Reinforcing a cello bridge

A method of inserting carbon-fibre rods to ensure the bridge retains its shape

I

f not properly treated, a cello bridge can bend. It happens because players do not tend to notice it – and if they do, they don’t know how to push it back to the right position. Worse, if it remains unnoticed or ignored for too long, the bridge can break under the pressure of the strings.

The thinner the bridge, the more likely this will happen. While for amateur musicians a thick bridge might just work well enough, this is often not an option for professional cellists. The thinness of a bridge can make all the difference between an instrument that sounds good, and one that sounds terrific.

For this reason, I wanted to find a solution to reassure customers that the bridge, despite being thin, will never bend. With no better material to hand, I made the first test with 2mm steel rods in a cello bridge. My idea was to insert two rods in the upper part of the bridge in a ‘v’ position (see drawing). Because of the heavy material I expected the bridge to produce a muted sound, but there was no noticeable muting effect. For the carbonfibre reinforcement described below, I just needed a supplier of 2mmdiameter carbon-fibre rods, which I very quickly found on the internet.

The line marker is adjusted to half the bridge width (in this case 2.5mm)

a

Marking the centre line

 b

1 The first step is to adjust the bridge feet as normal, and then to cut the bridge down to its final height with an allowance of 0.1–0.2mm. At this point, the thickness at the top edge should not be trimmed down (it will usually be c.5.0–5.5mm thick). Using a line marker, made from a screw embedded in a piece of wood (1a), I mark the centre line of the bridge’s top edge (1b).

Marking the A and C string spaces

a

b

2 Then I use a divider to mark the spacing of the A and C strings (2a). Eventually, the rods will be hidden under these two strings. I then deepen the markings with a steel needle (2b). This is so that the drill won’t slip when I start making the hole.

The drill is mounted on a board 

a

The side view shows the slight angle of the support block (two degrees)

3 Then I mount a small high-speed drill on a board so that it’s perfectly horizontal (3a), and make a wooden support block for the bridge (3b).

b

Checking the drilling direction

4 For the initial drilling I set the drill bit as deep as possible into the chuck, to ensure it does not bend in the initial stages. For a 2mm carbon rod I use a 2.1mm drill. It is worth checking the actual diameter of the rod with a calliper, because it might differ a fraction from the measurement given by the manufacturer.

I set the rotation at medium high speed, and check the direction of the drilling hole with a ruler.

Drilling the first hole 

Both holes land slightly away from the centre line

5 In the initial stage I drill the hole only halfway through, until it is about 2cm deep. Then I pull the drill out and clean the debris off (very important!). Then I reset the drill bit in the machine to its maximum length, and drill the hole right the way through to the underside of the bridge.

The epoxy is pushed through the hole

Carbon-fibre rods pushed through both holes

6 When it comes to gluing, my first step is to push e poxy right the way through the hole, using the carbon-fibre rod. Then I know for sure that the glue has been distributed evenly through the hole.

Grinding down the rods

The remaining carbon fibre can now be taken off with scissors

7 After the epoxy has hardened (usually t he next day) the bridge can be thinned down to its final thickness and the final cutting done. Bridges reinforced in this manner can be made thinner than usual.

The reinforced bridge

The rods are hidden by the A and C strings

8 When the rods have been finished and trimmed, if the holes are placed correctly, they should be hidden under the strings.

NOTE It is much more difficult to apply this method to a violin bridge. A 1mm drill is much more likely to bend in the hole during the drilling, and it might go through the surface of the bridge. I am currently working on a failure-free method for violin bridges.