I received a King 2b trumpet today with a bent bell stem. It bends around 30 degrees to the left (so no serendipitous Gillespie) and the tubing is still completely round with no dents or kinks. I know for certain that if I bend it back, it will kink and distort. I may end up using cerrobend, but the tubing is large enough in that area that it would take a lot of force to bend the filler metal.
Can someone please explain the physics of this, and how to override this when returning it to its original shape?
If I try to bend a bell stem, it invariably kinks. If I get a horn with an accidentally bent bell stem, it's almost always round. Why is that?
Did Dizzy's first horn that was allegedly bent in an accident have a smooth bend, or a kink?
The physics of bell bending.
-
ttf_Euphanasia
- Posts: 0
- Joined: Sat Mar 31, 2018 11:58 am
-
ttf_paulyg
- Posts: 0
- Joined: Sat Mar 31, 2018 11:59 am
The physics of bell bending.
The material at the outer radius of the bend has been thinned by the bending process. The point directly opposite this acts as the 'fulcrum' during the bending.
Compressing metallic shells (any hollow structures) like instrument bells will not result in compressing the material as if it were the opposite of drawing. Instead the structure will buckle (crumple).
Compressing metallic shells (any hollow structures) like instrument bells will not result in compressing the material as if it were the opposite of drawing. Instead the structure will buckle (crumple).
-
ttf_Lawrie
- Posts: 0
- Joined: Sat Mar 31, 2018 11:58 am
The physics of bell bending.
As I'm not an instrument repairer this thought may be totally off the mark, but if you could access a suitable bell mandrel and perhaps apply a version of "shrinking" that panel beaters use? Reckon I'd test in on something other than a customers horn though...
http://cid.vcc.ca/p1-dl/instructions/moodle/demo/demo-shrink.htm
I would certainly use a propane torch rather than oxy for this one I think... and you would, of course, have uneven annealing now, but without this you already have uneven work hardening...
http://cid.vcc.ca/p1-dl/instructions/moodle/demo/demo-shrink.htm
I would certainly use a propane torch rather than oxy for this one I think... and you would, of course, have uneven annealing now, but without this you already have uneven work hardening...
-
ttf_timothy42b
- Posts: 0
- Joined: Sat Mar 31, 2018 11:57 am
The physics of bell bending.
Quote from: paulyg on Oct 03, 2017, 08:25AMThe material at the outer radius of the bend has been thinned by the bending process. The point directly opposite this acts as the 'fulcrum' during the bending.
Compressing metallic shells (any hollow structures) like instrument bells will not result in compressing the material as if it were the opposite of drawing. Instead the structure will buckle (crumple).
So are you suggesting that in the case of non-kinked bent bells, the bell was under tension on both sides? That kind of makes sense if you caught it on a doorway; not so much if you dropped it.
Compressing metallic shells (any hollow structures) like instrument bells will not result in compressing the material as if it were the opposite of drawing. Instead the structure will buckle (crumple).
So are you suggesting that in the case of non-kinked bent bells, the bell was under tension on both sides? That kind of makes sense if you caught it on a doorway; not so much if you dropped it.
-
ttf_BillO
- Posts: 0
- Joined: Sat Mar 31, 2018 11:58 am
The physics of bell bending.
Quote from: Euphanasia on Oct 03, 2017, 04:33AMI received a King 2b trumpet today with a bent bell stem. It bends around 30 degrees to the left (so no serendipitous Gillespie)
That's the way his brother Dozy liked 'em.
That's the way his brother Dozy liked 'em.
-
ttf_Blowero
- Posts: 0
- Joined: Sat Mar 31, 2018 11:53 am
The physics of bell bending.
Pics or it didn't happen.
-
ttf_BillO
- Posts: 0
- Joined: Sat Mar 31, 2018 11:58 am
The physics of bell bending.
Metal dynamics are not really my strong suit, but let me try to give an explanation with my limited understanding of this.
paulyg is partly correct. The outside radius has stretched and the inside radius was under compression. I think your real question though is why this happens when the bend is accidental and why it kinks when the bend is intentional.
I believe the answer lies in the speed of the bending. If it occurs accidentally with sufficient speed the stretching of the 'outside' metal heats it up substantially and if it happens quickly enough it will heat the metal such that it becomes more ductile and before the heat can dissipate away from the area it stretches without kinking. If the bending is too slow, like when you do it deliberately, the heat conducts away from the area and the metal does not heat up enough to provide enough tensile ductility, leading to a crease/kink rather than a stretch.
As to how to get it back, well, there are more experienced instrument techs here for that answer, but it will end up that the metal around the bend will be considerably thinner than before and the length of the bell section will be slightly longer.
paulyg is partly correct. The outside radius has stretched and the inside radius was under compression. I think your real question though is why this happens when the bend is accidental and why it kinks when the bend is intentional.
I believe the answer lies in the speed of the bending. If it occurs accidentally with sufficient speed the stretching of the 'outside' metal heats it up substantially and if it happens quickly enough it will heat the metal such that it becomes more ductile and before the heat can dissipate away from the area it stretches without kinking. If the bending is too slow, like when you do it deliberately, the heat conducts away from the area and the metal does not heat up enough to provide enough tensile ductility, leading to a crease/kink rather than a stretch.
As to how to get it back, well, there are more experienced instrument techs here for that answer, but it will end up that the metal around the bend will be considerably thinner than before and the length of the bell section will be slightly longer.
-
ttf_JohnL
- Posts: 0
- Joined: Sat Mar 31, 2018 11:59 am
The physics of bell bending.
Quote from: BillO on Oct 03, 2017, 01:14PMThat's the way his brother Dozy liked 'em.
I thought his name was Doozy.
I thought his name was Doozy.
-
ttf_john sandhagen
- Posts: 0
- Joined: Sat Mar 31, 2018 11:59 am
The physics of bell bending.
Metal bending is like women...mysterious at best. A small percentage of the time the deformation will go back without visible distortion. On thin material like slides it can bow the entire tube. On heavy material it's better and on items like bells you may deform the original shape, but be able to fool the eye by make it symetrical.
I had a tuning slide that wasn't replaceable that was crushed. Way too much time later I had it back into shape as far as the outside taper and bend...but it was 1/2" taller... the entire bell was apart anyway and now he had much better slide clearance
RE the bent TRUMPET , use a mandrel to support the inside bore and a burnisher or roller to coax it back to shape...it may be a 3 handed operation.
I had a tuning slide that wasn't replaceable that was crushed. Way too much time later I had it back into shape as far as the outside taper and bend...but it was 1/2" taller... the entire bell was apart anyway and now he had much better slide clearance
RE the bent TRUMPET , use a mandrel to support the inside bore and a burnisher or roller to coax it back to shape...it may be a 3 handed operation.
-
ttf_CharlieB
- Posts: 0
- Joined: Sat Mar 31, 2018 12:34 pm
The physics of bell bending.
Wow.
Thirty degrees is a lot of bend.
The bent bell stem may look round, but it's probably oval now.
When thin walled tubing is bent, the force changes the shape of the tube from round to oval. These forces cannot be reversed to return the tube to round as straightening is attempted, so a kink usually forms. Even if you get very lucky and manage to straighten the bell stem without kinking it, the shape of the bell will remain compromised.
Thirty degrees is a lot of bend.
The bent bell stem may look round, but it's probably oval now.
When thin walled tubing is bent, the force changes the shape of the tube from round to oval. These forces cannot be reversed to return the tube to round as straightening is attempted, so a kink usually forms. Even if you get very lucky and manage to straighten the bell stem without kinking it, the shape of the bell will remain compromised.
-
ttf_robcat2075
- Posts: 0
- Joined: Sat Mar 31, 2018 11:58 am
The physics of bell bending.
The essential problem is that it's much easier to pull something straight than to push it straight.
-
ttf_robcat2075
- Posts: 0
- Joined: Sat Mar 31, 2018 11:58 am
The physics of bell bending.
The essential problem is that it's much easier to pull something straight than to push it straight.