My home school project #4: The hard part

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Blog entry by Farrout posted 05-13-2013 07:44 PM 1444 reads 0 times favorited 9 comments Add to Favorites Watch
« Part 3: The top layer Part 4 of My home school project series no next part

I’m not sure what you call the pieces on the side that make the whole thing open, so I’ll call them struts.
I knew the whole thing is basically a bunch of parallelograms so I started with that in mind.
I drilled the three holes as accurately as I could in the center boxes.
The struts were made as identical as possible, the shorter ones being 4.5” center to center, the longer ones
are 9” center to center for the outside holes.
I mounted each strut to a center box hole then used an angle locator, the kind with a needle that always points straight up regardless of the angle of the side.
Each piece was positioned at 35 degrees, then an awl punched a hole in for the other holes. Tedious.
This is the result.

I’m going to take all the struts off and round the ends, then build a handle that comes up the middle.
I learned a lot about the various processes on this project, and I made a lot of mistakes. Isn’t that how you learn?

-- If we learn from our mistakes, I should be a genius!

9 comments so far

View PurpLev's profile


8539 posts in 3705 days

#1 posted 05-13-2013 07:57 PM

looks great!

-- ㊍ When in doubt - There is no doubt - Go the safer route.

View stefang's profile


15881 posts in 3390 days

#2 posted 05-13-2013 08:10 PM

That worked out quite well. I would call the ‘struts’ hinges, but no matter, they work and that is the most important thing. Congratulations!

-- Mike, an American living in Norway.

View JoeinGa's profile


7739 posts in 2063 days

#3 posted 05-13-2013 09:09 PM

Neat. My wife has one of those she bought in sewing store about 25 years ago. Hers has a hoop handle in the middle and sits on top of legs that are bout 10” high. I was always fascinated by how it opens and closes so smoothly.

-- Perform A Random Act Of Kindness Today ... Pay It Forward

View gfadvm's profile


14940 posts in 2746 days

#4 posted 05-14-2013 01:03 AM

What an impressive engineering feat! How did you figure out the 35 degree angle and the hole placement?

-- " I'll try to be nicer, if you'll try to be smarter" gfadvm

View Farrout's profile


185 posts in 3210 days

#5 posted 05-14-2013 11:26 AM

Trial and error actually. I did do a drawing in TuroCad, but my first attempt didn’t line up correctly when I just tried to do measurements. I turned each center box 90 degrees and started over using the method above.
It was a practice project after all.

-- If we learn from our mistakes, I should be a genius!

View wcndave's profile


6 posts in 2945 days

#6 posted 05-14-2013 04:12 PM

There is a simple mathematical way do to this, that I used on mine.

So just using the dimensions on the next pictures as an example, not what I did / you might do.

Let’s say we want a box that is 16” long, with 3 layers of 4,3,2” depth, and I want a movement of 5” to allow 3” overlap (you need support there).

So given the middle box (for example) will move 5” and be at the same height at the end of moving, the movement looks like this.

so X and Y are the same, and therefore ? = 2.5”.

So now some simple trigonometry gives you the angles or dimensions to drilling as required.

However let’s complete the example. In my case I want the angle to be 45deg and the middle layer to have the holes in the middle vertically. So plugging in known values:

For a 45deg hinge, clearly ? = 2.5 – 1.5 = 1”

by a happy coincidence, that means the top layer will have holes 1” from bottom, which is also the middle of that layer vertically.

Truth is with mine, I went the other way. top and middle layers to be centred vertically, (that’s how you spell centre in English…) means 2.5” between first two rows of holes, bottom level therefore I also set to be 2.5” away from middle row, and for a 45 deg angle, you need 2.5” horizontal separation between holes in a level.

I then knew that with 2.5”, my total movement was 5” leaving 3” overhang, which seemed perfect.

If you wanted 30deg angle hinge from horizontal you can work out that you have a triangle with X = 2.5” x tan 60 deg = 4.3” (who remembers SohCahToa?). So you need 4.3” horizontal distance. unfortunately that means for 3 holes, you have 8.6” which is longer than the 8” second layer box, so instead you’d have to move the holes closer to the edges (top and bottom edges of bottom and top layers respc.). 5” between outermost holes gave me a nice 1.5” at each side to play with.

So, too much maths? Ok, here’s the summary.

Assuming you want to line up your holes vertically and horizontally, like in the grid below, then:

- A must equal B.
- C can be any value where you can physically fit 2xC across a box, with spaces at the sides.
- If C = A/B you will get 45 deg angle.
- The opening distance = 2xC

If we stretch just the grid, we get bigger opening movement, and lower angles, see below.
However if I increased A and B we’d get back to 45 deg again…

One final thing. On mine i did the arms, then added rubber stick on pads to the bases, to make it quieter and the hinges were out of whack. I strongly suggest to assemble fully and even finish (I pre-finished so had to finish it first), before marking holes in the arms. Also label clearly which one is which. with 12 arms, they get mixed up.

Hope that helps a little bit anyone wanting to work out the relationship between the measurements, the angles, and the opening distance!

View gfadvm's profile


14940 posts in 2746 days

#7 posted 05-15-2013 12:31 AM

Oh how I envy people with math skills!

-- " I'll try to be nicer, if you'll try to be smarter" gfadvm

View wcndave's profile


6 posts in 2945 days

#8 posted 05-15-2013 09:26 AM

Well, thanks, although this has to be some of the easiest maths, 8-9 years I would think, that basically corner to corner of a square is 45 degrees :-P

What took me the time when first building, and it seems others too, is coming to the realisation that the maths is so easy, and not some complex piece of logic!

View Farrout's profile


185 posts in 3210 days

#9 posted 05-15-2013 11:10 AM

FYI I used wcndave’s design as the basis for mine. I found his project on Wood Talk.

-- If we learn from our mistakes, I should be a genius!

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