Burnishing-is it a black art?

[glacey]

In the past I've never been able to figure out how to burnish a pivot. So, instead, I've used wet and dry 320, 600, 1000 grit and got a clean, shiny pivot. However, with a current clock repair I would like to try to produce a harder pivot through burnishing.

I have read many articles by An Analysis of Two Pivot Polishing Techniques, A Quick Evaluation of Surface Finish of Pivots by Bob Whiteman and Burnishing by David LaBounty, searched NAWCC forums, read Gazely and various other books and searched the internet. The closest to detailed instructions was the article by Mr. LaBounty who says that it is almost easier to burnish than explain how to do it! He does, however, explain some of it but he must have a smarter student than me in mind.

I have practiced on a piece of mild steel on my lathe. I have cut the steel with a graver, I have smoothed out some of the cut lines with a #6 Grobet file. The file removes the deeper lines but in turn leaves its own behind. I have a Timesaver metal burnisher that I have flattened and scored with 320 grit W&D. I have applied the burnisher using a gentle pressure, slow turning, hard pressure, fast turning using both the scored part and an unscored portion of the burnisher. It hasn't removed all of the lines created by the file but somewhere along the way has put a nice shine on my practice pivot. The pressure that I have used would be too much for a small pivot.

Is anyone able/prepared to explain how this black-art is achieved and what I should be doing when?

Some articles say 320 grit is to be horizontally drawn across the burnisher, others 600.

Some say use a Jacot pivot polisher others say that you can use a lathe. I would like to use a lathe.

Some say, turn the lathe slowly others slowly, then fast.

Some say use gentle pressure, others press hard.

Some say move the burnisher back and forth, others are silent…….

I keep looking for a smooth finish, should I be looking for something else?

Thank you for reading this and thank you even more if you can shed some light. A video would be stupendous!

 

[gmorse]

Hi glacey,

The essential factor in burnishing is that the pivot, especially if it's a fine one, must be directly supported underneath to allow the burnisher to be applied with sufficient pressure to move the metal without any risk of breakage. This is how the Jacot tool works, and also the Jacot drum accessories for lathes.

Both the burnisher and the pivot must be kept moving; in a lathe the pivot is continuously rotating fairly fast but the burnisher is kept moving relatively slowly. The beds in the Jacot tool or drum in which the pivots sit must be very smooth, scrupulously clean and kept lubricated with a thin oil. In the Jacot watch tools, they're made of dead hard steel, and the burnisher itself must be very hard as well, dressed across its width as you describe, with a coarse oilstone or wet & dry paper of similar grit.

Regards,

Graham

 

[R. Croswell]

In the past I've never been able to figure out how to burnish a pivot. So, instead, I've used wet and dry 320, 600, 1000 grit and got a clean, shiny pivot. However, with a current clock repair I would like to try to produce a harder pivot through burnishing.

I have read many articles by An Analysis of Two Pivot Polishing Techniques, A Quick Evaluation of Surface Finish of Pivots by Bob Whiteman and Burnishing by David LaBounty, searched NAWCC forums, read Gazely and various other books and searched the internet. The closest to detailed instructions was the article by Mr. LaBounty who says that it is almost easier to burnish than explain how to do it! He does, however, explain some of it but he must have a smarter student than me in mind.

I have practiced on a piece of mild steel on my lathe. I have cut the steel with a graver, I have smoothed out some of the cut lines with a #6 Grobet file. The file removes the deeper lines but in turn leaves its own behind. I have a Timesaver metal burnisher that I have flattened and scored with 320 grit W&D. I have applied the burnisher using a gentle pressure, slow turning, hard pressure, fast turning using both the scored part and an unscored portion of the burnisher. It hasn't removed all of the lines created by the file but somewhere along the way has put a nice shine on my practice pivot. The pressure that I have used would be too much for a small pivot.

Is anyone able/prepared to explain how this black-art is achieved and what I should be doing when?

Some articles say 320 grit is to be horizontally drawn across the burnisher, others 600.

Some say use a Jacot pivot polisher others say that you can use a lathe. I would like to use a lathe.

Some say, turn the lathe slowly others slowly, then fast.

Some say use gentle pressure, others press hard.

Some say move the burnisher back and forth, others are silent…….

I keep looking for a smooth finish, should I be looking for something else?

Thank you for reading this and thank you even more if you can shed some light. A video would be stupendous!

I do not consider myself qualified to instruct anyone on the "black arts" of burnishing or using a graver, so I'll just comment briefly on what I see that may be a problem. As for pivot polishing, I believe that you are only halfway there. 1000 grit wet or dry paper is ok for an intermediate step but 2000 or finer should be the final grit. After that, turn the paper over and use the back side at high speed. That can be followed with a fine abrasive polish such as Semichrome. The goal is a mirror bright surface that looks somewhat like black ice in the correct light. At that point the surface smoothness should be comparable to a properly burnished pivot.

As for burnishing, perhaps you are asking too much from the burnisher. Burnishing is a final surface treatment, it is not intended to remove grooves, scratches, and tool marks. The pivot must have a "pretty good" and uniform surface and have absolutely level and parallel sides to begin with. The burnisher must have a flat surface and it must be in contact with the entire length of the pivot. Preparing a pivot to have level parallel sides using a graver, if not impossible, requires a great deal of skill.

RC

 

[shutterbug]

The theory behind the burnishing step is that it flattens ridges, fills valleys and hardens the steel. A good polishing technique will eliminate the need for the first two, and the third is only work hardening at best. However, there is evidence that burnished pivots perform better and wear longer, so there must be something to it. I burnish. It's a simple process that can be learned quickly. You need a good burnisher and a way to spin the pivot rapidly enough to work harden it.

 

[Dick Feldman]

My take is that every time one uses abrasives, those particles are left behind. Embedded particles of abrasives are sharp and will contaminate the pivot surface.
An extreme example would be to run a clock train and introduce a bit of sand to smooth out and "wear in" the surfaces.
Microscopic or other means surely could detect small bits of abrasive left behind and caught in the pores of the metal. My feeling is that washing will not remove all of the abrasive particles.
As sharp as a good butcher keeps his/her knives, they constantly "steel" the knife edge to smooth out tiny imperfections, to improve the knife edge shape and to harden that edge. I have always thought of that as a form of Burnishing.
Like so many processes with clock repair, the process may make the clock repair person feel better but may do harm to the long-term operation of the clock.
Mr. Morse, I feel made good points in the discussion. The burnishing should be properly done and no short cuts should be taken.
That is how I feel.
Dick

 

[LaBounty]

Hi glacey-

It took me months of practicing, several hours a day, before I felt I was proficient in polishing/burnishing pivots and prepping a burnisher. The craftsman must develop a feel for the process that can only come from doing it repeatedly. You've read the article "Burnishing", which has a brief overview of the process, and maybe I can expand on some details of the process here...

1. I start with a pivot file and, using a small sawing motion and supporting the pivot file from underneath with my left index finger, work the surface of the pivot until it is flat and without wear grooves. I use moderate pressure and will often allow the file to load up with filings in order to speed up the filing process.

2. Once the pivot surface is flat (not tapered) and the wear grooves have been removed, I use lighter pressure to feather out the scratches left by the pivot file. During this process, I repeatedly clean the pivot file in order to remove any particulates which may mar the pivot surface. The proper "soft" appearance of the pivot is something gained through experience but there shouldn't be any noticeable scratch lines on the pivot surface. Please note, the surface isn't shiny at this point. I'm not using a sawing action during this phase but gradually work from the base of the pivot file towards the tip. This keeps the particulates in an area of the pivot file which isn't in contact with the pivot. I will often feather the pivot surface with the pivot file by working from the shoulder of the pivot out to the tip, using very light pressure. This technique is identical to burnishing with the exception of varying the pressure and lubrication during the burnishing process.

3. Now, on to burnishing... It is important to have a properly prepped burnisher. Any vertical scratch lines will be transferred onto the pivot surface. The surface of the burnisher is lubricated and the pivot dressed as in #2 above. I gradually increase the pressure against the pivot until I achieve the proper finish. It is important to keep the pivot away from the dirty oil as the fine particulates will score the pivot surface.

As you can imagine, hard steel is more difficult to remove wear but easier to burnish while soft steel is easier to remove wear but more difficult to burnish. I practiced on the pivots of American mantel clocks and, once I felt I had it down, moved on to French pivots. Each movement will require slightly different techniques in order to obtain the proper pivot finish, due to the metal makeup, age, and consistency.

I'm sure there are many other techniques for this process and you should use the one which best suits you!

Regards,

D.

 

[glacey]

What a fantastic response! Thank you all for your time spent and the many details given to me. LaBounty thank you for your expanded details.

In reading and rereading I realise just how many nuggets of information have been given and how much experience has taken so much time to acquire.

I will renew my burnishing practice! Thank you again for the very many details.

Regards

G

 

[Chris.K]

I use a thin piece of red oak and put the gear in my drill press on the slowest speed and rub the wood with the oak to polish and burnish it then just flip the gear over and do the other pivot. It does take time but seems to improve the freeness of motion of the gear.

 

[R. Croswell]

I use a thin piece of red oak and put the gear in my drill press on the slowest speed and rub the wood with the oak to polish and burnish it then just flip the gear over and do the other pivot. It does take time but seems to improve the freeness of motion of the gear.

Chris, I'm sorry but you lost me. I assume that you put the arbor (shaft) on which the wheel (gear) is mounted in the drill press and rotate it slowly, but I don't understand "rub the wood with the oak"? Do you mean that you apply the red oak to the rotating pivot to "burnish" it? If so, I doubt that you are achieving the full desired effect. The primary reason for burnishing rather than polishing is that it is believed to harden the surface of the pivot. I'm afraid that with one end in a drill press and the other unsupported, you may bend the arbor before reaching the required pressure. I've never heard of anyone accomplishing a burnish using wood, perhaps it is possible, but I suspect your technique is doing more polishing than burnishing.

RC

 

[Chris.K]

Chris, I'm sorry but you lost me. I assume that you put the arbor (shaft) on which the wheel (gear) is mounted in the drill press and rotate it slowly, but I don't understand "rub the wood with the oak"? Do you mean that you apply the red oak to the rotating pivot to "burnish" it? If so, I doubt that you are achieving the full desired effect. The primary reason for burnishing rather than polishing is that it is believed to harden the surface of the pivot. I'm afraid that with one end in a drill press and the other unsupported, you may bend the arbor before reaching the required pressure. I've never heard of anyone accomplishing a burnish using wood, perhaps it is possible, but I suspect your technique is doing more polishing than burnishing.

RC

I will make a short demo vid of what I was talking about.

 

[R. Croswell]

I will make a short demo vid of what I was talking about.

That will be helpful.

 

[Tbucket]

I'm posting this YouTube link which is a burnishing demo done by a company called Cogsdill, which make & sell burnishing tools for production machining. I have purchased & used these tools over the last 45 years & thought that this gives a good explanation of what can be expected when used properly. Obviously they are not burnishing clock parts. Hope you find this helpful/interesting in showing what burnishing can achieve.

 

[R. Croswell]

I'm posting this YouTube link which is a burnishing demo done by a company called Cogsdill, which make & sell burnishing tools for production machining. I have purchased & used these tools over the last 45 years & thought that this gives a good explanation of what can be expected when used properly. Obviously they are not burnishing clock parts. Hope you find this helpful/interesting in showing what burnishing can achieve.

Interesting vide. It had a link to a video on "roller burnishing" that was also interesting that used several thousand psi pressure.

 

[Tbucket]

Interesting vide. It had a link to a video on "roller burnishing" that was also interesting that used several thousand psi pressure.

That's a good one also. I always used the roller style when burnishing internal bores. They were a little tricky to set up. First you established the proper pressure then had to adjust the part finish before burnishing to obtain the Ra value that the part called for. If the part pre-finish was too fine the burnisher had little effect as there wasn't enough material to flatten. If it was to rough then the burnisher couldn't flatten it enough too achieve the final finish. The Cogsdill catalogs or Tech support would recommend the pre-finish Ra value based on the material you were burnishing.

 

[AlpineTime]

Beginning in the October 2009 NAWCC Bulletin, Stephen Nelson began a series of Technical Tidbits that included a lengthy discussion of burnishing and burnishers. He prepares pivots with a graded series of stones, not files (Tidbit #1), and works under a microscope (Tidbit #14). Per his instructions, I made my own burnisher from water-hardening tool steel (Tidbit #11). It is glass hard and after burnishing hundreds of pivots, the microscope still shows no wear to its fine diagonal graining. He recapitulates his whole pivot restoration method in Tidbit #19.
European regulators whose pivots I've prepared by Nelson's method have run for over a decade now on half the driving weight they came with. You might find is useful to go back and look it up. He's an iconoclast, for sure, but his system has worked well for me.
Other Tidbits (there are 24 in all) cover a variety of horological technical topics from movement assembly to tools and include a deep and sometimes surprising dive into the properties of oils.