Hooping Around The Garage

After the last post, Joonyah sent me an email in which he said I had to use the stress/strain relationship to determine the deformation, and that he estimated that with the EMT, yield stress would be reached at about 23% elongation. So I did a little reading on Hooke’s Law, and ended up feeling like I was sitting through a sermon in a foreign language.

Thinking that maybe I could just find a practical application idea somewhere out there, I located a whole bunch of video clips on tubing bending - stuck some of the links in my bookmarks because they may come in handy someday - but all of them dealt with nice sharp bends in short distances. So I then got the brillig idea to search using “bending greenhouse hoops”. Sometimes the obvious is the key to success. Up popped numerous YouTube videos of people bending greenhouse hoops. Doh.

There were even places selling bending tools for greenhouse hoops.

Typical commercial bender

They were priced anywhere from about $70 to $180. Being cheap, but more than that, wanting this to be a project that could be done anywhere in the world, conceivably, and having already started with a building board, I figured that the thingamajig could be built with the scraps at hand.

First though, to lay out the arcs. I needed to set up a trammel point on a centerline. Back to geometry class, where we erect a perpendicular bisector to a line. An arc was drawn from each end of the building board and a chalk line snapped down the center.

Then, allowing for a bit over an inch of space at the edge of the board, the radii were marked off from the table for arcs for the 20, 25, and 30 foot arcs (6’ 4-3/8”, 7’ 11-1/2”, & 9’ 6-5/8” radii), and 1/8” holes drilled to take the nail that would provide the trammel pivot.

The short white chalk marks are at the pivot points.

The three radii were scribed out on the building board. The green arc in the center is the one I am using for the 15’ 11” wide greenhouse.

At the same time as each arc was being scribed, I scribed the arc on a length of 1x12 about 3 feet long for the center section of the bending tool.

I took the one with the 7’ 11-1/2” radius and scribed off another arc 7/8” inside the bend.

The table saw was used to cut the arc to make the template for the “cheek” pieces.

That arc was transferred to 2x stock and then the inner arc was cut out and smoothed off. That would be the portion of the tool that the tubing would actually be bent on. The pieces were lined up and holes drilled for index pins because I anticipated taking it apart and tweaking things, and I wanted everything to line back up every time.

In one end of the tool I notched and recessed a 3” length of 1” black pipe that would hold the fixed end of the tubing while it was being bent. A piece of pipe strapping was screwed over that to hold it in place.

When it was all put together, I bent the first 10’ length. Bummer. Springback wasn’t 23% but it didn’t quite line up with my arc on the building board.

The outside edge of the bend should have been on the green arc.

So, apart it came. I guessed that if I planed off 1/4” from the edge of the arc that it might correct the springback. Guessing is a very important part of solving problems.

Then the “cheek” pieces had to be adjusted by the same amount. One thing that I found in the original bending exercise was that the sides did not need to be very high. I reduced them to 7/16” and planed the edges so that instead of a narrow groove I had a gentle trough. That makes the bending require much less effort. A new notch had to be made for the 1” pipe.

Note the index pin. I used gutter spikes for that.

Finally, I refastened the jig to the building board, placing it toward the center. The stacked 1x6s help keep the bent tube in the same plane through the entire operation.

That corrected the springback. A bit too much, but it is easier to bend the tubing out than it is to bend it in. I set it on the floor, pressed down in the center a little, and the bend now follows the green arc on the building board.

Time to go get the rest of the 3/4” EMT and bend the rest of the hoop sections. That is for tomorrow, Lord willing.