Hairspring Correction Guide
While it is doubtlessly one of the most intricate and challenging tasks that an amateur watchmaker can undertake, hairspring correction is also one of the most valuable and rewarding skills to add to your vintage watchmaking arsenal. There are already quite a few helpful guides and tutorials online that cover hairspring correction, many of which were created by professional watchmakers with significantly more experience, but I wanted to share my own approach, thoughts and recommendations, developed following many hours of practice, experimentation, successes, and failures. I hope you find it useful!
Introduction
One of the more challenging aspects of vintage watch restoration and repair is the unfortunate prevalence of balances with damaged hairsprings. This doesn't happen spontaneously, but hairsprings are extremely delicate, and it doesn't take more than a momentary indiscretion when handling a watch balance to create a permanent deformation in the spring, which can severely affect the function and performance of the watch movement.
The "parts and repair" watch market is full of examples with balances that have kinked or distorted hairsprings, likely caused by mishandling during a previous repair attempt. This sort of hairspring defect is often not disclosed by sellers (many are probably unaware of the issue), and can also be difficult or impossible to spot in photographs of the movement. Of course, balance replacement is an option in these circumstances, but this can be quite expensive, and finding a suitable replacement balance for vintage watches isn't always easy.
Common Hairspring Designs
In order to correct a hairspring properly, it helps to understand its design features, geometry, and how it is designed to interact with the other components in the balance assembly. This is particularly important when the hairspring is badly distorted, as it may not be immediately obvious how the spring needs to be reshaped without this understanding.
Flat Hairsprings
The most basic and common hairspring design commonly seen on vintage watches, flat hairsprings coil in a single geometric plane, with the innermost coil attached to the hairspring collet, which fits around the balance staff, and the outermost coil attached to a stud, which is fixed to an attachment point on the balance cock or bridge.
Some flat hairsprings are simple spiral designs, while others have various forms of terminal curve or bend in the outermost coil. Often this terminal curve is intended to provide additional clearance for the regulator while allowing the inner coils of the spring to "breathe" freely.
The key thing to observe about these designs is that they are intended to operate in a single, two-dimensional plane perpendicular to the balance's rotational axis. Therefore, corrections tend to be focused on achieving correct radial symmetry and positioning within this plane, although any distortions that prevent the hairspring from sitting flat will need to be corrected first.
Overcoil Hairsprings
In overcoil hairsprings, including the famous Breguet design, the terminal curve of the spring is deliberately shaped to extend above the other hairspring coils, forming a fully three-dimensional shape, with the hairspring stud elevated above the plane of the other coils. In addition to being three-dimensional, this terminal overcoil curve is also typically asymmetrical, with specific angles and shapes, such as the Phillips curve. This design is intended to improve the concentricity of the hairspring's breathing as the balance oscillates relative to a flat design, which can improve isochronism and timekeeping performance (at the cost of greater manufacturing complexity and additional vertical clearance requirements).
This additional design complexity and three-dimensional nature makes overcoil hairsprings significantly more challenging to correct properly. In addition to ensuring correct radial symmetry and positioning, the vertical positioning and angles of the terminal curve must also be carefully preserved or restored, making for a significantly more complex problem than with flat hairsprings. Moreover, the fact that the terminal curve of these hairsprings sits above the other coils makes them physically more challenging to manipulate. Early in my hairspring correction journey, I would generally "write off" damaged overcoil hairsprings as lost c
Tools and Workspace
Hairspring correction involves making very fine physical adjustments to a small and extremely delicate component. In order to maximise your chances of success, effective magnification, good quality tools, and a suitable work surface are all essential.
Magnification
Unless you have superhuman vision, high quality magnification is a must-have for hairspring work. While you can get by with a loupe (and, in fairness, many professional watchmakers have for many, many years), using a high quality microscope with strong magnification and good lighting makes inspection and adjustment of hairsprings considerably easier. A binocular or trinocular microscope is highly recommended, as the depth perception they provide really helps when attempting to grasp tiny hairspring coils with tweezers or other tools, especially when working with overcoil hairsprings, where correct vertical positioning is critical.
Tweezers
Adjusting hairsprings is generally a two-handed operation. You grasp the hairspring (carefully!) with one hand, typically using a pair of fine pointed tweezers, and then use a second tool in your other hand to make the actual adjustment. What to use for this second tool is largely a matter of preference, and there are specialized tools like hairspring picks available, but I find that using a second pair of high quality tweezers is the most versatile option. I prefer to use a pair of bent nose tweezers, as they put my wrists in a relaxed position, which I find improves stability and control, but this is also largely a matter of personal taste.
Regardless, fine motor control is particularly important when you are working with hairsprings, so make sure to choose the specific tool materials that you find to be comfortable and confidence-inspiring. Personally, I like to use titanium tweezers for their light weight and feel. The fact that they are non-magnetic is also a bonus when working with hairsprings.
Work Surface
In theory, any clean, smooth, flat surface can serve as a work area for hairspring correction. However, there are a few things to keep in mind. First, you'll want to make sure your work surface has good lighting and sufficient contrast with the hairspring itself to provide a clear view. The surface also needs to be sufficiently smooth to avoid snagging the hairspring or the tips of your tweezers when you're manipulating it. Glass is a traditional option here, although its reflectivity may cause visibility issues depending on your lighting and magnification setup.
Personally, I usually perform hairspring work on the smooth steel surface of my bench block. While it doesn't always provide a lot of contrast with the hairspring, I find that visibility is still good. Moreover, it is very flat, smooth, stable, and non-reflective. Finally, the holes in the block allow me to make some adjustments to the hairspring while it is still mounted on the balance (see below). One downside of using a large steel object like this as a hairspring work surface is the potential to induce magnetism, so I always demagnetize the block beforehand, and demagnetize the balance when I am done.
Technique
Adjusting a hairspring is not an easy task, and it's quite easy to attempt an adjustment that actually makes things worse. While you can certainly try to "feel" your way through the process, a thoughtful, deliberate and systematic approach is highly recommended. This helps to minimise risk by correcting the hairspring geometry with the smallest possible number of individual adjustments and helps to ensure that each adjustment has maximum positive impact.
Inspection
First things first, it's important to try to understand what's going on with the hairspring, so it's important to start with a careful inspection. Start by studying the hairspring while it is still installed in the watch movement. Here are some things to check and note:
Looking from above, are the coils of the hairspring concentric, and are they centered around the balance staff? If not, the hairspring is likely kinked or distorted radially. Make note of the direction of the offset or distortion.
Note the radial separation between the balance staff axis and both the stud carrier and regulator pins. This will be important to help assess whether the hairspring geometry is correct later on.
Next, examine the hairspring from the side. Is it sitting on a flat plane, or is it angled or cone-shaped? At this point you will want to check, and possibly adjust, the vertical position of the hairspring stud in the stud carrier to see if that resolves the issue. If not, a vertical hairspring correction will likely be required.
Check clearances between the hairspring and other components, including:
The balance cock or bridge
The balance wheel spokes
The regulator
The center wheel, if there is an overlap
Other nearby components
If the hairspring is an overcoil type, note the vertical separation between the hairspring where it attaches to the collet and where it attaches to the stud.
Next, it's a good idea to inspect the hairspring again after removing the balance from the cock or bridge:
Examine the hairspring stud:
Does it sit parallel to the balance staff axis? If not, the hairspring is likely twisted.
Does the radial separation between the stud and the balance staff axis match the separation between the balance staff axis and the stud carrier? If not, when the balance is installed, it will tend to pull the hairspring off-center, and a radial correction will be required to bring it back to the correct position.
If the stud is at the correct radial location, does the path of the hairspring leaving the stud line up with the radial position of the regulator pins? If not, again, some radial correction may be required to center the hairspring properly between the pins.
Does the hairspring sit flat when viewed from the side? If the hairspring is an ovecoil design, does the stud sit at roughly the correct height relative to the collet, based on your measurements taken earlier (note: it may sit a bit lower due to gravity when the balance is removed from the movement)? If not, a vertical correction will be required.
Examine the hairspring carefully from above. Study the overall geometry and note any obvious irregularities, kinks, or distortions that may require correction. Obviously, some deviations from a perfect, concentric spiral may be part of the hairspring's design, so it's helpful to be aware of the design features of the specific hairspring you are working with. It can be helpful in some cases to compare the hairspring to reference images of similar designs online.
Correction Strategies
Hairsprings are made from spring steel with a ribbon cross-section. As a practical matter, this constrains the manner in which they can be adjusted or corrected. There are generally two possible types of correction that you can make, radial and vertical:
Radial Corrections: Radial corrections are the easiest to understand, and involve adjusting the path of the hairspring in the plane perpendicular to the balance staff axis. This is typically accomplished by grasping the hairspring with your tweezers immediately adjacent to the adjustment point, and then gently bending or shaping the hairspring in the horizontal plane using your other tool. Note that unwanted kinks in the horizontal plane, or overcorrections, can sometimes be smoothed out by carefully "ironing" the hairspring between the tips of your tweezers, applying gentle pressure while sliding the tweezers along the hairspring coil.
Vertical Corrections: Vertical corrections are generally more challenging, both to plan and to execute. Because of the ribbon cross-section of the hairspring, coupled with the natural spiral or curved geometry, vertical adjustments are made by twisting the hairspring slightly about its own axis. This is typically accomplished by grasping the hairspring with your tweezers at the adjustment point, then applying the second pair of tweezers with a slight rotation (parallel to the horizontal plane) to create the twist. A relatively small amount of twist in the spring can have a significant impact on its vertical geometry, so be conservative with your adjustments. I find that it's generally easier to make small corrections by rotating the second pair of tweezers to the desired angle before grasping the hairspring, rather than afterwards; this allows you to simply squeeze the second pair of tweezers shut briefly to apply the twist and avoids the need to try to rotate them while holding the hairspring.
Gentle and Steady
Like any delicate piece of metal, hairsprings don't respond well to sudden or overly forceful adjustments. Every hairspring correction tends to introduce small distortions, and too many repeated adjustments can weaken or distort the steel and compound into a condition where the spring no longer performs reliably despite an outwardly correct geometry. Therefore it's important to take things slowly, be gentle, plan your corrections carefully, make small adjustments, and try not to overcorrect or overshoot your adjustment target. It helps to have a steady hand, so try to adopt a position where you can brace your forearms, wrists or hands to help stabilize your tools when they're in the proximity of the hairspring.
Hairspring correction is one of those watch tasks that can be quite frustrating, and even under the best of circumstances it may take multiple attempts to get things right. Don't be discouraged if your first adjustments don't achieve the results that you're looking for. The degree of focus required to precisely manipulate tiny and delicate components like this is surprisingly fatiguing, so take breaks as needed to rest your eyes and hands. Finally, remember to be patient with both the process and yourself. Rushing your corrections is almost guaranteed to end badly.
On/Off Balance Corrections
The "classic" approach to hairspring correction usually involves completely removing the hairspring from the balance prior to making any adjustments. While this certainly provides maximum access and visibility, I find that in practice it's possible to make most corrections with the hairspring still mounted on the balance, as long as you have a work surface with a hole that is the right size to contain the roller table so the balance wheel can sit flat on the surface with the hairspring on top. The main advantage of this approach is that it facilitates frequent testing of your corrections without the need to constantly remount and demount the hairspring, which can be time-consuming and risks further damage.
Planning Your Corrections
Based on your careful inspection, you hopefully have developed a clear mental picture of what needs to be changed or corrected in the overall hairspring geometry. Before you pick up the tweezers, it's a good idea to take a moment to plan out your correction strategy, helping ensure that every adjustment you make is purposeful and makes a positive contribution towards restoring the hairspring to its correct geometry.
Where to Correct
Sometimes, careful visual inspection of the hairspring will reveal an obvious kink or distortion that can be addressed directly with a corresponding correction at that location. However, in many cases hairspring distortions are more subtle and may be challenging to identify even under close inspection. Here are a few places to focus your attention when studying the hairspring to identify the most effective correction points:
Most hairspring distortions occur when the balance is mishandled during installation or removal from the movement. This type of distortion naturally tends to manifest at or near one of the three fixed attachment points of the hairspring:
The hairspring stud
The hairspring collet
The regulator pins
Look for coils that appear to be spaced unevenly relative to the other coils. These areas often indicate the presence of a radial kink or distortion somewhere along the radius of the coils in question.
In hairsprings with some form of terminal curve, including overcoil hairsprings, the points where the terminal curve is formed and the spring rapidly changes direction are more vulnerable to stress, and thus common locations for distortions to occur.
Similarly, for overcoil hairspring designs in particular, the points where the spring angles upward from the primary plane then flattens again to form the overcoil are particularly prone to distortion.
Order of Corrections
When planning your correction strategy, it's a good idea to consider the order in which you will make your adjustments. Here are some general guidelines to help you prioritise your corrections effectively:
In general start with the most significant distortions first. This is because less significant distortions can be difficult to assess if a larger distortion is throwing off the overall geometry.
Address vertical distortions before radial distortions. This is because it's difficult to make vertical adjustments without affecting the radial geometry, while careful radial adjustments typically do not impact vertical geometry.
Prioritize adjustments that are closest to the collet before moving outwards toward the stud. It is easier to assess the impact of your adjustments if you move outward along the hairspring rather than inward.
Testing and Tweaks
After making a planned correction, it's important to assess the impact, determine whether further adjustments are necessary, and plan your next steps. This process of testing and tweaking is often iterative, requiring multiple rounds of adjustments and assessments before the hairspring geometry is fully restored. Testing and assessment can progress through three main stages as the hairspring geometry approaches the desired state:
Initial Inspection: This involves a careful visual inspection of the hairspring geometry immediately following correction to see whether the geometry appears correct, or whether there are any obvious issues that still need to be addressed.
Test Fit: If the hairspring appears to be in good shape following initial inspection, the next step is to reinstall the hairspring on the balance (if it was removed) and perform a test fit on the balance cock/bridge, checking the relative placement of the balance staff, stud, and the portion of the hairspring that passes through the regulator to see if those radial geometries are correct. It's not necessary to fully attach the balance to the cock/bridge to do this; you can simply hold the balance upright with the upper pivot of the balance in the jewel setting, then observe if the stud aligns with the stud carrier and the hairspring aligns with the regulator pins with the hairspring in a relaxed state. This is also a good time to check if the alignment of the impulse jewel is at least in the correct ballpark, as it's relatively easy to adjust this before the balance is fully secured in the cock/bridge. In movements where the design allows, I will typically align the regulator with the pallet fork pivot jewels so I can use it as a reference to assess the position of the impulse jewel.
Full Test: Assuming the test fit looks good, it's time to move on to a full test with the balance fully installed in the cock/bridge and the watch assembled to the point where the balance is free to oscillate. This allows you to observe the hairspring's behavior in its operational environment, check clearances and regulator alignment, and plan any final tweaks or corrections. You can further progress this from a "dry" test, with the pallet fork uninstalled so that balance can turn freely, to a full running test with the pallet fork installed and the mainspring wound up to deliver power.
When you get to the full test phase, you may notice that you are "almost there," but that there are still small tweaks to be made before the hairspring geometry is perfect. For example, the hairspring may not be perfectly centered in the regulator. At this point, if you are confident in your hairspring manipulation skills, you may want to attempt those small (emphasis on small) corrections while the balance is still installed, but proceed with caution as your access to the hairspring with the balance installed is less than ideal, and it's very easy to make a mistake when making adjustments this way. The payoff is that you can avoid the tedium and risks of repeatedly removing the balance, and you get immediate and direct feedback on the impact of your adjustments.
Conclusion
Hairspring correction is undoubtedly one of the most challenging tasks in vintage watch repair, but it is also one of the most rewarding skills to add to your watchmaking repertoire. With careful inspection, thoughtful planning, and patient execution, even severely distorted hairsprings can often be restored to proper function, saving the need for costly balance replacements. Remember to take your time, be gentle, and approach each correction with a clear plan in mind. Like any difficult skill, you'll experience plenty of setbacks along the way, but when the alternative is either full balance replacement of a non-functioning watch, it's almost always worth a try, and as you gain experience, your success rate will improve significantly.