Stainless tubes bending radius.

PuffTheMagicDragon

PuffTheMagicDragon

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I intend to make a bimini for my boat by next summer. I shall be using stainless tubes, diameter 22mm x 1,5mm wall thickness. I shall also be trying to make the bends myself, the intention being to make a wooden former (quadrant) and bending the tubes over it. I plan to insert one of those long springs that are used when bending electrical conduit to prevent the tubes from collapsing.

My question is this: What would be a realistic radius for the corners at the top of the structure (where there will be the fabric) given that the only available power is one old(ish) man and a lever?

TIA for any comments, hints or pointers.
 
>What would be a realistic radius for the corners at the top of the structure (where there will be the fabric) given that the only available power is one old(ish) man and a lever?

Get some bendable wire and curve it to the size of bimini you want the then bend the tube to that shape. Do the bends very slowly so you don't overbend.
 
I think you will be very lucky to bend 22mm stainless tubing using a spring. I recently used a conduit bender to put a very slight bend in a tubular stern ladder, 25 mm, 1.5 mm wall thickness. We needed an additional length of pipe about 2 metres long to get enough leverage. Speedy Hire do them for £55 per week, could maybe negotiate for a shorter hire? They do a variety of radii and diameters.
 
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Hello,
With 1.5mm wall thickness you will be fine bending it with a bender shown in the post above. I tried using a plumbers pipe bending spring and lost the stainless tube and spring in the attempt they just are not strong enough.
I have done my own bending using 1.5mm wall stainless tube however I tried it with some cheap 1.2mm wall and it kinked. I also had one bend break due to the seam in the tube failing and I was advised to find the join and have that on the inside of the bend (if that makes sense?) As for the amount of bend you can do I think that is down to the design of the bender as some are more flexible in accomodating the tails. But I have done a complete U.
You can of course hire a bender for 22mm quite easily.

Good luck, Col.
 
go to a tool hire co like dale . hire a hydraulic tube bender for a couple of days
Do not bother with an electricians conduit bender. they do not have enough power.
I have a manual vertical pipe bender that does up to 38mm copper tube & that is a struggle, plus with a long curve there is not enough room to clear the ground. A horizontal pipe bender solves that problem & avoids sudden kinks
 
PuffTheMagicDragon said:
I intend to make a bimini for my boat by next summer. I shall be using stainless tubes, diameter 22mm x 1,5mm wall thickness.
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I'm feeling mathematical ...

To put a permanent bend in anything you need to apply a moment greater than the maximum elastic moment, which is the elastic section modulus Ze times the yield stress. For a tube of diameter 22mm and thickness 1.5mm, Ze = 537 mm3 (http://en.wikipedia.org/wiki/Section_modulus) and for 316 stainless the yield stress is typically 200 MPa = 200 N/mm2, so the minimum bending moment needed would be 537 x 200 = 107,400 N mm = 107 Nm. The theoretical maximum bending moment would be the plastic section modulus (Zp = 631 mm3 times the yield stress, which would be 126 Nm.
 
JumbleDuck said:
I'm feeling mathematical ...

To put a permanent bend in anything you need to apply a moment greater than the maximum elastic moment, which is the elastic section modulus Ze times the yield stress. For a tube of diameter 22mm and thickness 1.5mm, Ze = 537 mm3 (http://en.wikipedia.org/wiki/Section_modulus) and for 316 stainless the yield stress is typically 200 MPa = 200 N/mm2, so the minimum bending moment needed would be 537 x 200 = 107,400 N mm = 107 Nm. The theoretical maximum bending moment would be the plastic section modulus (Zp = 631 mm3 times the yield stress, which would be 126 Nm.
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So, unless I am missing something, since I will be using a steel pipe around 2 metres long as a lever, 126Nm should be well within my capabilities, no? (Centreline radius will probably be 100-110mm and it will be 90-degree bends).
 
Unless you are using a mandrel with a snug fit on the outside of the pipe (of half the pipe circumference) you will probably find it kiks.
You really need to move it through the mandrel to avoid the bend being concentrated at the apex. SS tube is a bit of a sod.
I was doing pipe over an inch in diameter and using a borrowed hydraulic pipe bender with a few tons capacity.

If I were doing it again I would take some templates to a s/s specialist, let them supply the tube, and let them have the grief.
 
Lakesailor said:
Unless you are using a mandrel with a snug fit on the outside of the pipe (of half the pipe circumference) you will probably find it kiks.
You really need to move it through the mandrel to avoid the bend being concentrated at the apex. SS tube is a bit of a sod.
I was doing pipe over an inch in diameter and using a borrowed hydraulic pipe bender with a few tons capacity.

If I were doing it again I would take some templates to a s/s specialist, let them supply the tube, and let them have the grief.
Click to expand...

Actually I was thinking of using a bending spring on the inside of the tubes and bending the tube (supported by the internal 'spring') over a secured wooden quadrant. Something like this:

20mm%20orange%20conduit%20bending%20spring.JPG


I am trying to avoid going to a specialist because transportation of the bent frame would be a bit of a problem on my car; if I manage to do the bending at home I can then carry the completed thing to the boat.
 
sailorbaz said:
I am also making a Bimini for my boat using 22 mm 316 tube. The local welding shop has a hydraulic bender and they say max centre line radius they can achieve is 150 mm.
Sailorbaz
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That's good to know although 6.8 times the diameter does sound a bit excessive; perhaps it is erring on the side of caution.
 
I
PuffTheMagicDragon said:
Actually I was thinking of using a bending spring on the inside of the tubes and bending the tube (supported by the internal 'spring') over a secured wooden quadrant. Something like this:

20mm%20orange%20conduit%20bending%20spring.JPG


I am trying to avoid going to a specialist because transportation of the bent frame would be a bit of a problem on my car; if I manage to do the bending at home I can then carry the completed thing to the boat.
Click to expand...

Buy plenty of them because if you are bending to much more than 75degs you won't get them out of the tube!
 
Lakesailor said:
It won't work. It's hard enough with the right gear.
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As a metal fabricator of 25 years experience, this would be my prediction also.
I would use 25x2mm. Although 1.5mm would probably do, 2mm will bend better with less chance of the tube collapsing. Both need the proper tooling anyway.

A 6m length of super mirror polished 25x2mm 316 would probably be near on £100, expensive learning curve...
 
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