Repivoting Arbors - Slip Fit & Loctite Method Also “The Polishing and Burnishing of Arbors”

Repivoting Arbors - Slip fit & Loctite Method Also “The Polishing and Burnishing of Arbors”

Another of the more advanced clockmaking tasks is the repivoting of arbors to replace worn or broken pivots. As with many aspects of clockmaking, there are a couple of approaches to this task. Ultimately, the replacement pivot needs to be concentric and properly burnished and this article discusses the “slip fit & Loctite method” which is the approach I used to successfully pass that part of my AWCI Certified Clockmaker exam.

SECTION I: Repivoting Arbors Step 1: Determine Original Dimensions: Measure pivot diameter, pivot length, arbor shoulder to shoulder. Goal is to replicate the original dimensions for the certification test. This is not as critical in real practice as one can re-bush the pivot hole. a. Use the Vernier calipers to measure the shoulder to shoulder dimension. b. Use the micrometer for pivot diameter c. Use 1/100th or 1/64th ruler for pivot length

Step 2: Check Lathe for Truth: Place male centers in headstock and tailstock to check for alignment. If they are not perfectly aligned, Loosen headstock & tailstock, realign until the male centers are pointed directly at each other and then snug up the headstock & tailstock. Recheck the male centers and if aligned, remove the male centers. If not aligned, which can happen after snugging them up, repeat the aforementioned re-alignment process.

Step 3: Check Wheel for Truth: Fit up wheel in lathe & check wheel, arbor and remaining pivot for truth. Oppose arbor and pivot against a male center in the tailstock and adjust them as necessary.

Step 4: Determine "Right Size" Drill and Plug Diameter: Using original pivot diameter dimension, calculate "right size " drill size and plug diameter. For best results in determining “right” sizes, use the “right size worksheet”. It accommodates all the variables and provides one a place to “hold” their calculations. Believe it or not, it is easy to forget your starting and intended end points particularly for short term memory folks such as myself.

Worksheet Narrative: Determining the right sizes is an iterative process whereby one compares and contrasts closest drill size and available plug material sizes. Machinist drill size chart is the reference guide of choice for this process. When determining these sizes there are three variables to consider:

1. Pivot's finish dimension equals same diameter as the original pivot. This goal is primarily for the certification test. In actual practice, while the original dimension is the target, it is not critical as one can rebush the pivot hole to the new finish dimension.

2. Leave enough material for polishing & burnishing. If the plug diameter will need to be machined, then add to the original pivot diameter approx. .003" for polishing. If no machining of the plug will be necessary, meaning you have the exact "right size" plug material, add .001", as little polishing should be necessary and burnishing removes very little material, generally not more than .0005”.

3. Leave the plug undersized of the hole for the slip fit. The goal is something less than .001" undersized. Achieving a “right size” slip fit is a case by case, try and retry process caused by three factors. a. Moving from a non-fit to the right sized sticky fit goes very, very quick when you are talking in terms of .0001”. Sometimes a quick touch of the India stone to the plug is all you’ll need to get that just right sticky fit. Pay attention!! The good news, if you take too much material off and have a sloppy fit, you can easily remake a new plug. It only takes a few minutes to do. b. A drill will make a larger hole than the actual drill size. Just how much, is case by case. This is due to the nature of the drilling operation removing slightly more than its actual size and to the lathe runout. While watchmaker lathes are very accurate, they are not perfect and therefore they have some miniscule amount of runout. Collectively these two factors combine to make the hole larger than the original drill size. Just how much, is case by case. c. The plug material deflects when being machined by the lathe cutting tool. This is particularly true on the plug end farthest away from the headstock. Therefore it will machine more material off then the readings are calling for. This is problematic when you are working with .0001” tolerances and material removals. Ergo why I recommend below stopping the use of lathe cutting tools when you are at .001” oversize of the right sized diameter and to changeover to the India stone.

Step 5: Determine "Right Size" Plug Length:  Pivot length finish dimension needs to equal the original pivot length. Overall plug length is the length of the original pivot x 2 for American T/S arbors and x 2 ½ for modern German movements + scant 1/64” in each instance to allow for final finishing i.e. leave the plug length long until after the plug is secured in the hole.  For best results use the “right size” worksheet. Again, this is a great place to “hold” your calculations.

2 Step 6: Remove Damaged Pivot:  There are several methods for removing the damaged pivot. Each of them requires the right tools and technique and it is always best to practice until a satisfactory technique is repeatable. Unless one is skilled with gravers…Removal of damaged pivot is best done with the wheel fit up in the lathe, having the pivot turning in reverse and using a jewelers saw. Using any other method risks damage to the shoulder. Fitting up the wheel into the lathe and cutting the pivot off without the arbor turning works poorly. While the saw will cut smoothly, invariably, the jewelers saw will cut at an angle and cut into the shoulder, a very poor result. Perhaps this has something to do with the narrowness of the jewelers saw making it difficult to avoid human error. Cutters and nippers pinch/squeeze off the arbor and will leave you with a considerable uneven, rough surface to deal with.  Removing the pivot with a jewelers saw while the lathe is turning in reverse will achieve the flatter, smoother surface one is looking for. I believe this has to do with the pivot revolving while cutting and the reverse direction that the arbor is spinning. As the pivot is turning, it is being cut all the way around at faster rate of speed then one could do by hand and the reverse direction is pulling the saw straighter. While you’ll likely end up with a “pip” by using the saw, the result will be flatter and smoother and the pip can be readily removed by stoning or facing it off.  Place the lathe motor in the reverse position i.e. switch in the down position.  Fit up the wheel in the lathe, securing with steady rest, if necessary.  Choice of jewelers saw blade: the rule of thumb, according to Archie Perkins, is you want 3 to 4 teeth engaged with the material at all times, in this instance the pivot diameter. This is a bit of a balance as you are starting a larger cut and going to smaller i.e. the pivot material gets cut smaller and smaller.  Lay the jewelers saw blade onto the pivot next to the shoulder and turn on the lathe with a medium speed and using a moderate sawing motion proceed to saw the pivot off with the arbor turning.  After pivot is removed, likely you’ll left with a pip and perhaps some fraze. You’ll need to stone it or face it flat. Either works. The important part is to end up with a flat surface to bring the center drill up to and achieve a centered hole. o If using the India stone, bring it up to the shoulder and eyeball it square against the shoulder and proceed to stone any pip off and bring it flat.

3 Step 7: Drill Hole in Arbor: Choice of Drill: Drill is oversized – Plug is undersized Depth of hole is the same as the length of the original pivot for all arbors except for modern German movements which should be 1 1/2 times the length of the original pivot. A nice, consistent technique for achieving this is to use the adjustable stop located on the chuck holding spindle in the watchmaker lathe’s tailstock. Loosen this stop and locate it on the mid part of the spindle and then snug up the stop. Loosen the spindle and with a machinist rule situated behind, move the spindle with the fixed stop to the desired depth and snug up the spindle. Insert the “right sized” drill into an appropriate collet, bring the drill up to the entry point made by center drill (see below) and snug up the tailstock. The right sized drill will be something between .0005” and .001” larger than the right sized pivot diameter. When ready to drill, loosen the spindle and begin. Drill until the stop makes contact with the tailstock housing. Typically one is using HSS drill, therefore the RPM’s can be high.

Start with center drill. The right size center drill is not so much measured by its point as it is by the cone part of the center drill. It is the first part of the cone diameter that should be close to the drill size as you want the center drill to enter the arbor until the cone just begins to enter. The goal is to create a slight cone in the face of the arbor. This creates a chamfered entry point for the right sized pivot drill. Finish drilling the hole with the "right sized" pivot drill. A # 0 center drill works for many pivots.

Eccentric (off center) Hole: If after drilling, the hole is eccentric (off center) you have introduced a fourth variable - the need to make the pivot concentric. In this situation, one will need to re-calculate the plug diameter to add enough material to true up the pivot. Just how much material will depend on how off center the hole is and experience will be the guide here. Oftentimes, .005” + or – will suffice.

In addition you will need to deal with the hole that has already been drilled. you will either need to drill a larger hole or machine a stepped plug,

Step 8: Reduce Plug Rough Size Drill Rod Material to Right Size Diameter: One needs to approach this step with caution, attentiveness and the right amount of respect for the required workmanship. Again the good news is if you over machine the plug, discard it and start a new one.  Start by machining off the plug diameter to within .001” oversize of the right sized diameter, try the plug in the arbor hole, likely it won’t fit at this point, but check anyways as it provides your first point of reference and gives you control over the process.  Using an India stone, remove additional material as necessary, continuing to try and retry the plug in the arbor hole until the plug slips in with a sticky fit and bottoms out.  Somewhere between .001” oversize and .0005” undersize, the sticky fit will occur.

4  Final fit is a try and retry process, India stoning off material until the plug is a “sticky” fit into the newly drilled hole.  To check and see if you have done it correctly. Place the wheel upright in a staking block, place the pivot plug into the arbor hole. The plug should “click” or “snap” into place and you should be able to pick the wheel up by the pivot plug and the plug stay secured in the hole.

Step 9: Secure Plug in Hole:  Clean the plug and arbor hole with alcohol to remove any oil residue.  Use “Loctite 680” to secure the plug. However, as with many horological repairs, there is a “trick” to everything. In this step, do not attempt to place Loctite in the hole and then insert the plug as most likely a “hydraulic action” will occur such that it will prevent the plug from bottoming in the hole. Instead, place a drop or two on a note card or wax paper and roll the plug in the Loctite to coat its surface. Then insert the plug into the hole, tapping it down to secure if necessary. Invariably also, applying to much Loctite will cause it to squish out of the hole, and as it has to go somewhere, it will dribble down and collect at the shoulder. Leaving the possibility of the Loctite getting on one’s expensive pivot file. Not a good result. So only one drop is necessary. o When inserting the plug into the hole, if you do feel the need to tap the plug with a hammer to insure it is fully seated, support the wheel on a staking block in such a fashion so as to not inadvertently dismount the wheel.  Insert the plug and twist it around to fully distribute the Loctite on all surfaces and then press the plug all the way into the hole, turn it around, etc. until it bottoms out in the hole and is securely set. With Loctite 680, let the plug set for about 15 minutes (if using Loctite 609 – must set for 24 hours) and then it is ready to be machined whether it needs to be trued-up or just polished and burnished.  Check secured plug for concentricity. o Fit up wheel in lathe, secured plug in collet. o Fit up male dead center in tail stock opposing the good pivot and check for concentricity. Adjust secured pivot as necessary. o Determine if secured pivot needs to be faced off to be concentric. If so, fit up cross slide and face material off of secured pivot until it is concentric.

Step 10: Reduce Secured Plug to Finish Length and Rough Round Off End:  Under no circumstances use a jewelers saw to reduce the pivot plug to finish length. There is no forgiveness if the saw bounces out of the cut onto your good pivot surface. Without a doubt, it will leave a gouge in your good surface that will need to polish out. Not good if you are trying to stay within a certain tolerance as is required by the certification test.  With wheel fit up into lathe and arbor turning, use a # 2 & # 4 hand file to file off end. Checking and rechecking the finish length with the 5” machinist rule.  This is the point to finish the pivot end to mimic the original. Before the polishing and burnishing begins. Use a # 2, # 4 file and India stone to round off the end.

SECTION II: Polish and Burnish the Plug/Finish Round off the End:

This task presents its own challenge as well.

 One of those being, to achieve a flat result. For several reasons, it is very easy to polish a taper on one end or the other. One reason being, the plug pivot end will flex, deflect more at its end then nearer the shoulder. A second reason is the pressure one applies may be uneven. In any case while polishing, until I develop the necessary hand eye coordination, I will need to constantly check with the micrometer jaws my progress and adjust as necessary to achieve a flat result.

 A second challenge while polishing is achieving a square shoulder without removing too much material. Removing more than .0005” from the shoulder is considered too much. o Here’s where it pays to know your tools. In examining the pivot/burnisher you will see a square edge and a radius edge. o Avoid using the radius edge as it will leave you with an undesirable radius at the shoulder. o Instead, use the square edge and avoid pressing too hard against the shoulder. o Also, hold the pivot file squarely, otherwise the side of the pivot file may catch the arbor shoulder and again remove an undesirable amount of material. o Notice also the burnisher has a slight cutting edge along its length that is radiused. Avoid using this cutting edge area of the burnisher, otherwise the cutting edge may again remove shoulder material. o In case you are wondering, the pivot file/burnishers are designed with these radius edges so as to polish and burnish curved areas such as a balance staff would have. o If adhering to the above cautions and one is still removing too much material. Roll the edge of the pivot file/burnisher on 600 grit wet dry sandpaper.  Prepare pivot burnisher o Dave LaBounty recommend medium grit emery cloth on the course side and crocus 1000 grit on the fine side. This resulted in only modest success. Ultimately Dave recommends preparing a carbide tool with diamond grits. o John Losch recommended using course India stone for one side and the fine India stone for the other side. This worked good even better. Ultimately though, I made a carbide tool burnisher, and that really is the best tool.  Start with the pivot file end of the file/burnisher tool to remove scratches and tool marks. Do not add any lubricant to the file surface.  When you have removed the scratches and surface marks and are within .0005” of the finished size, switch to the burnisher end of the pivot/burnisher using a lubricant on the burnisher’s surface course and fine surfaces. When surface is mirror like switch to the fine buff sticks to achieve a mirror polish.

6 SECTION III: Email Discussions with Dave LaBounty:

I've been practicing polishing and burnishing. Some questions:

Q. 1. The AWCI certification exercise for repivoting calls for holding the exact dimensions of the original for the shoulder to shoulder, pivot length and diameter. I am able to repeatedly keep the original dimensions for pivot length and diameter, however, not for the shoulder to shoulder. Inevitably, I take off .010" to .015" more than the original dimension as a result of the polishing to get a square shoulder and interestingly enough, my my burnisher wants to take off some of the shoulder as well. So questions: a) how does one take nothing off the shoulder and get a square shoulder? In day to day clockmaking, should one expect to take a bit off the shoulder? and this is just a certification item to content with? b) should I ease the edge of my burnisher so that it does not cut into the shoulder. I've read where burnishing really should not be taking any material off. When burnishing one is just flattening the steel, hardening the steel. For reference, I use the left hand pivot file/burnisher commonly sold. I think it is a Grobet. I prepare it with the medium grit emery paper for course side and 1000 grit automotive paper for the fine side. I have yet to get the 1000 grit crocus, though I have finally found it for sale at McMaster Carr and plan on purchasing.

Hey Dave- It sounds like your pivot file and/or your burnisher is cutting the shoulder a bit too aggressively. Yes, you can expect to remove some material off of the shoulder but it will be very minimal; on the order of 0.0005". If you are having trouble maintaining the shoulder-to-shoulder length in your repivoting exercise, you could always machine the arbor back a bit and install an oversized plug. That would give you plenty of room for error and you could sneak up on the desired dimensions.

7 Q. 2. How to avoid tapering the new pivot is my next question. It is quite common for me to taper the pivot - either at the end or at the shoulder. Outside of practice, practice, practice which I intend to do - what do you credit your ability to keep the pivot from tapering? I notice you keep your finger under the pivot file and move the file in a sawing motion. Is keeping the finger in that position part of it? To date, I've been leaving myself about .003" of material to remove during polishing? I could bring that down to .001" or .002" to minimize material removal and chance of tapering, though that doesn't leave me much to work with. I use O-1 drill rod for my American T/S and modern Hermle wheels. In day to day clockmaking I understand that I could just burnish a .003" size and broach the pivot hole larger in the plate, but in the certification test I have to keep the original diameter. Appreciate your good feedback.thx. DM

Avoiding a taper takes a great deal of hand-eye coordination. You will need to get to the point where you are able to see that the pivot is perfectly flat while you are filing. Then, if you notice the pivot is becoming slightly tapered as you file, apply slight pressure by twisting the file to correct the taper and bring the surface back to flat. It is pressure and visualization... and yes, the finger under the file helps a lot :-).Great questions!D.

Q. 3. Are ww collets available in wire sizes? Or only in the metric sizes? I have all of Sherline's metric and inch collets and had moved to the Hermle modern arbors to practice repivoting - I'm working with the 2nd wheels with their pivot diameters of .070" and .078". I like to add a few thousandths more for polishing and burnishing and found no collets near these sizes. I have the .073" drill sizes for example, but no collet sizes. My Internet search didn't find any wire sizes - so I'm guessing none are made but thought I'd ask. At this point, my plan is to machine down a .078" drill rod to fit the .0748" collet for the .070" pivot and machine a larger size drill rod down to fit the .0827" collet for the .078" pivot. Both of these are a little heavy and will require more filing than normal. I can machine them down on the Sherline if the repivot is on the pinion end. The other pivot hangs out and requires a steady rest which eliminates doing it on the Sherline and I'm not yet set up on the watchmaker's lathe with the cross slide to do that type of repair. That brings me to another question regarding these pivots - is it correct to conclude that most often, the wear is on the pivot near the pinion end? where the driving force is? and, if so, in the certification test, presumably one would be asked to repair where the normal wear would be?? -DM

Hey Dave- I don't believe WW collets come in wire sizes but 0.070" is a #18 and 0.78" is a #20. When I'm repivoting Hermle second wheels, I'll go with 0.070" all the way around even though some were originally larger. And then I'd use a #51 carbide drill bit for the hole. The idea is to do it fast and with the least amount of work. And batch them out! You should be able to do all six repivots, start to finish, in under an hour.

8 A couple of comments on the exam...-If you are dealing with plated steels, I'd automatically repivot all second wheels regardless if wear. Yes, you will find most wear on the pinion end of the arbor but sometimes the pinion is in the middle :-).-Part of the exam I took was to repivot to a specific diameter and length of pivot. If that's the case, you must go a bit larger and work down to the desired size. So, you wouldn't necessarily go with a 0.070" pivot diameter to start with.

Q. 4. Hi Dave - I'm practicing my repivoting and using my watchmaker lathe for drilling. Couple of questions. a) If I've aligned my headstock/tailstock and after drilling with center drill/pivot drill, it appears at times that my result is off center. Could it be that I'm pushing too hard on the tailstock when drilling, not clearing chips often enough?

Hey Dave- If you are starting the hole true and it is getting off as you drill... Yes, you are a bit too aggressive with your feed (pushing too hard). Tickle the material off and clear your chips as soon as they are produced.

b) What is the "acid test" for determining if the hole is on center (concentric) or not? I fit up the repivoted wheel in the wheel and can't detect any of the "wobble" as it is referred to. Is it all by eye? or is there a regular setup/practice one uses to check whether the repivot is concentric? I'm trying to replicate what a certification evaluator would do. thx.DM

My acid test is to just watch for runout. If you are drilling the hole and the drill bit wobbles, the hole isn't true and the pivot plug won't run true. When the pivot plug is inserted, it should run true with the opposite pivot, pinion, and wheel. It doesn't necessarily need to run true with the arbor. So, chuck up on the pivot and see if the other one runs true. If there's no run-out on the pivot, wheel, or pinion... you got it right :-).D.

Q. 5. I'm practicing my pivot polishing/burnishing with my Vallorbe pivot file/burnisher and getting to recognize the nuances of the tool i.e. three square corners and one radius corner. Do you know the purpose of this radius edge? I've found an article or two which implies this is for a radius finish on say a balance staff??

The file end has a radius along the full length of one corner that is machined with a cutting edge and the burnisher end has a radiused corner along its full length with a lesser cutting edge. Actually the cutting edge ends about halfway down the corner and sort of goes smooth. So first question - do you know what the purpose of this design is? Would I spoil the tool any if I smoothed some of the cutting edge on the burnisher end?

9 The root of this inquiry is my need to end up with a square shoulder without removing more than .0005" of shoulder material. I'm still falling short on this quest. Invariably, when I use the radius corner to remove pivot diameter material, my shoulder distances stay intact and I'm well within the .0005" limits, however it doesn't leave me with a square shoulder. When I use the square edge to remove pivot diameter material, I obtain a nice square shoulder but remove too much material and the shoulder to shoulder distances.

So, would it spoil the tool to round off the square edge some? I'm guessing I could stroke this with one of my stones.thx. DM

Hey Dave- Yes, the rounded edges of the file and burnisher are designed to match the curve of a balance staff pivot. They obviously won't work well for polishing pivots with a square shoulder :-).

If you are finding your burnisher is actively cutting the shoulder, like you were machining it, then you need to knock the edge off a bit. I'd roll it on some fine sandpaper. It will take some trial and error until you get the desired results but the theory is the edge should burnish rather than cut.

And if you are removing too much material from the shoulder as you are filing, don't push so hard against the shoulder :-).Regards,D.

Q. 6. D - What is more critical? a result with a square shoulder or maintaining the shoulder to shoulder distance?

It is more critical to have a square shoulder than to maintain shoulder-to-shoulder distance. A rounded shoulder would reduce the end shake and could cause a bind. D.