Cat cut- a quick alternative?

[sodastat]

Hello All,
I was looking at the Excel program for making cat cut tarps and I had an idea and thought I would send it out to see if anyone has done this. The program gives you a list of x and y coordinates used to transfer the shape of the cut. It also gives a sag dimension that shows what I think is the deepest part of the cut at the center of the dimension "a". Instead of marking all these coordinates and connecting them, what if you took a piece of wood, 1/4" x 3/4" maybe pine or cedar, and held them at the corners and then found the center and raised it to the "sag" dimension and traced the line onto the pattern? You would get a fair curve and it would only take seconds to lay it out. You might need a little help holding the ends down but would this give a true cat cut? Would it work and has anyone tried this?
Dave

[Rat]

Catenary cuts aren't that difficult, I never saw the need for a spreadsheet, but obviously it has helped some people. Here is how I make mine.
1) Hang material (or pattern material) on a wall, you need a long wall, it is harder to find than you think. I used my work shop but you may be able to use an outside wall of your house.
2) Use thumb tacks or tape to hang the material
3) Use a small ball chain (like on dogtags) and string it up between the corners. This will give you a nice big curve from corner to corner.
4) Mark the middle of the material and measure down for your deepest point of the cat cut.
5) Lower the chain so the curve just reaches the mark you just made.
6) Resecure chain on the end you just used for adjustment
7) Use sharpie to mark along the chain, that is your catenary curve. Cut, hem and sew.

Here's a hint, use water based contrasting colored spray paint to mark you cut mark if making a pattern. Just lightly spray it over the chain, remove chain and the outline will still be visible on the material. I used Tyvek for my patterns.

[gargoyle]

Cat cuts have been done before using a long piece of wood (somewhere in the gallery) and justjeff (I think). Or just do as hogn8r said, you can use a string to sag and make your marks. But you still gotta allow for hem's.

[amac]

Hogn8r,
This sounds like an excellent technique. I am continually impressed with the ingenuity of the members of this forum. One question, how do you know what the "deepest point of the cat cut" should be?

[Ramblinrev]

I am now waiting with baited breath for one of the quants here to expound on the difference between a Cat cut and a parabolic cut. The wood method (which is my way of choice although I use an old discarded tent pole) does not produce a true cat cut. But after reading the quant's explanation and factoring in the UFF (universal fudge factor) I can't for the life of me figure out what the essential significant difference is in this application

[Ramblinrev]

Hogn8r,
This sounds like an excellent technique. I am continually impressed with the ingenuity of the members of this forum. One question, how do you know what the "deepest point of the cat cut" should be?

you read the experiments on the forums and then pick a number, any number and try it out. I like about 6" but that's me.

[GrizzlyAdams]

I am now waiting with baited breath for one of the quants here to expound on the difference between a Cat cut and a parabolic cut.

You rang?

For the shallowness of a cut involved in a tarp, or a bridge hammock, the difference between a spreadsheet cat cat, a hanging chain curve, a parabola, or what you get with a flexible rod is smaller than the error inherent in my ability to measure, mark, cut, and hem.

In any case, in my understanding the key thing is that there be a continuous and gradual change in the cut depth (and here's the part RamblinRev was wanting to see)---probably want it to be convex, but after that the exact nature of the derivatives doesn't much matter.

The wood method (which is my way of choice although I use an old discarded tent pole) does not produce a true cat cut. But after reading the quant's explanation and factoring in the UFF (universal fudge factor) I can't for the life of me figure out what the essential significant difference is in this application

Well, you lose bragging rights 'round the campfire if you do the spreadsheet method, for being a quantitative pinhead, beyond that, see above.

Grizz the quantitative pinhead

[Rat]

For the cat cut allowances I used 1 inch of depth for every linear foot of run. So a 10 foot side of the tarp would have a 10 inch deep cat cut. Having deeper cat cuts will give you a taughter (?) pitch. I found that I was able to reduce the depth of my cat cuts with edge binding versus just a rolled hem. Now, having said all that, on my short cat cuts, like the 5 foot runs, 5 inches just didn't look like enough so I added a couple of inches just for good measure. So there's a place to start, but you will proly end somewhere else!

[Coffee]

I am now waiting with baited breath for one of the quants here to expound on the difference between a Cat cut and a parabolic cut. The wood method (which is my way of choice although I use an old discarded tent pole) does not produce a true cat cut. But after reading the quant's explanation and factoring in the UFF (universal fudge factor) I can't for the life of me figure out what the essential significant difference is in this application

This engineer is staying out of that arguement. True cat cut no, close enough most likely.

It's really hard to get a true cut. I think it's even harder to get it sewn just right. A hint is and wrinkles and you didn't get it. I would rather make a quilt than a tarp anyday.

[oldgringo]

Boat builders have been using "bendy" battens to scribe arcs for centuries. I guess nobody told them about spreadsheets.