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.

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 …

Midwinter Musings

Valentines Day. Half-way through February and about 4 weeks until Spring arrives. Too early to start the tomato, pepper, and cucurbit seedlings, but not too early to dream.

Some follow-up on the lettuce. The AC lettuce (leaf lettuce) was exhibiting some strange growth; the leaf edges and centers were turning black.

Whatever it was did not seem to affect the Buttercrunch or Romaine. A little bit of research turned up the possibility that the nutrient solution was too concentrated - too high a concentration of sodium in the water (we are using salt-softened water) can inhibit calcium uptake in lettuce, and if the leaves are deprived of enough calcium at a critical stage, they become deformed and never recover. I will be reducing the nutrient concentration in future batches, and am giving up on raising the AC lettuce hydroponically since it seems so sensitive.

At any rate, we are now in full production, with a new tank of 9 heads going roughly each week. (That's sweet basil growing in the coffee cans.)

Following the instructions of Mike McGroarty (Mike’s Backyard Nursery on YouTube), I built a propagation chamber out of some of the scrap cedar that came off the roof 23 years ago.

Roughly 2 feet by 4 feet, sand and peat moss in the bottom, and I have several hundred cuttings in there to be rooted. Some apple, grapes, weigela, blueberries, upright yews, and emerald spire arbor vitae.

Then, because I am greedy, I built 2 more, about 3 feet by 4 feet, for starting more cuttings - planning more yews, arbor vitae, azaleas, whatever.

I have to finish the lids for those by stapling on the plastic.

Back in March of 2009 I began building my greenhouse (“Mo' Ham, Maude”, 3-20-09; “It's a CLOUDYhouse, Mr. Dirt”, 4-1-09; & “Clearly an Improvement”, 10-19-09).

Over the years I have learned several important lessons.

1. 8 feet x 8 feet is too small.

2. Ventilation is critical.

I have long dreamed of building a new and improved model, but having been impressed in the wrong direction by the performance of the polyethylene cover and seeing the way the polycarbonate (Sun-Tuf™) has held up, my dreaming always ran into the cost of the glazing. The corrugated polycarbonate runs about $1.25 per square foot, plus there would be the cost of the treated wood framing. A 16 x 25 greenhouse with 8’ walls and a moderately pitched roof would need about $1500 in polycarbonate alone.

However, after seeing what Bobby Smith (mhpgardener on YouTube), SSLFamilyDad (on YouTube), and Grow Appalachia (on YouTube) have done with high tunnel poly houses, I think I’m ready to try that. Having gone through 2 winters with my Harbor Freight woodshed, I’n not quite so fearful regarding polyethylene, especial UV resistant varieties. Also, my Better Half has repeatedly told me that I over-engineer things, and keeping that in mind, the lighter framing used in the high tunnels seems to be adequate.

My thoughts are these: Based on the videos from SSLFamilyDad, I think I can cobble together a frame made from 3/4” EMT. Utilizing the support and ventilation ideas shown by Bobby Smith and the Grow Appalachia folks at Berea College in Berea KY, and scouring the net for material prices, I think I can put together a 16 x 24 high tunnel for right around $800 plus some of my other odds and ends on hand (like the vinyl triple-track windows we tore out of Mike’s house on Tampa when he replaced all his windows 15 years ago). Maybe I will finally even get my shed cleaned out!

Here is what I envision. Each hoop will be a 25’ arc (2 - 10’ and 1 - 5’ length of EMT, the sections joined with 6” lengths of 1” EMT as sleeves) which should provide 15.92’ of width and 7.96’ of height. The hoops will be mounted in 1” EMT driven into the ground along the sides at 4’ intervals with about 2’ exposed, to give a total interior height of just under 10’. The 2’ high side sections will be closed with 2x8 treated center-match lumber (which is needed because of the slope of the site). The plastic and wiggle-wire channels for mounting it will run about $350.

The biggest challenge will be forming the arcs. A tubing roller from Harbor freight runs about $180, but looking at the tool reviews and considering the lengths and multiple passes involved to provide an 18” deep arc in a 10' tube, there would be a great risk of twisting the arc out of the plane. I will need to build a bending form, using 3 - 10’ 2x8s, some additional scrap lumber to hold everything together, and a bottle jack. Once the bending form is made, it can be used to duplicate the 16’ wide tunnel numerous times (hint, hint, chilluns). If I can build that bending form, then the only other obstacle is money. Stay tuned for more adventures of Grandpa Jim.