ulmadoc@gmail.com


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## markba633csi (Jan 5, 2018)

It may be a combination of stress from the fall and a bit of "zinc pest" that slipped through
Mark


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## KDLaun (Jan 5, 2018)

Thanks to all for your input. I did take another look this morning under much lower blood pressure conditions.  
Here's a few additional points for your consideration:

I pulled the machine out from the wall and good a better look at the back gear mechanism. A couple of things to note. The cover hinges are just stubs as the originals were broken off in the earlier tipover. However, the bolts and castings for the back gear shaft are fully intact, no cracks. I did note that the lower bolts on both sides are _not installed_. Did someone remove the assembly at some point? I don't know. Furthermore, I can not see any trace of adjustment shims. It gives a bit more credence to the idea that someone took it apart at one time in the past. The back gear shaft has about 0.250" play left to right. It doesn't let it foul into the pulley, but it allows the  back gears to come out of perfect alignment with the spindle gears. Looks like the collars on the shaft need to be adjusted to limit the movement. As I mentioned in my original post, the gear mesh is extremely tight. In my non-expert opinion, too tight. The teeth bottom out with force when the lever is fully engaged, the chuck can barely be wiggled.. To me, it seems that the backlash is where it should be when the lever is about vertical. Is there a spec for setting this somewhere in the manual? I tried the card stock in the teeth hack and it coincides closely with the vertical lever position. 

I might go along with the idea that there was something already in the teeth, but I had JUST finished meticulously cleaning each gear. There was some kind of congealed grease that I brushed out with a wire brush and then went through with the point of a plastic scraper wrapped in a shop tower and wiped out each groove. I went around each gear several times.

I also did some other websurfing and found that at least one other person had a similar problem, although their lathe was under power at the time. Someone offered up that those die-cast alloys often don't age well. It might explain why only one gear out of 4 lost teeth and the metal looks so brittle and crystalized.


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## markba633csi (Jan 5, 2018)

Definitely sounds like zinc pest although Atlas was pretty good at avoiding it 
Mark


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## wa5cab (Jan 6, 2018)

First, it sounds to me like the factory put shims under the back gear journals and a PO removed them.  You should be able to feed a strip of brown wrapping paper or 24# bond through the meshed gear pairs without significantly increasing the torque required to turn the chuck.  The "IN" position should not be governed by how far you can push the engagement lever back.  The failure could be attributable to Zink Pest, in whole or in part.   The fact that the other gears aren't failing could be just a consequence of Atlas not practicing FIFO on their inventory.  They could be from different batches.


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## KDLaun (Jan 6, 2018)

Also of note, it appears the spindle has been removed before based on the marks on the threaded collar. The current gear may not even be the production mate for the others on the machine. Maybe I will learn more when I get it apart. It's been "interesting".


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## Superburban (Jan 6, 2018)

Is it possible a degreaser weakened the zinc?


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## francist (Jan 6, 2018)

I don't know that degreaser would do that although I'm not an expert. A strong caustic chemical is bound to have some effect although I've not been able to successfully reproduce any characteristic failure to date. 

I wonder though about repeated stress failure? In the OP picture you can see at the extreme ends of the gear teeth as well as along the sharp corners of the teeth there is what looks like a delamination occurring. I've seen this before, most recently on a previously unused pulley (either NOS or reproduction, not sure) as well as on this example of a change gear. 

In the case of the pulley, I was trying to true it up on the lathe and it just continued to fall away at the edges until it ultimately split in half. This gear though is really the dramatic example. The metal has disintigretated to the point it is expanded on the one side giving a substantial crown to the gear. It's quite amazing, really, I keep it around just to look at.










It could be that some of these failures will become more common, I don't know. Age related due to the alloy, perhaps, and we will be seeing it crop up now for the first time since the parts were produced. Don't know, like I said, I'm not an expert. Just curious.

-frank


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## Superburban (Jan 6, 2018)

I think you are on to something there Frank. I always thought zmak was suposed to be the cure for zinc pest, but it could be quite likely it just gave the items more life, and not a vaccination. My first 618, had gears like the OP, I thought it was from a combination of weak metal, and **** poor operator. Luckily back in the 70's, a new set of gears was not that expensive from Atlas.

I learned the hard way not to use CLR on nickle plated tools.


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## wa5cab (Jan 7, 2018)

What is CLR?


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## Superburban (Jan 7, 2018)

Calcium, Lime, Rust remover.


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## wa5cab (Jan 7, 2018)

OK.  Thanks.


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## markba633csi (Jan 7, 2018)

The gear pictured in post #11, I would say that is certainly an example of zinc pest, but as far as I know it was rarely seen from Atlas, batches may have occasionally slipped through with impurities in the zinc.  I've seen it also in small die-cast fasteners and metal toys. It's quite a disaster for collectors of the aforementioned toys since some of them command high prices- when they start falling apart...yikes
Fortunately Atlas gears are still in good supply on the used market.
Mark


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## wa5cab (Jan 8, 2018)

Yes, as well as new gears.  I've bought I think four within the past year.  However, a new bull gear is probably a bit more expensive.


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