Bending tube

[Graham_Wright]

For my pulpit I need a semi-circular bend with a 10" radius. My own attempts with a home made bender have not been a great success. This uses a half pulley with a 1" groove and two side plastic pulleys also with 1" grooves. The result is not bad but there are slight wrinkles on the inside.

The ideal tool is a ring bender and there are two in my region but neither can pull the required radius.

Suggestions have been filling with wet sand, filling with dry sand and filling with (perfectly) dry sand and welding up the ends.

The problem with an ordinary bender is that the outside is stretched and the inside compressed. A ring bender overcomes this by stretching the outside of the bend and not compressing the inside.

Any other suggestion would be very welcome.

The tube is 1" OD with a 2mm wall.

 

[Lakesailor]

SS tube is very tricky (as you have found) Maybe worth getting it done by specialists http://www.cooneymarine.co.uk/pages/tube-bending.html

Not a million miles from you.

 

[Avocet]

The sand should work. I've done it before. You just need to make sure it is absolutely shaken down and compacted as much as possible so it's pretty incompressible. Being dry is better but I always leave an air gap when welding the end plates so that any evaporating moisture doesn't turn it into a pipe bomb when heated! Haven't done it with stainless, but I know it will polish up (with a lot f work afterwards). Worth having longer "tails" on the bend than necessary to cut off the blanking plugs afterwards.

 

[Lakesailor]

Haven't done it with stainless,

Stainless is an absolute bugger for kinking. I borrowed an hydraulic pipe bender to put a swan neck bend in a tiller. I had to keep moving the pipe in the mandrel when it looked like it was about to kink.

 

[Graham_Wright]

when heated!

Does the "heating" refer to the welding or are you suggesting I gas heat the tube to make it more ductile?

Metallurgically, what are the mechanics of the shape change? What does the sand actually do?

Others have suggested using withies.

I wonder if a bar of plastic would do the trick? That has provoked a thought about lead. Anybody tried that?

 

[Graham_Wright]

Stainless is an absolute bugger for kinking. I borrowed an hydraulic pipe bender to put a swan neck bend in a tiller. I had to keep moving the pipe in the mandrel when it looked like it was about to kink.

Now, there's an idea. If I plug the ends (one end welded the other taking a fitting) and fill it with oil with a plug to compress it, would that stop the kinks?

I could even fill any voids that resulted by winding the plug in.

 

[rogerthebodger]

What you need to get a good and smooth bend in thin wall tube is a mandrel bender like this

http://www.youtube.com/watch?v=QoUyFcDA27M

I did design a pneumatic operated one a long time ago when I worked for tube investments while I was living in the Birmingham UK

 

[Graham_Wright]

What you need to get a good and smooth bend in thin wall tube is a mandrel bender like this

http://www.youtube.com/watch?v=QoUyFcDA27M

I did design a pneumatic operated one a long time ago when I worked for tube investments while I was living in the Birmingham UK

Given a few weeks, I could probably make one of those - but! Very impressive. That is what my colleague has (professionally built) but all his mandrels bend to too tight a radius.

 

[30boat]

What you need to get a good and smooth bend in thin wall tube is a mandrel bender like this

http://www.youtube.com/watch?v=QoUyFcDA27M

I did design a pneumatic operated one a long time ago when I worked for tube investments while I was living in the Birmingham UK

Great rig.I suppose I could easily knock one one off in a weekend or so....

 

[rogerthebodger]

1 inch tube at a radius of 10 inch could be done without a mandrel but then the formers must be quite a tight fit with overlap the tube and you may still get some wrinkles.

The old type of electrical conduit bender may also bend that size of tube but again the quality of bend would not be up to that of a mandrel bender.

 

[myquest]

Now, there's an idea. If I plug the ends (one end welded the other taking a fitting) and fill it with oil with a plug to compress it, would that stop the kinks?

I could even fill any voids that resulted by winding the plug in.

What about plugging one end, filling the tube with water and freezing it. That would give you a solid filling without voids.

 

[Buck Turgidson]

My basic knowledge of physics tells me that the moment you stretch the outer diameter of the bend the internal volume increases so the idea of filling with sand, plastic etc. doesn't help.
I may be wrong as I'm not a bender by trade ;-)

 

[jerrytug]

For my pulpit I need a semi-circular bend with a 10" radius. My own attempts with a home made bender have not been a great success. This uses a half pulley with a 1" groove and two side plastic pulleys also with 1" grooves. The result is not bad but there are slight wrinkles on the inside.

The ideal tool is a ring bender and there are two in my region but neither can pull the required radius.

Suggestions have been filling with wet sand, filling with dry sand and filling with (perfectly) dry sand and welding up the ends.

The problem with an ordinary bender is that the outside is stretched and the inside compressed. A ring bender overcomes this by stretching the outside of the bend and not compressing the inside.

Any other suggestion would be very welcome.

The tube is 1" OD with a 2mm wall.

If you are making the pulpit yourself, but you can't get it bent properly, then perhaps it might be better to change the design. Why do you need a curve, what about a short athwartships section at the front.
Are you welding it yourself?
I can think of advantages to NOT having a curve, easier to attach things to just for starters.
After all, let's face it, most manufacturers make pulpits to look nice, not to be functional, useful or strong!

 

[trying]

When I replaced mine (which is 7/8" dia 1.6mm wall) I did it on our small hydraulic bender..

no shrinkage wrinkles etc.

The bender is similar to this...

http://www.machinemart.co.uk/shop/p...ulic-pipe-bender?da=1&TC=SRC-hydraulic bender

Need to make sure the former is a snug fit to the tube...


You could try filling the tube with sand and plugging the ends, it does work... Or similarly you could you a bending spring inside though it will probably be single use!

 

[Avocet]

Does the "heating" refer to the welding or are you suggesting I gas heat the tube to make it more ductile?

Metallurgically, what are the mechanics of the shape change? What doe the sand actually do?

Others have suggested using withies.

I wonder if a bar of plastic would do the trick? That has provoked a thought about lead. Anybody tried that?

You definitely have to apply heat (other than the welding). The sand is there to prevent the tube from going oval when bent. It behaves a bit like an incompressible fluid. The theory is that when you bend the tube, the outside radius parts of the tube go into tension and stretch, the inside bits compress, and the neutral axis stays where it is. IF the sand is in there, tightly packed, the inside radius can't compress so the outside radius has to stretch instead. This should stop the kinks forming on the inside edge (it does with mild steel, but I've never tried stainless). Yes, you have to heat the tube (especially the outside "half" that you want to stretch. It goes blue / bronze coloured but that will (eventually!) polish out. I've polished plenty of welded stainless things before, just haven't sand-bent any.

I think any plastic that was stiff enough in compression to prevent the kinks forming (or indeed, the tube going oval) probably wouldn't come out again afterwards.

 

[John the kiwi]

I have just - as in at the weekend - succeeded in bending 1 inch ss316 tube in a bender i made out of plywood.
Smooth bends without kinking. No Sand or forming springs and absolutely minimal flattening of the tube.
I am so pleased with the result!
Having failed with a hydraulic bender due to wrong formers , I was inspired by the you tube video here.

http://www.youtube.com/watch?v=lcTIDlDevog

I made mine in a similar fashion to this.
I believe the secret is to get the profile of the groove just right ie a perfect 12.7 mm radius.
I made my big forming wheel/mandrel a radius of 150mm and i made two smaller wheels to feed in, so larger radius will be easy

Let me know if you would like more detail.
Good luck.

 

[Graham_Wright]

What about plugging one end, filling the tube with water and freezing it. That would give you a solid filling without voids.

That is a technique still used in the aviation industry apparently. There may be a problem domestically freezing the water inside a 1 metre long tube let alone anything longer!

 

[Graham_Wright]

My basic knowledge of physics tells me that the moment you stretch the outer diameter of the bend the internal volume increases so the idea of filling with sand, plastic etc. doesn't help.
I may be wrong as I'm not a bender by trade ;-)

I have sympathy with that logic but the technique is reported by many as successful. Perhaps the sand is reluctant to move due to friction.

 

[Graham_Wright]

If you are making the pulpit yourself, but you can't get it bent properly, then perhaps it might be better to change the design. Why do you need a curve, what about a short athwartships section at the front.
Are you welding it yourself?
I can think of advantages to NOT having a curve, easier to attach things to just for starters.
After all, let's face it, most manufacturers make pulpits to look nice, not to be functional, useful or strong!

At the moment, I have a piece with two ~50 mm radii bends with a straight length in between and, to be honest, it doesn't look bad. (The small radius bends were produced by a colleague with his professional kit). This section ends with parallel arms which engage with the side rails and are pinned in place and supported from underneath. The idea is that it can be removed for access over the bow. (Maybe it could also shorten the LOA for mooring fee reduction).

 

[Graham_Wright]

You definitely have to apply heat (other than the welding). The sand is there to prevent the tube from going oval when bent. It behaves a bit like an incompressible fluid. The theory is that when you bend the tube, the outside radius parts of the tube go into tension and stretch, the inside bits compress, and the neutral axis stays where it is. IF the sand is in there, tightly packed, the inside radius can't compress so the outside radius has to stretch instead.

As has been pointed out by another, the tube volume must increase if the outside length increases. So why does not the sand move? I accept the technique works and maybe friction is the reason.

Yes, you have to heat the tube (especially the outside "half" that you want to stretch. It goes blue / bronze coloured but that will (eventually!) polish out. I've polished plenty of welded stainless things before, just haven't sand-bent any.

After welding, I remove the discolouration with pickling paste. Would that work for heat stains as well?

I think any plastic that was stiff enough in compression to prevent the kinks forming (or indeed, the tube going oval) probably wouldn't come out again afterwards.

I agree. Maybe something like HDPE could be melted out?

How about lead?