Hatz Biplane Association

OK, so I guess bending tubing will be my achilles heel. I'm trying to bend the seat frames and have tried the conduit bender, bending an empty tube around a form with heat, bending a tube filled w/sand around a form with heat... all to no avail. I've succeeded in "bending" the tubes, but not in a fashion that I would call acceptable.

So, for those of you out there who have built the Classic seats according to plans, how did you do it? I'm getting ready to spring for one of those expensive tubing benders next... like this...
http://www.northerntool.com/shop/tools/ ... _200256024

Thoughts?

Hi Doug,
I tried the tubing bender without success too and ended up welding an angle rather than trying to make a radius. It made it easier to design the seat cushions and our seats are comfortable. I tried to add a photo but the file size (457KB) always seems to be too large.
John Hanson

Doug

I made a steel jig and welded it to the top of one of our steel tables. Then using heat and a series of radius faced clamps slowly started to tighten the clamps. It is important to keep the heat isolated to where you want the bend. We use wet rags to "block in' the heat. See pic.

Did you get the DWG disk?

Michael
N838MM

Thanks Mike.
Yes, I did receive the CD.

I cut the radius out of 3/4" plywood and bent the tube against it with heat. Yes I burned up the plywood, but it worked just fine.

Nick

Hi Nick,
A long time ago I was shown a method for bending heavy walled hydraulic tubing. The trick was restraining the tube from flattening. For the bend that was needed, all it took was a vise with smooth jaws. The tube was heated and bent by hand between the jaws of the vise and a very acceptable bend was made.
I used the same method with a wooden form with flanges to do aluminum, but for your 4130 a steel fixture would be needed. Still not sure it would work on your size of tube.
Good luck,p
Al

I bent tubing in two places... the rudder/vertical stabilizer and the horizontal stabilizer.
The 1/2" tubing was bent cold. I made a form with a radius, drew lines on the table for the outline and went at it. I would bend a small section, hold it up to the line and adjusted as necessary. When I was done, I laid it on the table and it wasn't flat... tweeked it a little until it was.
For the 3/4" tube I put sand inside, packed it as best I could and plugged the ends so it wouldn't leak out. I made a form out of plywood and scewed it to the table top and added an anchor where the bend started so the end was trapped and couldn't move. I heated the tube all around the diameter as best I could and slowly bent the tube around the form. Fight the temptation of only heating the outside of the radius. If you do, that area will stretch and become thinner than the rest. Use the color of the tube to guide you. Any color brighter than a dull red will soften the material and cause kinks. I also left the tube as long as I could to help with leverage and heat distribution.
Jeff Moore

Jeff your method sounds just like mine minus the sand. I just used a lot of patience, took my time, and the bends worked out very nicely with no kinking or flattening. Probably try it with sand the next time.

Nick

I built my own bender out of scrap mat'l around my shop----it only does 3/4" but very well. I filled the .065" tube with fine sand & tamped it in 4''--6" increments, then plugged with a wooden plug.

The white pulley was an accessory drive pulley from an old Cummins 250 diesel engine. I contoured the groove to a 3/8" radius and mounted it on a heavy steel shaft with 2 flats to grip in a heavy vise--it is a slip fit in the pulley--you drop a bolt thru a hole near the rim of the pulley down between the jaws of the vise to keep it from rotating..you may have to drill several holes if you don't have much walking room around the vise.--bend a little, rotate pulley a little, bend a little more.
Oh, there is /was a drilled hole in the top of the heavy shaft to pivot the angle iron bar on.

The small red wheel is the INNER race of a large (worn) ball bearing that used 3/4" balls--(making the groove a perfect fit
around the tube.) A smaller ball bearing with an OD the same as the ID of the inner race is then bolted to a 6 ft± 2X2X1/4" angle iron..Unless you have a very stout vise & bench, you'll need a helper with a bar or pipe stuck thru the opening between the jaws of the vise to balance the PULL you exert on the angle Iron.

And this is the secret to success--the tube is supported inside & out by closely fitting wheels--the only gap is ~~3/16" wide , and it falls near the neutral axis of the bend.--this thing makes beautiful bends-COLD with a nice constant radius. Jerry†

Old heavy truck transmissions-(manual) use bearings with large balls.

There is a short pc of 7/8" tube cut on an angle to slip over the tubing to protect the outer side from being dented by the
heavy bolt that holds the tube in the starting position

Well, I seem to have finally solved this problem... I bought a Holsclaw bender off of Craigslist and have successfully bent one seat. Before bending the seat I did a couple tests and learned pretty quickly that packing the tube with sand makes a huge difference. Below are a couple picks. One is of the bender, the other is of two test pieces. The test piece on the right was bent empty... without sand. The one on the left was packed with sand prior to bending. As you can see, the one without sand flattened quite significantly (it's worse that the picture shows) and also kinked in three places. The one WITH sand came out very nice.

Thanks for all the photos and ideas everyone.