Tutorial to make your own bag - one piece, no seam on bottom

Post #1 made 4 years ago
hello all!

I just decided to move toward all grain, and decided on the beer in a bag (BIAB) method. So I bought a Tallboy 8 gallon pot, but needed a bag. Although there is a lot of great information online, and a lot on here, I found that it was quite hard to piece it all together, and took me a while to figure out how to make the bag I wanted. So, I will condense what I learnt into a tutorial below on how to make a bag as I did.

This is an easy but time consuming project. I had no idea how to sew, so started reading my partner's machine's manual. I learnt what a bobbin is, and that needles don't thread themselves. So, if you are a complete beginner, you can do this. Also, this entry is VERY long because I didn't want to leave anything out. Don't let the length fool you.

My bag is a single piece bag with no seam on the bottom so there is less pressure there. Basically, imagine that you are making a box with no top. However, since it's cloth, the bag shape molds to fit the round pot, AND more importantly, the bottom sinks out to add height - the grain sits here, NOT on a seam!
My technique is based off of one I found online, but I did the basic math to help you make it fit your pot (I copied from http://imgur.com/a/upDJ6 but added a lot more details!) The bag should completely line the pot with a lot of extra wrinkly bag kicking around. Not tight, and, not hanging in the pot.

You need a pot. Measure your pot and jot down the height and circumference. In all of your measurements you probably want to be a little liberal as more room is better than not enough. Can calculate the circumference from the diameter by multiplying by 3.14. BE SURE to measure the widest part, e.g., the lip on the kettle, because the bag has to go over this.

You will need a square piece of cloth. In this technique, once you have a square, you can change your measurements to make the bag deeper (thus make it narrower circumference) or make the bag have a bigger circumference (but make the bag shorter), but if the square is too small to start with there is only so much you can shift things. For example, if you make your circumference bigger but the bag is too short you should have started with a bigger piece of cloth. I will talk more about this later, but basically work through the following simple calculations. I am working in CM but you can change all to inches and it all should work.

We have three sets of measurements
- the measurements of your pot
- the target height and circumference of the bag we will make
- the measurements of the "box" we will make that will sag into a bag. This is important to setup our cloth and cutting.
We also have to build in seam allowance - wasted space to sewing. I am liberal here (you should be), as it makes life easier later when you sew. trust me. I have my seam allowance at 4cm. Take more if you can and find out you have enough fabric.

SeamAllowance (SA) = 4

PotHeight (PH) in cm
PotCircumfrence (PC) in cm

<targets - what we want>
BagHeight (BH) = PH + 6cm (drawstring allowance) + 20cm (bag overhang on pot allowance)
BagCircumference (BC) = PC + 10 (for good measure and working room)

<box dimensions pre sewing>
box width (BxW) = BC/4 + 2*SA
box height (BxH) = BH - (BxW-2*SA)/4 (for those following, the /4 is an estimate of how much "height" the sagging adds, quarter the length)

square fabric width and height = 2*BxH+BxW

----Measurement example
Let me run through an example. My pot measured 40cm high and 110cm circumfrence

BH=40+6+20 = 66cm
BC=110+10 = 120cm
BxW=120/4+2*4 = 38cm
BxH=66-(38-8)/4 = 58.5cm
required fabric square = 2*58.5+28 = 155cmx155cm square.

NOTE: my store carries fabric in 1.5M widths so I could not make this work! So what do I do? Oh no! I need to drop this somehow to 150x150 square. You can basically make two adjustments: the width of the box - determines the circumference of the bag - or the height of the box - determines the height of the bag. In my case, I took a piece of string, measured it, considered that circumference. I was worried about it so I dropped my BxH to 52cm since it seemed a lot.

so now required square = 2*52+38 = 142cm, which gives me some headroom. If you really do need a bigger square, make one by making a rectangle and sewing a piece on using the sewing ideas below. This will create a seam but will still keep it off the bottom of the bag.

MAKE sure to keep these box measurements as they will tell you where to cut your fabric.

If you decide to eyeball it and skip these measures - LIKE I DID THE FIRST TIME - you'll end up with a bag with a ridiculously large circumference and very shallow which doesn't fit well, or the opposite.. The first one is fixable, go back and read what I wrote.

So now that you have a bag plan you need to go shopping.
  • Fabric - look for Voile. SUPER FINE MESH makes your window screen look wide open. VERY thin (shear). Often in curtain section but ask someone working they can help you. Warning: Voile is cheap, Voille is expensive lace. They may get confused.
  • Fabric - get polyester. Nylon probably is fine (according to forums) but polyester has a higher melting point and is more robust against rot. If you have to get nylon, not a biggie, but prefer polyester
  • Fabric - get more than you think you need. An extra few metres of fabric is probably cheaper than an extra trip and time. AND you can use the leftovers for a bunch of stuff.
  • Thread - get polyester thread. I bought the cheap regular stuff - heavy duty can be expensive. I went through three spools after errors, etc., and its cheap so get a lot.
  • String - for your draw string. Cheap so get like 4x what you think you need. Useful for making hop socks
  • "webbing" (a.k.a. seat belt material) for your lifting strap and support. I got 1" wide. Buy like 4m or more. I bought nylon because polyester at my store was $4 a meter!!
you're set!

------ CUTTING
PRACTICE ON A PIECE OF PAPER FIRST to be sure it works as planned.

First you need to make a square. Fold the cloth into a triangle and line it up, and cut it off. When unfolded, makes a square
Folding: (as per images)
fold square on the diagonal to make a triangle.
Do it again (keep all the "open" ends on the same side)
do it again (all "open" ends on the same side).
You did three folds. Practice with paper.
Now, conveniently, you can get away with one cut. You will cut inward from the single fold edge (all folds together) with the open ends down, as shown.
Now that you've done it with paper, let's look at how to do it with fabric to your measurements. I found "binder clips" to be super helpful as this fabric is nasty and slippery.
Now, with cloth, you need to be careful where you cut. If you (and I!) did your calculations right, you should measure again in from the main fold with the open ends toward you. Measure in half your box width. If you go straight up from here you should have your box height. I used a big ruler to help me get this straight, and clamped with binder clamps. You can only line this stuff up so well, just do your best and don't sweat it too much if it's a bit off.
Measure twice! Mark it off! cut! When I did this the fabric slipped all over the place and I got a messy cut. In the end it didn't matter, just rough is fine.

Unfold (did you practice on paper first?) You should have the following shape. Now you are ready to sew!! NOTE: my shape as shown here was way off and my circumference was massive. I had to go back and do the math and re-cut. Yours will likely have different ratios.
Also, note that you cut out nice squares that are good for making hop socks! Great!
Now is a good time to check the bag in the pot as a sanity check. My bag shown here was WAY off as I mentioned. Too wide around. I went back, figured the math out, and re-did the bag. Don't tell anyone that I didn't notice until sewing 4 sides...

---- Sewing
This fabric is nasty. I found my sewing machine pulled so hard it pulled holes in it. I had to use a super low tension. Also, I used a decorative wide stich on my machine for strength. If you don't have one may want to double up seams. SEAMS come undone! google how to finish a seam. I mixed reversing over it, going over twice, tying ends. etc. I have no idea what works. I hope my bag doesn't unravel.

Grab a scrap of cloth and play with your machine's stich settings until it's reasonable.
The first phase is to make the box. This is where we hit our first problem. If your fabric is like mine, it RUNS like crazy:
This scares me. Why do all this work if it'll unravel in a year!? So we have a plan that fit's in well with making REALLY strong seams. ALSO, we'll have a smooth seam inside the bag so no bits of milled grain can get caught anywhere. A win all around.

We will make two seams side by side, with the unraveling end still open. THEN, we'll fold the fabric over itself and sew a third seam to tuck those away.

First seam - leave a lot of room on the outside (>2cm?) as this will make your life easier. Trust me.
Do the first seam on all four corners of the box. Now is a good time to toss it into your pot to make sure it fits OK. You will add more
seams so it will get a little smaller but not much.

Second seam - can be closer to the first, but on the inside of the bag. This is the easy part. Mine was about 2cm apart from the first, maybe less.
Third seam- FOLD the outside over in almost a full circle so that the unraveling end is tucked away. This is probably the hardest part
of the job but it'll get easier after the first 20cm or so.
One problem with this - the bottom corners. If you were not careful (like I wasn't) then you have little pockets where grain can get caught in, a pain to clean. Sew the bottom corners to close this off
Yay! You have a bag! now do the top of the bag. Again you want no rough ends. Particularly in the draw string, string moving against those edges will fray like crazy
- sew a lip on the bag, outside. That is, fold the fabric over itself just a little bit and sew it closed so the edge is hidden. I used a single seam here instead of the broad decorative one.
- now you have a lip, fold it OUTSIDE again, probably a good 2 or 2.5 cm over itself. You are making a pocket for the drawstring. Sew this all the way around the bag. ALMOST! I actually sewed it shut so I couldn't put string in. Don't sew all the way around, stop about 2cm from the end so you can get string in there.
- use a knitting needle, tape your string to the needle, and thread it through the hole. You have a draw string!
Next, you need your lifting strap. This is easy. Put it right below the draw string. I put a wide decorative stitch through the center of the webbing ALL the way around. Be careful with the ends. Again you don't want them to fray, so tuck them over and sew as shown. I then added 2 single-stitch seams at the top and bottom of the strap. That is, it is on with three seams now. The two single seams are nearly impossible to see, but they're in the bottom part of the pic.
Now, put on the lifting handles. I put them on with 26cm length but you can choose what you want. I did a square with a diagonal, again being careful to tuck the fraying ends away.
WHEW you are done! Wash the bag well before using.

Well, before you put all the equipment away, go make hop socks from those squares you cut out to make the bag. You will thank me later! I made mine with drawstrings and all, now that I'm a sewing pro ;)
Last edited by jimyoung999 on 26 Sep 2014, 21:23, edited 1 time in total.

Post #5 made 4 years ago
Thanks for the feedback! I actually seriously considered just cutting out a single piece of fabric and finishing the end, but the more I read and saw, I just wanted to try to see if I could do something more challenging :) I am happy to hear that the simpler bag is working out. I'll also say that, after using this one, the straps are not as necessary as I expected, either. And, with a draw string on the one-piece bag, it can help it to keep shape even with the bunching.

Post #6 made 4 years ago
jimmy, welcome to the forum :salute:,

To me that is a beautiful, clever, very elegant design. Please post pics of the bag in action and I'm sure the admin here will post a link to this thread in one of the main stickies.

I'm trying to catch up tonight on forum threads so haven't time to read every single word above but from what I have read, you have certainly made my day/week. It is very rare a new member will come aboard and give information. Most new members come here for information and that is all good and fine but to see an new member starting a thread like this, I think, is a real treat.

:thumbs: :clap: :salute: :champ:,
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