Bridge Fitting
Bridge Fitting
Bridge Fitting
By Lars Kirmser
• Belt Sander
• Stanley 9 1/2 Block Plane
• Violin Knives (5mm, 10mm, 15mm)
• String Height Projection Sticks (complete set)
• Bridge Templates (complete set)
• Slotting Files (or equivalent)
• Rat Tail File
• Mill Bastard File (med.)
• Bridge Holding Jigs (set)
• Sanding Papers
• Linseed Oil (opt.)
• Ammonia (opt.)
• Parchment Paper
• Alaphatic Resin (Carpenter's) Glue
Bridges on instruments of the Violin Family are made of a unique grade of maple.
This select maple is precisely cut on the quarter, and as the result, reveals a
distinctive pattern known as "flake". Bridges on the violin family are freestanding,
never being affixed or glued to the instrument's top, but rather being held in place
by the tension of the strings passing over them.
The bridge, of course, is the first and one of the more important links in the
transmission of vibrations directly from the strings to the hollow enclosure of the
instrument. The precision with which the bridge is cut and fitted to the specific
instrument will largely determine the character and quality of that instrument's
voice. Much of the process will be very specific where a number of important
criteria must be carefully attended to, however, there will be elements within the
process that are considered to be arbitrary, and offers the luthier an opportunity to
assign his own "signature" to each bridge he installs. As a repair technician, the
installation of a bridge is one of the rare occasions where you will be allowed to
project your own signature to your work, as long as it is consistent with good taste,
and only after all technical criteria are successfully met.
Usually, only manufacturers of better quality bridge blanks will offer you an
adequate range of bridge sizes. And, since only instruments of sufficient quality will
require this consideration, you need not be overly concerned with having to
maintain an extensive inventory of bridges, unless of course you want to specialize in
the repair of artist quality instruments. On most occasions a standard bridge will be
quite adequate, so on student and intermediate quality instruments where a less
expensive blank is called for, this consideration will be, for the most part, academic.
To be sure, it is usually possible to trim out a standard size bridge in such a manner
as will effectively suit the requirements of a particular instrument.
When you have determined the size bridge blank you require, you must then select a
blank with the appropriate physical qualities. Two considerations must be made
here, the first being the type of wood, the second being the cut of the blank. Wood,
being the natural material that it is, will exhibit a wide range of physical
characteristics. For example, it can vary from being tremendously hard to quite
soft, flexible to stiff, highly figured to plain. Its largely a matter of experience for
one to be a good judge of wood types. As a consequence, it will probably be rather
confusing to you at first, but my suggestion would be for you to begin by carefully
examining and testing the bridges which you have in your inventory as well as those
that routinely come across your work bench. Begin by flexing them between your
fingers and thumbs to get a sense of their relative strength. Drop them on a hard
bench and listen to the relative ping they make as they rebound off the hard surface.
Those with a defined ringing will be harder (more dense) then those that exhibit
more of a thump. Take note of how they feel under your plane and knife as you
work with them. Later, when we begin our acoustical studies we will attempt to
make a systematic analysis of many of the variables, but for now simply try to gain a
general feel for their relative physical qualities.
Finally, in selecting a bridge (and also when purchasing your inventory) you should
be aware of the cut of the blank itself. Remember, you can't put wood where no
wood exists, so if your blank is too thin or too narrow in an area which we consider
to be important, then you will simply be wasting time and money by trying to work
with it. As you gain experience and begin to have an idea of the general type of
blank you like to have in stock, you can check with various suppliers to locate what
you need, or if you want you can even work with a manufacturer to produce the
style you desire. Naturally, you will be expected to pay a premium price for such a
service.
Next, you must determine which side will be the front of the bridge. The front is the
side that will face the fingerboard; the back is the side which will face the tailpiece.
The best way to establish this is to look at the edge of the bridge to see which way
the white wood rays lean. The rays will be nearly vertical, but they will usually run
out toward one face more than the other. Make the side they run out-to be the back
of the bridge. When you assign this side as the back, there will be a tendency for the
wood to have more resistance to warping forward, which can be a very common
problem, especially on the 'cello and double bass. As it turns out, the surface pattern
of the back side will appear as a figure of smaller, more defined round flecks, while
the front side will have more elongated ones. Incidently, the backside of the bridge
should be the side with the trademark, so this whole decision becomes mute if the
manufacturer has already made this decision for you. On the other hand, you can
sand off the trademark and reassign the back side if you desire, as long as the
trademark is unimportant to the client.
The next step will be to plane the feet of the bridge on the front side of the blank to a
thickness just a bit thicker than your anticipated finished thickness. The finished
thickness for violin is 4.4mm, viola 4.7mm, 'cello 11.6mm, and double bass 21mm.
Plane generally across the feet as indicated by the arrows in the illustration above
and check the thickness of the feet from time to time with your dial caliper. Plane as
parallel to the opposite surface as you can, thus limiting your planing to the feet
portion only, and not the entire front face.
If the instrument is not already marked out for a sample bridge template, do so now.
A sample bridge template is made "by the numbers" so to speak and will allow us to
get some preliminary indications of how our blank will relate to the unique
dimensions of the instrument at hand. You will prepare this template according to
the standard measurements of the model being worked on. The template usually
consists of a full blank with flexible feet (i.e. de Jacques type) that will easily
conform to the contour of the top, and have a dark line drawn across its face to
indicate the finished profile as determined by the fingerboard to bridge string
projections. Eventually, you will want to have a sample template for each nominal
size of all the instruments in the violin family. The sample bridge template will allow
you to predict with relative accuracy the compensations that must be made when
cutting the bridge blank so that you can adjust for any irregularities unique to the
instrument we are working on.
Hold the sample bridge template in place and sight down the fingerboard. You will
first note the relative height of the fingerboard with respect to your dark line,
secondly, you will note how well it is centered with respect to your sample bridge.
Carefully noting these two factors will allow you to accurately determine exactly
how you will need to cut the feet and/or crown of your bridge blank.
Ideally, you will want the heart of the finished bridge to be approximately in the
center of the main body of the bridge. So, if the fingerboard is high with respect to
the sample bridge, it will be necessary to leave a corresponding amount of meat on
the feet of the blank (to raise the body of the bridge higher). On the other hand, if
the fingerboard appears low with respect to the line on the sample bridge, you will
want to take off a corresponding amount of wood from the blank's feet, thus
bringing the body of the blank into an acceptable range with respect to the
fingerboard.
Similarly, if the angle of the neck is off to one side, you will want to bring the body
of the bridge into alignment with it as much as is possible by removing more wood
off the foot on the side toward which your need to move. The reason here is that the
principle of symmetry is very important to the balanced good sound of an
instrument. We already know that the bass bar and soundpost are balanced evenly
on each side of the center of the instrument, so naturally, we will want to keep the
bridge centered as well. In this manner we are able to adjust the crown of the bridge
relative to fingerboard and at the same time keep the feet evenly and equally spaced
on the top of the instrument.
FITTING THE VIOLIN AND VIOLA BRIDGE
We shall begin by making the initial trim on the feet of the bridge blank. If a
substantial amount of wood must be removed from the blank, then it may be
advisable for you to use the belt sander. Before sanding the feet, make sure that the
belt sander table is exactly perpendicular to the face of the belt. It is important to
have the back side (trademark) of the blank facing down when sanding the base of
the feet since the back side of the finished bridge should be at an exact 90 degree
angle with respect to the belly of the violin. Prior to sanding, make a light pencil line
on each foot allowing for 2 or 3mm more wood than you expect your finished
thickness will be. If only a small amount of wood is required to be removed then you
may want to remove it with your violin knife (12 - 15mm), as the knife offers more
precision than the belt sander. When using a knife, I prefer to use one with a slightly
wider angle at the tip, but this is my own preference and you may prefer a more
pointed blade. Hold the bridge blank such that the bottom of the feet are facing
upwards, and begin by making a paring cut from outside of the first foot towards
the inside. Hold the blank such that you are able to comfortably (and safely!) make
the cut towards yourself. Be sure to offset your thumb slightly with reference to you
knife blade (when your making a paring cut) so that you wont cut into your thumb
if the knife slips. Remember, ANYTHING that is in front of any sharp blade (knives
- chisels - gouges) is fair game as far as the blade is concerned -- blades have an IQ
of approximately 0 and are unable to discriminate! So, think twice before cutting.
OK, continuing on . . . . Throughout this whole process we must always keep in
mind that our goal is first of all, to trim the feet such that they fit against the top
plate precisely where all 4 edges of each foot set firmly and evenly (no gaps!),
secondly, we want to end up with the feet at precise thicknesses, and thirdly, we
must maintain the 90 degree angle at which the bridge will sit with respect to the
tailpiece side of the instrument's top. The importance of this 90 degree angle cannot
be underestimated as it will allow for approximately equal angles on each side of the
bridge with respect to the strings passing over. And, as a consequence will provide
an evenly balanced downward pressure on the instrument's belly, thus avoiding the
propensity of warping later on. When correctly placed, the bridge will appear to be
leaning slightly backwards toward the tailpiece when sighting from the side, and a
perfect right angle will be present with respect to the top on the tailpiece side of the
bridge (see illustration>. As we proceed later on, the thinning-out of the bridge will
come from its front side, that is, the side with the elongated-shaped flake (lacking
the trademark). When the bridge angle is, in fact, correct there will be appear to be
a slight backward leaning effect, while the back side of the bridge should remain
absolutely perpendicular to the top plate. When trimming the feet, an attempt
should be made to maintain these relationships.
As you are making your cuts, it is advisable to position the bridge on the instrument
from time to time to monitor and evaluate your progress. This, of course, requires a
critical examination of both the front and backside of the bridge noting where gaps
exist or adjustments must be made. For example, if the E foot of a violin bridge
looks like this:
This indicates that material needs to be removed from the outside of the foot; both
front and back, but more on the front side. Your next cut should remove material
like this:
In the beginning, aim towards getting both feet to fit roughly, then start making
more precise finish cuts. A slight lick of the tongue to moisten the bottom of the foot
will allow you to remove paper-thin slices of wood. Since the grain runs straight
across the bridge, and the feet will angle upwards and inwards for the arching, you
are unable to cut from inside to outside, since this would be going against the grain
and may very well cause you to tear out small chunks:
However, at the very outside of the bridge feet, where it is difficult to make fine cuts
towards the center, you can cut against the grain by taking a VERY thin cut with a
lot of slicing movement. DO NOT wet the wood for this kind of cut as this would
only further the tendency to split along the grain. Needless to say, you will have to
maintain the edge of your knife.
Continue cutting until both feet exhibit a perfectly flush fit. Make sure that your
alignment of the bridge crown is not "wandering" as you make slight adjustments
on the feet. This is often easier said than done, since it is entirely impossible to fit
each foot individually without affecting the other, and consequently the final
alignment with respect to the fingerboard/neck alignment. When complete, there
should be an absence of "rocking" on the free standing bridge, and, with a very
light downward pressure you should be able to cause each foot to become perfectly
flush with respect to the top; a fairly large order for the neophyte, however, the
more bridges you work with, the easier this whole process becomes.
Having done this, take the old bridge (if one exists) and place it on top of the front
side of the new blank in such a way that you can scribe the feet curvatures on it,
allowing for the desired foot thickness. If an old bridge is not available, you may
place the new bridge in position and draw a pencil line along the bottom of each foot
using the adjacent curvature of the top to guide your pencil. Or better yet, if you
have a Roth de Jacques (self-adjusting swivel feet) handy, you may use it as a
template for transferring these cut lines. It would be my recommendation to acquire
a de Jacques bridge for every nominal size available (all violin family) to be used in
this manner.
Once these reference lines have been established, you may use a bandsaw to remove
the excess wood allowing a little extra material to remain for adjustment purposes.
Again, be sure to place the bridge with its backside down to establish the 90 angle
that must exist with respect to the instrument's belly.
You may now proceed as described earlier with respect to the violin and viola
bridge. After getting the feet to fit closely, and having removed all the saw marks,
you may now isolate any "high spots" present on the bottom of the feet by drawing
a small rectangular piece of carbon paper beneath each foot (carbon side up of
course). This will allow you to easily see those spots on the feet which must be
carefully pared down with your violin knife to form a precision fit. Continue this
technique until the carbon marks pretty much cover the entire base of each foot.
You may then further precision adjust the fit with a scraper. Complete this step by
removing all traces of carbon paper and carefully check the fit. A feeler gauge,
similar to that which is used in woodwind repair may be used to indicate the
precision of your fit.
Be sure to plane from the bottom to the top as shown below, and set the plane for a
very fine cut and a very small throat opening. Test your plane on some scrap maple
to establish the adjustment. Be careful that you do not make the center (just below
the heart) of the bridge too thin as this will likely lead to poor tone quality and be
prone to warpping. Since the bridge is tapered, and the sides of the crown (arch) are
lower than the middle, it will be necessary for you to plane more material from the
outside edges where it is thicker A good method is to plane radially from the center
of the bridge outwards.
• The ears are trimmed from basic rectangular lobes to a more pointed shape:
• The feet on the inside are trimmed mostly on the top of the foot (which
should not slope too much)
• Measure the internal distance from leg to leg to make sure that you get as
close as you can to the standard measurement there. Do not make the bottom
arch too rounded. The outside of the feet are trimmed with a curve as
follows:
It is in trimming the foot and leg that you are able to do a little shifting in the total
bridge width. If you want a narrower bridge, do most of the leg trimming from the
outside. It will then be necessary to trim a little from the outside of the feet. Cut the
length of each foot down from the inside to a MINIMUM of: violin - 11.5mm, viola -
12.5mm, 'cello - 23.5mm, and double bass - 43mm.
FINAL FINISHING
Complete all of the fine details remaining at this point, then check the bridge as a
whole, focusing on the lines of your curves. The bridge derives a finished look from
all the subtile decorative touches, but derives a much more fundamental elegance
from the smoothness and balance of your curves as well as from a general
perception of style which is consistently executed throughout the bridge. In your
first attempts, work on acquiring the general knife skills so you are able to cut
where you want, in the manner you want. As you acquire skills in the functional
aspects of bridge making, you may then begin to concentrate on the elements of its
form, that is, to work on developing its beauty within the limits of strength and good
acoustical properties. . . . .one that is uniquely your own.
When the trimming-out is essentially complete, use your sample bridge template or
calipers to mark out the string notches, spacing them evenly on the bridge. Install
the strings and bring them up to a moderate degree of tension. Verify each string's
clearance at the end of the fingerboard, and if any adjustments must be made, insert
a bridge jack beneath the strings so that you can remove the bridge and make the
required adjustments. Replace the bridge, check again, and continue this process
until the strings are at their correct heights with respect to the fingerboard. As a
final step, place the bridge jack in position and remove the bridge for the last time
so you may put the final finishing touches on it. Begin by sanding the surface with
fine #320 sandpaper until all marks and blemishes are gone. If the bridge is without
an ebony insert, you must provide parchment reinforcement over the treble string
slots to help keep them from wearing the string slot prematurely. To prepare the
parchment begin by soaking some small pieces in plain water for a few minutes.
Parchment will be placed over the slots for the E string on the violin, A string on the
viola, and the A and D strings on the 'cello. The small rectangular pieces of
parchment will be glued into position with a small dab of Titebond (alaphatic resin)
adhesive. Allow the freshly glued parchment to dry in place while you give the
bridge a light coating of linseed or lemon oil. Some luthiers will hang the bridge in a
jar of ammonia to artificially age the wood. If you use this method, do not allow the
bridge to submerge in the ammonia. Instead, pour a small quantity of ammonia in
the bottom of a jar and suspend the bridge from a wire running through the lid. Be
very careful of the ammonia fumes as they can be quite dangerous if inhaled! Work
in an area with good ventilation. The bridge should now be complete and ready to
be placed on the instrument.