Sewing tolerances – inaccuracy in cutting and seam width

[Alex N]

Hi everyone, first post here and first of all I’d like to say a big thanks to all of you who have contributed to this forum, there is a wealth of information here, which I have already benefitted from.

I’m planning my first big project having only really made drybags and little bits and pieces in the past. I run group backpacking trips and I want to make a fly only tent based fairly closely on the Hilleberg Altai. (I realise this is a totally insane first tent, and that I shouldn’t even consider attempting it)

I’m fairly confident with the design aspects of this, I was an aeronautical engineer in the past, but I am certainly new to sewing and have been practicing my flat felled seams (with basting tape).

Having cut the triangular roof panels from 1.6 silpoly from ripstop by the roll there is some variance in panel dimensions, perhaps a total range of plus and minus 5mm on ‘roof’ triangles that measure roughly 2m (6ft) by 1.4m (4ft). I am not sure that I could have done it more accurately to be honest!

How good my flat-felled seam is is mostly dependent on how accurately I baste the two panels together. I was a bit casual with how I did this on my test on a bit of scrap. Over the 1m length the 1cm wide seam “swallows” plus or minus 5mm of material (this is basically because when I basted the two together the overlap varied plus or minus 2mm which then gets doubled or more when you fold the material over to form the seam) I could probably half this if I am careful, but this does lead me to a question…

How important are tolerances like this generally? I realise it is highly dependent on the design and size of the tent, but should I be trying as hard as possible for perfection? What kind of tolerances are you guys able to achieve? Are your cut panels all almost exact? Are your seam tolerances so small that you don’t even think about it?

Thanks!

Alex

 

[Jerry Adams]

if the seam swallows 5 mm + or – along 1 meter, your tent when assembled will have wrinkles in the panels when you erect the tent.  Mostly just aesthetic.

Also, as you sew along a seam, one piece of fabric will stretch a little relative to the other piece so they won’t line up when you’re at the end of the seam

if you do a “faux flat felled seam” it’s easier

Draw a line on each panel where you want the edge of the finished panel to be.  And mark the begin and end of the seam.  If you want the edge of those panels to be 2 meters, measure that precisely on both.

Then, use pins (or I use hand stitches) to sew the panels together at the beginning and end of the seam and at several places along it.

Then sew the first row of stitches.  As you sew along, you want to carefully keep the top panel from slipping relative to the bottom panel.  Then fold over the selvage and sew a row of stitches to finish the faux flat felled seam.  Pull the fabric apart as you go so both rows of stitches have equal tension on them.  Practice this with a couple test pieces just to get the technique down.

This is for a home sewer.  If you commercially made tents you’d probably want to have a machine that sews a flat felled seam in one operation.  Laser cut the fabric and do everything relative to the edge of the fabric…  Time is money.  I don’t really know about this though.

I think a true flat felled seam is theoretically stronger than a faux flat felled seam, but in practice other things will cause the tent to fail before this.

[Alex N]

Thanks Jerry for taking so much time to respond.

Its good to know that those inaccuracies will likely only mean aesthetic issues, although I suppose I was hoping you would say “1cm here and there won’t make a bit of difference”!

I’ve read about the faux flat felled seam from you and others on this forum so I did try that at first. The problem I had was layers slipping (as you suggested) and whilst pinning did work I needed a lot of pins and found it cumbersome. The basting tape on the other hand seems very easy and completely prevents the layer slippage (though the water-soluble tape is then left inside the seam). Once the two layers were taped together I actually found it easier to just roll the seam over and do a flat felled seam rather than a faux seam, and so since that seam was also stronger, I chose that as the way forward.

But it does have the drawback of multiple tolerance issues (the cut and the seam itself) whereas the method you propose would seem to only have one error source and so that might be a better solution!

Thanks so much!

Alex

[Jerry Adams]

what I like about BPL is many techniques mentioned.  A reader can try what works for them.

In order to avoid slipping I’ll have a hand stitch every couple feet.  When I get to one hand stitch, I’ll pull the fabric by the next hand stitch a couple feet out, then pick an intermediate point and squeeze together with other hand fingers to align it, then sew up to that point, repeat.  It takes time to do more hand stitches so I do as few as necesary.

[Alex N]

Well to follow up on this, having now sewn a couple of 2m long thin triangles together to make one of the roof panels I can certainly see more problems with the flat felled seam. I ended up taping AND pinning. On my practices I was sewing together 1m long strips, but this didnt simulate all the additional issues I would have forming the seam with much larger panels.

The two panels I have made up so far are OK, but the seam width isn’t as consistent as I would like and a little bit of puckering actually shortens the panel by 25mm (1inch) over that 2m length, although a small amount of tension does get it back to full length.

I’m now wondering just how much more challenging this is going to get as the panels get larger! (particularly doing the final seam!)

[Jerry Adams]

try sewing together two strips of scrap fabric 2 meters long

mark the beginning and end to make sure one isn’t slipping

repeat several times

[Alex N]

Yeah the shortening is on both pieces due to a slight puckering I’ve dropped the thread tension right down and I’m applying tension to the material throughout, which does help.

Fortunately the basting tape completely solves the issue with panel alignment

[Roger Caffin]

I use pinning in the seam allowance (lots of pins) and good lengthwise tension while sewing.
More work, but I have never had any problems that way.
I haven’t tried basting tape because I found it does not stick to silnylon very much.

Cheers

[Alex N]

Thanks Roger – yes it doesnt stick brilliantly, but it helps a lot when it comes to slippage. I’ve ended up using more and more pins the more seams I do (I have started with the easiest ones so that I can get better) but honestly I think doing a proper flat felled seam was a bad idea and a faux felled seam probably makes more sense with the volume of fabric I am dealing with!

As for tension, that definitely helps, but its also a major skill doing it well, a skill I have yet to acquire!

 

[Jerry Adams]

“but its also a major skill doing it well”

that’s what I’ve found – after a couple projects I’ve perfected it more

you could shortcut that a bit by sewing 2 meter strips of scrap fabric

lengthwise tension is the key

[Diane “Piper” Soini]

Wonder clips or small binder clips are also really helpful. Pins introduce some of the imprecision. Clips can hold without deforming. Doesn’t work everywhere always.

[Roger Caffin]

wonder clips
I learn something new every day.
Thank you Diane.

[Tim (Slowhike) Garner]

A side question, I am not familiar with basting tape but it looks like a helpful thing.

These days I’m mostly sewing small projects with Dyneema but soon hope to make another hammock using 1.8 Airwave Hybrid Nylon Ripstop.

I had glanced at basting tape at some point but thought it might have sticky stuff that would cause problems with the needle, but apparently not.

So, I’m wondering what types of basting tape would work best for the 1.8 Airwave Hybrid Nylon Ripstop?

Sewing Dyneema is easy peezy.

[Roger Caffin]

sticky stuff that would cause problems with the needle
I found it gums the needle up something awful. Hard to clean.

Cheers

[Tim (Slowhike) Garner]

Thanks Roger.

[Bill Budney]

I’m just a padawan at sewing, but I’ve been using 1/4″ basting tape from SailRite. It is narrow enough that I don’t sew through it.

[Tim (Slowhike) Garner]

Good point Bill.

 

[Roger Caffin]

Bill
That 1/4″ tape. Just wondering. If you are not sewing thru it, could you peel it off later?

Cheers

[Bill Budney]

Perhaps, assuming that you don’t sew it into a seam, and the fabric is strong enough. I have pulled it off of Alpha Direct (to reposition it). I would say that it slightly compromised the fabric, but it was within the seam allowance so I didn’t care.

Dritz makes wash-away 1/4″ tape that dissolves in the laundry. I have a roll of that but have not tried it yet.

 

[Alex N]

The basting tape I am using is 5mm and I don’t sew through it. I’m going to switch to faux felled seams, the precision is far greater. For those seams I can use the basting tape to perfectly position everything and prevent slippage for the first stitch. Then I just peel it off, its massively helpful to me and the adhesion is just enough to be useful

[Roger Caffin]

Interesting.
Photos?

Cheers

[Bill Budney]

Here are some tips from SailRite on using basting tape. Some highlights:

• Use narrow tape at edge of fabric to avoid sewing through the tape.
• Use wider tape if you want to sew through for water resistance. (The tape is slightly self-healing).
• They recommend against trying to remove the tape.
• A gummed needle can be cleaned with alcohol on cotton swab (or adhesive remover).
• Some tightly woven fabrics self-clean the needle so that it doesn’t need to be cleaned as often as with knit or loosely-woven fabrics.
• They have four different tapes with different adhesives:
  • Regular (Sailmaking and Vinyl) has less adhesive, for lighter fabrics and less gumming.
  • Canvas for better adherence to coated fabrics.
  • Mylar for DCF and other laminates with an exposed mylar side.
  • 3M™ Super Seamstick for even more aggressive stick. Can replace stitching for mylar-on-mylar bonds.

SailRite has many tutorials and videos about various techniques and tips. They regularly use seam tape, which is where I got the idea. Tape was the only way that I could make a successful seam on Alpha Direct (probably because I’m a novice).

 

[Moab Randy]

Alex, If you can get basting tape to stick to your fabric you are way ahead. I’ve been working with sil/sil silnylon and nothing sticks to that, not Sailrite, not Dritz. If you can baste, I don’t see why you would not want to do the true flat felled seam and not build your tent with something “faux.”

Fabric “shortening”: it’s the stitching itself that bunches up fabric slightly. (Example: if you want to sew a curved zipper, you can pre-curve it by first sewing a stitch line on the zipper tape.) The fewer stitches per inch the less shortening. Stretching tension as you stitch, as mentioned, also helps a bit to minimize.

Fabric slippage (top vs. bottom): I truly don’t believe any practical amount of pinning or clips will solve this problem; you’ll just periodically be sewing over small rucks. First, use a roller foot for better results. Second, yes you have to pull some tension as you sew. Third, if I’m sewing a long seam, I usually start in the middle and sew to the outer end, then switch and sew the other direction for the other half. At least then any amount of inequality will be distributed to both ends instead of all on one end. Either way, these home sewing projects are never going to be precise like a factory job and you have to allow for adjustments on the fly. Fabric panels are never cut out to plan (or if they are, how did you do it??? Is someone computerized laser cutting this for you on a giant table?). On a tent as large as you are making, 5mm tolerances will be plenty precise.

 

[Alex N]

To answer your question Randy, the basting tape works well enough to completely eliminate slippage, but not enough to reliably hold the material together. So as I bunch the fabric up the two sides can separate, and pins (lots of them) are needed. But as the material gets close to the needle the two sides are sticking together to the point that the sheer load is carried between the two layers and so there is absolutely no slippage. So for a flat felled seam the width of the seam is just too hard to keep consistent. You are completely reliant on a lot of pins and really accurate folding (though for sure the basting tape makes it dramatically easier, because otherwise I dont think I would even be able to do the folds because the silpoly doesnt hold the folds well at all). My process for the flat felled seams ended up as:
1: Mark on panel 1 where the edge of panel 2 needs to go
2: Apply basting tape to panel one between the marked line and the edge.
3: Place second panel on top in the exact right position.
4: Pin to make sure the two layers do not come apart.
5: Fold the seam removing and replacing pins (at far more regular intervals as I go)
6: Sew

This process was just far too labour intensive and fiddly for long seams and big panels requiring lots of bunching (even though on my 1.5m seams with very narrow strips it was very easy and precise).

So I tried the faux felled seam which is far far easier and infinitely more precise because the first line of stitches determines the panel size so the tolerances are much smaller. The basting tape is incredibly useful for this process as well and can be completely removed. Below is the process I am using at the moment and it seems fairly manageable to make a clean seam even with my incredibly cheap home sewing machine!

As for the shortening. I think I have gone as far as I can with my machine in trying everything, including stitch length, smaller sharper needles etc. I’m now using a sew-all thread because it seems plenty strong enough to me and with tension it almost eliminates the shortening.

My view is the same as your regarding slippage. Without the tape I would as you say be sewing over rucks no matter how many pins I use. I also find the fabric so slippy that placing pins alone is a skill in itself! Yes I have wondered about a roller foot of even splashing out on a walking foot, but now that slippage issue is reduced to the seam itself, which I think makes it purely aesthetic, so I will ignore it and carry on with the tape!

“Third, if I’m sewing a long seam, I usually start in the middle and sew to the outer end, then switch and sew the other direction for the other half. At least then any amount of inequality will be distributed to both ends instead of all on one end. ”

This is a very good idea, which could come in handy if the tape isnt playing ball!

Good to know you think 5mm tolerances won’t matter much. I think I just have to crack on and see how it comes out. I just wish I hadnt tried the flat felled seams initially because four of the 8 roof panel triangles are now a bit innaccurate, but you live and learn and I don’t want to waste the material!

Thanks everyone for your help!

[Alex N]

Oh and as an aside, I know many people have said that using the faux-felled seam as above might have a problem with hole elongation under stress creating a potential leak point. I think that might be true to a small extent (though you do have to load the seam very heavily). But it obviously depends on the primary load path direction, and this particular ‘yurt’ style design has most forces aligned with the seam direction, and almost all of the seams on the roof are aligned. So it then becomes a bit of a moot point. If I were to choose a seam for a ridge of a highly loaded tarp I might be more inclined to consider something else, but obviously even in that scenario people on this forum have used the faux-felled seam to great success.

 

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