6 mins
Fitting a bridge to a violin belly
A detailed account of a process that revisits a fundamental skill for luthiers
Makers reveal their special techniques
Luthier based in Hudson, NY, US
Fitting a bridge to a violin is an everyday shop activity that has an enormous effect on the overall success of the set-up and sound of the instrument. We know from experiments that the bridge is responsible for a broad peak in the violin’s upper frequency response – roughly around 2.5kHz, known as the ‘bridge hill’. Although what happens in the higher frequency range of a violin is extremely complex, the importance of a properly fit bridge is self-evident, for the energy of the bowed string to pass from the bridge to the corpus with maximum efficiency.
When it comes to many operations in my workshop, I tend to follow the principle of Occam’s Razor. In this article I look at the fundamental concepts of the task from a traditional perspective, disregarding the number of complicated alignment and sanding jigs that there are on the market.
My fundamental belief is that the traditional methods are still a reliable way to perform our daily work, and I hope that this article might spark some conversation about why our craft is often resistant to innovation.
Finding the precise location of the bridge
ALL PHOTOS CAITLIN XANTHA VON GRAF
1 I begin by ensuring that a well-fitted soundpost is installed in a desirable location. I also like to check that the violin is level in my cradle by measuring to the top edge at both the neck and the saddle ends, and adjusting accordingly. Performing this simple step allows me to judge more effectively the vertical axis for the initial phase of orienting the bridge.
Then I define a precise location for the bridge (see photo). I need to find the centre line of the top; in many cases, finding the centre between the f-holes is adequate. I like to mark the back corners of the feet, so I add half the thickness of my finished bridge (in this instance I’m using an average thickness of 4.4mm) to the standard stop length of 195mm (so, 197.2mm), and measure south from the upper edge adjacent to the neck mortise. From the centre line/ back of bridge intersection, I measure out 20.75mm in both directions to establish the standard bridge width of 41.5mm.
Before committing to the final bridge position, I check the bass-bar is located 1.0–1.5mm inside the bass foot. Sometimes compromises will need to be made. Various methods of marking can be used, but I find blue ‘painter’s tape’, weakened slightly with saliva, to be both safe and accurate.
2a
The medullary rays of the front appear as dots
2b
Those of the back are elongated
2 Now that the bridge position is located, it is time to choose a blank and begin the fitting. I choose the highest-quality blank I can, and decide which side is the front (the side facing the fingerboard) and the back (facing the tailpiece). Typically, the front is the side with truncated medullary rays, which appear as ‘dots’ (2a). The back will present elongated medullary rays, which appear as long, vertical lines (2b). This is the preferred orientation because the back of the bridge remains mostly flat and the long medullary rays add strength.
3a
The bridge and the cut-down pencil
3b
Marking the inside of the feet
3 Now we can finally begin the job. First, I plane the blank down to 0.1mm above the final thickness (in this case 4.5mm). I leave the extra tenth for the final clean-up. I have made a jig for transferring the arch on to the blank. I have also cut a standard pencil to around a third of its original length, sharpened it to a fine point (3a) and then planed one side of it down, until the point marks roughly half the height at the inside of the feet (3b).
The bridge may appear to lean forward slightly
4 I make some initial cuts using a chisel, slowly a pproaching the pencil marks. A little planning is required at this point, as I want the bridge’s lower arch to be low while keeping about 8–10mm of wood around the heart. Some of this depends on the blank but also on the projection of the neck. Either way, it is important to know how much material to remove in order to meet these requirements.
Another variable that needs attention at this point is the vertical orientation of the bridge. Typically, the back of the bridge is made ‘plumb’ to the horizontal axis of the belly. I will admit to occasionally breaking this rule. If I think the bridge will be under increased pressure, usually owing to the neckset, I will orient the bridge so that a theoretical vertical line passes through the centre of the top and feet (see photo). This will appear to have a slightly forward lean, but may help to offset any load that is larger than normal. However, straying from the standard orientation is less than ideal, and should only be done with good reason. Both of these orientations can be checked with a small engineer’s square laid on the top edge for a visual reference.
5a
Using soft graphite on the top
5b
The feet are fully covered in graphite
5 When I am satisfied I am on the right track, I begin to refine the fit with a knife, always cutting from the outside towards the centre of the bridge. I prefer my knife to have a slight convex curve to it, allowing me to isolate areas to be removed.
When I judge the fit to be about 75 per cent, I move on to a further refinement. I apply some soft graphite to the top to check the final phase of fitting (5a). I slightly moisten the feet to help pick up the graphite, being careful to move the bridge laterally as minimally as possible. This process is repeated until the feet are fully covered in graphite (5b).
Checking for rolling or tipping
6 Once I am happy with the fit, I remove the graphite either with a light b uffing compound or with mineral spirits. The graphite will have built up a slight thickness, which can result in a less than perfect fit; so I take the fitting one step further. I use a pointer of some kind (an awl works well) and press the four outside corners of the feet to observe any rolling or tipping (see photo). Depending on the movement, I take very small amounts of material away until the bridge remains solid when the corners are pressed. The trick is to choose the appropriate spots based on the movement.
This step usually results in the fit I am looking for. The tactile clues at this point are a pronounced ‘surface tension’ when the bridge is put in place. There should also be a ‘clicking’ feel when the bridge is rocked back and forth on the belly.
I make a final check to ensure the bridge is still standing upright in the orientation of my choosing, and also not canting off to either side. Any corrections are made at this point. Otherwise, I consider the fitting to be done. Then I go on to cut the bridge to height and carve it in the usual manner, with whatever particular acoustic considerations in mind.
Someone once told me that a well-fitting bridge should look as though it has grown out of the top as a tree grows from the ground. I think this is a great analogy and I carry this thought in my mind as I finish every bridge I fit.