• The NAWCC Museum and Library & Research Center are currently open. Please check the Visiting Schedule for Days and Hours at the bottom of the Visit Page.

Polish pivot holes?

murphyfields

Registered User
Jun 24, 2020
137
17
18
Westminster, CO
Country
Region
I am trying to consolidate my learning here, and there is something that is not connecting for me...do you polish pivot holes, and if so, how? This is for both new bushings and for old holes with minimal wear

I see and understand recommendations to use bushings with the right sized hole rather than broaching because parallel sides on the hole are good, they should match the pivot. I assume the same would be true of a smoothing broach...it will only smooth/polish/burnish the very edge of the hole, not the full length. Now, if you do use a cutting broach then I guess the smoothing broach will smooth the entire length of the hole.

So if you don't use both a cutting broach and a smoothing broach as a pair, how do you smooth/polish/burnish the holes and get a good polish on the entire depth of the hole?
 

Kevin W.

Registered User
NAWCC Member
Apr 11, 2002
22,889
466
83
63
Nepean, Ontario, Canada
Country
Region
I peg my holes out, sometimes with a small hand held dremel small one, works fast and does a decent job. I dont use smoothing broaches.
 

Altashot

Registered User
Oct 12, 2017
276
76
28
45
Country
The proper technique, in my opinion, with broaches is to use the cutting broaches from the inside first then to the same depth on the outside. This will create a slight hour glass shape. Then use the matching smoothing broach in the inside and outside with a wobble, as if you wanted to wallow out the hole.

My understanding is that the wobble action will push the waist of the hour glass into a more cylindrical shape as well as burnishing the entire length of the hole.

This is the method I use with great success. I can really see the difference in the finish inside the hole, especially on larger ones.

Oh! and I do this to new bushings too, they’re not always finished to my liking.

M.
 
  • Like
Reactions: SuffolkM

Vernon

Registered User
NAWCC Member
Sponsor
Dec 9, 2006
921
117
43
Country
Region
I don't do much polish/burnish to a pivot hole anymore. I focus on making sure they are clean by pegging with toothpicks and acetone. If I'm not happy, I will make a very light cut with a cutting broach but that rarely occurs. The thing is, dirt/metal can get imbedded into the soft brass and needs to be removed.
 
  • Like
Reactions: Kevin W.

woodlawndon

Registered User
NAWCC Member
Jan 18, 2017
730
187
43
Woodlawn, Ontario
Country
Region
I've been using a smoothing broach on old and new bushings for awhile now. Pegging works just fine for sure, but it was something Willie X said that got me doing it. He was describing a low-powered French movement he was working on that just wasn't quite right with the power. He said he took the movement apart, smooth broached the bushings with a bit of clock oil and all was well afterward. Does it make a difference, who knows, but it only takes a couple of extra minutes.
Don
 

Jerry Kieffer

Registered User
NAWCC Member
May 31, 2005
2,842
474
83
wisconsin
Country
Movements that have been manufactured on machine tools for the last 170 years having straight round brass holes and straight round steel pivots that are polished. The brass holes have never seen a cutting broach or smoothing broach or anything other than the punch or drill that made the hole. These movements in general, have run flawlessly for many decades. Why would one want to change that proven approach.

Jerry Kieffer
 

Vernon

Registered User
NAWCC Member
Sponsor
Dec 9, 2006
921
117
43
Country
Region
How right you are Jerry. I should move the broaches away from the bench or at least out of arms reach. Everything else is done by using drills and reamers, why drop the ball on this?
 

wow

Registered User
NAWCC Member
Jun 24, 2008
4,675
411
83
75
Pineville, La. (central La.)
Country
Region
I use a pivot gauge to determine the size bushing I need. With the gauge, you can select a bushing that allows a 5% wobble. Once installed, I peg the hole with a toothpick. I see no need to mess up the smooth straight hole in the new bushing. Polished pivots are the most Important thing. In holes that do not need a new bushing, a smoothing broach is a good idea, IMHO.
 

shutterbug

Moderator
Staff member
NAWCC Member
Oct 19, 2005
45,077
1,520
113
North Carolina
Country
Region
As Murphy stated, the pivot only contacts a small area of the bushing. That area will wear itself smooth fairly quickly regardless of whether it was smoothed or not. After that, it will run comfortably for many years. If the walls of that area are parallel, you'll have a better bearing surface to start with, and the pivot won't have to work as hard to correct the irregularities.
 

Phil G4SPZ

Registered User
NAWCC Member
Oct 18, 2018
561
89
28
67
Bewdley, Worcestershire, UK
Country
Region
...a low-powered French movement that just wasn't quite right with the power... smooth broached the bushings with a bit of clock oil and all was well afterward.
I normally only smooth-broach new bushings, but recently I was struggling to get a low-powered 400-day clock to run properly, and as a last resort I smooth-broached all the pivot holes. It did the trick.
 
  • Like
Reactions: woodlawndon

D.th.munroe

Registered User
Feb 15, 2018
759
260
63
39
BC Canada
Country
Region
I peg all holes and polish some it depends on the clock if I use broaching or not.
As Jerry mentions clocks have been made by machines with parallel pivot holes for a long time, the best ways are to recreate that with machines and proper tools.
However if you do it by hand the chance of getting it perfect is lower, broaching seems kind of a cheat for this if the bushing after installing is a bit out or off upright, or the plates flex when adding power and the train binds, broaching will give it freedom.
Is that really a good idea? In my opinion, for my customers clocks, No, except for some cheap junk and modern cuckoos, but it does work.