The Plot Bends

Or maybe it thickens. Whatever.

I didn’t see any greenhouse seeds lying around, so I figured I had better work on some plans. Following the idea that the simplest way to craft what I want would be to use 3/4” EMT for the hoops, I began to research the mathematics involved. I didn’t find an arc angel, but I did locate a calculator at a website called “handymath.com”, and using that, cobbled together a table which provides data for various sized hoops using 10’ lengths of EMT.

The green shaded rows are what I believe would be optimal. The red shaded rows I think would be less desirable from the standpoint of wasted material.

The basic idea for what I want to do looks like this:

In order to bend the EMT, some sort of tool is needed. A tubing bender would require dozens of discrete kinks in the tubing to achieve a smooth arc. As mentioned earlier, a tubing roller might work, but from the reviews, that would also need multiple passes. I decided to try to build a simple press which would bend the 10’ length into one smooth curve in a single pass.

The table data is critically important for designing the bending form. There are also some other considerations. First, the arc height measurement, which is the distance the tube must be bent, is for the outside of the curve. The inside of the form has to account for the 7/8” outside diameter of the tubing. That I think can be accommodated by routing a 7/8” deep channel into the male section of the form.

Secondly, EMT has a certain amount of “spring” to it. Some method of making sure the tube does not spring back to a slightly larger than desired arc needs to be incorporated. I can’t find any tables addressing that, so I will need to take a wild guess and make the arc height slightly greater than what is in the table, in the hope that the mildly more curvaceous segment would spring out to what I actually want. Tricky stuff, because as can be seen from the table, a change of 1/2” in the arc height can make a half-foot difference in the width of the greenhouse.

So, Tuesday I went to Lowe’s to get the project started. The arc height is between 18-19 inches; I figured that gluing three 10’ 2x8s ($9.13 each) together would give me a chunk of wood 21-3/4” wide. Unfortunately, all that I could find were 2x8 bananas. The 2x6s ($6.22 each) were nice and straight though, and 4 of those would give a 22” wide plank, and would actually cost $2.51 less for the project. Not a tough decision. I also picked up a length of 3/4” ($6.20) EMT for a test run. If I buy those in batches of 10, they are $5.27 each. Plus sales tax. The Guv always has his hand out.

Four boards and a tube.

First glue joint.

Last glue joint.

First time I ever saw a 10 foot long 2X22!

So now I am ready to make the form. I think I will go with the 1’ 6-3/4” arc height to account for spring-back, and hopefully any minor adjustments to the hoops can be accomplished by brute force when they are being inserted into the 1” base tubes. I looked, now I need to leap. Until next time …