The mark of a True Watchmaker- Forming Hairsprings Part 2- Vibrating the spring using a Luthy Tool

My first instalment about forming a hairspring detailed how to form the inner hook, pin the spring to the collet, flatten and centre the spring and finally the process of fitting it to a balance wheel. The next step is to count the vibrations of the spring using a luthy tool in order to find the hairsprings timing point, the result is hopefully a spring that vibrates at the right frequency.

The Luthy Tool has a master balance in its base that vibrates at a set frequency, in this case 18,000 BPH, and a set of jaws above it designed to hold a hairspring thats fitted to a balance wheel. A lever under the base rotates the tool in order to get the master balance and attached balance oscillating. The aim is to get both balances oscillating in sync for at least 20 seconds or more which in turn shows they are both vibrating at the same frequency.

Luthy Tool…


The timing point can vary in position depending on the type of spring that is being used, so far we have worked on two types of springs, one with the timing point on the outer end of the spring and one with it at 180 degrees from the outer end of the spring. With this in mind, the hairspring is placed between the jaws on the luthy tool at one of the approximate locations above and the balances are oscillated. The balances are both observed, if you get lucky they will be perfectly in sync first time, but this doesn’t happen very often so some adjustment is needed.

Balance in position…

Before making any adjustment its important to determine which balance is moving the quickest because moving the jaws along the spring in one direction increases its frequency and the opposite direction decreases it. So if the master balance was moving faster the spring would need to increase its frequency and vice versa. This can be a long and frustrating process but things become slightly easier after weeks of practice because you know the approximate location of the timing point.

Checking the timing point…

However, what has just been found is only the initial timing point, there is more work to be done! Next the outer portion of the hairspring is cut exactly one coil out from the timing point. This portion is removed to make it easier to find this initial timing point later on while still leaving enough hairspring for forming the terminal curve and pinning it to the stud. The hairspring can now be removed from the luthy tool and using a pair of hairspring collet levers it can also be removed from the balance wheel.

One of my previous posts detailed poising the balance wheel and you may remember that it is important for the balance to be in poise in order for it to keep good time, particularly in the vertical positions. We regard something as being in poise when its weight is evenly distributed around its axis of rotation. Similarly, the hairspring can also be poised by ensuring that its timing point lines up with the point at which the spring is pinned to the collet. If these points lie on the same line, the active length of the hairspring should consist of whole coils and therefore its mass should be more or less evenly distributed.

In order to check the two points aline we use a protractor like scale on a laminated piece of card. The hairspring is placed on this card and the pinning point at the collet is aligned with the zero angle, the angle to the timing point can then be read off the scale. We are given a tolerance of +/- 15 degrees either side of the points aligning and if the result is within this tolerance no material needs to be removed from the hairspring. If however the result falls outside the tolerance, the resulting figure is used to determine how much of the hairspring must be cut off the inner coil in order to re pin it and achieve ‘poise’. Taking this figure forward we use a formula devised by Pierre Le Roy to determine the amount of hairspring to cut off.

Checking the points align (In this picture they do)…

The Pierre Le Roy formula is relatively simple and delivers a good result providing all the variables are accurate and the cutting of the spring is done at the precise point the formula dictates. The formula is as follows: A + (A÷3) = B

A is the angle between the timing point and pinning point. B is the number of degrees to be cut off from the intial pinning point. The minus of 60 degrees leaves room to form a new hook and re pin the hairspring.

Using the formula, the amount of spring to be cut off B is accurately measured using the laminated scale once again and cut away, a new hook of 60 degrees is then formed and the hairspring re pinned as before. The process of re pinning the spring has to be completely repeated including flattening and centring it at the collet. The spring must be placed back on the balance and the process of finding the timing point started again as it will have moved, normally by the same amount as the length of spring removed from the centre. Suffice to say that making sure the timing point and pinning point align at the beginning are a massive time saver in the whole process. The ways to achieve this alignment are only found through hours of practice and experimentation with forming the initial hook.

This part of the process can be quick and easy at times but if things don’t go to plan it can a real nightmare and very stressful. I have lost count of the number of times I have spent hours at this point with no success but after starting again with another spring I’m done within 5 mins! In that respect the exam is a bit worrying at this point because some days things just click in to place and other days it all goes horribly wrong. Something in the middle of the two would be fine on exam day but I still have a number of weeks to practice yet so fingers crossed.

The next step is to prepare the spring for forming the terminal curve but I’ll cover that in Part 3 as I think its easier to digest this subject one stage at a time. I’m sure the majority of people reading this will be surprised at the amount of effort required to form one hairspring without a machine. It just goes to show why the cost of restoring a vintage timepiece with obsolete parts is so time consuming and expensive in todays world. Anyway I hope you have enjoyed this post and keep your eyes peeled for the next instalment!

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