Dent Removal

I'm wondering whether you use something like the Ferree's Dent machine (English wheel) in your shop, or whether you have different tools to do the same job.

Reply #3 - Oct 12th, 2005,

So, let’s talk about dents. They are easier to make than to remove. I will explain a little about the removement, and the (hand) tools used.


The most important tools are the balls. A lot of balls are required in many sizes with small steps. The balls are somewhat barrel shaped, and all have a hole in the middle, allowing them to be mounted on a mandrel.

Mandrels in different sizes and lengths are needed. For a dent to be removed from the base of the bell of a trombone. The diameter of the (straight) tube has been found, and a suitable ball mounted on the mandrel. The ball must be just able to pass the point with the dent. A little Vaseline oil is rubbed on the ball surface.



The bell is put over the mandrel, and with a small magnet the position of the ball in the tube is verified. Then the instrument's tube is gently rubbed over the ball, with pressure directed towards the dent. With a small, rather soft dent, the same ball could do the whole job. With larger dents the ball must be changed to larger sizes to get the right final radius of the tube.

A vice mounted with the jaws angled 90 to the workbench is a nice help.



In cases where sharp bends have been made in the brass, annealing the brass will be necessary before starting the dent work. Even then, when the dent is gone, lines may be visible where the sharp bends were.



To remove these, a burnishing tool that compares to the goldsmith's polishing steel is used.



The brass could need to be annealed again, then a little Vaseline oil is put on the polishing steel, and the surface rubbed gently with a corresponding ball or mandrel inside the tube. Note that the surface of the polishing steel and the item must be very clean, or scratches will be made. Also inspect the polishing steel carefully for small scratches.



Mandrels are needed in many shapes. Straight mandrels may be used with a ball, or as they are (requires rounded ends). Straight Mandrels with different diameters are especially good when dents are to be removed from slides.



Curved mandrels are also useful when working with curved tubes. With smaller diameters and sharp curves, mandrels with a fixed round ball are best.



One example is the ever-present dent in the second valve loop on trumpets. A special curved mandrel, equipped with a ground ball at the end is used. With this mandrel every part of the slide is accessible from at least one of the sides. In most cases the mandrel can be mounted in a vice, and the slide can be rubbed with the ball inside. Don't forget the Vaseline oil.

In some cases, a comfortable hold on the slide is not possible, and especially with bad dents, there is a risk that the bow changes its distance between the tube ends. To avoid this, the bow could be soldered to a piece of brass (thickness at least 2mm) with holes for the mandrel, corresponding to the distance between the ends of the bow. This piece of brass is a very useful tool, with the bow mounted, you mount it in your vice, the vice must attach on the brass piece, not the bow! Then you begin to rub on the dent from inside with the ball ended mandrel handheld.



There are cases where you cannot get to a dent with a mandrel without disassembling the whole instrument. On the German egg-shaped tenor horns mandrels are good for nothing. Then the balls must do the work alone, almost at least. You find a ball that can pass the dent, always starting from the tube end with the largest diameter. This ball could be of copper or brass, but steel will do also. It will help you to back out the next ball that should stick in the dented place. Use a magnet to verify the location of that ball. Also check that the throw-out ball does not limit the movement of the working ball.



Then you take the next ball available with a (little) larger diameter and send it after the second ball. Now you use this ball to knock on the ball stuck in the dent, as you shake the instrument part gently in the direction that works best. Your ball will pass the dent, having raised it a little. If it can’t, a smaller ball must be used, and you use the first ball to knock it out.



If it worked, you check that there is room in the tube for the passed ball, and let the next ball act on the dent, as you put in a ball just a little larger again. Then you repeat the procedure, until the dent is raised as good as it comes. Please keep checking the number of balls you have at work in the tube!

When the dent is almost done, the last way is the worst. You will need to help your balls from outside with a hammer. Not just a hammer.



But one with polished surfaces, you will never use this hammer for knocking nails! Several hammer shapes are useful, here are some of mine.



You start to knock where your magnet tells you that the working ball is, as you find the part of the tube that has least contact with the ball. It is where the tube has the largest diameter. you knock gently with your polished hammer on that tube part, as you spread your knocks a little. The hammer may not leave marks, if it does you knock too hard, or the hammer is scratched. You will notice that your working ball advances through the tube because you hold the tube so that gravitation causes the working ball to move downwards, eventually helped by the weight of more balls from above. Still, remember a little oil on the balls, and that annealing would be necessary with larger dents, or dents with sharp edges.



Also remember that possessing the tools does not mean being familiar with their use. It takes time to learn, especially the hammers and polishing steels.



Reply #5 - Oct 14th, 2005

I have barrel dent balls up to about 1 in. and then balls in greater graduations up to 2". Smaller diameters and straight tubing are manageable enough, but in situations like the bottom bow on a tuba that has been crushed seem to be beyond me to get them smooth again. I can get them to a wavy approximation of how they should be. And, I don't really want to go the expense of a dent machine, which is how I believe the dents in the above tuba were removed.


I think you did not anneal your bow sufficiently before raising the dent. On a tuba bow I would recommend some annealing under the process. The brass will get softer if you shock cool it with water after each annealing but beware of steam and boiling water in the tube, it could splash around.



Honestly, the last bow of a tuba is one of the trickier things, I would solder the bow free and thus be able to use the balls on a mandrel!




Unrelated, but sometimes when trying to anneal some old silver horns, the surface sort of bubbles on me. Some were German silver, but one appeared to be a plated though it was also old. Have you ever experienced that?





Reply #8 - Oct 16th, 2005,

Be careful with silver plated items, if you anneal too hot, the silver will be damaged. The tube need not be more than dark red, and that will not harm the plating, if it is done well (the plating!)