After riveting and poising five balances it was time for us to learn how to fit the pallet staff to the pallet frame, how to fit and secure the pallet jewels and finally how to set the correct depth of lock and run to banking. The pallets are a key part of setting the escapement of a watch up correctly and are therefore a crucial part of the exam. The setting of the pallets have an effect the unlocking of the escapement and if set incorrectly can have a detrimental effect on how the watch runs.
We were first shown how to fit the pallet staff into the pallet frame, this is done with the aid of the staking tool and is fairly easy if a little fiddly. The pallet staff is placed into position in the pallet frame and pressed lightly down with a pair of tweezers. The staking tool is then used to press the staff down into the frame and into its correct position.
Fitting the staff is the easy part and we were now ready to take on the hard part, fitting the pallet jewels. The process involves fitting pallet stones, checking the depth of lock and run to banking and shellacking the stones into position. This would also allow me the chance to use another tool that I had recently made, a brass pallet warmer.
The pallet stones look identical to an untrained eye, especially when there is mixed pot containing a few hundred of them, but they are in fact slightly different. The exit stone (which sits closest to the outside of the movement) has a face with much steeper slope than the entry stone. This difference is easier to spot when the two stones are next to each other, which makes the task putting them in the frame the right way simpler.
6498 Pallet Frame and Pallet Stones ready to be inserted…
The pallet frame, with the staff now fitted, is placed on a staking block with the staff located in a hole of the correct size and held in position with a pair of brass tweezers. The pallet stones are then slotted into the correct slots and manoeuvred into what is believed to be the correct position. The stones stay in position thanks to the precise tolerances of modern manufacturing but on a lot older watches this isn’t the case.
6498 Complete Pallet Fork…
The next part is probably the most difficult thing we would need to learn and also the most important, getting it wrong can effect the running of the whole escapement. We would be learning how to set the correct depth of lock on the pallet stones, this involves checking the drop to lock, run to banking and the total lock.
The depth of locking is the amount the stones penetrate into the escape wheel and must be precisely set. This is done by moving the stones into or out of the slots in the pallet frame. The total lock value is measured from the centre of the balance staff and should be roughly 1.5 to 2 degrees. The total lock includes both the drop to lock and run to banking.
Diagram of Total Lock…
Drop to lock is the position of the of the escape wheel tooth when it falls onto the locking plane of the pallet stone. Run to banking is the point at which the escape wheel tooth slides along the locking plane and comes to a stop. There are generally three rules when setting the pallet stones, they are:
1) All teeth must drop safely onto the locking face of the stone.
2) Run to Banking must be equal on both entry and exit stones.
3) Total lock should be as small as possible.
When making any adjustment to the stones, its important to remember that adjusting one stone will affect the other stone as well. The run to banking depends on the position of the opposite pallet and remembering this makes adjustment of the lever escapement easier. Run to banking is reduced as the opposite pallet stone moves outwards from its slot, this is because the tooth of the escape wheel leaves the pallet later.
In order to check the depth of lock the pallets are fitted into the watch and the ratchet wheel is turned a couple of clicks so the mainspring can release a small amount of its power. There is no need to fully wind the mainspring as too much power makes it difficult to see the drop to lock and run to banking. A piece of peg wood is then used to move the lever from one banking pin to another. This impulse allows the escape wheel to advance and drop onto the other stone, further movement shows the run to banking. Its generally easier to check the run to banking is even on both sides by checking the distance the lever can move from the drop position until it touches the banking pin.
With the stones set in the correct position they must now be secured using shellac. Shellac is a resin secreted by the female lac bug on trees in the forests of India and Thailand. It is processed and sold as dry flakes that need to be heated to form an adhesive and then left to cool. The brass pallet warmer is used in conduction with an alcohol heater to warm the shellac enough to make it liquid and almost honey like in consistency. The pallet warmer is basically a piece of brass with a small hole to accommodate the pallet staff and a wooden handle that doesn’t conduct the heat.
The pallet fork is put in position on the pallet warmer and then a small piece of shellac is on top of the stone. The pallet warmer is then placed over the alcohol heater until the shellac becomes sticky. A small pin is used to drag the shellac into the required shape, its then heated until its liquid like and left to cool. Its important to get the shellac hot enough so that it runs around the stone and fill the gaps otherwise the stone can work itself loose.
Moving the shellac around can sometimes alter the position of the stones so they need to be checked in the watch once again. Its also now important to check the lock on every tooth of the escape wheel on both stones. The escape wheel can sometimes be eccentric causing some teeth to lock deeper than others. Any necessary adjustments should be made by repeating the above processes.
Reheated for some minor adjustment…
Its also important to get the right amount of shellac, enough to hold the stone but not too much so that it runs down the sides of the frame and stone. We were shown some examples, below, but it took a lot of practice to get it right. The face of the stone should also be cleaned to make sure there is no shellac that will affect it functioning.
We then learnt how to remove the stones and the shellac, the pallet fork is placed over a hole on the staking block and downward pressure applied on the stone to break the shellac. The stone can be removed and the stone/pallet frame can me be soaked in meths to dissolve the majority of the shellac. Any remaining shellac can be chipped off.
Once again we had to complete the whole process five times, with our tutor checking the lock of pallets in the watch and then the strength, quantity and neatness of our shellac. As with everything, practice is key and you just get quicker every time. I was able to set the stones in almost the correct position at the first attempt after some practice which reduces the amount of adjustment required. Applying the correct quantity of shellac can be tricky at first but eventually you can apply the right size of flake to begin with which makes things a lot easier. The whole process has not only given the me the confidence to take on adjusting pallets but it has also helped with my understanding of how the escapement functions.