Bending Tubing
Another job that is often challenging and takes time and practice to accomplish satisfactorily and
predictably is bending tubing, including bell stems.  Making wide curves with small diameter tubing
such as cornet and French horn mouthpipes and trumpet bell stems is fairly easy, but some of the
information below will make even these more attractive and you can move on to more challenging
projects.  The top photo is of the first keyed bugle that I made for a customer in about 1985.  I
had bent a lot of tubing by that time, but I hadn't yet figured out how to bend such large diameter
tubing in as small a curve as I wanted, resulting in a very wide keyed bugle.  Even with such a
wide curve, I recall that there was much rippling of the metal on the inside of the curve that had to
be hammered down.  I experimented with different materials for filling the tubing, including pitch
(which I use most often), lead and Cerrobend (a tin based metal formulated for this purpose which
is available from
McMaster-Carr and other industrial suppliers).  The results were similar with all
of these filling materials, although there are a few pointers for each that will help.  First, be
cautions when working with hot liquids.  Any of these materials can cause very bad burns.  When
using pitch, temperature when bending is important.  It will likely vary, depending on the
formulation of the pitch, but I get best results at between 60 and 70 degrees Fahrenheit.  With
both lead and cerrobend, it is important to coat the inside of the tube with oil first, any sort of
motor oil will do.  Cerrobend must be quenched or cooled very quickly after pouring.  Have a
large enough container of cold water ready to lower the part in as quickly as possible.  For larger
diameter tubing, I put plenty of ice in the water.  This is not necessary with lead and can be
dangerous if the molten lead splashes in the water.  With both lead and cerrobend, the temperature
during bending is less important than pitch, but emptying must be done in an oven.  Using a torch
will get spots of the brass too hot resulting in a loss of surface tension of the oil and the filler metal
will stick to your tube.  You will have a very hard time getting it all out in this case.  I highly
recommend using an oven for emptying pitch as well.  It is much easier to do it safely and the
residual pitch can be dissolved and washed out with a solvent such as kerosene.

The next photo is 1/2 inch outside diameter tubing, typical size for trumpets, cornets and French
horns.  The shape of the curve is the result of bending by hand around a round form, which is the
most common practice in repair shops.  The shape isn't as round as might be predicted, with the
smallest radius in the center of the curve.  This is caused by the leverage being the greatest at that
point.  There is nothing wrong with this shape acoustically and some trumpet makers will try to
convince you that it is a superior shape.  To my aesthetic view, it looks sloppy, but I suppose this
comes from the amount of effort that I have spent in avoiding this shape.  In a high quality repair
or restoration, such as replacing the mouthpipe on an early Bach cornet, most would expect the
replacement to both function and appear very close to the original.  As to acoustics in this
instance, I find that any well designed Bb cornet mouthpipe, such as those available from
Allied
Supply or makers such as Kanstul and others can be very satisfactory.  Towards this goal, careful
attention must be paid to the internal dimensions of the replacement.  I match the diameter at the
small end of the replacement to the original and have had no complaints (I am always up front  
with the fact that the replacement was not made by Bach).  In order to get the nice square bends
as in the originals, it is a great help to make wooden forms as seen in the third photo.  The middle
of the curve will still receive more bend, not hugging the form unless it is held against it at that
point.  The next, more challenging bend will illustrate this.  These are bells made for pocket
trumpets or cornets and demonstrate what I learned on the very next keyed bugle that I made after
that illustrated above.  I found that if I bent the bell about half way, emptied it, annealed it again
and refilled it, I was able to make these tight bends with minimal rippling and no tearing of the
metal and thus less distortion of the tube and less work involved.  The fourth photo shows the first
half of this accomplished and the fifth photo shows the finished bend.  You can see that I have the
form clamped to a steel plate with many holes drilled in it.  One wedge holds the flare end of the
bell in place and a second wedge holds the middle of the curve against the form, moving the
maximum leverage to the point between that wedge and the bending lever, which is seen at the
bottom of this photo.  A pad of soft wood or leather prevents damage to the tube.  The form lets
the tube be bent slightly farther than the finished bend, allowing for some spring back.  You can
also remove it from the jig and give it a little more bend if necessary.  In the next photo is seen the
whole lever.  In some of the videos on YouTube, you will see well made bending jigs that are
designed to bend one part in one shape.  While these are instructive in general, I must find
techniques for bending the widest range of parts in many shapes.  I made the lever hefty enough
to bend tuba and euphonium branches.  If you are only going to work on smaller instruments, I
would suggest making a lighter version of it.  The seventh photo shows what can be done when
the tubes are too short to bend.  In most cases, you will need extra length to push against and very
often the piece at hand is too short.  I silver solder a piece of scrap brass tubing on the end before
annealing and filling the tube.  In this case, I was using an existing Olds Recording model cornet
bell to make a pocket version for
Nick DeCarlis.  I wanted all of these pieces to retain the red
brass "Rey-O-Loy" in the finished instrument, so I cut the bell into three pieces, the straight ones
to be bent and the original bell curve had to be manipulated to a different shape to fit.  This
technique extends the possibilities using existing parts and materials at hand.  The last photo shows
a relatively sharp bend being accomplished using a form made of Jett Sett Fixturing Compound.  
This is a plastic material that contains ceramic to make it stronger and can be formed or molded at
temperatures between 150 and 180 degrees farenheit and becomes very hard at room
temperature.  It is available from
Rio Grande.  Ideally, you would need an existing tube of the
correct diameter and slightly tighter curve to shape this form, but with a little ingenuity, you can
think of other methods.

Click on images for larger views.