What is the function of the roller/impulse jewel?
Before replacing the impulse jewel, it is first important to understand its function. When the balance wheel turns the Impulse jewel which is attached to the roller table swings back and forth as it passes through the horns on the pallet fork. It’s this action that makes the pallet fork move back and fork, unlocking the pallet stones which allows the escape wheel to advance one tooth at a time and regulates the escapement of the watch. This type of repair is best done under a high magnification as the parts you are dealing with are super small, even for watchmaking.
When should a roller/impulse jewel be replaced?
Impulse jewels should be replaced when they are broken, chipped or if it is to narrow inside the horns of the pallet fork. If the roller jewel is broken off, a quite common problem, the balance wheel will rotate back and forth but does not engage with the pallet fork and the watch will not run. If the impulse jewel is chipped it can cause wear on the pallet fork horn which will ultimately lead to a fault in timing. If it is to narrow for the horns, there will be too much side shake which reduces the power between the pallet and escape wheel.
This reduction in transmitted power is one source for the loss of amplitude that can be hard to detect with newer watchmakers.
Roller jewel types
There are 3 types of impulse jewels. The first is shaped like a triangle, one is oval shaped and the most common shape you will see is D-shaped. The shape you need to use is solely based on the shape of the hole in the roller table. The actual width needed is determined by the width of the pallet fork horns.
Measuring for a New Roller Jewel Length
There are 2 ways to determine the length of a Impulse jewel. The easiest way to measure the length is to use a pair of calipers and measure from the underside of the roller table to the tip of the jewel. This is fine if you are replacing a jewel that is chipped or too narrow. Unfortunately most Impulse jewels you need to replace are going to be broken off.
When the roller jewel is broken or missing you are presented with a different problem. It is almost impossible to get any type of measuring device into the area where the balance staff and pallet fork horns are. What I do in these situations is to measure the thickness of the Impulse Roller Table and then I measure from the top of the Horns and Notch to the top of the guard pin. I add these two numbers together which gives me a general idea on the length needed. As you can see from the picture below there is a little bit space for a longer jewel. This is important to know for a couple of reasons. Sometimes the width listed by the supplier does not include the length of the jewel. If I am going to need to shorten the jewels, I want to be able to do it before they are set into the roller table. In most size 16-18 pocket watches, the Impulse Jewels will fall between 1.07-1.42 mm. in length but the smaller the size of the watch, the smaller the jewel.
Measuring for the roller jewel width
There are essentially 3 ways to measure for the roller jewel width. You can measure the existing roller jewel with it with a micrometer or calpers. This is the least desirable method as you are assuming that the jewel is the correct size.
There are also vintage Pallet fork gauges. These are basically a thicker style of feeler gauge used to determine the inside width of the pallet horns which in turn tells the the size of the impulse jewel you will need. Insert the gauge between the pallet fork horns. start smaller and move up in sizes until you you have a snug fit. The number on the feeler gauge represents the width you need to order. one of the reasons I am not a fan of this method is that a lot of the gauges I’ve bought over the years are damaged or bent.
My preferred method is to use a pin gauge. Pin gauges are perfectly accurate machined rods that are sized in .01 mm widths. This set that I use gives me extremely accurate measurements. I use it for many other tasks but find that it is perfect for measuring inside a pallet fork.
Gently insert the pin in between the horns, starting with a smaller size and move up in size .01 mm at a time until the pin just slips in. You don’t want to force it in, but the fit should just be snug. I find these easier to work with and I already use the pin gauge for other measuring needs like pivot holes and do not really see the need to have an extra tool.
Once you have the gap width measured, subtract .02 mm and that’s the width of the roller jewel you need to order. As the width of Impulse jewels increases so does the length. I mention this because some supplies don’t list the length
Does the roller table need to be removed?
You can really do it either way. Some people prefer to remove the roller table from the balance staff and that is fine to do but I would not recommend it. The main reason is that you always run the risk of damaging the balance staff or throwing the balance out of poise. I just remove the hairspring to get it out of the way so that I don’t damage it.
Holding the Roller Table
Since the roller table must be heated to release the old jewel from the shellac and then reheated to install the new jewel there is a special tool. Do not heat the roller table directly as you run a risk of overheating the balance wheel and possibly warping it. The preferred tool is often called a roller jewel warmer or roller jewel setting tool.
When the jaws are opened there is a slight V groove in the jaws that are used to hold the roller table and the long wing is where the flame is applied to transfer the heat to the roller table. I like to place the roller table into the clamp with the roller jewel as close to the opening of the jaws as possible allowing my tweezers to easily get to the roller jewel hole.
Removing the old roller jewel
Removing the old roller jewel is straight forward. I usually start by dropping the balance wheel in a jar of lacquer thinner for a couple of minutes to soften the shellac. Then I take a pair of strong tipped tweezers and put the bottom tip on the side of the roller table with the long part of the roller jewel sticking out and the other tweezer tip on the bottom of the roller jewel and simply push it out.
If it still feels a little snug, try heating the tip of the tweezer and then rest it on the bottom of the jewel just long enough for the jewel to warm enough to break it free.
Sometimes they will really be stuck in the hole and you will need to install it in the clamp to remove it. With the roller table installed in the vise slowly heat the long wing of the clamp to heat up the roller table. With the point of a #2 tweezer or a needle apply light pressure to the underside of the roller jewel until enough heat has transferred to melt the old shellac allowing the jewel to slip out of the jewel hole.
Drop the roller table into a jar of lacquer thinner or denatured alcohol for a minute or so to dissolve any remaining shellac from the roller jewel hole and table. I use a needle and thin foam surgical swab to remove the rest of the remaining shellac. I then swirl the table around in clean IPA alcohol to remove any leftover residue on the roller table from the lacquer thinner. Make sure the Hole for the Impulse Jewel is completely clean of all old shellac before proceeding to the next step.
Shortening a roller jewel that is too long.
If your new roller jewel is going to be too long it will need to be trimmed down before its installed into the roller table. A roller jewel is to long when inserted into the jewel hole and it comes in contact with the guard pin of the pallet fork.
The roller jewel can stick up ever up slightly on the opposite side of the jewel hole, but this needs to be kept to a minimum as to much of the jewel sticking through can throw of the oscillation of the balance wheel.
To shorten a roller jewel, the first thing you have to be able to do is hold the jewel securely. The best way I have found to do this is to sharpen a piece of peg wood and insert a thin wire into it, making a small hole just big enough to be able to put the roller jewel into it.
Holding the tip of the peg wood over the alcohol lamp in one hand and a string of shellac in your other hand slowly move toward the flame to melt the shellac to lock the jewel in place. Add enough shellac around the jewel to so that it does not move around on the tip of the peg wood.
Once the roller jewel has been secured, place a piece of 2000 grit wet/dry sandpaper on a piece of glass or a flat bench block with no holes drilled in it. Apply a small amount of clock oil onto the sandpaper and slowly glide the tip of the jewel across the sandpaper until the proper amount has been removed.
Installing the new roller jewel
Now that you have the old roller jewel removed, the jewel hole cleaned, and the new roller jewel the proper length we are ready to put it in. I like to use the roller jewel tool to hold it while I use the tweezers to insert the roller jewel. Start by inserting the new roller jewel into the jewel hole. There are a couple of ways to do this.
One is to grab the roller jewel with a pair of strong tweezers and drop it into the jewel hole. Once its sitting in the hole, put the bottom tip of the tweezer on the edge of the roller table and the top tip of the tweezer onto the top of the roller jewel and push it in. Push far enough so that the bottom of the roller jewel is flush with the bottom roller table.
The other method is to open the tweezer and lick the top tweezer leaving a bit of spit. The top of the roller jewel will stick to the spit and allow you to position it over the hole with the bottom tweezer tip on the edge of the roller table. Slowly press the roller jewel into the jewel hole until it is fully seated. Check the roller jewel and make sure it is perpendicular to the roller table.
Both these methods require care or you will be on your knees looking for a roller jewel. I would suggest having a clean and uncluttered work bench in case the jewel goes flying.
Shellacing the Impulse Jewel
Now we are ready to add shellac. As usual there are several ways to apply the shellac. The goal for either method is the same, to completely fill in the space between the roller jewel and the jewel hole.
The first method uses flake shellac. Break of a small section about 1 ½ times the size of the bottom of the jewel. With the roller table mounted in the vise lay the shellac flake on the bottom of the roller table directly on the bottom of the roller jewel. Heat the wing tip of the warmer until the shellac melts into the bottom of the roller jewel. Whatever you do don’t use old shellac flakes bought from some old watchmakers stash. Only use fresh shellac .
I like to use shellac sticks. These are ½” x 6” solid sticks of shellac. I just heat up the end and grab the softened shellac and pull out a thread. What I’m left with is a thin thread of shellac stuck to the end of the tweezer.
As I heat up the roller table in the vise I just touch this shellac thread to the underside of the jewel. When the metal is hot enough to melt the shellac, capillary action draws it into the area between the roller jewel and the edge of the hole. If you have ever soldered copper pipes the process is basically the same.
After I am satisfied enough shellac has entered the gap between the roller jewel and the jewel hole, I allow the roller to cool off a couple minutes and then check the roller jewel with my tweezers to make sure it is solid.
Adjusting the newly installed roller jewel
If you find for whatever reason find that you need to adjust the position of the roller jewel, you can reinstall it into the roller jewel warmer and heat it up to soften the shellac and make the adjustment. I prefer to heat up my steel tweezers and grab the underside of the roller jewel and let the heat transfer to the jewel to make the slight adjustment.
If everything is lined up correctly and solid, reinstall the hairspring, install the balance and check for function.