Well I guess I’ll be taking my lathe saddle apart now.

[Redneck_Sophistication]

Seems like I can’t win these days!
Been such a busy year, I haven’t even been able to look at my last problem on the mill (electrical power feed)

I had just finished a couple small parts for the last of my smoker build and thought I’d mess around with the feeds on the lathe as I hadn’t figured them out yet (no labels)

Started playing with them and they are working at least. As I’m sitting there watching it I notice this gear rolling down the lead screw!

Now the saddle doesn’t move!

I can just see in the end and can just see the end of the shaft spin.

Now is the real downside of having zero info about my lathe on the interwebs.

I’m hoping it is a a simple shaft to have made and replace. The gear is slightly damaged, but since it only index’s with a feed gear I’m hoping I can clean it up and use it still.

 

[Susquatch]

Oh Boy, that's not good. The shaft failure appears to be fatigue followed by overload fracture. I can't see the other half. But basically there is way more cyclical load there than it is designed to handle. Replacing the shaft is only half the battle. You will still need to figure out why that happened.

The good news is that it is WAY easier to tear down and repair a saddle than a headstock. Prolly way easier to make/buy parts for it too.

 

[Redneck_Sophistication]

Well at least I got the headstock out of the way already!

Yeah I really don’t know what happened.

Tailstock was far away, didn’t go near the head, there was not bang or noise. Must have been right when I engaged the feed as the handle wasn’t spinning fast, I thought the saddle “took it out of gear”

There’s 3 leavers on my saddle. For feeds with one label that says EDIT crossfeed. Not neutral

 

[RobinHood]

If someone previously messed with the lead screw to feed lever interlock (ie disabled or removed it) then it would be possible to have both the threading half nuts AND the feed engaged at the same time. The weakest link then breaks…

Note: this is just conjecture on my part - not saying that that was THE reason for the failure.

Something obviously did not allow the saddle to move freely. Doubt that the saddle lock was engaged as you probably moved the carriage manually by its hand wheel.

 

[Tom O]

What lathe do you have?

 

[Susquatch]

If someone previously messed with the lead screw to feed lever interlock (ie disabled or removed it) then it would be possible to have both the threading half nuts AND the feed engaged at the same time. The weakest link then breaks…

Note: this is just conjecture on my part - not saying that that was THE reason for the failure.

Something obviously did not allow the saddle to move freely. Doubt that the saddle lock was engaged as you probably moved the carriage manually by its hand wheel.

Must have been right when I engaged the feed as the handle wasn’t spinning fast, I thought the saddle “took it out of gear”

I think Robinhoods speculation about the moment of failure is probably correct. That final overload of some kind is evident on the fracture surface.

But don't overlook the fatigue problem. Something was cycling that part with a fairly heavy load for a very long time. It wasn't just a problem that popped up recently.

If you ignore the fatigue problem it will bite you again in the future and lord knows what could happen when it does. It's one thing to replace a shaft. It's another to replace gears, racks, or even the saddle itself.

 

[Tom O]

Thing is though the breaks in the pic look too fresh and clean no oil stain either.

 

[Redneck_Sophistication]

What lathe do you have?

Fantastic question!

It’s labeled McDougall made locally in Galt.

However it was not a McDougall made machine.

Possibly the bed is but VDM of Germany bought the company at some point.
They put German headstocks on McDougall lathes
Then later added other components
Then later imported or assembled whole machines and labeled McDougall then they dropped the name entirely.

Was on redit and a local users father worked at McDougall up to 63 and says my lathe looks nothing like what they had there then.

 

[Redneck_Sophistication]

But don't overlook the fatigue problem. Something was cycling that part with a fairly heavy load for a very long time. It wasn't just a problem that popped up recently.

If you ignore the fatigue problem it will bite you again in the future and lord knows what could happen when it does. It's one thing to replace a shaft. It's another to replace gears, racks, or even the saddle itself

This old girl had a hard life for a long time I think.

Lots of dirty fingers messing with bits for a LONG time. (Welded gears in the two speed box in the base :/ )

 

[Redneck_Sophistication]

So I took A couple parts off.

Peeking in the hole and man there’s a lot of stuff going on in there!

So looks like I’ll need to drop the lower portion with all the gearing away from the saddle to do anything further.

 

[Susquatch]

Thing is though the breaks in the pic look too fresh and clean no oil stain either.

Fatigue surfaces are not always dirty or oily. It depends on where they are and how protected they are.

Of course, without a really nice closeup of the fracture surface I cannot tell for sure. It's just that I've seen so many in my life that I'm virtually certain that's what this is too.

But ya, I could be wrong.

@Redneck_Sophistication - could you please post a nice closeup photo of that failure surface you showed earlier. It would also be good if you included a photo with the light on an angle so the surface roughness creates tiny shadows. I'm mostly interested in the surface on the right of your earlier photo inside the red circle below.

It would be good to see what is on the backside behind the pin too.

 

[Tom O]

You could always extend the rod with a new piece like in Kieth Rucker’s video ( I think Abom has one too but threaded his join ) if push comes to shove.

 

[Redneck_Sophistication]

Took a bit to knock that pin out.

 

[Susquatch]

Took a bit to knock that pin out.

Great photos! Thank you!

The good news is that my first impressions were wrong. It isn't fatigue after all. It is a plain torsion shear failure. Probably exactly as @RobinHood suspects. Something locked up and the parts couldn't handle it.

That means you don't need to go looking beyond what happened when the failure occurred.

I know it's a mess and nobody wants to see that happen. But I think it's worth a small celebration that it isn't fatigue related after all!

 

[YYCHM]

Agree with @Susquatch , classic torsion shear failure.

Yikes..... Is that last gear pictured the one that meshes with the bed rack?

 

[Redneck_Sophistication]

Hey @Susquatch I’ll take what I can get!

Yes @YYCHM that’s the gear that engages the rack, it is external to the saddle gearbox so it fell away when it broke.

Hopefully I’ll get a better idea of the workings when I open it up further

 

[YotaBota]

How's the rack or have you got that far?

 

[Proxule]

Surprised no shear pins were installed or some sort of brass pin to aid in this exact issue.
gluck

 

[terry_g]

Was the pin that held the gear to the shaft tapered?

 

[Redneck_Sophistication]

Ok put some more work in

@YotaBota the rack is mostly ok. The is a slight amount of damage to a single tooth. I’m sure I can file it out.

@Proxule the gear was held on with a shear pin! A roll pin type. When I first saw the gear I thought it was a small cap head bolt because of the mushroomed end looks like a head(there’s a bunch on the lathe)
Was kinda pissed there wasn’t a shear pin, till I cleaned it up and noticed it was apparently the strongest shear pin of all time.

Maybe replace with a piece of aluminum tig rod?

@terry_g
No it wasn’t tapered. But it was bent in an interesting twist that made it hard to tap out. (Was going gentle to avoid further damage.


Pulled the gear box off the saddle. That was involved, with the feed rod, lead screw and what ever you call rod that has the fwd/rev leavers on it.

Was surprised to see a oil pump that actuates off of a cam one of the gears, works even! Of course I broke off a small piece of microscopically small copper tube that drips on the brass gear.

Was able to figure out all the leavers and selectors now that i can see what’s going on, and the fwd/neutral/rev is moving better (didn’t want to force before )

The very beefy half nut doesn’t want to move super smooth but I’ll get into that.

The end of the shaft you can see two different grain structures in the break.

Really tempting to weld or otherwise try and repair this :/