Tube bending

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geraldmorrissey

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I'm at the point that I have to bend the empennage outline tubes for my Bearhawk. The horizontal stab leading edge tube will be the toughest to bend. It's 7/8" x.035. I've been reviewing everything I can find on the subject and I wonder what others have done. The elevator and rudder trailing edge tubes are far smaller and I think I can muscle those around a suitable mandrel. A local tube bending firm will roll the 7/8" but the setup minimum is $350 plus the cost of the actual rolling. I have a conduit bender that fits the tube O.D. really well. Thought I would fill the tube with sand, cap it and give it a try. Harbor Freight has a pipe bender that "might" get the job done. But I'll probably never use it again.
 

wsimpso1

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I found and applied a method that worked great for bending tubing. You might not need to go this far, but if you have trouble, this is out there.


billski
 

PMD

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What is the radius you are trying to achieve? Can we assume material is 4130?

Way back in my VW powered airboat days (before high volume production) I used to make 1600cc direct drive engines. Exhaust was 1 3/8" 0.035 ERW and I can tell you THAT was a proper beeeattch to bend. I used nickel slag (iron sulphide) and steel dies with a radius around 3D. My bender used one fixed round die and one rolling round die - and I would not do it that way today. If your radius is large enough, just stuffing the tube and using a plywood template can work quite well, but going down to 5D or less you are going to need some very stout grooved dies to make it work - and even then you will find the tube deformation will tend to lock it into the die. You must also allow for and expect spring back. You might have to sacrifice some material to get some values, but I would suggest starting with speaking with other Bearhawk builders to see what has worked for them.
 

TFF

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It’s 7/8”! I have Bearhawk plans but I can’t get to them, I thought it was about 1/2”.
 

Pops

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In building 4 Bearhawks, Dallas and I have a hand crank bender that will go up to 1" in dia. Really works good. After building the jig for the complete fuselage we bent the first set of longerons and counted the turns of the crank on the bender. Repeated the amount of turns for the other 3 sets of longerons and they all fell into place in the jig. Also use it to make curved hand lifting handles on the bottom rear of the fuselage, etc.
For the large curves as in the tails, just hand bending over a plywood form and comparing to the full size pencil layout on the table works good.
The rudder in the hardest because its the longest. I also made a hand bender . 1/4" thick, 3" steel angle about 1.5 feet long bolted the top edge of the work table. Welded two 3/4" dia steel rods about 6" long on top of the angle sticking over the edge of the table. Make a straight mark down the length of your tube and make sure the mark it up when bending.
 

geraldmorrissey

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Curious, if a tube is levered between the two rods, doesn't that impose a point loading on the tube? Two cylinders at 90 degrees to one another. Seems like it would tend to put a dent in the tube. Did you fill the tube with anything? Am I missing somthing?
 

wsimpso1

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Curious, if a tube is levered between the two rods, doesn't that impose a point loading on the tube? Two cylinders at 90 degrees to one another. Seems like it would tend to put a dent in the tube. Did you fill the tube with anything? Am I missing somthing?

Who are you asking? We are all talking about contoured rounds for bend radii that are a small number of tube diameters, and some indicated that for large radii bends (compared to tube diameter) simple forms work. Just where the break point between them occurs is a point for more research and maybe some trials.
 

geraldmorrissey

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My inquiry was to Pops regarding his angle iron/rod tool. My bend radaii requirement is approximately 20" radaii (the curvature varies along the 4' length of the tube, more outbd, less inbd) for a 7/8" dia 4130 cond N tube of. 035 wall. Should have pointed that out in the initial post. Trials are expensive. 50 bucks plus shipping for each tube I turn into welding practice. This is a unique part, only two on the plane like it. So far I like the contoured plywood form tool with a filler material inside the tube. Cheap and easy to build.
 
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Pops

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IF you put a hard enough bend in one spot in will dent the tube. For mild bends mark a line down the top of the tube so you will not bend a spiral. Then mark in short intervals , ( say 1/2" ) and make a series of very small bends. The smaller dia of tubing the better it works. Using 3/8" dia 4130 for the trailing edge of the elevator or rudder, it works good.
I put the rods 6" apart. That means that the first small bend will at least 3" from the end of the tube.

Our hand crank tube bender will do 3/4"and down 4130 tubing with a 3" radius with no kinks.
Check out the Bearhawk seat frames and the hand lifting handle on the lower longeron.
Curved engine mount tube on the JMR Special.
 

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PTAirco

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The Harbor Freight pipe bender won't work for this. The dies are sized for commercial pipes, not actual diameter tube. I use one for all kinds of things but not that.
The curve seems gentle enough to use a plywood former to bend it around. Use an extension bar or tube (either internally or externally )and start at the end with the smallest radius. I tried filling with sand, results were so-so.
 

dog

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just re watched 2 videos on tube bending. in both cases its exhaust tube, one is titanium, both are done by hand. of note are the tube clamps that are used, and in the #2 fancy video there is point where the sand is removed from the tube, in there tight enough that it takes a whack to get it out, also looks like fine sand that has probably been sieved for an even size. Both use a big rosebud torch that looks like oxy/acetylene to me.
 
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Mike von S.

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I'm at the point that I have to bend the empennage outline tubes for my Bearhawk. The horizontal stab leading edge tube will be the toughest to bend. It's 7/8" x.035. I've been reviewing everything I can find on the subject and I wonder what others have done. The elevator and rudder trailing edge tubes are far smaller and I think I can muscle those around a suitable mandrel. A local tube bending firm will roll the 7/8" but the setup minimum is $350 plus the cost of the actual rolling. I have a conduit bender that fits the tube O.D. really well. Thought I would fill the tube with sand, cap it and give it a try. Harbor Freight has a pipe bender that "might" get the job done. But I'll probably never use it again.
This past week I built a DIY bending jig for 3/4" EMT conduit, which has an OD of .922. If that size works for you, you can get down to 6" radius. Up from there using a series closely spaced slight bends. The jig uses the head of a standard conduit bender, mounted on a stable platform. The pull handle is replaced by an offset lever (in my case a 5' piece of 1" square steel tubing). Instead of rolling the die over the tube, the tube is rolled around the die. I copped the design from Mario (link to his video below). The advantage of this set up is that it allows better precision, multiple 90 deg bends close together without the pull handle interfering with the tube, and bends in different planes (3D). Mario tried a single skateboard wheel as a roller, which did not work well. He replaced that with a roller he machined on a lathe out of ABS, giving it a U shaped groove to fit the tube. Instead of this, I mounted two rollerblade wheels with a small spacer between, using the gap between the two to keep the tube positioned. Rollerblade wheel bearings are designed for 8mm axles. (I used 5/16, which is a hair smaller). Over the course of 8 bends, my axle deformed somewhat, but still got the job done. Key is pulling on both the pull handle and the tube simultaneously.
 

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Mike von S.

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This past week I built a DIY bending jig for 3/4" EMT conduit, which has an OD of .922. If that size works for you, you can get down to 6" radius. Up from there using a series closely spaced slight bends. The jig uses the head of a standard conduit bender, mounted on a stable platform. The pull handle is replaced by an offset lever (in my case a 5' piece of 1" square steel tubing). Instead of rolling the die over the tube, the tube is rolled around the die. I copped the design from Mario (link to his video below). The advantage of this set up is that it allows better precision, multiple 90 deg bends close together without the pull handle interfering with the tube, and bends in different planes (3D). Mario tried a single skateboard wheel as a roller, which did not work well. He replaced that with a roller he machined on a lathe out of ABS, giving it a U shaped groove to fit the tube. Instead of this, I mounted two rollerblade wheels with a small spacer between, using the gap between the two to keep the tube positioned. Rollerblade wheel bearings are designed for 8mm axles. (I used 5/16, which is a hair smaller). Over the course of 8 bends, my axle deformed somewhat, but still got the job done. Key is pulling on both the pull handle and the tube simultaneously.
 

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geraldmorrissey

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Mike, I have a conduit bender exactly like the one you show. My question is the serrated hook that holds the tube in place at the start of the groove. Do the serrations scar up the tube?
 

proppastie

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Emt conduit is not considered "aircraft quality"..... you can not sub it for 4130 on existing plans built Aircraft.....If you have done proper strength calculations for your own design in order to use Emt you will have a very large weight penalty. This stuff is low strength soft steel.
 

Mike von S.

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Emt conduit is not considered "aircraft quality"..... you can not sub it for 4130 on existing plans built Aircraft.....If you have done proper strength calculations for your own design in order to use Emt you will have a very large weight penalty. This stuff is low strength soft steel.
Of course EMT is not appropriate for aircraft construction. It is also not something you would want to weld (toxic fumes from the zinc). I am only suggesting that if 0.922” is close to the diameter of the thin wall 4130 being used, something like this jig might work.
 

BBerson

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The hook on the end of the conduit bender will dent the tube. (Even with tape) It isn’t designed for large radius bends either.
A plywood form works best, but might take several tries to get the right radius including spring back.
 
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