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Locating the original location of a worn pivot and reaming it


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Worn pivot holes continue to be something that is causing some headache.  If a pivot is worn, it would be difficult to quickly tell which side of the "oval" is the original location.  Though I was thinking about it, and the orientation of the gears should provide some clue.  Let me know if anything here is off base.

I made this mockup of an imaginary gear train, with the red gear being the mainspring barrel.  Assuming Newton's 3rd law holds true here as it does everywhere else in the universe, then if the mainspring barrel pushes against the center wheel (red arrow), then the center wheel also pushes back on the mainspring barrel equally (top right green arrow).  This will have the effect of carving out the oval in the direction of the green arrow.

The center wheel pushes against the third wheel (left side green arrow), and the third wheel pushes back (right side blue arrow).  The combination of the force from the mainspring barrel and third wheel pushes the center wheel down and to the right (black arrow).

The same concept applies to the rest of the gear train.  Then it would seem intuitive that the original location of the pivot is wherever the opposite of the direction of the force is.  Again, please correct me if I'm wrong.

With that said, if I want to ream out a pivot to rebush or jewel it, how do I make sure that the reamer stays dead center on top of the original location?  The second that reamer touches down onto the oblong hole (sometimes exacerbated by being in a concave recess), it's going to want to walk from it's original position to the center of that oblong hole.

gear direction.jpg

Edited by GregG
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  • GregG changed the title to Locating the original location of a worn pivot and reaming it

I’m not sure about the physics/mechanics of your analysis of the forces on the pivot holes leading to oval shaped holes, but I think best practice is to use an existing bridge/mainplate location in which the pivot is still round to act as your true center.

As for recesses/oval-holes leading to the reamer drifting off-center, the only way to avoid this is to use a watchmakers lathe with faceplate, and optically center. For oval shaped holes, it may be possible to enlarge the hole in the opposite direction so that the reamer will find its true-center, but that seems a bit risky to me. For recesses/uneven thickness in plates and bridges causing the reamer to drift, you have to use a lathe.

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I‘m sorry to say that direction of forces acting in the bearings seems not to be right in your drawing. You have to start with the driving force on the two meshing teeths which is oriented perpendicular to their current contact plane. The bearing load of the barrel is exactly parallel to this force.

38527B7E-BAC6-4297-A933-3E98DABEB16B.thumb.jpeg.62bdba10ddcb7e2b15a1394ec99810a0.jpeg

https://www.drivetrainhub.com/notebooks/gears/strength/Chapter 1 - Force Analysis.html

Edited by Kalanag
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6 hours ago, Kalanag said:

I‘m sorry to say that direction of forces acting in the bearings seems not to be right in your drawing. You have to start with the driving force on the two meshing teeths which is oriented perpendicular to their current contact plane. The bearing load of the barrel is exactly parallel to this force.

38527B7E-BAC6-4297-A933-3E98DABEB16B.thumb.jpeg.62bdba10ddcb7e2b15a1394ec99810a0.jpeg

https://www.drivetrainhub.com/notebooks/gears/strength/Chapter 1 - Force Analysis.html

Ahhh, looks like I forgot about the tangential component.  Thank you!

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In most cases the pivot hole will be surrounded by an oil sink, which in all but the worst grade stuff (even then) will be pretty concentric to the original hole location (not necessarily true on really old antique stuff though). In a watch you can "drift broach", where you use a cutting broach while applying lateral force to bring the hole centered again. Then continue to broach to size, keeping an eye on the centering. In a clock it would be more common to file it back to center.

 

If there's no visual reference or you need it really really spot-on like an ultra thin watch, best is to use a faceplace, centering on what is hopefully and unworn opposing hole. Lacking that optical centering is great, like ifibrin mentions.

 

Many watch and clock makers just run the commercial broaches through counting on them to center as best they can. The hole location will end up somewhere between the worn point and the original point. Obviously not ideal, but it works. This can get hairy if one  wheel's location gets shifted, then down the road the next wheel in line shifts and when it gets broached to a new location things bind up.

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I'm thinking that if you can begin to cut into the hole without using the worn pivot as a guide, then that should suffice.  For example, if you take a domed staking punch, you might be able to center on an oil well by using just the oil well, not the pivot.  Imagine placing a ball into a cup whose diameter is smaller than the ball.  The ball will center itself to the cups center axis without actually using the bottom of the cup as a reference.

From there, assuming the reamer drifting isn't an issue, you can ream out the hole to the point where it encompassed the entire oblong pivot.  From there, you can either jewel it or stake it closed and ream it again.

Edited by GregG
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I was thinking if this was clock repair there'd be another way to do this. Oh wait the same tool that exists in clock repair does exist in watch repair.

here's a picture at the link of what the watchmaking variety it looks like. There is a caution though in your buying these in that they get dropped and then there out of alignment.

https://pennyfarthingtools.co.uk/watchmakers-depthing-tool/2019/03/07/

 

 

 

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12 minutes ago, JohnR725 said:

I was thinking if this was clock repair there'd be another way to do this. Oh wait the same tool that exists in clock repair does exist in watch repair.

here's a picture at the link of what the watchmaking variety it looks like. There is a caution though in your buying these in that they get dropped and then there out of alignment.

https://pennyfarthingtools.co.uk/watchmakers-depthing-tool/2019/03/07/

 

 

 

Lol ,  oh wait John your humour is really dry 👍 😂 do you have any Yorkshire ancestry ?

Edited by Neverenoughwatches
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Something I do. I align the mainplate on a stump by using the train bridge as a guide. Then, while applying light pressure, I hot glue the mainplate to the stump. I use as much glue as possible.  Then I remove the train bridge and move the stump to Seitz tool to start reaming. I also glue the stump to the tool base. Of course there is a little play between the stump and the hole so it would be perfect if you could center it perfectly before gluing. I haven't reamed horribly oval holes but I believe this method might work. Hot glue is still a bit elastic but at least you don't have to hold the plate with hand while reaming. Let me know if this sounds real silly 🙂

IMG_20220506_164335.jpg

IMG_20220506_165156.jpg

IMG_20220506_181603.jpg

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1 hour ago, Malocchio said:

Something I do. I align the mainplate on a stump by using the train bridge as a guide. Then, while applying light pressure, I hot glue the mainplate to the stump. I use as much glue as possible.  Then I remove the train bridge and move the stump to Seitz tool to start reaming. I also glue the stump to the tool base. Of course there is a little play between the stump and the hole so it would be perfect if you could center it perfectly before gluing. I haven't reamed horribly oval holes but I believe this method might work. Hot glue is still a bit elastic but at least you don't have to hold the plate with hand while reaming. Let me know if this sounds real silly 🙂

I like this idea.  The hot glue I can see as being a bit messy and I'm not sure how you would make sure that every trace of it is removed, but in principle it's good.  I think all the jewels on the plates and bridges should be friction fit, not shellacked (someone please correct me if I'm wrong).  But with that said, you could probably use cyanoacrylate superglue to really get that thing secure.  Then you could use acetone to clear the glue.  Since nothings shellacked, there shouldn't be any risk of loosening jewels.

Are you using a legit Horia tool or a Chinese clone?  I have a Horia clone and a legit Seitz tool, and I never figured to see if the stumps were compatible.

 

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15 minutes ago, GregG said:

I like this idea.  The hot glue I can see as being a bit messy and I'm not sure how you would make sure that every trace of it is removed, but in principle it's good.  I think all the jewels on the plates and bridges should be friction fit, not shellacked (someone please correct me if I'm wrong).  But with that said, you could probably use cyanoacrylate superglue to really get that thing secure.  Then you could use acetone to clear the glue.  Since nothings shellacked, there shouldn't be any risk of loosening jewels.

Are you using a legit Horia tool or a Chinese clone?  I have a Horia clone and a legit Seitz tool, and I never figured to see if the stumps were compatible.

 

Hot glue is nice because it bonds pretty well with metal but also peels off really easy. I remove any visible bits and then give it a good soak in acetone and also isoprophyl alcohol. I tried some super glue but it cracked when I rotated the parts. Maybe I should have used a lot more. I have the chinese Horia clone and real Seitz. The clone is pretty nice quality for the price. The stumps fit my Seitz which is 4mm. I also use Seitz bits in the "Horia". Mainly for cannon pinion tightening.

 

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5 hours ago, JohnR725 said:

it occurred to me when I saw the reference to a seitz tool I've seen something like this before? In case you don't have the users manual I've attached the PDF below and I have a image out of the manual that you might find interesting.

Seitz centering pump pusher to upright.JPG

seitz BOOK.pdf 3.68 MB · 4 downloads

That begs the question, which one is more likely to be good?  Theoretically I supposed it should be whichever side is opposite the pinion.  And what if neither are good?

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