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I'm sure this has been discussed, but I'm wondering how much better the DIY tarps are that have mathematically figured curves/cuts, rather than just 'eyeing' the job.
Assuming you're not hung up on looks, is the function (wind resistance, quietness, etc) much better than if you just jumped in and made the cuts?
I have my doubts about some of the mathematically-formulated tarp curves. ;)
just removeing enough of the loose edge material is all that really matters as far as the cut goes.
There is the theoretical answer and the practical real life answer. In theory one is more precise than the other. By the time you figure in the width of the marking line and inaccuracy of hand held scissors you have left the theoretical world in the far off distance.
I made a template out of cardboard, by hanging a string the length of the curve, I marked the bottom of the curve by, I think, .5" for every 1 foot of length. Then I hung the cardboard upright, and traced the curve of the string onto the cardboard. After I made the template, I used a roller cutter following the curve to make an exact cut. That being said, it all depends on how well and precise the material is hemmed too.
Dwight
That's what I say. I calulated it using a MATLAB program someone else wrote. I think sewing and cutting make a bigger difference. Mine had a couple small winkles sewn in. More from my error than anything else.Quote:
Originally Posted by Ramblinrev
If you do some searching there are countless threads dicussing Math vs a curved stick for cat cuts.
I have made a few ( more than 10 :scared:) cat cut tarps. I use the string through the tube method to create the arch. The one thing that I have learned is to make the arch less extreme than I want the final product.
yis,
Erin
I've got a question about cat cut edges on a tarp. When using the spreadsheet, does the calculation take into account the material needed for the rolled hem? I havn't looked at the file myself yet, but it would seem to me that your cuts could end up as much as an inch off if you don't allow for it. Is the true?
Dave
Dave,
Don't know the answer about the hem, but the formula is for catenary curve only (cable curve) and does not account for the effect of pulling the fabric on the bias. That factor blows all the calculations. The eyeball is the best judge. Bill Moss of Moss Tents was the great master of compensating for bias. He just eyeballed it and fiddled until he got it right. Then he ripped out the seams and used the prototype to make patterns. Different fabrics with different coatings vary in their distortion through the bias, and no algorithm can compensate for all that.
The first time I made a hex tarp with curves on the hems alone, it took me two tries to get enough offset in the curves. It was lots more than I had anticipated. The hems along the bias took a full 1.25 inch per foot offset whereas the hems aligned square to the weave took only 3/8 inch per foot. Offset refers to the distance at the center of a hem from the straight edge to the curved edge. Please note the caution above: it depends on the fabric and the angle of the hem to the weave. What I used may not work on your project.
When you fold (and sew) a curved edge, the folded edge does not line up precisely. Only with straight edges does that happen. You have to make the fabric do something it does not naturally want to do... and silynylon is slippery and cantankerous without asking it to do something unnatural. Experience is a big help.
If you look in some of the sewing books you will see they cut darts around neck lines to relieve the buckling(?) that naturally wants to occur. I wouldn't do that on these curved edges on tarps but you have to deal with it a little differently than when you hem straight edges.
Another technique the sewing books offer is to trim the seam allowances very close which makes it easier to "ease" the fabric around a curve. This is often done on arm holes, collar seams and the like. The smaller the seam allowance the less fabric there is to bunch and wrinkle.Quote:
Originally Posted by Youngblood
Yep. My problem with sewing smaller hems is that I don't have a straight stitch machine, it is 7 mm zig zag. I have a straight stitch presser foot but that doesn't solve my issues with the location of the feed dogs and the opening on the face plate. If I try to sew this light weight slippery fabric too close to the edge, bad things happen... the needle tends to pull the fabric into the machine. But I only sew proto types, so I just cuss and fuss and do the best I can.:DQuote:
Originally Posted by Ramblinrev
That's why the technique is shown as using the normal seam allowance and them trimming the seam. It is a pain to do on production which is why many manufactures use serging or marrow machines. Plus, they have fancy schmancy binding attaching machines and special heat sealing cutters. Frequently on mass commercial items, the seam binding is put right over a raw edge, specially in the really cheap lines of goods. The binding is put on as the seam itself is sewn.Quote:
Originally Posted by Youngblood
I did one tarp years ago with catenary ridgeline and edges where I just hot cut everything and used only folded hems, not rolled hems. I hot cut the catenary ridgeline with the panels laying on top of each other such that the edges melted together. It worked fine and was much easier to sew but my hot cutting technique was too much work. With DIY projects and low volume projects, when you weigh your options the best choices sometimes play out a little differently than with higher volume or more sophisticated operations. There is nothing wrong with hot cutting and using folded hems rather than rolled hems in many applications.Quote:
Originally Posted by Ramblinrev
If I was interpreted to disparage one technique over another I mis-spoke. Sorry for any confusion. You are correct. There is nothing wrong with any technique that works as long as it is successful in the application.
I was implying that at all, sorry if I came off that way. I was interested in your information and trust the conversation is of interest to the DIY folks on the forum.Quote:
Originally Posted by Ramblinrev
I believe I tried the hot cut, folded hem approach after wrestling with trying rolled hems on curves after I read some info on sewing techniques that I found on Stephenson's Warmlite site. I have had one their tents for a few years and have always admired the design, construction of it, and how well it does what it is suppose to do.
But back to the curved surfaces and rolled hems for a minute. The basic problem is the geometry, it just doesn't want to fit when you do a rolled hem. The bigger the hem and the deeper the curve, the more it doesn't want to fit. You can force it to happen but it likely will not look as neat as the rolled hems you see on straight edges.
That's why _I_ don't like to use grosgrain on tarps edges, particularly cat cuts. The geometry of grosgrain and I don't get along at all well. It loves straight lines. Seam binding or bias tape is made to take those curves with greater grace. I roll my hems simply because I use what I have on hand which often does not match the base fabric. So I roll twill tape into the hem to provide the strength and stabilize the curve. For _me_ that works better. If I had the machine that would stitch the seam and apply the binding all with only one pass through the machine I would use it and love it. But.... I would have a hard time trying to justify such an investment to my wife for the sheer purpose of "making gear."Quote:
Originally Posted by Youngblood
have you done tarp edges with a rolled hem without the twill tape inside? how does it differ?
no I haven't. I like having the stiffness... such as it is... to help guide the next roll. I have a whole industrial spool of 3/8" white twill tape so I am not about to run out of it in a hurry.
The tape gives me a guide to work with while easing the fabric. Otherwise I just end up with a horrible mushy mess. But that's me. YMMV
i've done that with ggr, it does make it easier to roll. it probably makes the rolled edge look better too. that's one reason i use the edge trim you, don't have to roll it.
Spinntex feels stiffer than silnylon. Does not seem to stretch as much. Would a cat cut be necessary? If needed would a reduced depth of the cut be effective?
Quote:
Originally Posted by koaloha05
maybe, it depends. i bet the spinntex still stretches on the bias.
cuben on the other hand doesn't really stretch in any direction so i'd think a tiny or no cat cut would be better. haven't seen the spinn up close yet so can't really say. what does oes do about the cut? same or less than the sil version?
You have too seen the Spinntex, you were just too busy showing off your new stuff and you forgot. :DQuote:
Originally Posted by warbonnetguy
I've got a MacCat sil standard at home. I'll lay them both out tonight and see if the curves are the same. Unless Brain logs in and answers the question first. I can't remember it stretching in any direction, but it may a bit on the bias.
Cannibal - I have been summoned by name, and so I come down from my dorm room and respond.
The SpinnUL fabric I use has very little to no stretch due to 'water-stretch', but it still stretches along it's bias as Brandon guessed. I cut the silnylon and SpinnUL versions of my MacCat tarps with the same cuts, as I have seen no to reason to change it - the design works just fine with the lighter fabric and the benefits of a cat cut are certainly still evident.
That's funny! I just thought as I was typing in another tarp thread that I needed to get my tarps out and look at them. :lol:
Thank you for coming down from on high o tarp master. :D
Now now, there's no need for fightin' words!:laugh:Quote:
Originally Posted by Cannibal
I would like to announce that with the blessing of my German professor (letting me take my final earlier), I am almost sure I will be at Traildays this year!:) I'm starting to get excited to finally meet at least a sliver of the hundreds of people that I have met online, especially here at HF!
I may also be doing an internship in Germany beginning early June, but that is a topic for later discussion.
OK. Lets see if I can even describe what I'm thinking here.
For the sake of making this point, lets say the hem is going to be 1/4 inch, and the length of the tarp is 10 feet. I measure in from the center point 10 inches and then 'eye' the cat cut curve to either tarp point.
Now I'm trying to sew my hem on the curved part. This is where the bunching up of material happens, right? Could you not sew several 1/4 inch (depth of hem) 'spot welds' perpendicular to the edge, starting at the center point, then the two 'new' center points, etc, until you have smaller, more manageable segments to sew along the entire long edge?
I know this would be a pain, but it would work? And would it be worth the trouble?
Use pins....
That's what they are made for and you can move them at will if you choose.