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  1. #11
    Senior Member Flash Grundelore's Avatar
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    N00b??z....

    How do you deal with the hem on an edge cat-cut for a tarp? Do you just fold it right over like on a straight hem?... or do you have to make slits to get the seam to go flat?
    >> Onward thru the fog...>>
    Find me on my blog Moosenut Falls https://moosenutfalls.wordpress.com/

  2. #12
    Senior Member ripstopbytheroll's Avatar
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    Quote Originally Posted by Flash Grundelore View Post
    N00b??z....

    How do you deal with the hem on an edge cat-cut for a tarp? Do you just fold it right over like on a straight hem?... or do you have to make slits to get the seam to go flat?
    You just kinda make do to be honest. I had the same question after doing my first cat-cut tarp. I wasn't fully satisfied with the hem on the cat-cut and thought there was maybe some trick I was missing. Talked to a vendor who said there really is no trick :/.

    I think lower depth cat cuts with silpoly is your best bet to get a prettier result.
    - Kyle

    www.RipstopbytheRoll.com | "The Best Fabrics on Earth. Guaranteed."

  3. #13
    Senior Member xxl_hanger's Avatar
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    Quote Originally Posted by ripstopbytheroll View Post
    Talked to a vendor who said there really is no trick :/.
    Ask better Ramblinrev. He knows almost any trick:
    Quote Originally Posted by Ramblinrev View Post
    A couple of sewing tips for doing cat cut hems. 1) use a shorter than normal stitch. Not tiny short unless you really want to spend a good chunk of time for each hem. The shorter the stitch the longer it takes to stitch each foot of length. _But_ the benefit is.. the shorter the stitch the easier it is to go around curves. So a short stitch is worth the additional time IMO. 2) Fold something into the hem when you get started. I use 3/8" twill tape. I would advise against gros grain for this because gros grain likes to go on straight. The weave makes it somewhat stiffer and so you may have to fight to get it around the curve. I know lots of people like using gros grain for binding the hem and I am not saying it is wrong to use. But I think it takes a more skillful or more patient stitcher. The benefits of rolling something inside the em is two fold (groan). It provides an extra bit of strength to the hem. Plus it acts as a guide to help keep the rolls in the hem consistent.
    Last edited by xxl_hanger; 05-13-2016 at 10:36.

  4. #14
    Senior Member Flash Grundelore's Avatar
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    Thx Kyle and XXL.... I kinda figgured it was "just do it".
    Fortunately, the cat cut I'm thinking of retro-fitting onto a tarp I made last year would probably have a fairly shallow curve. I just want tighten up the edges a little bit now that I'm hanging it a little higher. It was fine as a ground shelter. Now I'm figuring to use it more as a sunshade/windbreak/gear hut around camp.
    >> Onward thru the fog...>>
    Find me on my blog Moosenut Falls https://moosenutfalls.wordpress.com/

  5. #15
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    Quote Originally Posted by rammsteini View Post
    I wrote this a month or two ago and I can't remember where I found the pieces of the algorithm that I copied and pasted together. But for me it works. But feel free to send me some code - in almost any programming language. If it works, I will update my generator.
    It looks like the length went up significantly in the odd example while the distance remained constant. Under those parameters, I might expect a rope or cable to lay "on the ground" like that, assuming that negative values of 'y' are not allowed.
    Caminante, son tus huellas el camino y nada más... - Antonio Machado

  6. #16
    Senior Member xxl_hanger's Avatar
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    This picture with 50 points was my first concern:
    https://www.hammockforums.net/forum/...2&d=1463064825

    However, I just found out that the horizontal distance between all points is in all cases the same. What I first saw was an optical illusion only. The result is 100% correct.

    Further insights with this example:
    Distance: 297mm (Din A4 width)
    Sag: 200mm

    If I choose 1 point the software uses 2 points and the diagram shows the distance, but no sag at all. If I choose 30 points the software uses 31 points with a mid point. The diagram is pretty nice and smooth in this case. If I choose 29 points the software uses 30 points with no mid point. Insight: The software always adds one point. Example: If I divide 297 / 20 and choose 14 points I get a diagramm with 15 points -> 14 distances all 297 / 14 = 2.121cm width.

    I get very nice and easy to use laser printouts (100% correct cantenary curve templates) if I do not allow the printer to scale the print results.

    I said it already that the software is also able to calculate the length of any cantenary curve. This feature is especially helpful if you want to understand or adjust TheXringHunt's WBBB plans for example. It's a very powerful general-purpose tool. Now I can forget all my other complicated spreadsheet calculations where I struggled with a view months ago.

  7. #17
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    After some trail and error I made a new version with the "real" catenary formular y=a*cosh(x/a) from https://en.wikipedia.org/wiki/Catenary
    No need to think about numbers of points now. The first versions curve wasn't exactly a catenary, pleas forgive me.

    I also added a preview of the pdf (perhaps ist doesn't work in every browser, but Chrome and Firefox in Windows 10 works)

    https://dl.dropboxusercontent.com/u/...narycurve.html

    1.png

  8. #18
    Senior Member xxl_hanger's Avatar
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    It's better now, because nobody needs points. But now you have a big bug in your calculation.

    Make the following realistic example for a silpoly winter tarp:

    Distance 93" * 25.4 = 2362mm
    Sag = 93/12/2" * 25.4 = 98mm

    and look what happens.

  9. #19
    New Member
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    thanks, fixed it. The problem is to solve the equation:

    a*(cosh(1/a)-1)=2*sag/distance

    for "a"

    I don't know how to do this, so I try a lot of values in a loop and take the best. This is not very efficient, not very precise and slow. But it should work now.




    Quote Originally Posted by xxl_hanger View Post
    It's better now, because nobody needs points. But now you have a big bug in your calculation.

    Make the following realistic example for a silpoly winter tarp:

    Distance 93" * 25.4 = 2362mm
    Sag = 93/12/2" * 25.4 = 98mm

    and look what happens.

  10. #20
    Senior Member
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    Superb, rammsteini! I've written programs that work the same way, by iterating until it "approaches" the correct value within some threshold of error, or with a limited number of iterations. It works just fine, and your program seems nice and responsive without enough of a delay to even be noticeable. I could, if you like, ask a mathematician friend of mine how to solve for a? But, I don't think it's really necessary.

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